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This paper systematically examines the performance of contemporary wavefunction and density functional theory methods to identify robust and cost-efficient methods for predicting gas-phase anion binding energies. This includes the local coupled cluster LNO-CCSD(T) and DLPNO-CCSD(T), as well as double-hybrid DSD-PBEP86-D3(BJ) and various hybrid functionals M06-2X, B3LYP-D3(BJ), ωB97M-V, and ωB97X-V. The focus is on dual-hydrogen-bonding anion receptors that are commonly found in supramolecular chemistry and organocatalysis, namely, (thio)ureas, deltamides, (thio)squaramides, and croconamides as well as the yet-to-be-explored rhodizonamides. Of the methods examined, M06-2X emerged as the overall best performing method as the other functionals including DSD-PBEP86-D3(BJ) and the local coupled cluster DLPNO-CCSD(T) method displayed systematic errors that increase with the degree of carbonylation of the receptors. Hybrid ONIOM models that employed semiempirical methods (PM7, GFN1-xTB, and GFN2-xTB) and "threefold"-corrected small-basis set potentials (HF-3c, B97-3c, and PBEh-3c) were explored, and the best models resulted in 50- to 500-fold reduction in CPU time compared to W1-local. These calculations provide important insight into the structure-binding relationships where there is a direct correlation between Brønsted acidity and anion binding affinity, though the strength of the correlation also depends on other factors such as hydrogen-bonding geometry and the geometrical distortion that the receptor needs to undergo to bind the anion.

Nanoconfinement offers opportunities to tune physical properties of molecular entities by altering their assembled structures. This also applies to acene-based molecules with potentially rich π–π interactions. Unlike most of the previous cases with acene-based guests directly incorporated into hosts, we take a further step by oligomerizing a fluorescent anthryl monomer, 9-vinylanthracene, inside nanochannels of a metal–organic framework, which is a pillared three-dimensional kagome net of [Zn2(bdc)2(dabco)] (bdc2– = 1,4-benzenedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane). The fluorescence emission of the guest can be significantly enhanced after oligomerization, which is likely due to the suppressed nonemissive interaction between the oligomerized molecules in the nanospace and the MOF wall. The case we have demonstrated for fluorescence enhancement via confined oligomerization provides inspiration for the design of luminescent composites and is encouraging for further exploration of molecules in a nanoconfined space.

We have examined the use of systematic bond-separation reactions and purposely constructed chemistry-preserving isodesmic reactions for the thermochemical calculation of aromatic hydrocarbon species. The bond-separation approach yields somewhat disappointing accuracy even when the reaction energies are obtained with generally robust composite and double-hybrid (DH) density functional theory (DFT) methods. In contrast, for the purposely constructed reactions, we find a dramatic improvement in the accuracy for energies calculated with all methods examined. Notably, for medium-sized aromatic hydrocarbons, we find that an effective approach for formulating a well-balanced reaction is to split the target species into two halves with an aromatic overlapping region. Overall, the G4(MP2)-XK, MPW2PLYP, MN15, PBE, and DC-DFTB3 methods are reasonable within their respective classes of methods for the calculation of bond-separation as well as chemistry-preserving isodesmic reactions. We have further computed per-carbon atomization energy (AE) for a series of D6h benzene-type molecules, and thus obtained a formula for extrapolation to the graphene limit [AEn = 711.5 × (1 − 1/n0.640) kJ mol−1, where n = number of carbons]. It suggests that nano-graphene with a length larger than 10 nm would resemble properties of bulk graphene, and conversely, downsizing a nano-graphene beyond this point may lead to considerably altered properties from the bulk.

Innovative and robust photosensitisation materials play a cardinal role in advancing the combined effort towards efficient solar energy harvesting. Here, we demonstrate the photocathode functionality of a Metal–Organic Framework (MOF) featuring cofacial pairs of photo- and electro-active 1,4,5,8-naphthalenediimide (NDI) ligands, which was successfully applied to markedly reduce the overpotential required for CO2 reduction to CO by a well-known rhenium molecular electrocatalyst. Reduction of [Cd(DPNDI)(TDC)]n (DPNDI = N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide, H2TDC = thiophene-2,5-dicarboxylic acid) to its mixed-valence state induces through-space Intervalence Charge Transfer (IVCT) within cofacial DPNDI units. Irradiation of the mixed-valence MOF in the visible region generates a DPNDI photoexcited radical monoanion state, which is stabilised as a persistent species by the inherent IVCT interactions and has been rationalised using Density Functional Theory (DFT). This photoexcited radical monoanion state was able to undergo charge transfer (CT) reduction of the rhenium molecular electrocatalyst to effect CO generation at a lower overpotential than that required by the discrete electrocatalyst itself. The exploitation of cofacial MOFs opens new directions for the design philosophy behind light harvesting materials., The photocathode functionality of a Metal–Organic Framework (MOF) featuring cofacial photo- and electro-active ligands provides a new approach to CO2 reduction via charge transfer with a rhenium electrocatalyst.

