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The structure of aqueous ammonia solutions is investigated through classical molecular dynamics (MD) and ab initio molecular dynamics (AIMD) simulations. We have preliminarily compared three well-known classical force fields for liquid water (SPC, SPC/E, and TIP4P) in order to identify the most accurate one in reproducing AIMD results obtained at the Generalized Gradient Approximation (GGA) and meta-GGA levels of theory. Liquid ammonia has been simulated by implementing an optimized force field recently developed by Chettiyankandy et al. [Fluid Phase Equilib. 511, 112507 (2020)]. Analysis of the radial distribution functions for different ammonia concentrations reveals that the three water force fields provide comparable estimates of the mixture structure, with the SPC/E performing slightly better. Although a fairly good agreement between MD and AIMD is observed for conditions close to the equimolarity, at lower ammonia concentrations, important discrepancies arise, with classical force fields underestimating the number and strength of H-bonds between water molecules and between water and ammonia moieties. Here, we prove that these drawbacks are rooted in a poor sampling of the configurational space spanned by the hydrogen atoms lying in the H-bonds of H2O⋯H2O and, more critically, H2O⋯NH3 neighbors due to the lack of polarization and charge transfer terms. This way, non-polarizable classical force fields underestimate the proton affinity of the nitrogen atom of ammonia in aqueous solutions, which plays a key role under realistic dilute ammonia conditions. Our results witness the need for developing more suited polarizable models that are able to take into account these effects properly. [ABSTRACT FROM AUTHOR]

Conducting a comprehensive molecular-level evaluation of a photoacid generator (PAG) and its subsequent impact on lithography performance can facilitate the rational design of a promising 193 nm photoresist tailored to specific requirements. In this study, we integrated spectroscopy and computational techniques to meticulously investigate the pivotal factors of three prototypical PAG anions, p-toluenesulfonate (pTS−), 2-(trifluoromethyl)benzene-1-sulfonate (TFMBS−), and triflate (TF−), in the lithography process. Our findings reveal a significant redshift in the absorption spectra caused by specific PAG anions, attributed to their involvement in electronic transition processes, thereby enhancing the transparency of the standard PAG cation, triphenylsulfonium (TPS+), particularly at ∼193 nm. Furthermore, the electronic stability of PAG anions can be enhanced by solvent effects with varying degrees of strength. We observed the lowest vertical detachment energy of 6.6 eV of pTS− in PGMEA solution based on the polarizable continuum model, which prevents anion loss at 193 nm lithography. In addition, our findings indicate gas-phase proton affinity values of 316.4 kcal/mol for pTS−, 308.1 kcal/mol for TFMBS−, and 303.2 kcal/mol for TF−, which suggest the increasing acidity strength, yet even the weakest acid pTS− is still stronger than strong acid HBr. The photolysis of TPS+-based PAG, TPS+·pTS−, generated an excited state leading to homolysis bond cleavage with the lowest reaction energy of 83 kcal/mol. Overall, the PAG anion pTS− displayed moderate acidity, possessed the lowest photolysis reaction energy, and demonstrated an appropriate redshift. These properties collectively render it a promising candidate for an effective acid producer. [ABSTRACT FROM AUTHOR]

Spectroscopic exploration of sulfur-centered hydrogen bonding involving a thiol group (S–H) as the hydrogen bond donor is scarce in the literature. Herein, we have investigated 1:1 complexes of 2-fluorothiophenol (2-FTP) with methanol (MeOH) and ethanol (EtOH) in the gas phase to examine the physical characteristics and strength of the S–H⋯O hydrogen bond. Structures, conformations, and the strength of the S–H⋯O interaction are investigated by measuring the electronic and Infrared (IR) spectra of the two complexes employing resonant two-photon ionization, UV–UV hole-burning, and IR–UV double resonance spectroscopic techniques combined with quantum chemical calculations. Three conformers of 2-FTP⋯MeOH and two conformers of 2-FTP⋯EtOH have been detected in the experiment. A comparison of the IR spectra obtained from the experiment with those of the low-energy conformers of 2-FTP⋯MeOH and 2-FTP⋯EtOH predicted from the theory confirms that all the observed conformers of the two complexes are primarily S–H⋯O hydrogen bonded. The IR red-shifts found in the S–H stretching frequencies in 2-FTP⋯MeOH and 2-FTP⋯EtOH concerning that in 2-FTP are ∼76 and ∼88 cm−1, respectively, which are much larger than that was reported earlier in the 2-FTP⋯H2O complex (30 cm−1). The strength and physical nature of different noncovalent interactions, including the S–H⋯O hydrogen bond existing in the complexes, are further analyzed using natural bond orbital analysis, quantum theory of atoms in molecules, and localized molecular orbital-energy decomposition analysis. The current investigation reveals that the S–H⋯O hydrogen bond can be strengthened by judicial choices of the hydrogen bond acceptors of higher proton affinities. [ABSTRACT FROM AUTHOR]

The current opinion manuscript posits that not only Piezo2 voltage block, but also proton affinity and availability in relation to Piezo2, a mechanically gated ion channel, may count in the mediation of pain and its sensitivity. Moreover, this paper argues that autonomously acquired Piezo2 channelopathy on somatosensory terminals is likely the initiating peripheral impaired input source that drives the central sensitization of spinal nociceptive neurons on the chronic path as being the autonomous pain generator. In parallel, impaired proprioception and the resultant progressive deficit in neuromuscular junctions of motoneurons might be initiated on the chronic path by the impairment of the proton-based ultrafast proprioceptive feedback to motoneurons due to disconnection through vesicular glutamate transporter 1. The irreversible form of this autonomously acquired Piezo2 ion channel microdamage, in association with genetic predisposition and/or environmental risk factors, is suggested to lead to progressive motoneuron death in addition to loss of pain sensation in amyotrophic lateral sclerosis. Furthermore, the impairment of the proton-based ultrafast long-range oscillatory synchronization to the hippocampus through vesicular glutamate transporter 2 may gain further importance in pain modulation and formation on the chronic path. Overall, this novel, unaccounted Piezo2-initiated protonic extrafast signaling, including both the protonic ultrafast proprioceptive and the rapid nociceptive ones, within the nervous system seems to be essential in order to maintain life. Hence, its microdamage promotes neurodegeneration and accelerates aging, while the complete loss of it is incompatible with life sustainment, as is proposed in amyotrophic lateral sclerosis. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Chinese Mass Spectrometry Society is the property of Journal of Chinese Mass Spectrometry Society and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The properties (geometry, bond-dissociation enthalpies and proton affinities) of three azoles, pyrrole, imidazole and pyrazole, with twenty-two N-substituents R covering a significant part of the periodic table [1 (lithium group, alkaline), 2 (beryllium group, alkaline earth), 13 (boron group, triel), 14 (carbon group, tetrel), 15 (nitrogen group, pnictogen), 16 (oxygen group, chalcogen) and 17 (fluorine group, halogen) of the periods 2, 3 and 4 plus the hydrogen] have been calculated with the G4 composite ab initio method. These three properties were discussed with regard to the azole and to the group R using as model compound the amines H2N–R. The large set of compounds and their consistency allowed finding many equations that related different calculated properties. General properties such as bond-dissociation enthalpies, BDH, N–R bond lengths and proton affinities were tested in search of simple equations that explain the calculated properties. [ABSTRACT FROM AUTHOR]

