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An NNP created by the active learning scheme was used to assist the exploration of the conformational space of sodiated pyranoses. As a result, more than 17 000 distinct local minima at the DFT level were located and an NNP with an accuracy of 1 kJ mol−1 was created.

A self-adapting first-principles exploration to understand the mechanism of collision-induced dissociation (CID) in mono-saccharides assisted with neural network potentials (NNP).

We propose a new method that utilizes the database of stable conformers and borrow the fragmentation concept of many-body-expansion (MBE) methods in ab initio methods to train a deep-learning machine learning (ML) model using SchNet.

This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for H Al co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Collision-induced dissociation of fructose is studied with experiments and first-principles kinetic modeling. The preference for dehydration cannot be easily predicted by the relative orientation of the OH groups as done for aldohexoses.

In this work, we analyze the vibrational spectra of ammonium, methylammonium, and dimethylammonium ions solvated by either water molecules or bisulfate anions using anharmonic vibrational algorithms. Rich and complicated spectral features in the 2700-3200 cm

The infrared (IR) spectra of the O-H stretching vibrations of pyridine-water clusters (Pyd)m(H2O)n, with m, n = 1-4, have been investigated with infrared-vacuum ultraviolet (VUV) spectroscopy under a jet-cooled condition. The time-of-flight mass spectrum of (Pyd)m(H2O)n+ by VUV ionization at ∼9 eV showed an unusual intensity pattern with very weak ion signals for m = 1 and 2 and stronger signals for m ≥ 3. This unusual mass pattern was explained by a drastic structural change of (Pyd)m(H2O)n upon the VUV ionization, which was followed by the elimination of water molecules. Among the recorded IR spectra, only one spectrum monitored, (Pyd)2+ cation, showed a well-resolved structure. The spectrum was analyzed by comparing with the simulated ones of possible stable isomers of (Pyd)2(H2O)n, which were obtained with quantum-chemical calculations. Most of the calculated (Pyd)2(H2O)n clusters had the characteristic structure in which H2O or (H2O)2 forms a hydrogen-bonded bridge between two pyridines to form the π-stacked (Pyd)2, and an additional H2O molecule(s) extends the H-bonded network. The π-stacked (Pyd)2(H2O)n moiety is very stable and is thought to exist as a local structure in a pyridine/water mixed solution. The Fermi resonance between the O-H stretch fundamentals and the overtones of the O-H bending vibrations in (Pyd)m(H2O)n was found to be less pronounced in the case of (Pyd)m(NH3)n studied previously.

The hemibond is a nonclassical covalent bond formed between a radical (cation) and a closed shell molecule. The hemibond formation ability of water has attracted great interest, concerning its role in ionization of water. While many computational studies on the water hemibond have been performed, clear experimental evidence has been hardly reported because the hydrogen bond formation overwhelms the hemibond formation. In the present study, infrared photodissociation spectroscopy is applied to (H2O-Krn)+ (n = 1-3) radical cation clusters. The observed spectra of (H2O-Krn)+ are well reproduced by the anharmonic vibrational simulations based on the hemibonded isomer structures. The firm evidence of the hemibond formation ability of water is revealed.

Vibrational spectra of 1:1 clusters of 2-pyridone (2PY) with water, ammonia, and other hydrogen bond-forming molecules have been measured by several experimental groups over the past two decades. Complex vibrational signatures associated with the N-H stretching fundamental at 3 μm are often observed. Several anharmonic coupling schemes have been proposed; however, the origin of these commonly seen complex features remains unclear. In this work, we present our theoretical analysis on the structure and vibrational spectra of these clusters using ab initio random search and ab initio anharmonic algorithms, respectively. Low-energy conformers were found to be hydrogen-bonded clusters and their vibrational spectra at 3 μm were simulated with ab initio anharmonic algorithms. We demonstrate that simple anharmonic mechanisms of Fermi resonance (FR), coupling between NH stretching modes, and overtone/combinations of skeleton modes of 2PY can lead to the complex vibrational signatures observed experimentally. Since this vibrational coupling scheme is inherent to the cis-amides with adjacent N-H and C═O groups when a hydrogen bond is formed with 2PY as the donor and acceptor, we believe that such a phenomenon is general to other hydrogen-bonding systems with the same functional group.