Electroactive metal-organic frameworks (MOFs) are an attractive class of materials owing to their multifunctional 3-dimensional structures, the properties of which can be modulated by changing the redox states of the components. In order to realise both fundamental and applied goals for these materials, a deeper understanding of the structure-function relationships that govern the charge transfer mechanisms is required. Chemical or electrochemical reduction of the framework [Zn(BPPFTzTz)(tdc)]·2DMF, hereafter denoted ZnFTzTz (where BPPFTzTz = 2,5-bis(3-fluoro-4-(pyridin-4-yl)phenyl)thiazolo[5,4-d]thiazole), generates mixed-valence states with optical signatures indicative of through-space intervalence charge transfer (IVCT) between the cofacially stacked ligands. Fluorination of the TzTz ligands influences the IVCT band parameters relative to the unsubstituted parent system, as revealed through Marcus-Hush theory analysis and single crystal UV-Vis spectroscopy. Using a combined experimental, theoretical and density functional theory (DFT) analysis, important insights into the effects of structural modifications, such as ligand substitution, on the degree of electronic coupling and rate of electron transfer have been obtained.

Conjugated dienes underwent Ni-catalyst-promoted 1,2-hydrocarboxylation in a 1:1 ratio with carbon dioxide under atmospheric pressure in the presence of diisobutylaluminum hydride (DIBAL-H) to give the corresponding β,γ-unsaturated carboxylic acids, without dimerization or oligomerization of the conjugated diene.

We present an approximate approach for the simulation of UV/vis spectra using conventional [non-time-dependent (non-TD)] DFT computations. It uses Kohn-Sham orbitals and orbital energies to estimate both the excitation energies and the associated oscillator strengths. For a wide range of systems from small molecules to large molecular dyes used in electrochromic and solar-cell applications, reasonable UV/vis spectra are generated, each with just two conventional DFT computations. The accuracy is generally comparable to what one would expect from TD-DFT calculations. In comparison to TD-DFT, the protocol of the present study provides an intuitive and notably more rapid means for simulating electronic absorption properties. It enables efficient screening of materials for a wide range of relevant applications., The Journal of Physical Chemistry Letters, 11(18), pp.7882-7885; 2020

We have used quantum chemistry computations, in conjunction with isodesmic-type reactions, to obtain accurate heats of formation (HoFs) for the small fullerenes C20 (2358.2 ± 8.0 kJ mol-1), C24 (2566.2 ± 7.6), and the lowest-energy isomers of C32 (2461.1 ± 15.4), C42 (2629.0 ± 20.5), and C54 (2686.2 ± 25.3). As part of this endeavor, we have also obtained accurate HoFs for several medium-sized molecules, namely 216.6 ± 1.4 for fulvene, 375.5 ± 1.5 for pentalene, 670.8 ± 2.9 for acepentalene, and 262.7 ± 2.5 for acenaphthylene. We combine the energies of the small fullerenes and previously obtained energies for larger fullerenes (from C60 to C6000) into a full picture of fullerene thermochemical stability. In general, the per-carbon energies can be reasonably approximated by the "R+D"model that we have previously developed [ Chan et al. J. Chem. Theory Comput. 2019, 15, 1255-1264 ], which takes into account Resonance and structural Deformation factors. In a case study on C54, we find that most of the high-deformation-energy atoms correspond to the sites of the C-Cl bond in the experimentally captured C54Cl8.In another case study, we find that C60 has the lowest value for the maximum local-deformation energy when compared with similar-sized fullerenes, which is consistent with its "special stability". These results are indicative of structural deformation playing an important role in the reactivity of fullerenes., The journal of physical chemistry. A, 124(33), pp.6688-6698; 2020

A new simple and conceptual theoretical scheme is proposed for estimating one-electron excitation energies using Kohn-Sham (KS) solutions. One-electron transitions that are dominated by the promotion from one initially occupied orbital to one unoccupied orbital of a molecular system can be expressed in a two-step process, ionization, and electron attachment. KS with long-range corrected (LC) functionals satisfies Janak's theorem and LC total energy varies almost linearly as a function of its fractional occupation number between the integer electron points. Thus, LC reproduces ionization energies (IPs) and electron affinities (EAs) with high accuracy and one-electron excitation energies are expressed as the difference between the occupied orbital energy of a neutral molecule and the corresponding unoccupied orbital energy of its cation. Two such expressions can be used, with one employing the orbital energies for the neutral and cationic systems, while the other utilizes orbital energies of just the cation. Because the EA of a molecule is the IP of its anion, if we utilize this identity, the two expressions coincide and give the same excitation energies. Reasonable results are obtained for valence and core excitations using only orbital energies.

By using lignin model compounds with relevant key characteristic structural features, the reaction pathways of α-O-4 aryl ether linkages under hydrothermal conditions are elucidated. Experimental results and computational modeling suggest that the α-O-4 linkages in lignin undergo catalyzed hydrolysis and elimination to give phenolic and alkenylbenzene derivatives as major products in subcritical water. The decreased relative permittivity of water at these high temperatures and pressures facilitates the elimination reactions. The alkyl group on the α-carbon and the methoxy groups on the phenyl rings both have positive effects on the rate of conversion of α-O-4 linkages in native lignin.

A new metal-organic framework (MOF) containing a Mn(II) salen complex (BET surface area = 967 ± 6 m2 g-1) undergoes a reversible crystalline-to-amorphous transformation. Experimental studies and computational calculations show that the MOF is stable to a one-electron reduction at more anodic potentials than the corresponding discrete complex.

In this study, the extractive desulfurization of model fuel oil with ionic liquids (ILs) has been studied in an attempt to gain insights into the dominant forces controlling the extraction efficien...