Natural ion‐transporters in cellular membranes play a critical role in maintaining cell homeostasis. Synthetic ion‐transporters are attractive model systems for understanding and addressing dysfunction of natural transporters. Herein, a simple amide derived from azobenzene and m‐aminobenzoic acid achieves photoregulated ion transport across lipid membranes. The amide forms pores or channels that selectively co‐transport H+/X− across the lipid membrane. Photoisomerization from the trans to cis form results in a 2‐fold increase in ion‐transport rates due to the higher proton affinity of the cis isomer. [ABSTRACT FROM AUTHOR]

DFT calculations were carried-out for some of novel designed structures with basicity and superbasicity potentials. In this research, two series of cyclic phosphorus-containing compound were designed inspiring from 1,3,5,7-tetraazaadamantane and 1,4-diazabicyclo[2,2,2]octane. The proton affinity (PA) in the gas phase was calculated for each of the structures 1–14. Most of the designed structures were found to have higher basicity than that of 1,8-bis(dimethylamino) naphthalene (DMAN) as the threshold of superbasicity (3, 4, 7, 10–14). The effect of heteroatom and substituents type in the ring as well as of internal bond angles on the proton affinity were evaluated. It was found that the structure 14 possesses highest PA value of 1198.27 kJ mol−1 in the gas phase. According to the obtained results, the PA values were significantly amplified by substitution with electron releasing groups on the molecular framework. Also, the limitations created by the ring for the electron donation of the substitutents in the neighborhood of the main phosphazene group are discussed. [ABSTRACT FROM AUTHOR]

Rationale: Compounds like caffeine metabolites with more than one proton acceptor site can produce a mixture of isomeric protonated ions (protomers) in electrospray ionization and atmospheric pressure chemical ionization (APCI) ion sources. Discrimination between the protomers is of interest as the charge location influences ion structure and chemical and physical properties. Methods: Protonation of caffeine in an APCI ion source was studied using ion mobility spectrometry. The hydronium ions, H3O+(H2O)n, are the main reactant ions in the APCI ion source. Different dopant gases including NO2, NH3, and CH3NH2 were used to produce new reactant ions NO+, NH4+, and CH3NH3+, respectively. Density functional theory was employed to explain the experimental results and calculate the energies of the ionization reactions. Results: The ion mobility spectrum of caffeine showed three peaks. In the presence of NO2 dopant and NO+ reactant ion, caffeine was ionized via charge transfer and formation of M+ ion. As NH3 and CH3NH2 are stronger bases than H2O, the reactant ions NH4+ and CH3NH3+ selectively protonated the more basic site of caffeine, that is, the imidazole nitrogen. Using these dopants, we could attribute the first ion mobility peak to M+ ion, the second peak to the protonation of caffeine at the carbonyl oxygen atom, and the third peak to the protonation of the imidazole nitrogen atom. The calculated collisional cross‐sections of M+ and the protomers of caffeine confirmed the peaks' assignment. Conclusions: The criterion for the selection of an appropriate dopant is that its proton affinity (PA) should be between those of the proton acceptor sites of the molecule studied. [ABSTRACT FROM AUTHOR]

Human voltage-gated proton (hHv1) channels are crucial for regulating essential biological processes such as immune cell respiratory burst, sperm capacitation, and cancer cell migration. Despite the significant concentration difference between protons and other ions in physiological conditions, hHv1 demonstrates remarkable proton selectivity. Our calculations of single-proton, cation, and anion permeation free energy profiles quantitatively demonstrate that the proton selectivity of the wild-type channel originates from its strong proton affinity via the titration of the key residues D112 and D174, although the channel imposes similar kinetic blocking effects for protons compared to other ions. A two-proton knock-on model is proposed to mathematically explain the electrophysiological measurements of the pH-dependent proton conductance in the conductive state. Moreover, it is shown that the anion selectivity of the D112N mutant channel is tied to impaired proton transport and substantial anion leakage. [ABSTRACT FROM AUTHOR]

The proton affinity (PA) and gas phase basicity values of previously reported cyclic allene series are investigated using high-level quantum chemical calculations. Some of the studied structures possessed more than one protonation site. All protonation types were explored for each structure and the compounds with PA value more than ≈1030 kJ mol−1 were considered superbasic structures. Cyclic allene's basicity strength was affected by ring size, adjacent heteroatom's presence and resonance existence in protonated form. [ABSTRACT FROM AUTHOR]

Norpinonic acid has been known as an important α -pinene atmospheric secondary organic aerosol (SOA) component. It is formed in the reaction of α -pinene, β -pinene or verbenone with atmospheric oxidizing reagents. In the presented study, tandem mass spectrometry techniques were used to determine the exact norpinonic acid fragmentation pathway in the gas phase. The precursor anion – deprotonated norpinonic acid (m/z 169), generated in an electrospray (ESI) source – was introduced into the collision cell of the mass spectrometer and fragmented using the energy-resolved collision-induced dissociation (ER-CID) technique. The experimental energy values of degradation processes were determined via analysis of the breakdown curves. Quantum chemical calculations of the reaction models were also constructed, including calculation of all transition states. Comparison between the experimental and the theoretical threshold energies calculated at a ω B97XD/6-311 + G(2d,p) theoretical level has shown a good correlation. Two basic pathways of the fragmentation of the parent anion [M-H]- (m/z 169) were observed. Firstly this leads to the decarboxylation product (m/z 125) and secondly to the loss of a neutral molecule (C4H6 O), together with the formation of the anion m/z 99. On the other hand, the breakdown of the anion m/z 125 gives rise to the m/z 69, 57 and 55 ions. To confirm structures formed during ER-CID experiments, the gas-phase proton transfer reactions were examined of all norpinonic acid anionic fragments with a series of neutral reagents, characterized by proton affinity (PA) values. Based on PA difference analysis, the most possible chemical structures were proposed for the observed fragment anions. [ABSTRACT FROM AUTHOR]