Elucidating the dynamic couplings of hydrogen bonds remains an important and challenging goal for spectroscopic studies of bulk systems, because their vibrational signatures are masked by the collective effects of the fluctuation of many hydrogen bonds. Here we utilize size-selected infrared spectroscopy based on a tunable vacuum ultraviolet free electron laser to unmask the vibrational signatures for the dynamic couplings in neutral trimethylamine-water and trimethylamine-methanol complexes, as microscopic models with only one single hydrogen bond holding two molecules. Surprisingly broad progression of OH stretching peaks with distinct intensity modulation over ∼700 cm-1 is observed for trimethylamine-water, while the dramatic reduction of this progression in the trimethylamine-methanol spectrum offers direct experimental evidence for the dynamic couplings. State-of-the-art quantum mechanical calculations reveal that such dynamic couplings are originated from strong Fermi resonance between the stretches of hydrogen-bonded OH and several motions of the solvent water/methanol, such as translation, rocking, and bending, which are significant in various solvated complexes commonly found in atmospheric and biological systems.

The aggregates of monofluorinated phenylacetylenes in the gas-phase, investigated using the IR-UV double resonance spectroscopic method in combination with extensive structural search and electronic structure calculations, reveal the formation of liquid-like clusters with a π-stacked dimeric core. The structural assignment based on the IR spectra in the acetylenic and aromatic C–H stretching regions suggests that, unlike the parent non-fluorinated phenylacetylene, the substitution of a F atom on the phenyl ring increases the dipole moment, leading to robustness in the formation of a π⋯π stacked dimer, which propagates incorporating C–H⋯π_{Ar/Ac} and C–H⋯F interactions involving both acetylenic and aromatic C–H groups. The structural evolution of fluorophenylacetylene aggregates in the gas phase shows marginal effects due to fluorine atom position on the phenyl ring, with substitution in the para-position tending towards phenylacetylene. The present study signifies that the π⋯π stacked dimers act as a nucleus for the growth of higher clusters to which other molecular units are added predominantly via the {Ar}_C–H⋯π_{Ar} type of interaction and the dominant interactions present in the crystal structures gradually emerge with increasing cluster size. Based on these features, gas-phase clusters of fluorophenylacetylene are hypothesized as “liquid-like clusters” acting as intermediates in the generation of various polymorphic forms starting from a π⋯π stacked dimer as the core molecular unit.

Studies have debated what is a favorable cluster size in liquid methanol. Applications of the quantum cluster equilibrium (QCE) model on a limited set of cluster structures have demonstrated the dominance of cyclic hexamers in liquid methanol. In this study, we examined the aforementioned question by integrating our implementation of QCE with a molecular-dynamics-based structural searching scheme. QCE simulations were performed using a database comprising extensively searched stable conformers of (MeOH)n for n = 2-14, which were optimized by B3LYP/6-31+G(d,p) with and without the dispersion correction. Our analysis indicated that an octamer structure can contribute significantly to cluster probability. By reoptimizing selected conformers with high probability at the MP2 level, we found that the aforementioned octamer became the dominant species due to favorable vibrational free energy, which was attributed to modes of intermolecular vibration.

The methylammonium ion (CH3NH3+, or noted as MA–H+) is one of the smallest organic ammonium ions that play important roles in organic–inorganic halide perovskites. Despite the simple structure, the vibrational spectra of MA–H+ exhibit complicated features in the 3 μm region which are sensitive to the solvation environment. In the present work, we have applied the ab initio anharmonic algorithm at the CCSD/aug-cc-pVDZ level to simulate the IR and Raman spectra of the solvated methylammonium ion, MA–H+⋯X3, where X denotes the solvent molecules, to understand the Fermi resonance mechanism in which the overtones of NH bending modes are coupled with the fundamentals of NH stretching modes. The spectral features of the solvated clusters with proper solvent species resemble those observed in the perovskite crystal, indicating that they have similar solvation environments and hydrogen bond interactions. Therefore, a linkage between the gas-phase cluster models and the condensed-phase materials can be established, and our simulations and anharmonic analyses help in interpreting the spectral assignments of the observed IR and Raman spectra of perovskites reliably. Furthermore, we have extended this approach to the SFG spectra to demonstrate the selective appearance of bands depending on both the beam polarization configurations and the symmetry of vibrational modes.