Photo-chemistry provides a non-intuitive but very powerful way to probe kinetically limited, sometimes thermodynamically non-favored reactions and, thus, access highly specific products. However, reactivity in the excited state is difficult to characterize directly, due to short lifetimes and challenges in controlling the reaction medium. Among photo-activatable reagents, rhodamine dyes find widespread uses due to a number of favorable properties including their high absorption coefficient. Their readily adaptable synthesis allows development of tailor-made dyes for specific applications. Remarkably, few studies have directly probed the chemical reactivity of their triplet excited state. Here we present a new conceptual approach to examine the specific chemistry of the triplet excited state. We have developed a pump (488 nm) – probe (600 nm) strategy to examine the gas-phase lifetime and reactivity of the triplet cation of Rhodamine 6G (3Rh6G+) in an ion trap mass spectrometer. The confounding effects of solvent, aggregation and formation of other reactive intermediates is thus avoided allowing fundamental reactivity to be explored. In the presence, in the ion trap, of helium seeded with 1% of nitric oxide (˙NO) (∼ 60 ion/˙NO collisions per second), the triplet lifetime is shortened from 1.9 s to 0.7 s. Simultaneously, the reaction products [Rh6G-H]˙+ and [Rh6G-H + NO]+ are observed. Reaction of 3Rh6G+ with ˙NO2 yields [Rh6G-H]˙+, [Rh6G-H + NO2]+ and [Rh6G-2H]+. None of these products are observed for the singlet, 1Rh6G+. DFT calculations suggest a stepwise mechanism only allowed from 3Rh6G+, in which H atom abstraction by ˙NOx (x = 1 or 2) yields [Rh6G-H]˙+ which, then, reacts with another ˙NOx molecule. This illustrates the power of light to initiate specific chemical reactions, and the relevance of gas-phase ion–molecule reaction approaches to understand stepwise reaction mechanism from specific excited states.

The core electron binding energies (CEBEs) and core-level excitation energies of thymine, adenine, cytosine, and uracil are studied by the Kohn-Sham (KS) method with long-range corrected (LC) functionals. The CEBEs are estimated according to the Koopmans-type theorem for density functional theory. The excitation energies from the core to the valence π* and Rydberg states are calculated as the orbital energy differences between core-level orbitals of a neutral parent/cation and unoccupied π* or Rydberg orbitals of its cation. The model is intuitive, and the spectra can easily be assigned. Core excitation energies from oxygen 1s, nitrogen 1s, and carbon 1s to π* and Rydberg states, and the chemical shifts, agree well with previously reported theoretical and experimental data. The straightforward use of KS orbitals in this scheme carries the advantage that it can be applied efficiently to large systems such as biomolecules and nanomaterials.

We present a systematic assessment of the density functional tight binding (DFTB) method for calculating heats of formation of fullerenes with isodesmic-type reaction schemes. We show that DFTB3-D/3ob can accurately predict Δf H values of the 1812 structural isomers of C60 , reproduce subtle trends in Δf H values for 24 isolated pentagon rule (IPR) isomers of C84 , and predict Δf H values of giant fullerenes that are in effectively exact agreement with benchmark DSD-PBEP86/def2-QZVPP calculations. For fullerenes up to C320 , DFTB Δf H values are within 1.0 kJ mol-1 of DSD-PBEP86/def2-QZVPP values per carbon atom, and on a per carbon atom basis DFTB3-D/3ob yields exactly the same numerical trend of (Δf H [per carbon] = 722n-0.72 + 5.2 kJ mol-1 ). DFTB3-D/3ob is therefore an accurate replacement for high-level DHDFT and composite thermochemical methods in predicting of thermochemical stabilities of giant fullerenes and analogous nanocarbon architectures.

Previously proposed theoretical schemes for estimating one-electron excitation energies using Kohn-Sham (KS) solutions with long-range corrected (LC) functionals are applied to the charge-transfer (CT) excitations of the ethylene···tetrafluoroethylene (C

In this paper, the performance of a wide range of DFT methods is assessed for the calculation of interaction energies of thermal clusters of a solute in water. Three different charge states (neutral, proton transfer transition state and zwitterion) of glycine were solvated by 1 to 40 water molecules as sampled from molecular dynamics simulations. While some ab initio composite methods that employ insufficiently large basis sets incurred significant errors even for a cluster containing only 5 water molecules relative to the W1X-2 benchmark, the DLPNO-CCSD(T)/CBS and DSD-PBEP86 (triple zeta basis set) levels of theory predicted very accurate interaction energies. These levels of theory were used to benchmark the performance of 16 density functionals from different rungs of Jacob's Ladder. Of the Rung 4 functionals examined, the ωB97M-V and ωB97X-V functionals stood out for predicting absolute interaction energies in 40-water clusters with mean absolute deviations (MAD) ?4 kJ mol-1. The B3LYP-D3(BJ) functional performed exceptionally well with a MAD ?1.7 kJ mol-1 and is the overall best performing method. Calculations of relative interaction energies allow for cancellation of systematic errors, including basis set truncation and superposition errors, and the ωB97M-V and B3LYP-D3(BJ) double zeta basis set calculations yielded relative interaction energies that are within ?3 kJ mol-1 of the benchmark. The ONIOM approximation provides another strategy for accelerating the calculation of accurate absolute interaction energies provided that the calculations have converged with respect to the size of the "high-level-layer"., Physical chemistry chemical physics : PCCP, 22(7), pp.3855-3866; 2020