Rationale: With the development of matrix‐assisted laser desorption/ionisation (MALDI) mass spectrometry (MS) in spatial localisation omics research on small molecules, the detection sensitivity of the matrix must increase. However, the types of matrices suitable for detecting acidic small molecules in (−) MALDI‐MS mode are very limited and are either not sensitive enough or difficult to obtain. Methods: More than 10 commercially available benzimidazole and benzothiazole derivatives were selected as MALDI matrices in negative ion mode. MALDI‐MS analysis was performed on 38 acidic small molecules and mouse serum, and the matrix effects were compared with those of the common commercial matrices 9‐aminoacridine (9AA), 1,5‐naphthalenediamine (DAN) and 3‐aminoquinoline (3AQ). Moreover, the proton affinity (PA) of the selected potential matrix was calculated, and the relationships among the compound structure, PA value and matrix effect were discussed. Results: In (−) MALDI‐MS mode, a higher PA value generally indicates a better matrix effect. Amino‐substituted 2‐phenyl‐1H‐benzo[d]imidazole derivatives had well‐defined matrix effects on all analytes and were generally superior to the commonly used matrices 9AA, DAN and 3AQ. Among them, 2‐(4‐(dimethylamino‐phenyl)‐1H‐benzo[d]imidazole‐5‐amine (E‐4) has the best sensitivity and versatility for detecting different analytes and has the best ability to detect fatty acids in mouse serum; moreover, the limit of detection (LOD) of some analytes can reach as low as ng/L. Conclusions: Compared to 9AA, DAN and 3AQ, matrix E‐4 is more effective at detecting low‐molecular‐weight acidic compounds in (−) MALDI‐MS mode, with higher sensitivity and better versatility. In addition, there is a clear correlation between compound structure, PA and matrix effects, which provides a basis for designing more efficient matrices. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Basrah Researches (Sciences) is the property of Republic of Iraq Ministry of Higher Education & Scientific Research (MOHESR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract The study investigates the molecular structure of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives in the gas phase using B3LYP and M06-2X functional methods. Intermolecular interactions are analyzed using natural bond orbital (NBO) and atoms in molecules (AIM) techniques. NO2-substituted TEMPO displays high reactivity, less stability, and softer properties. The study reveals that the stability of TEMPO derivatives is mainly influenced by LP(e) → σ ∗ electronic delocalization effects, with the highest stabilization observed on the oxygen atom of the nitroxide moiety. This work also considers electron density, atomic charges, and energetic and thermodynamic properties of the studied NO radicals, and their relative stability. The proton affinity and gas-phase basicity of the studied compounds were computed at T = 298 K for O-protonation and N-protonation, respectively. The studied DFT method calculations show that O-protonation is more stable than N-protonation, with an energy difference of 16.64–20.77 kcal/mol (22.80–25.68 kcal/mol) at the B3LYP (M06-2X) method. The AIM analysis reveals that the N–O…H interaction in H2O complexes has the most favorable hydrogen bond energy computed at bond critical points (3, − 1), and the planar configurations of TEMPO derivatives exhibit the highest EHB values. This indicates stronger hydrogen bonding interactions between the N–O group and water molecules.

The accurate description of nuclear quantum effects, such as zero-point energy, is important for modeling a wide range of chemical and biological processes. Within the nuclear–electronic orbital (NEO) approach, such effects are incorporated in a computationally efficient way by treating electrons and select nuclei, typically protons, quantum mechanically with molecular orbital techniques. Herein, we implement and test a NEO coupled cluster method that explicitly includes the triple electron–electron–proton excitations, where two electrons and one proton are excited simultaneously, using automatic differentiation Our calculations show that this NEO-CCSDTeep method provides highly accurate proton densities and proton affinities, outperforming any previously studied NEO method. These examples highlight the importance of the triple electron–electron–proton excitations for an accurate description of nuclear quantum effects. Additionally, we also implement and test the second-order approximate coupled cluster with singles and doubles (NEO-CC2) method as well as its scaled-opposite-spin (SOS) versions. The NEO-SOS′-CC2 method, which scales the electron–proton correlation energy as well as the opposite-spin and same-spin components of the electron–electron correlation energy, achieves nearly the same accuracy as the NEO-CCSDTeep method for the properties studied. Because of its low computational cost, this method will enable a wide range of chemical and photochemical applications for large molecular systems. This work sets the stage for a variety of developments and applications within the NEO framework. [ABSTRACT FROM AUTHOR]

In this review, the principles of gas-phase proton basicity measurements and theoretical calculations are recalled as a reminder of how the basicity PA/GB scale, based on Brønsted–Lowry theory, was constructed in the gas-phase (PA—proton affinity and/or GB—gas-phase basicity in the enthalpy and Gibbs energy scale, respectively). The origins of exceptionally strong gas-phase basicity of some organic nitrogen bases containing N-sp3 (amines), N-sp2 (imines, amidines, guanidines, polyguanides, phosphazenes), and N-sp (nitriles) are rationalized. In particular, the role of push–pull nitrogen bases in the development of the gas-phase basicity in the superbasicity region is emphasized. Some reasons for the difficulties in measurements for poly-functional nitrogen bases are highlighted. Various structural phenomena being in relation with gas-phase acid–base equilibria that should be considered in quantum-chemical calculations of PA/GB parameters are discussed. The preparation methods for strong organic push–pull bases containing a N-sp2 site of protonation are briefly reviewed. Finally, recent trends in research on neutral organic superbases, leaning toward catalytic and other remarkable applications, are underlined. [ABSTRACT FROM AUTHOR]

Brønsted basicity is a fundamental chemical property featured by several kinds of inorganic and organic compounds. In this Review, we treat a particularly high basicity resulting from the mechanical entanglement involving two or more molecular subunits in catenanes and rotaxanes. Such entanglement allows a number of basic sites to be in close proximity with each other, highly increasing the proton affinity in comparison with the corresponding, non-entangled counterparts up to obtain superbases, properly defined as mechanically interlocked superbases. In the following pages, the development of this kind of superbases will be described with a historical perusal, starting from the initial, serendipitous findings up to the most recent reports where the strong basic property of entangled molecular units is the object of a rational design. Brønsted basicity can be greatly enhanced by the mechanical entanglement of two or more interlocked molecular subunits within catenanes and rotaxanes. Here, the authors discuss the development of such mechanically interlocked superbases, and outline challenges and opportunities for future directions of research. [ABSTRACT FROM AUTHOR]

Context. The CO2-dominated thick atmosphere of Venus coexists with an ionosphere that is mainly formed, on the dayside, via the ionization of atmospheric neutrals by solar extreme ultraviolet and soft X-ray photons. Despite extensive modeling efforts that have reproduced the electron distribution reasonably well, we note two main shortcomings with respect to prior studies. The effects of pro-tonation and Coulomb interaction are crucial to unveiling the structure and composition of the Venusian ionosphere. Aims. We evaluate the role of protonated species on the structure of the dayside Venusian ionosphere for the first time. We also evaluate the role of ion-ion Coulomb collisions, which are neglected in many existing models. Methods. Focusing on the solar minimum condition for which the effect of protonation is expected to be more prominent, we constructed a detailed one-dimensional photochemical model for the dayside Venusian ionosphere, incorporating more than 50 ion and neutral species (of which 17 are protonated species), along with the most thorough chemical network to date. We included both ion-neutral and ion-ion Coulomb collisions. Photoelectron impact processes were implemented with a two-stream kinetic model. Results. Our model reproduces the observed electron distribution reasonably well. The model indicates that protonation tends to diverge the ionization flow into more channels via a series of proton transfer reactions along the direction of low to high proton affinities for parent neutrals. In addition, the distribution of O2+ is enhanced by protonation by a factor of nearly 2 at high altitudes, where it is efficiently produced via the reaction between O and OH+. We find that Coulomb collisions influence the topside Venusian ionosphere not only directly by suppressing ion diffusion, but also indirectly by modifying ion chemistry. Two ion groups can be distinguished in terms of the effects of Coulomb collisions: one group preferentially produced at high altitudes and accumulated in the topside ionosphere, which is to be compared with another group that is preferentially produced at low altitudes and, instead, depleted in the topside ionosphere. Conclusions. Both protonation and Coulomb collisions have appreciable impacts on the topside Venusian ionosphere, which account for many of the significant differences in the model ion distribution between this study and early calculations. [ABSTRACT FROM AUTHOR]