In this work, we probed the lithium and sodium intercalation properties in monolayer-stacked NbSe2 bilayer homostructure configurations for their potential application as anode materials in lithium and sodium ion batteries. Similar to known monolayer transition metal dichalcogenides, such as VS2, the structural phase transition barrier of NbSe2 from 1H to 1T is strengthened by lithium and sodium adsorption, implying that it is robust under multiple charging and discharging processes. As multi-layer, stacked 2D materials are more relevant to experiments and their intended applications, four bilayer homostructure stackings were constructed based on the alignment of Nb and Se. All four bilayer homostructure stackings were found to significantly enhance the binding of lithium and sodium at the van der Waals interface, and thus potentially increase the theoretical specific energy capacity of NbSe2via bilayer stacking. In terms of ionic diffusion, it is observed that for all of the bilayer homostructure configurations the diffusion energy barrier for lithium and sodium generally increased compared to the monolayer case. The nature of the stacking appears to affect the diffusion energy barrier with a value of as high as 1.94 eV in the case of sodium for the AB full stacking (compared to 0.08 eV for the monolayer). It is inferred that although the bilayer homostructure stacking of NbSe2 could significantly increase the theoretical specific energy capacity for both lithium and sodium, its drawback is the slowing down of the ion kinetics at the van der Waals interface, which are also important in the charging and discharging processes of a battery system.

To explore the impact of fluorination on the hydrogen bond networks of protonated alkylalcohols, infrared spectroscopy and theoretical computations of protonated 2,2,2-trifluoroethanol clusters, H

Recently, superconcentrated electrolytes have been successfully employed in Li-ion batteries because of their enhanced reductive stability and formation of an excellent solid electrolyte interphase...

Complex vibrational features of solvated hydronium ion, H3O+, in 3 μm enable us to look into the vibrational coupling among O-H stretching modes and other degrees of freedom. Two anharmonic coupling schemes have often been engaged to explain observed spectra: coupling with the OH bending overtone, known as Fermi resonance (FR), has been proposed to account for the splitting of the OH stretch band at ∼3300 cm-1 in H3O+···Ar3, but an additional peak in H3O+···(N2)3 at the similar frequency region has been assigned to a combination band (CB) with the low-frequency intermolecular stretches. While even stronger vibrational coupling is expected in H3O+···(H2O)3, such pronounced peaks are absent. In the present study, vibrational spectra of H3O+···Kr3 and H3O+···(CO)3 are measured to complement the existing spectra. Using ab initio anharmonic algorithms, we are able to assign the observed complex spectral features, to resolve seemingly contradictory notions in the interpretations, and to reveal simple pictures of the interplay between FR and CB.

The structure and vibrational spectra of protonated Ar clusters ArnH+ (n = 2-3) are studied using potential energy surfaces at the CCSD(T)/aug-cc-pVTZ level and basis set. Ar binding energies, as well as position isomerism in Ar3H+, were investigated. In our previous work, the spectra of Ar2H+ reveal a strong progression of combination bands, which involves the asymmetric Ar-H+ stretch with multiple quanta of the symmetric Ar-H+ stretch. In this work, insights on the origin of such progression were examined using an adiabatic model. In addition, contributions from mechanical and electrical anharmonicity on the progressions' intensities were also examined. Comparison of the calculated spectrum for the bare and Ar-tagged ions reveals that the reduction of the symmetry group, from D∞h to either C∞v or C2v, results in a richer vibrational structure in the 500-1700 cm-1 region. When compared with previously reported action spectra (D. C. McDonald III, D. T. Mauney, D. Leicht, J. H. Marks, J. A. Tan, J.-L. Kuo, and M. A. Duncan, J. Chem. Phys., 2016, 145, 231,101), it appears that the position isomers, because of the binding of the weakly bound Ar messenger, are needed to account for the additional bands in the infrared photodissociation spectrum for Ar3H+. These findings demonstrate the active role of the messenger atom in relaxing some of the selection rules for the bare ion's vibrational transitions - resulting in an augmentation of the bands in the action spectrum.