The synthesis of four novel crystalline zeolitic imidazolate framework (ZIF) structures using a mixed-ligand approach is reported. The inclusion of both imidazolate and halogenated benzimidazolate-derived linkers leads to glass-forming behavior by all four structures. Melting temperatures are observed to depend on both electronic and steric effects. Solid-state NMR and terahertz (THz)/far-IR demonstrate the presence of a Zn-F bond for fluorinated ZIF glasses. In situ THz/far-IR spectroscopic techniques reveal the dynamic structural properties of crystal, glass, and liquid phases of the halogenated ZIFs, linking the melting behavior of ZIFs to the propensity of the ZnN4 tetrahedra to undergo thermally induced deformation. The inclusion of halogenated ligands within metal-organic framework (MOF) glasses improves their gas-uptake properties.

In the present study, we have examined a range of quantum chemistry methods for the calculation of conformers for lactose, maltose, and sucrose. We find that the DSD-PBE-P86/aug'-cc-pVTZ//B3-LYP-D3BJ/6-311+G(2d,p) protocol yields good relative energies in comparison with reference CCSD(T)/CBS//B3-LYP-D3BJ/maug-cc-pVTZ values. We have surveyed a total of ∼550 conformers for the three disaccharides with the chosen DSD-PBE-P86 method in conjunction with continuum aqueous solvation. In each case, the lowest free energy conformer is characterized by hydrogen bond(s) between the two rings. Another finding is that the major contributors to the overall variations in aqueous free energies are the electronic energies and the solvation energies. To facilitate investigations of larger systems, we have compiled the DSCONF set of conformers for the three disaccharides, and we have assessed lower cost methods with this set. We find MS1-D3/6-31+G(2d,p) to be cost-effective and accurate for both geometry optimization and the calculation of relative energies for disaccharides. In addition, we note that MS1-D3 has previously been found to yield good relative energies for the WATER27 set of water clusters. We thus deem this method to be appropriate for the study of saccharide conformations in both gas phase and aqueous solution.

Polyproline sequences are highly abundant in prokaryotic 10 and eukaryotic proteins, where they serve as key components of 11 secondary structure. To date, construction of the proline?proline motif 12 has not been possible owing to steric congestion at the ligation junction, 13 together with an n ? ?* electronic interaction that reduces the 14 reactivity of acylated proline residues at the C-terminus of peptides. 15 Here, we harness the enhanced reactivity of prolyl selenoesters and a 16 trans-?-selenoproline moiety to access the elusive proline?proline 17 junction for the ?rst time through a diselenide?selenoester ligation? 18 deselenization manifold. The e?cient nature of this chemistry is 19 highlighted in the high-yielding one-pot assembly of two proline-rich 20 polypeptide targets, submaxillary gland androgen regulated protein 3B 21 and lumbricin-1. This method provides access to the most challenging of ligation junctions, thus enabling the construction of 22 previously intractable peptide and protein targets of increasing structural complexity.

Understanding the nature of charge transfer mechanisms in 3-dimensional metal-organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn(II) frameworks containing cofacial thiazolo[5,4- d]thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus-Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both in situ and ex situ electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative.

Accurate double-hybrid density functional theory and isodesmic-type reaction schemes are utilized to report accurate estimates of the heats of formation (ΔfH) for all 24 isolated-pentagon-rule isomers of the third most abundant fullerene, C84. Kinetic stabilities of these C84 isomers are also considered via C–C bond cleavage rates (Pcleav) calculated using density functional theory. Our results show that the relative abundance of C84 fullerene isomers observed in arc discharge synthesis is the result of both thermochemical and kinetic factors. This provides timely insight regarding the characterization of several C84 isomers that have been obtained experimentally to date. For instance, the established assignments of C84 isomers of (using the Fowler–Manolopoulos numbering scheme) 22 [D2(IV)], 23 [D2d(II)], 19 [D3d], 24 [D6h], 11 [C2(IV)], and 4 [D2d(I)] are consistent with the relative ΔfH and Pcleav values for these structures. However, our thermochemical and kinetic stabilities of Cs isomers 14, 15, and ...

A new iron-based hydrogenase mimic, [Fe(C5(p-C6H4Br)5)(CO)2Br], was synthesised and characterised using X-ray crystallography. Two analogous catalytic precursors, [Fe(C5Ar5)(CO)2Br] (Ar = C6H5; p-C6H4Br), were also analysed electrochemically in order to investigate how bulkier and more electron withdrawing C5Ar5 ligands affect the catalytic generation of hydrogen from trichloroacetic acid. Compared to previously reported iron-based cyclopentadienyl analogues, these complexes were found to have smaller overpotentials and larger turnover numbers. The [Fe(C5(p-C6H4Br)5)(CO)2Br] complex was found to be the most efficient of the two catalytic precursors with a turnover number of 25, an overpotential of around 310 mV and a faster rate of reaction compared to [Fe(C5Ph5)(CO)2Br].