The study investigates the molecular structure of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and its derivatives in the gas phase using B3LYP and M06-2X functional methods. Intermolecular interactions are analyzed using natural bond orbital (NBO) and atoms in molecules (AIM) techniques. NO2-substituted TEMPO displays high reactivity, less stability, and softer properties. The study reveals that the stability of TEMPO derivatives is mainly influenced by LP(e) → σ∗ electronic delocalization effects, with the highest stabilization observed on the oxygen atom of the nitroxide moiety. This work also considers electron density, atomic charges, and energetic and thermodynamic properties of the studied NO radicals, and their relative stability. The proton affinity and gas-phase basicity of the studied compounds were computed at T = 298 K for O-protonation and N-protonation, respectively. The studied DFT method calculations show that O-protonation is more stable than N-protonation, with an energy difference of 16.64–20.77 kcal/mol (22.80–25.68 kcal/mol) at the B3LYP (M06-2X) method. The AIM analysis reveals that the N–O...H interaction in H2O complexes has the most favorable hydrogen bond energy computed at bond critical points (3, − 1), and the planar configurations of TEMPO derivatives exhibit the highest EHB values. This indicates stronger hydrogen bonding interactions between the N–O group and water molecules. [ABSTRACT FROM AUTHOR]

This study implements the full multicomponent third-order (MP3) and fourth-order (MP4) many-body perturbation theory methods for the first time. Previous multicomponent studies have only implemented a subset of the full contributions, and the present implementation is the first multicomponent many-body method to include any connected triples contribution to the electron–proton correlation energy. The multicomponent MP3 method is shown to be comparable in accuracy to the multicomponent coupled-cluster doubles method for the calculation of proton affinities, while the multicomponent MP4 method is of similar accuracy as the multicomponent coupled-cluster singles and doubles method. From the results in this study, it is hypothesized that the relative accuracy of multicomponent methods is more similar to their single-component counterparts than previously assumed. It is demonstrated that for multicomponent MP4, the fourth-order triple-excitation contributions can be split into electron–electron and electron–proton contributions and the electron–electron contributions ignored with very little loss of accuracy of protonic properties. [ABSTRACT FROM AUTHOR]

Thermodynamic and spectroscopic data on the formation and protonation of furans and their nitrile-disubstituted derivatives are missing from the literature. These data are very important in astrophysics and astrochemistry for the detection of new species in the interstellar medium (ISM). The discovery of these species in the ISM has not been reported. The formation of dicyanofurans and their protonation under ISM conditions of pressure and temperature were investigated using computational models (G2MP2, G3B3, G3 and G4). To identify the preferential site of protonation, the proton affinity (PA) was calculated at different sites of molecules. The enthalpy change (ΔrH), entropy change (ΔrS), and change in Gibbs free energy (ΔrG) of protonation were calculated at different temperatures (5 K, 10 K, 150 K and 298.15 K) at a pressure of 1 atm. The nitrogen atom of the furthest –CN functional group from the heteroatom was identified as the preferential site of protonation of dicyanofurans. On the basis of the negative values of ΔrH, ΔrS and ΔrG, spontaneous reactions producing these species under ISM conditions of temperature and pressure were suggested. The quadrupole hyperfine structures and vibrational spectra (essential tools for the characterization and identification of interstellar molecular species) were predicted in the region where brightest lines fall for different temperatures. We expect the results reported herein to assist astrophysicists and astrochemists in the discovery of new chemical compounds in the ISM environment. [ABSTRACT FROM AUTHOR]

Electronic structure and proton affinity of Schwesinger bases (Pn bases where n=1–4 indicates the number of phosphorous atoms) were studied using quantum mechanical calculations carried out at M06/def2‐TZVPP//BP86‐D3(BJ)/def2‐TZVPP level of theory. Each phosphorous atom is in the ligating environment of four tricoordinated/dicoordinated nitrogen atoms having one/two lone pairs. While the lone pairs on the tricoordinated/dicoordinated nitrogen atoms act as hyperconjugative donors, the P−N σ* orbitals act as hyperconjugative acceptors. These phosphazene bases are highly stabilized by synergetic hyperconjugative interactions which can accordingly be termed as counterpoise hyperconjugation. The number and the extent of pseudo‐π‐delocalization resulting from hyperconjugative interactions get enhanced as P1 changes to P4. Moreover, additional pseudo‐π‐delocalization starts to appear as P1 changes to P4. The phosphazene P2 has one Ndi(σ)−P−Ndi(σ)−P pseudo‐π‐delocalization which is similar to that of classical π‐delocalization in 1,3‐butadiene. On the other hand, phosphazene P3 has two pseudo‐π‐delocalization similar to that of 1,3‐butadiene and one pseudo‐π‐delocalization similar to that of 1,3‐pentadienyl cation. There are three pseudo‐π‐delocalization similar to that of 1,3‐butadiene and another three pseudo‐π‐delocalization similar to that in 1,3‐pentadienyl cation in P4. Consequently, pseudo‐π‐delocalization is present in all the tetrahedral symmetry planes of P4. Hence, P4 can be considered as a three‐dimensional pseudo‐π delocalized system. Even though the lone pair orbitals are stabilized by hyperconjugative interactions, they are susceptible to protonation. Among the dicoordinated and tricoordinated nitrogen atoms, the proton affinity of the dicoordinated nitrogen atom connected to the tBu group is much higher (257.4 kcal/mol–291.6 kcal/mol), and the proton affinity values increase on going from P1 to P4. The very high proton affinity of this dicoordinated nitrogen atom indicates its superbasic character. [ABSTRACT FROM AUTHOR]