In this study, we have examined the adsorption properties of hydrogen on pristine Sc2C monolayers by DFT calculations. Based on these calculations, we have proposed a thermodynamic model to estimate the hydrogen storage capability within the typical ranges for the operating temperature and pressure. Our thermodynamic modeling has shown that the maximum uptake of usable hydrogen could reach up to 7.2 wt% under cryogenic conditions. When calculating the usable hydrogen uptake, we have taken into consideration that, under realistic operating conditions, not all hydrogen adsorbed on pristine Sc2C can be desorbed from the surface, as some surface-adsorbate interactions are too strong. On the other hand, the interaction between the usable hydrogen and Sc2C appears to be too weak to reach the targets for the year 2025 set by the US Department of Energy (5.5 wt% at operating temperatures between 233 K and 358 K and delivery pressures of up to 12 bar). According to the modeling results, one needs to decrease the temperature to 120 K to reach 5.5 wt% hydrogen uptake at 12 bar. The results obtained with the thermodynamic model have been confirmed with a kinetic Monte Carlo simulation, which has also been used to estimate the time scale of the hydrogen adsorption and desorption processes. In addition, we have also evaluated the changes in the electronic structure of the Sc2C monolayer upon adsorbing hydrogen. As the band gap of Sc2C changes significantly upon adsorbing H2, Sc2C may have more potential as a hydrogen detector instead of as a hydrogen storage material.

A series of Zn/Cu Zeolitic Imidazolate Frameworks (ZIFs) ZIF-202, -203, and -204 are systematically investigated by Density Functional Theory (DFT) with and without van der Waals (vdW) corrections. The elastic constants for non-solvent structures indicate that ZIF-202 and -204 are mechanically stable while ZIF-203 is unstable, which arises from the stiffness along the x-axis under a uniaxial strain in the PBE-D3 method. By considering the presence of solvents in ZIF-203, a structural phase transformation from a monoclinic to a triclinic structure is found which could be explained by the Jahn-Teller distortion. From the chemical bonding point of view, it is found that vdW interactions and hybridization between d-orbitals (copper) and p-orbitals (imidazolate) are the main-driving forces in stabilizing ZIF-202 and -204, respectively. The electronic structure calculations predict the presence of two optical transitions in the visible region in agreement with the experimental observation for ZIF-204 both without and with water. The DFT simulations reveal that CO2 molecules prefer to locate near imidazolate and water in dry and hydrated ZIF-204, respectively. The analysis of Canonical Monte Carlo (GCMC) simulations reveals that Coulomb interaction between CO2 and H2O molecules is mainly responsible for the enhanced CO2 uptake and selectivity under humid conditions compared to dry ones.

Strong coupling between stretching fundamentals and bending overtones of vibrational modes, known as Fermi resonance (FR), has been observed for proton motions in the protonated trimethylamine-water cluster. To investigate the role of FR, we examined the vibrational spectra of other three protonated ammonia/amine-water clusters, including the NH4+ ion and its mono- and di-methylated analogues, respectively, with and without argon tagging. In these systems, a simple frequency-scaled harmonic oscillator model will predict only one strong band between 2600 and 3200 cm-1 uniquely due to the hydrogen-bonded NH stretching fundamental for a given conformer. In the experimental vibrational spectra, however, multiple sharp bands were observed. Such a discrepancy often leads to the notions of the coexistence of multiple conformers and/or the appearance of an overtone state as a result of FR. In this work, we applied a discrete variable representation (DVR) implementation of ab initio anharmonic algorithms and demonstrated how one N-H+ stretching fundamental can lead to multiple bands as a result of intrinsic anharmonic couplings. A prominent effect of tuning these FR bands and lighting up dark overtone states in this wide frequency range was investigated by changing the number of methyl groups in the protonated amine moiety. The effect of Ar-tagging was also analyzed and decent agreement between the experimental and simulated spectra certified the above-mentioned simple pictures. We also found that the coupling constant for trimethylamine is the largest among these protonated amine-water clusters, and the overall coupling strength decreases as the hydrogen-bonded NH stretching frequency redshifts in the order of dimethylamine, methylamine, and ammonia.