Ionic liquids (ILs) have been widely promoted as functional replacements of volatile organic solvents; however restricting their use to conventional solvents constrains their potential. One means of enhancing their utility is through the development of dual purpose ILs, capable of acting as both solvent and catalyst. A series of bromozincate ILs was synthesised, and the Lewis acidities probed spectroscopically. Furthermore, the crystal and molecular structure of [C2Py]2[ZnBr4] was determined through single-crystal X-ray diffraction. The ILs were then used in a model reaction, aimed at probing effects of both IL Lewis acidity and structural organisation on the reaction outcome.

In a recent computational study, we found that hydrogen bonding/partial deprotonation facilitates subsequent electron transfer from amides to HO•. We have now analyzed these and related reactions with a glycine derivative as a model peptide, investigating not only reaction energies but also barriers for the individual steps. We find that partial deprotonation not only assists subsequent electron transfer (a sequential proton-loss electron-transfer (SPLET)-type reaction pathway) but also promotes sequential hydrogen-atom transfer (HAT, in a sequential proton-loss hydrogen-atom-transfer (SPLHAT)-type process), both being potential alternatives to direct HAT as routes for peptide oxidation. Each of these alternative pathways is calculated to have energy requirements that make them accessible and competitive. These oxidative processes may produce α-carbon-centered peptide radicals that, through deprotonation, are readily oxidized to the corresponding imines. We have also examined the possibility of competing ...

This paper reports additional properties of the electron donor molecule triselenathiafulvalene (TSTF) and its synthesis via a new route involving the precursor tetracarbomethoxytriselenathiafulvalene (TCMTSTF). The structural, electronic and computational properties of TSTF are fully profiled, and discussed in the context of the closely related archetypal electron donor molecule tetrathiafulvalene (TTF). TSTF was also incorporated into a charge transfer (CT) complex with the well-known electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ), to generate TSTF–TCNQ. In order to establish its position in the greater TXF–TCNQ series (where TXF = a tetrachalcogenafulvalene), spectral, electrochemical, magnetic, conductivity and computational studies were performed on TSTF and TSTF–TCNQ to extend what is known about the highly interesting and useful new donor molecule.

The kinetics of many reactions are critically dependent upon the barrier heights for which accurate determination can be difficult. From the perspective of attaining such quantities using computational quantum chemistry, it is important to appropriately validate routine and efficient methodologies such as density functional theory (DFT) procedures. In the present study, we embark on the journey of establishing diverse databases using a consistent high-level quantum chemistry procedure, against which new and existing methodologies can be assessed. Thus, we have used the composite protocol W3X-L to provide more than 100 refined reference values for existing databases [e.g., Y. Zhao and D. G. Truhlar, J. Phys. Chem. A, 2005, 109, 5656] and additionally establish benchmark data that are of interest to atmospheric and combustion chemists. While our endeavor has just begun, assessment of various DFT methods with our existing results lends support to the use of MN15 as an adequate method for general kinetics applications. We also recommend the use of less-costly W2X and WG composite protocols for obtaining adequately accurate reference thermochemical values for larger molecular systems., Physical Chemistry Chemical Physics, 20(16), pp.10732-10740; 2018

Using both experimental and computational methods, focusing on intermediates and model compounds, some of the main features of the reaction mechanisms that operate during the hydrothermal processing of lignin were elucidated. Key reaction pathways and their connection to different structural features of lignin were proposed. Under neutral conditions, subcritical water was demonstrated to act as a bifunctional acid/base catalyst for the dissection of lignin structures. In a complex web of mutually dependent interactions, guaiacyl units within lignin were shown to significantly affect overall lignin reactivity.

We have developed the WnX series of quantum chemistry composite protocols for the computation of highly-accurate thermochemical quantities with advanced efficiency and applicability. The W1X-type methods have a general accuracy of ~3–4 kJ mol−1 and they can currently be applied to systems with ~20–30 atoms. Higher-level methods include W2X, W3X and W3X-L, with the most accurate of these being W3X-L. It can be applied to molecules with ~10–20 atoms and is generally accurate to ~1.5 kJ mol−1. The WnX procedures have opened up new possibilities for computational chemists in pursue of accurate thermochemical values in a highly-productive manner.

We have obtained benchmark data for a set of small molecular systems of Cu, Ag, and Au using coupled-cluster methods. Using this collection of reference data (that we termed the CUAGAU set) for assessing DFT-type methods, we find the MN15-L nonhybrid DFT to be cost-effective for geometry optimization [mean absolute deviation (MAD) in bond length = 0.20 Å], with an accuracy that is comparable to that for the double-hybrid (DH) DFT method DSD-PBEP86 (MAD = 0.19 Å). For the computation of thermochemical properties, among "conventional" (non-MP2-based) DFT methods, the best performance is found for the global-hybrid meta-GGA functional MN15, with an MAD of 11.4 kJ mol

Salens are well-known for their ability to catalyse electrochemical transformations; however, despite their rich history, the fundamental reduction chemistry of these systems remains relatively unexplored. This work reports the design and synthesis of eight discrete, functionalised Mn(iii) pyridyl salen metal complexes, in which the diamine is varied. The electrochemical properties of the complexes were examined using cyclic voltammetry (CV), spectroelectrochemical (SEC) techniques and Density Functional Theory (DFT) computational modelling to explore the mechanisms that underly Mn salen reduction chemistry. We briefly examine the electrochemistry of these complexes in the presence of CO2. These complexes represent potential ligands for incorporation into both discrete and extended metallosupramolecular assemblies.