Norpinonic acid has been known as an important α-pinene athmospheric SOA (Secondary Organic Aerosol) component. It is formed in the reaction of α-pinene, β-pinene or verbenone with atmospheric oxidizing reagents, such as ozone (O3) and hydroxy radicals. In the presented studies, tandem mass spectrometry techniques were used to determine the exact norpinonic acid fragmentation pathway in the gas phase. The precursor anion – deprotonated norpinonic acid (m/z 169) generated in an electrospray (ESI) source were subjected into the collision cell of the mass spectrometer and fragmented using CIE (Energy-Resolved Collision Induced Dissociation) technique. Based on the analysis of the breakdown curves, the experimental energy values required to initiate the gas – phase degradation processes were determined. Quantum chemical calculations of the reaction models for observed fragmentation processes were also constructed, including calculation of all transition states presented in the reaction mechanism. Comparison between the experimental and the theoretical threshold energies calculated at ωB97XD/6-311+G(2d,p) level of theory has shown a very good correlation. Two basic pathways of the fragmentation of the parent anion [M-H]− (m/z 169) were observed. A first, lead to the decarboxylation product (m/z 125) and second to the loss of neutral molecule (C4H6O) together with the formation of anion m/z 99. Loss of C3H6 or C2H4O molecules and formation of the anion m/z 41, together with anion m/z 55, were found for fragment anion m/z 99. Further breaks down of anion m/z 125 give a rise of 69, 57 and 55 m/z ions. To confirm structures formed during ER-CID experiments, the gas-phase proton transfer reactions were examined of all Norpinonic acid anionic fragments with a series of neutral reagents, characterized by different Proton Affinity (PA) values. It was found that only m/z 55 and m/z 69 anionic fragmentation products have higher PA values and accept the proton from all neutral reagents. Based on PA differences analysis, the most possible chemical structures were proposed for the observed fragment anions. [ABSTRACT FROM AUTHOR]

The detection of gaseous elements (He, Ne, Kr, and Xe) and molecules (CO2, CH4, and NH3) using inductively coupled plasma-tandem mass spectrometry (ICP-MS/MS) is described. In this study, sample gasses were introduced into a collision/reaction cell, and the sample gas components were ionised either by charge-transfer or by protonation reactions in the reaction cell through an ion reaction with the primary ions produced in the ICP. Ions originating from He and Ne, elements whose first ionisation potentials are greater than that of Ar, were not detected, whereas Kr and Xe were ionised through collision with Ar+. This implies that the ionisation can proceed basically by charge transfer reactions under equilibrium. The background counts for these elemental or molecular/polyatomic ions originating from either air engulfment into the ICP or from gas impurities in Ar can be effectively removed by the first quadrupole mass filter. The magnitude of softness/hardness of the ionisation can be controlled by selecting the primary ions (i.e., H+, Ar2+, and Ar+, in descending order of softness), and the resulting signal intensities for [M − H]+, M+, or [M + H]+ ions changed as reflecting a proton affinity or electronegativity of atoms, or polarity of molecules. Overall transmission efficiencies through ionisation, ion transport, mass separation, and ion detection processes for CO2+ and CH3+ were about 3.6 × 10−10%, 1.6 × 10−10%, and 3 × 10−8%, respectively. The technique described here can become a rapid and on-line analysis technique for both gaseous or volatile organic compounds (VOCs). [ABSTRACT FROM AUTHOR]

Proton transfer reaction mass spectrometry has a so far little exploited potential for the analysis of high-purity process and inert gases. In order to protonate impurities like saturated hydrocarbons, precursor ions with very low proton affinity must be used, which in turn leads to a large energy release upon protonation of molecules with high proton affinity, potentially causing fragmentation of the product ions. To explore the potential of low-proton affinity precursor ions like ArH+ and N2H+ for analytical purposes, we studied their gas-phase reactions and kinetics with acetone by Fourier transform ion cyclotron resonance (FT–ICR) mass spectrometry. The dominant product ion in both cases is protonated acetone, but fragment ions formed include C2H3O+, C3H5+ and CH3O+, which are formed in higher abundance with ArH+ compared to N2H+. The reaction efficiencies are determined to be close to 100%. Quantum chemical calculations reveal energetically favorable reaction pathways and explain the loss of water leading to the formation of C3H5+, as well as loss of methane to yield C2H3O+via dissociative proton attachment, which corresponds to the main product in the EI mass spectrum of acetone. Formation of protonated formaldehyde CH3O+ involves rearrangement of the C–C bonds to eliminate ethylene. [ABSTRACT FROM AUTHOR]

Quantum chemical computations at M06-2X/def2-TZVPD level of theory were employed to compute structural and thermodynamic properties of gaseous donor-acceptor complexes IX·LB [X = I, Cl; LB = trimethylamine, triethylamine, pyridine, 2-aminopyridine, 4,4′-bipyridine, pyrazine, 1,2-bis(4-pyridyl)ethylene, 1,2-bis(4-pyridyl)ethane]. Iodine monochloride is a stronger Lewis acid compared to the molecular iodine. With respect to homogeneous gas phase dissociation into components, the complex of ICl with triethylamine is the most stable and the complex of I2 with pyrazine is the least stable. It is shown that dissociation enthalpies do not correlate with proton affinity of the Lewis base, but there is a good correlation between dissociation enthalpies and I–X bond lengthening upon complex formation. This allows to estimate the stability of the complex based on its structural data. [ABSTRACT FROM AUTHOR]

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The nuclear–electronic orbital (NEO) framework provides a practical approach for directly incorporating nuclear quantum effects and non-Born–Oppenheimer effects of specified nuclei, typically protons, into quantum chemistry calculations. Multicomponent wave function based methods, such as NEO coupled cluster singles and doubles, and multicomponent density functional theory (DFT), such as NEO-DFT, require the appropriate selection of electronic and nuclear basis sets. Although a wide array of electronic basis sets are available, systematically developed nuclear basis sets that balance accuracy and efficiency have been lacking. Herein, a series of nuclear basis sets are developed and shown to be accurate and efficient for describing both ground and excited state properties of multicomponent systems in which electrons and specified protons are treated quantum mechanically. Three series of Gaussian-type nuclear basis sets, denoted PB4, PB5, and PB6, are developed with varying levels of angular momentum. A machine-learning optimization procedure relying on the Gaussian process regression method is utilized to accelerate the optimization process. The basis sets are validated in terms of predictions of ground state energies, proton densities, proton affinities, and proton vibrational excitation energies, allowing the user to select the desired balance between accuracy and efficiency for the properties of interest. These nuclear basis sets will enhance the tractability of NEO methods for applications to a wide range of chemical systems. [ABSTRACT FROM AUTHOR]