The methylammonium ion (CH

The structures of the formyl ion (HCO

In this work, we investigate the effects of water on the structural stability of Mn

The hemibond is a nonclassical covalent bond formed between a radical (cation) and a closed shell molecule. The hemibond formation ability of water has attracted great interest, concerning its role in ionization of water. While many computational studies on the water hemibond have been performed, clear experimental evidence has been hardly reported because the hydrogen bond formation overwhelms the hemibond formation. In the present study, infrared photodissociation spectroscopy is applied to (H

Motivated by the fundamental difference in the reactivity of hexoses and N-acetylhexosamines under collision-induced dissociation (CID) mass spectrometry conditions, we have investigated the CID of...

We combined spectroscopic ellipsometry, Raman scattering spectroscopy, and first-principles calculations to explore the optical properties of YBaCuFeO5 single crystals. Measuring the optical absorption spectrum of YBaCuFeO5 at room temperature revealed a direct optical band gap at approximately 1.41 eV and five bands near 1.69, 2.47, 3.16, 4.26, and 5.54 eV. Based on first-principles calculations, the observed optical excitations were appropriately assigned. Analysis of the temperature dependence of the band gap indicated anomalies in antiferromagnetic phase transition at 455 and 175 K. Additionally, a hardening in the frequency of the Eg phonon mode was observed at 175 K. The value of the spin–phonon coupling constant was 15.7 mRy/Å2. These results suggest a complex nature of spin–charge–lattice interactions in YBaCuFeO5.

Dissociation of HCl embedded in dimethyl sulfoxide (DMSO) clusters was investigated by projecting the solvent electric field along the HCl bond using B3LYP-D3/6-31+G(d) and MP2/6-31+G(d,p) levels of theory. A large number of distinct structures (about 1500) consisting of up to five DMSO molecules were considered in the present work for statistical reliability. The B3LYP-D3 calculations reveal that the dissociation of HCl embedded in DMSO clusters requires a critical electric field of 138 MV cm-1 along the H-Cl bond. However, a large number of exceptions wherein the electric field values much higher than the critical electric field of 138 MV cm-1 did not result in dissociation of HCl were observed, in addition to several cases wherein the HCl dissociates with an electric field less than the critical electric field. On the other hand, the MP2 level calculations reveal that the critical electric field for HCl dissociation is about 181 MV cm-1 with almost no exceptions. A comparison of calculations carried out using the MP2 and the B3LYP-D3 levels suggests that the dissociation of HCl embedded in DMSO clusters is bistable at the B3LYP-D3 level, which is an artifact, suggesting that care must be exercised in interpreting the processes of proton transfer. The answer to the question raised as the title of this paper is NO.

The dissociation of HCl embedded in DMSO clusters was investigated by projecting the solvent electric field along the HCl bond using B3LYP-D3/6-31+G(d) and MP2/6-31+G(d,p) levels of theory. The B3LYP-D3 calculations reveal that the dissociation of HCl embedded in DMSO clusters requires a critical electric field of 138 MV cm–1 along the H–Cl bond. However, a large number of exceptions wherein the electric field values much higher than the critical electric field of 137 MV cm–1 did not result in dissociation of HCl. On the other hand, the MP2 level calculations reveal that the critical electric field for the HCl dissociation is about 181 MV cm–1 with almost no exceptions. The B3LYP-D3 calculations suggest that the dissociation of HCl embedded in DMSO clusters is bistable, which is an artefact. The answer to the question raised as the title of this paper is NO.