In the present study, we have obtained geometries and frequency scale factors for a number of double-hybrid density functional theory (DH-DFT) procedures. We have evaluated their performance for obtaining thermochemical quantities [zero-point vibrational energies (ZPVE) and thermal corrections for 298 K enthalpies (ΔH298) and 298 K entropies (S298)] to be used within high-level composite protocols (using the W2X procedure as a probe). We find that, in comparison with the previously prescribed protocol for optimization and frequency calculations (B3-LYP/cc-pVTZ+d), the use of contemporary DH-DFT methods such as DuT-D3 and DSD-type procedures leads to a slight overall improved performance compared with B3-LYP. A major strength of this approach, however, lies in the better robustness of the DH-DFT methods in that the largest deviations are notably smaller than those for B3-LYP. In general, the specific choices of the DH-DFT procedure and the associated basis set do not drastically change the performance. Nonetheless, we find that the DSD-PBE-P86/aug'-cc-pVTZ+d combination has a very slight edge over the others that we have examined, and we recommend its general use for geometry optimization and vibrational frequency calculations, in particular within high-level composite methods such as the higher-level members of the WnX series of protocols. The scale factors determined for DSD-PBE-P86/aug'-cc-pVTZ+d are 0.9830 (ZPVE), 0.9876 (ΔH298), and 0.9923 (S298).

We have used computational quantum chemistry to investigate the thermochemistry of α-hydrogen abstraction from the full set of amino acids normally found in proteins, as well as their peptide forms, by •OH and •SH radicals. These reactions, with their reasonable complexity in the electronic structure (at the α-carbon), are chosen as a consistent set of models for conducting a fairly robust assessment of theoretical procedures. Our benchmarking investigation shows that, in general, the performance for the various classes of theoretical methods improves in the order nonhybrid DFT → hybrid DFT → double-hybrid DFT → composite procedures. More specifically, we find that the DSD-PBE-P86 double-hybrid DFT procedure yields the best agreement with our high-level W1X-2 vibrationless barriers and reaction energies for this particular set of systems. A significant observation is that, when one considers relative instead of absolute values for the vibrationless barriers and reaction energies, even nonhybrid DFT procedures perform fairly well. To exploit this feature in a cost-effective manner, we have examined a number of multilayer schemes for the calculation of reaction energies and barriers for the abstraction reactions. We find that accurate values can be obtained when a "core" of glycine plus the abstracting radical is treated by DSD-PBE-P86, and the substituent effects are evaluated with M06-2X. Inspection of the set of calculated thermochemical data shows that the correlation between the free energy barriers and reaction free energies is strongest when the reactions are either endergonic or nearly thermoneutral.

We have carried out large-scale computational quantum chemistry calculations on the K computer to obtain heats of formation for C60 and some higher fullerenes with the DSD-PBE-PBE/cc-pVQZ double-hybrid density functional theory method. Our best estimated values are 2520.0 ± 20.7 (C60), 2683.4 ± 17.7 (C70), 2862.0 ± 18.5 (C76), 2878.8 ± 13.3 (C78), 2946.4 ± 14.5 (C84), 3067.3 ± 15.4 (C90), 3156.6 ± 16.2 (C96), 3967.7 ± 33.4 (C180), 4364 (C240) and 5415 (C320) kJ mol(-1). In our assessment, we also find that the B3-PW91-D3BJ and BMK-D3(BJ) functionals perform reasonably well. Using the convergence behavior for the calculated per-atom heats of formation, we obtained the formula ΔfH per carbon = 722n(-0.72) + 5.2 kJ mol(-1) (n = the number of carbon atoms), which enables an estimation of ΔfH for higher fullerenes more generally. A slow convergence to the graphene limit is observed, which we attribute to the relatively small proportion of fullerene carbons that are in "low-strain" regions. We further propose that it would take tens, if not hundreds, of thousands of carbons for a fullerene to roughly approach the limit. Such a distinction may be a contributing factor to the discrete properties between the two types of nanomaterials. During the course of our study, we also observe a fairly reliable means for the theoretical calculation of heats of formation for medium-sized fullerenes. This involves the use of isodesmic-type reactions with fullerenes of similar sizes to provide a good balance of the chemistry and to minimize the use of accompanying species.

The use of an organic donor–acceptor polymer containing a viologen electron acceptor and triarylamine electron donor as a platform in the development of multifunctional materials is presented. The highly robust porous organic polymer (POP) system allows for exploration of the interplay between electronic and host–guest interactions in the synthesized polymers, POP-V1, which contains a redox-active triarylamine core and POP-V2, which contains a redox-inactive benzene core, where each of the redox states present can be reversibly accessed. The degree of charge transfer in addition to the H2 and CO2 gas adsorption properties of the polymer are able to be tuned as a function of the electronic state which has important implications for the potential applications of these polymers in optical, electrochromic and solar cell devices.

Six different bromide salts – tetraethylammonium bromide ([N2,2,2,2]Br, 1Br), 1-ethyl-1-methylpiperidinium bromide ([C2MPip]Br, 2Br), 1-ethyl-1-methylpyrrolidinium bromide ([C2MPyrr]Br, 3Br), 1-ethyl-3-methylimidazolium bromide ([C2MIm]Br, 4Br), 1-ethylpyridinium bromide ([C2Py]Br, 5Br), and 1-(2-hydroxyethyl)pyridinium bromide ([C2OHPy]Br, 6Br) – were studied in regards to their capacity to form polybromide monoanion products on addition of molecular bromine in acetonitrile solutions. Using complementary spectroscopic and computational methods for the examination of tribromide and pentabromide anion formation, key factors influencing polybromide sequestration were identified. Here, we present criteria for the targeted synthesis of highly efficient bromine sequestration agents.