Ammonium perchlorate (AP) is a commonly used oxidizer in solid propellant compositions. As a result, numerous experimental and theoretical studies have been carried out in order to better understand the behavior of its decomposition in both the liquid and gas phases. In this work, the first step of Ammonium perchlorate (AP) decomposition in the condensed phase has been investigated using quantum mechanics‐based calculations. The process of proton transfer from ammonium ion (NH4+) to perchlorate ion (ClO4−) was investigated in‐depth as it is thought to be the preferred reaction for the initiation of AP decomposition in the condensed phase. The calculations were carried out using density functional theory (DFT) at the B3LYP level in conjunction with the 6‐311G++(d,p) basis set. The current study revealed that proton transfer from ammonium cation to perchlorate anion takes place in two steps. The first step involves the decomposition of ClO4− to ClO2− and O2 followed by the second step which is H+ transfer from NH4+ to ClO2− giving NH3 and HClO2. In order to better understand protonation and deprotonation, it is essential to know the proton affinity (PA) of the involved species. In this study, the CBS‐QB3 method is used to determine the PA of various energetic ionic compounds. These compounds include guanidinium azotetrazolate, triaminoguanidinium azotetrazolate, guanidinium nitrate, ammonium nitrate, and ammonium perchlorate. Based on the results of the calculations, it is concluded that direct proton transfer process does not occur in the condensed phase because the proton affinities of all cations are significantly higher than those of the corresponding anions. In conjunction with experimental studies, this modelling study can help in the development of a better understanding of AP decomposition in the condensed phase. [ABSTRACT FROM AUTHOR]

Small, but important, differences in the structure–property relationships between ionomers composed of amidinium or imidazolinium groups with alkylcarbamate or alkyldithiocarbamate counterions have been examined experimentally by us previously. To unravel the sources of these differences, DFT calculations are conducted here for ion‐pair complexes (IPs) of these systems and their corresponding uncharged base and acid components (NPs). Calculations include IPs and NPs in which the amidine/amidinium and imidazoline/imidazolinium groups are anchored to a dimethylsiloxane pentamer. A surprising dependence of proton transfer on the dielectric constant (ε) of the medium is found for the systems: Whereas interconversion between the NPs and IPs is strongly dependent on medium dielectric in systems with an alkylcarbamate, none of the initial IP forms with an alkyldithiocarbamate transforms to an NP. Although the calculations do not include individual solvent–solute molecular interactions, they do probe how the components sense their bulk environments. The lack of detectable reversible proton transfer from the amidinium or imidazolinium cations to the dithiocarbamates is consistent with the "principle of proton affinity/pKa equivalence": Because the acidity of dithiocarbamates is higher than that of carbamates, the gap between their proton affinities (ΔPA) is decreased, favoring stronger electrostatic‐based H‐bonds in the ion‐pair complexes of the dithiocarbamates. The differences can also be estimated from the Deuri–Phukan nucleophilicity index scale, which is based on DFT calculations and suggests a strong dependence of proton transfer on the nucleophilicity of the base and the dielectric constant of the solvent. Predictions from these calculations on some important experimental systems are mentioned. [ABSTRACT FROM AUTHOR]

The first in situ preparation and single‐crystal structure identification of pure 1‐(pyridin‐4‐yl)‐4‐thiopyridine (PTP), C10H8N2S, a simple and basic derivative of mercaptopyridine, from a crystallization mixture is described. The same PTP was found in two multicomponent crystal forms with 3,5‐dinitrobenzoic acid as a classic two‐component cocrystal, namely, 1‐(pyridin‐4‐yl)‐4‐thiopyridine–3,5‐dinitrobenzoic acid (1/1), C7H4N2O6·C10H8N2S, and with 2‐hydroxy‐3,5‐dinitrobenzoic acid as a salt formed via proton transfer from the hydroxy group of the acid to the pyridyl N atom of PTP, namely, 4‐(4‐sulfanylidene‐1,4‐dihydropyridin‐1‐yl)pyridin‐1‐ium 1‐carboxy‐3,5‐dinitrophenolate, C10H9N2S+·C7H3N2O7−. The protonation energy of PTP is 944.64 kJ mol−1, indicating slightly greater N‐basicity compared to pyridine, a well characterized and very basic chemical reference. A variety of molecular interactions can be observed in the three new crystal structures of PTP, which are all discussed in detail. Our findings confirm those of previous studies, indicating that PTP and 4‐mercaptopyridine may, under suitable conditions, be chemically converted to one another, and that this process can be stimulated by light (UV–Vis). [ABSTRACT FROM AUTHOR]

Electrochemical proton storage plays an essential role in designing next‐generation high‐rate energy storage devices, e.g., aqueous batteries. Two‐dimensional conjugated covalent organic frameworks (2D c‐COFs) are promising electrode materials, but their competitive proton and metal‐ion insertion mechanisms remain elusive, and proton storage in COFs is rarely explored. Here, we report a perinone‐based poly(benzimidazobenzophenanthroline) (BBL)‐ladder‐type 2D c‐COF for fast proton storage in both a mild aqueous Zn‐ion electrolyte and strong acid. We unveil that the discharged C−O− groups exhibit largely reduced basicity due to the considerable π‐delocalization in perinone, thus affording the 2D c‐COF a unique affinity for protons with fast kinetics. As a consequence, the 2D c‐COF electrode presents an outstanding rate capability of up to 200 A g−1 (over 2500 C), surpassing the state‐of‐the‐art conjugated polymers, COFs, and metal–organic frameworks. Our work reports the first example of pure proton storage among COFs and highlights the great potential of BBL‐ladder‐type 2D conjugated polymers in future energy devices. [ABSTRACT FROM AUTHOR]

Natural flavonoids are renowned for their exceptional antioxidant properties, but their limited water solubility hampers their bioavailability. One approach to enhancing their water solubility and antioxidant activity involves the use of cyclodextrin (CD) inclusion. This study investigated the impact of CD inclusion on the three primary radical scavenging mechanisms associated with flavonoid antioxidant activity, utilizing apigenin as a representative flavonoid and employing density functional theory (DFT) calculations. Initially, the optimized geometries of CD−apigenin inclusion complexes were analyzed, revealing the formation of hydrogen bonds between CD and apigenin. In less polar environments, the inclusion complex strengthened the bond dissociation enthalpies of hydroxyl groups, thereby reducing antioxidant activity. Conversely, in polar environments, the inclusion complex had the opposite effect by lowering proton affinity. These findings align with experimental results demonstrating that CD inclusion complexation enhances flavonoid antioxidant activity in aqueous ethanol solutions. [ABSTRACT FROM AUTHOR]

Four methods of computational quantum chemistry are used in a study of hyperconjugation in protonated aromatic molecules. Benzene, benzenium, toluene, and four isomeric forms of toluenium are examined using the self-consistent field level of theory followed by configuration interaction and coupled cluster calculations, as well as density functional theory. Results for proton affinities, geometric parameters, atomic populations, dipole moments, and polarizabilities are reported. The calculated results are in good agreement with previous computational studies and with experimental data. The presence of hyperconjugation is evident from the shortened carbon–carbon bond lengths in the aromatic ring and concomitant changes in dipole moments and polarizabilities. The proton affinities of benzene and toluene compare well with experimental values. The examination of all of the toluenium isomers reveals that the position of the methyl group has a minor impact on the strength of hyperconjugation, although the most stable isomer is found to be the para form. Mulliken population analyses indicate that the addition of a proton contributes to aromatic hyperconjugation and increases the strength of π-bonds at the expense of σ-bonds.