Matrix isolation experiments have been successfully employed to extensively study the infrared spectrum of several proton-bound rare gas complexes. Most of these studies have focused on the spectral signature for the H

Vibrational spectra of the methyl groups in mono-methylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) monomers and their clusters were measured in three experimental set-ups to capture their complex spectral features as a result of bend/umbrella-stretch Fermi resonance (FR). Multiple bands were observed between 2800 and 3000 cm-1 corresponding to the methyl groups for MMA and DMA. On the other hand, the corresponding spectrum of TMA is relatively simple, exhibiting only four prominent bands in the same frequency window, even though TMA has a larger number of methyl groups. The discrete variable representation (DVR) based ab initio anharmonic algorithm with potential energy surface (PES) at CCSD/aug-cc-pVDZ quality is able to capture all the experimentally observed spectral features across all three amines, and the constructed vibrational Hamiltonian was used to analyze the couplings that give rise to the observed FR patterns. It was observed that the vibrational coupling among CH stretch modes on different methyl groups is weak (less than 2 cm-1) and stronger vibrational coupling is found to localize within a methyl group. In MMA and DMA, the complex feature between 2850 and 2950 cm-1 is a consequence of closely packed overtone states that gain intensities by mixing with the stretching modes. The simplification of the spectral pattern of TMA can be understood by the red-shift of the symmetric CH3 stretching modes by about 80 cm-1 relative to MMA, which causes the symmetric CH3 stretch to shift outside the FR window.

Collision-induced dissociation (CID) of α-xylose and β-xylose were studied using mass spectrometry and quantum chemistry calculations. Three dissociation channels, namely loss of metal ions, dehydration, and cross-ring dissociation were found. The major dissociation channel of sodium adducts is the loss of sodium ions, and the minor dissociation channels are dehydration and cross-ring dissociation. By contrast, dehydration and cross-ring dissociation are the major dissociation channels of lithium adducts, and the corresponding dissociation mechanisms can be used to determine the anomericity and linkages of xylose in oligosaccharides. These mechanisms include (1) the dehydration branching ratio can be used to differentiate the anomericity of xylose and xylose in oligosaccharides because α-xylose has a larger branching ratio of dehydration than β-xylose, (2) various cross-ring dissociation reactions can be used to identify linkage positions. The oligosaccharide with xylose at the reducing end is predicted to undergo 0,2X, 0,3X, and 0,2A cross-ring dissociation for the 1 → 2, 1 → 3, and 1 → 4 linkages, respectively. Application of these mechanisms to determine the anomericity and linkage positions of xylobiose was demonstrated.

The appearance of multiple bands in the N–H stretching region of the infrared spectra of the neutral methylamine dimer and trimer is a sign of NH bend–stretch anharmonic coupling. Ab initio anharmonic calculations were carried out in a step-wise manner to reveal the origin of various bands observed in the spectrum of the methylamine dimer. A seven-dimensional potential energy surface involving symmetric and asymmetric stretching and bending vibrations of both the hydrogen bond donor and the acceptor along intermolecular-translational modes was constructed using the discrete variable representation approach. The resulting spectrum of the dimer shows five bands that can be attributed to the symmetric stretching (νsymD), asymmetric stretchin (νasymD), and bending overtone (2νbendD) of the donor moiety. These appear along with the combination band arising out of bending vibrations of the donor and acceptor (νbendD + νbendA) and with the combination of the intermolecular translational mode over the donor bending overtone (νtrans + 2νbendD). The spectrum of the trimer essentially consists of all the features seen in the dimer with marginal changes in band positions. The analysis of the experimental spectra based on the two-state deperturbation model and ab initio anharmonic calculations yield a matrix element of about 40 cm−1 for the N–H bend–stretch Fermi resonance coupling. In general, the IR spectra of the hydrogen-bonded amino group depict three sets of bands that arise due to bend–stretch Fermi resonance coupling.