The reaction of H + phenol and H/D + toluene has been studied in a supersonic expansion after electric discharge. The (1 + 1') resonance-enhanced multiphoton ionization (REMPI) spectra of the reaction products, at m/z = parent + 1, or parent + 2 amu, were measured by scanning the first (resonance) laser. The resulting spectra are highly structured. Ionization energies were measured by scanning the second (ionization) laser, while the first laser was tuned to a specific transition. Theoretical calculations, benchmarked to the well-studied H + benzene → cyclohexadienyl radical reaction, were performed. The spectrum arising from the reaction of H + phenol is attributed solely to the ortho-hydroxy-cyclohexadienyl radical, which was found in two conformers (syn and anti). Similarly, the reaction of H/D + toluene formed solely the ortho isomer. The preference for the ortho isomer at 100-200 K in the molecular beam is attributed to kinetic, not thermodynamic effects, caused by an entrance channel barrier that is ∼5 kJ mol(-1) lower for ortho than for other isomers. Based on these results, we predict that the reaction of H + phenol and H + toluene should still favour the ortho isomer under elevated temperature conditions in the early stages of combustion (200-400 °C).

Owing to the increased proton affinity that results from additional negative charges, multiply-charged anions have been proposed as one route to prepare and access a range of new and powerful “superbases”. Paradoxically, while the additional electrons in polyanions increase basicity they serve to diminish the electron binding energy and thus, it had been thought, hinder experimental synthesis. We report the synthesis and isolation of the ortho-diethynylbenzene dianion (ortho-DEB2−) and present observations of this novel species undergoing gas-phase proton-abstraction reactions. Using a theoretical model based on Marcus–Hush theory, we attribute the stability of ortho-DEB2− to the presence of a barrier that prevents spontaneous electron detachment. The proton affinity of 1843 kJ mol−1 calculated for this dianion superbase using high-level quantum chemistry calculations significantly exceeds that of the lithium monoxide anion, the most basic system previously prepared. The ortho-diethynylbenzene dianion is therefore the strongest base that has been experimentally observed to date.

We introduce a database of 14 accurate bond dissociation energies (BDEs) of noble gas compounds. Reference CCSD(T)/CBS BDEs are obtained by means of W1 theory. We evaluate the performance of contemporary density functional theory (DFT), double-hybrid DFT (DHDFT), and composite ab initio procedures. A general improvement in performance is observed along the rungs of Jacob’s Ladder; however, only a handful of functionals give good performance for predicting the bond dissociation energies in the NGC14 database. Thus, this database represents a challenging test for DFT methods. Most of the conventional DFT functionals (71%) result in root-mean-square deviations (RMSDs) between 10.0 and 82.1 kJ mol−1. The rest of the DFT functionals attain RMSDs between 2.5 and 8.9 kJ mol−1. The best performing functionals from each rung of Jacob’s Ladder are (RMSD given in parenthesis): HCTH407 (30.9); M06-L (5.4); PBE0 (2.8); B1B95, M06, and PW6B95 (2.7–2.9); CAM-B3LYP-D3 (5.4); and B2T-PLYP (2.5 kJ mol−1).

We have synthesized a perfluoro analog of dibutyl viologen by replacing the side –(CH2)2-CH2CH3 groups with –(CH2)2-C2F5 groups. The effect of fluorination on the aqueous solution electrochemistry was investigated using experimental voltammetric measurements and DFT computations. The formal potential of one-electron transfer redox couple, viologen radical monocation/dication, became 128 mV less negative upon fluorination, and the diffusion coefficient of the oxidized form was lowered by ca. 12%. Calculations indicated that the shift of the formal potential is largely caused by strong electron-withdrawing nature of the perfluorinated groups despite a separation of (CH2)2 from the viologen core. The effect is counter balanced by an opposite but smaller effect from the change of solvation free energy due to perfluorination. This perfluoro viologen is water-soluble in both oxidation states and is expected to be used as a redox probe at electrified oil-water and fluorous phase-water interfaces., Journal of Electroanalytical Chemistry, 856, art. no. 113691; 2020

The effect of adding explicit water molecules to the neutral (N) and zwitterionic (Z) forms of the glycyl radical has been examined. The results show that a minimum of three water molecules is required to stabilize the Z radical as a local minimum, with an energy gap of 123 kJ mol

In the present study, we have examined in detail literature data of deviations for a wide range of (mainly) DFT methods for the extensive MGCDB82 set (∼4400 data points) of main-group thermochemical quantities. We use the data and standard statistical techniques (lasso regularization and forward selection) to devise the MG8 model for linearly combining assessment results of a collection of small data sets to accurately estimate the MAD of MGCDB82. The MG8 model contains a total of 64 data points representing noncovalent interactions, isomerization energies, thermochemical properties, and barrier heights. It is thus well suited for rapid evaluation of new quantum chemistry procedures. We propose that a value of ∼4 kJ mol–1 for an estimated MAD by the MG8 model (EMADMG8) to be an initial indicator of a highly robust quantum chemistry method, with large deviations occurring mainly for properties (such as heats of formation) that are known to be difficult to accurately compute. For methods with larger EMADs, ...