In this review, we discuss gas phase experimentation centered on the measurement of acidity and proton affinity of substrates that are useful for understanding catalytic mechanisms. The review is divided into two parts. The first covers examples of organocatalysis, while the second focuses on biological catalysis. The utility of gas phase acidity and basicity values for lending insight into mechanisms of catalysis is highlighted. [ABSTRACT FROM AUTHOR]

Series of new thirteen annulated heterocyclic were prepared and screened for their antioxidant activities. The ethyl-4(2-cyanoacetamido)benzoate as a multifunctional component which possess both electrophilic and nucleophilic centers that were utilized in organic synthesis of different candidates. These new compounds studied as antioxidants at different concentrations based on DPPH and ABTS assays using ascorbic acid as a standard antioxidant. The most potent compound was 5 that recorded as the best antioxidant compound with IC50 equal 0.006 mg/ml for DPPH results. While for ABTS study, compound 4 anticipated the best result with IC50 equal 27.46 µM. Different reactive and thermodynamic descriptors were calculated as a preliminary step for a comparative thermodynamic study for antioxidant property. The results clarified that the potent compounds showed dependence on SET–PT mechanism than HAT or SPLET mechanism. These mechanisms depended on different parameters as proton affinity and electron transfer energy that were studied. SET–PT mechanism was dependent on two descriptors: IP and BDE which were studied and observed that were less-valued in site A for two potent compounds 4 and 5. [ABSTRACT FROM AUTHOR]

The hydride ion transfer and proton transfer are the key steps in the reactions of (de)hydrogenation, dehydrocoupling, production of H2, and reduction of CO2 with the participation of transition metal hydrides; complexes with bifunctional ligands often act as catalysts for these transformations. The aim of this work was to study the hydride-donating properties of pincer palladium(II) hydride (PNP)PdH (1; PNP is bis(2-diisopropylphosphino-4-methylphenyl)amide). For this purpose, its reaction with Lewis acids (BF3·Et2O, B(C6F5)3) was studied using IR and NMR spectroscopies combined with quantum chemical calculations (DFT/M06/def2-TZVP). Correlations between electrochemical reduction potentials of the corresponding cations and thermodynamic hydridity of the metal hydrides proposed in the literature were also applied. [(PNP)Pd(MeCN)][BF4] undergoes an irreversible two-electron reduction in acetonitrile ( = –1.82 V). The use of the obtained potential in correlations gives an overestimated value of the hydride donating ability It was found that the reaction of 1 with boron-containing Lewis acids unexpectedly leads to the protonation of the nitrogen atom of the pincer ligand with an impurity of water, rather than the reaction with the hydride ligand. According to DFT calculations, the proton affinity of the nitrogen atom is much higher than that of PdH, which determines its higher activity in protonation processes. [ABSTRACT FROM AUTHOR]

Herein we report the reduction of R‐EX2 (E=P, Sb) with two equivalents of KC8 in the presence of silylene (LSiR; L=PhC(NtBu)2) to give Trip‐P=SiL(C6H4PPh2) (1), TerPh−P=(tBu)SiL (2) and TerPh−Sb=(tBu)SiL (3). The last (3) belongs to a new class of heavier analogues of Schiff bases (>C=N−), containing a formal >Si=Sb− double bond. The theoretical calculations suggest that lone pairs on the dicoordinated group‐15 centers are stabilized by hyperconjugative interactions resulting in pseudo‐Si−P/Si−Sb multiple bonds which are highly reactive as indicated by the high first and second proton affinities. [ABSTRACT FROM AUTHOR]

Photocatalytic water splitting is a promising approach to generating sustainable hydrogen. However, the transport of photoelectrons to the catalyst sites, usually within ps‐to‐ns timescales, is much faster than proton delivery (∼μs), which limits the activity. Therefore, the acceleration of abstraction of protons from water molecules towards the catalytic sites to keep up with the electron transfer rate can significantly promote hydrogen production. The photobasic effect that is the increase in proton affinity upon excitation offers means to achieve this objective. Herein, we design photobasic carbon dots and identify that internal pyridinic N sites are intrinsically photobasic. This is supported by steady‐state and ultrafast spectroscopic measurements that demonstrate proton abstraction within a few picoseconds of excitation. Furthermore, we show that in water, they form a unique four‐level lasing scheme with optical gain and stimulated emission. The latter competes with photocatalysis, revealing a rather unique mechanism for efficiency loss, such that the stimulated emission can act as a toggle for photocatalytic activity. This provides additional means of controlling the photocatalytic process and helps the rational design of photocatalytic materials. [ABSTRACT FROM AUTHOR]

Context: Proton transfer in acid–base systems is not well understood. Some acid–base reactions do not proceed to the extent that is expected from the difference in the pKa values between the base and acid in aqueous solutions, yet some do. In that regard, we have computationally studied the process of proton transfer from the acids of varying strength (benzenesulfonic acid (BSu), methansulfonic acid (MsO), and sulfuric acid (SA)) to the amines with different numbers of propyl substituents on the nitrogen atom (propylamine (PrA), dipropylamine (DPrA), and tripropylamine (TPrA)) upon complexation. Density functional theory calculations were used to thoroughly examine the energetic and structural aspects of the molecular complexes and/or ionic pairs resulting from the acid–base interaction. The potential energy curves along the proton transfer coordinate in these acid–amine systems were analyzed. The change in free energies accompanying the molecular complexes and ionic pair formations was calculated, and the relationship between the energy values and the ΔРА parameter (difference in proton affinity of the acid anion and amine) was established. The larger ΔРА values were found to be unfavorable for the formation of ionic pairs. Using structural, energy, QTAIM, and NBO analyses, we determined that the hydrogen bonds in the molecular complexes PrA-MsO and PrA-BSu are stronger than those in their corresponding ionic pairs. The ionic pairs with the TPrA cation possess the strongest hydrogen bonds of all the ionic pairs being studied, regardless of the anion. The results showed that hydrogen bonding interactions in the molecular complexes contribute significantly to the energies of the acid–base interaction, while in the ionic pairs, the most important energy contribution comes from Coulomb interactions, followed by hydrogen bonding and dispersion forces. The ionic pairs with propylammonium, dipropylammonium, and tripropylammonium cations have stronger ion–ion interactions than tetrapropylammonium (TetPrA)-containing ionic pairs with the same anions. This effect rises with the order of the cation: TetPrA → TPrA → DPrA → PrA, and the sequence of anions is SA → BSu → MsO. The results obtained here expand the concept of acid–base interaction and provide an alternative to experimental searches for suitable acids and bases to obtain new types of protic ionic liquids. Methods: All quantum-chemical calculations were carried out by using the DFT/B3LYP-GD3/6-31++G(d,p) level as implemented in the Gaussian 09 software package. For the resulting structures, the electron density distribution was analyzed by the "atoms in molecules" (QTAIM) and the natural bond orbital (NBO) methods on the wave functions obtained at the same level of theory by AIMAll Version 10.05.04 and Gaussian NBO Version 3.1 programs, respectively. [ABSTRACT FROM AUTHOR]