Complex vibrational features of solvated hydronium ion, H

Studies on the vibrational spectra of various ammonium-centered clusters under different solvation environments have raised interest over the last thirty years. The gas-phase infrared photodissociation spectroscopy (IRPD) experiments showed that these NH4+⋯Xn clusters exhibit rich spectral features from 2600 to 3400 cm−1. In this work, we have simulated the vibrational spectra and analyzed couplings among vibrational quantum states in the aforementioned frequency range using ab initio anharmonic algorithms. Originating from the anharmonic couplings between NH stretching fundamentals and bending overtones, Fermi resonance (FR) is a common feature in these spectra, and its extent is determined by the magnitude of couplings and the energy matching conditions between relevant states, which are governed by the proton affinity, number, and bonding configuration of the solvation species. For weakly bound clusters consisting of rare gas atoms, FR is insignificant but not negligible; for strongly bound clusters, such as ammonium–water clusters, the hydrogen-bonded NH stretching fundamentals redshift and reach a better resonance condition, and thus light up the bending overtones as prominent FR bands. Our simulated spectra are in good agreement with previous experimental reports of these ammonium-centered clusters and provide a better understanding of the vibrational coupling behind the spectra of the NH stretching region.

The microscopic mechanism of the H2 adsorption of two Mg-MOF-74 isoreticular frameworks, one with a benzenedicarboxylate (BDC) linker and the other with a dihydroxyfumarate (DHF) linker, were studied on the basis of density functional theory (DFT) method. Possible adsorption sites on the internal surface of the two MOFs were detected using ab initio molecular dynamics (AIMD) annealing simulations. The simulations were able to reproduce all adsorption sites which have been experimentally observed for the BDC-based M-MOF-74 frameworks with M = Ni and Zn. In descending order of binding strengths, they are the adsorption sites primarily induced by the open metal sites P1, the oxygen atoms of the oxido groups P2 and the aromatic rings P3. The H2–framework binding strengths were properly evaluated by taking into account the vibrational zero-point energy (ZPE) contribution. An additional type of adsorption sites induced by the oxygen atoms of the carboxyl groups P4 is predicted for the Mg-MOF-74 framework. Two types of adsorption sites primarily induced by the open metal sites P1 and oxygen atoms of the carboxyl groups P2 were predicted for the DHF-based Mg-MOF-74 framework. Detailed analysis of the electron density showed that the electrostatic interaction of the H2 molecule with the charge distribution of the local framework environment within a radius of ∼3.5 A is a key factor to define adsorption positions and binding strength. The absence of the P4 sites in the BDC-based Zn-MOF-74 framework is caused by the lower charge density at the oxygen atoms induced by less electro-positive metal. The substitution of the nonaromatic DHF linker for the aromatic BDC linker reduces the binding strength at the metal induced adsorption sites by 1.45 kJ mol−1 due to the absence of the aromatic ring.

The infrared spectra of the C–H stretching vibrations of (pyridine)m, m = 1–3, and the N–H stretching vibrations of (pyridine)m–(NH3)n, m = 1, 2; n = 1–4, complexes were investigated by infrared (IR)–vacuum ultraviolet (VUV) spectroscopy under jet-cooled conditions. The ionization potential (IP0) of the pyridine monomer was determined to be 74 546 cm−1 (9.242 eV), while its complexes showed only smooth curves of the ionization thresholds at ∼9 eV, indicating large structural changes in the ionic form. The pyridine monomer exhibits five main features with several satellite bands in the C–H stretching region at 3000–3200 cm−1. Anharmonic calculations including Fermi-resonance were carried out to analyze the candidates of the overtone and combination bands which can couple to the C–H stretching fundamentals. For (pyridine)2 and (pyridine)3, most C–H bands are blue-shifted by 3–5 cm−1 from those of the monomer. The structures revealed by random searching algorithms with density functional methods indicate that the π-stacked structure is most stable for (pyridine)2, while (pyridine)3 prefers the structures stabilized by dipole–dipole and C–H⋯π interactions. For the (pyridine)m–(NH3)n complexes, the mass spectrum exhibited a wide range distribution of the complexes. The observed IR spectra in the N–H stretching vibrations of the complexes showed four main bands in the 3200–3450 cm−1 region. These features are very similar to those of (NH3)n complexes, and the bands are assigned to the anti-symmetric N–H stretching band (ν3), the symmetric N–H stretching (ν1) band, and the first overtone bands of the N–H bending vibrations (2ν4). The anharmonic calculations including the Fermi-resonance between ν1 and 2ν4 well reproduced the observed spectra.