The reaction of seventeen borane derivatives of five-membered N-heterocyclic rings (BNHRs) with CO2 has been studied by means of DFT calculations. Several non-covalent complexes between the BNHRs and CO2 which evolve through a TS in a single adduct for each BNHR have been identified. The calculated IRC of the TS has allowed to identify the non-covalent complex involved in the reaction in each case. The stationary points of the reactions have been analyzed with the distortion/interaction partition model. In addition, empirical models have been attempted to correlate the acid (fluoride ion affinity) and basic (proton affinity) properties of the isolated BNHR with the TS barriers and adduct energies. The energetics of the reactions are influenced by the number of nitrogen atoms in the ring. [ABSTRACT FROM AUTHOR]

Understanding the adsorption behavior of base probes in aluminosilicates and its relationship to the intrinsic acidity of Brønsted acid sites (BAS) is essential for the catalytic applications of these materials. In this study, we investigated the adsorption properties of base probe molecules with varying proton affinities (acetonitrile, acetone, formamide, and ammonia) within six different aluminosilicate frameworks (FAU, CHA, IFR, MOR, FER, and TON). An important objective was to propose a robust criterion for evaluating the intrinsic BAS acidity (i.e., state of BAS deprotonation). Based on the bond order conservation principle, the changes in the covalent bond between the aluminum and oxygen carrying the proton provide a good description of the BAS deprotonation state. The ammonia and formamide adsorption cause BAS deprotonation and cannot be used to assess intrinsic BAS acidity. The transition from ion-pair formation, specifically conjugated acid/base interaction, in formamide to strong hydrogen bonding in acetone occurs within a narrow range of base proton affinities (812–822 kJ mol−1). The adsorption of acetonitrile results in the formation of hydrogen-bonded complexes, which exhibit a deprotonation state that follows a similar trend to the deprotonation induced by acetone. This allows for a semi-quantitative comparison of the acidity strengths of BAS within and between the different aluminosilicate frameworks. [ABSTRACT FROM AUTHOR]

Small ring nitrogen heterocycles, azabicyclobutanes and azirines, were investigated by computational methods in order to address the discrepancy between their regioisomers 1‐ and 2‐azabicyclobutane and 1H‐ and 2H‐azirines. Both 1‐azabicyclobutane and 2H‐azirine are well known synthetic starting points to larger nitrogen heterocycles whereas 2‐azabicyclobutane and 1H‐azirine and their derivatives have yet to be reported as isolable compounds. Calculated parameters such as structure, base strength (proton affinities), NICS values and enthalpies of formation from which strain energies are derived are reported. The destabilization of the less stable regioisomers is attributed to homoantiaromaticity in 2‐azabicyclobutane and antiaromaticity in 1H‐azirine. Two stereoisomers exist for 2‐azabicyclobutane with the endo‐ stereoisomer being more stable. This phenomenon is indicative of the hydrogen bond acceptor properties of the neighboring cyclpropane and the π‐bond character of the central bond in 2‐azabicyclobutane. [ABSTRACT FROM AUTHOR]

The nuclear-electronic orbital (NEO) framework enables computationally practical coupled cluster calculations of multicomponent molecular systems, in which all electrons and specified nuclei, typically protons, are treated quantum mechanically. In addition to energies, computing accurate proton densities is essential for the calculation of reliable molecular properties, including vibrationally averaged geometries and vibrational frequencies. Herein, the Lagrangian formalism for the multicomponent coupled cluster with single and double excitations (NEO-CCSD) method is derived and implemented. The multicomponent coupled cluster with double excitations method using optimized Brueckner orbitals, denoted as NEO-BCCD, is also developed. Both of these methods are used to compute the proton densities for two molecular systems. The results illustrate that orbital relaxation effects, which can be included either indirectly with the NEO-CCSD method or directly with the NEO-BCCD method, are critical for computing even qualitatively accurate proton densities. Both methods are also able to provide accurate proton affinities and vibrationally averaged optimized geometries. This Lagrangian formalism will enable the calculation of other properties such as analytical nuclear gradients and Hessians with NEO coupled cluster methods. Moreover, the accuracy of these methods may be improved systematically by the inclusion of higher-order excitations. Thus, this work provides the foundation for a wide range of future methodological developments and applications within the NEO framework. [ABSTRACT FROM AUTHOR]

Multicomponent Density Functional Theory (MDFT) is a promising methodology to incorporate nuclear quantum effects, such as zero-point energy or tunneling, or to simulate other types of particles such as muons or positrons using particle densities as basic quantities. As for standard electronic DFT, a still ongoing challenge is to achieve the most efficient implementations. We introduce a multicomponent DFT implementation within the framework of auxiliary DFT, focusing on molecular systems comprising electrons and quantum protons. We introduce a dual variational procedure to determine auxiliary electron and proton densities which leads to a succession of approximate energy expressions. Electronic and protonic fitted densities are employed in (i) electron-electron, proton-proton, and electron-proton classical Coulomb interactions and (ii) electron exchange-correlation, proton-proton exchange, and electron-proton correlation (EPC) potentials. If needed, exact exchange among electrons or among protons is computed by the variational fitting of the corresponding Fock potential. The implementation is carried out in deMon2k. We test various electron proton correlation functionals on proton affinities. We find that auxiliary densities can be safely used in electron-electron, proton-proton, and electron-proton classical Coulomb interactions, as well as in EPC, albeit with some precautions related to the choice of the electronic auxiliary basis set that must be flexible enough. Computational tests reported indicate that introduction of density fitting in MDFT is clearly advantageous in terms of computational effort with good scaling properties with respect to the number of electrons and protons treated at the DFT level. [ABSTRACT FROM AUTHOR]

Singlet carbenes are extensively studied compounds capable of electrophilic, nucleophilic or ambiphilic behaviour. The ambiphilic reactivity of singlet carbenes has been conventionally observed in orthogonal planes. Here, we report a detailed bonding and reactivity study of a homobimetallic carbon complex [(MCp*)2(μ‐NPh)(μ‐C)] (1M, M=Fe, Ru, Os) that shows ambiphilicity in the same direction. The structure of this complex can be considered as two fused three‐membered M−C−M and M−N−M rings. The bonding analysis suggests that these 17 valence electron homobimetallic complexes have one formal M−M bond with a bridging carbene centre featuring a high‐lying spn‐hybridised lone pair. Accordingly, the carbene centre shows high proton affinity and act as a good 2e− donor to Lewis acids and transition metal fragment. Apart from the transition metal non‐bonding electrons, the π‐framework of M−C−M and M−N−M arms can be best described as 3c–2e− bonds. The two transition metals in the four‐membered skeleton generate many low‐lying, virtual orbitals. These low‐lying virtual orbitals induce electron excitation from the spn‐hybrid orbital in presence of H− and other 2e− donor ligands such as PMe3, NHC and CO. Hence, the spn‐hybrid lone pair orbital shows σ‐hole reactivity in presence of Lewis bases. [ABSTRACT FROM AUTHOR]