Ambient temperature spin crossover with wide hysteresis has been achieved in 2D Hofmann-type materials, where removal of guest molecules optimises ligand–ligand interactions, resulting in increased cooperativity., Molecule-based spin state switching materials that display ambient temperature transitions with accompanying wide thermal hysteresis offer an opportunity for electronic switching, data storage, and optical technologies but are rare in existence. Here, we present the first 2D Hofmann-type materials to exhibit the elusive combination of ambient temperature spin crossover with wide thermal hysteresis (ΔT = 50 and 65 K). Combined structural, magnetic, spectroscopic, and theoretical analyses show that the highly cooperative transition behaviours of these layered materials arise due to strong host–host interactions in their interdigitated lattices, which optimises long-range communication pathways. With the presence of water molecules in the interlayer pore space in the hydrated phases, competing host–host and host–guest interactions occur, whilst water removal dramatically increases the framework cooperativity, thus affording systematic insight into the structural features that favour optimal spin crossover properties.

In the present study, we use the ONIOM strategy of Morokuma and coworkers to examine the various CH bond dissociation energies (BDEs) of a small peptide (2ONW) and compare these with values obtained for its component individual amino acid residues. To evaluate suitable methods for ONIOM-based geometry optimizations, we test an "internal consistency" approach against full B3-LYP//B3-LYP results, and find B3-LYP/6-31G(d):AM1 to be appropriate. We find that the BDEs at the α-carbon in 2ONW are generally larger than the corresponding values for the individual residues on their own. This is attributed to the constraints of the peptide backbone leading to conformations that are not ideal for captodative stabilization of the resulting α-radicals. At the more flexible β- and γ-positions, the differences between the BDEs in 2ONW and the individual residues are smaller. Overall, the α-BDEs are smaller than the β- and γ-BDEs in most cases. Thus, to rationalize the inertness of peptide backbones with respect to α-hydrogen abstraction that is frequently found experimentally, it is necessary to consider alternative protection mechanisms such as the polar effect. © 2018 Wiley Periodicals, Inc.

Coordination frameworks containing DCNQI2– (DCNQI = N,N′-dicyanoquinonediimine ligand) are produced by deprotonation of DCNQIH2 in the presence of a metal center and a co-ligand. This approach has yielded two-dimensional (2D) sheet compounds [Cd(DCNQI)(L)2] (where L = pyridine (py) or isoquinoline (isoquin)) that can be partially oxidized via solid-state electrochemical and in situ spectroelectrochemical methods to materials that contain DCNQI as its radical monoanion. The new frameworks display charge-transfer bands that are indicative of interligand charge-transfer interactions as supported by TD-DFT computational calculations. The redox-state dependent spectral properties of the frameworks have been probed using a newly developed solid-state spectroelectrochemical cell. Coupled with computational calculations, the experimental data provide an understanding of the fundamental charge-transfer processes that may underpin long-range functional properties such as conductivity in framework materials.

In the present study, we have reparametrized the HISS exchange functional. The new "reHISS" exchange provides a balance between short- and mid-range Hartree-Fock exchange (HFX) and a large total HFX coverage, with a fast convergence to zero HFX in the long range. The five parameters in this functional (according to equations 3 and 4 in the main text) are cSR = 0.15, cMR = 2.5279, cLR = 0, ωSR = 0.27, and ωLR = 0.2192. The combination of reHISS exchange with a reparametrized B97c-type correlation functional (Chan et al., J. Comput. Chem. 2017, 38, 2307) and a D2 dispersion term (s6 = 0.6) gives the reHISSB-D method. We find it to be more accurate than related screened-exchange methods and, importantly, its accuracy is more uniform across different properties. Fundamentally, our analysis suggests that the good performance of the reHISS exchange is related to it capturing a near-optimal proportion of HFX in the range of interelectronic distance that is important for many chemical properties, and we propose this range to be approximately 1-4A. © 2018 Wiley Periodicals, Inc.

In this study, we explore the effect of supplementing the DuT spin-component-scaled double-hybrid density functional method with post-second-order Møller-Plesset-type theory (MP2) correlation terms. We find that the inclusion of additional MP3 correlation energies has almost no effect on the performance. Further addition of correlation effects from MP4 generally leads to a small improvement in the performance. However, we find that the inclusion of the higher-order perturbative correlation effects does not rectify some major shortcomings of DuT for more challenging systems, and the use of MP4, in fact, leads to a significant deterioration in the performance in some cases. We also find that the use of correlation energies from CCSD(T) instead of those from MP3 and MP4 does not lead to a substantial improvement over the MP4-based method, both in general and in some difficult cases that we have examined. An additional observation is that, for large systems that are dominated by noncovalent interactions, DuT and the two MPn-based post-MP2 double-hybrid density functional theory (DFT) procedures all benefit from the inclusion of dispersion corrections. Overall, our investigation suggests that the current generation of MP2-based double-hybrid DFT methods may already be providing close to the optimal performance that can be achieved with the double-hybrid methodology paired with spin-component-scaling. Development of even better double hybrids is an active research field, and we hope that our study provides valuable insights. We recommend the continuing use of existing MP2-based double-hybrid methods as a bridging level between hybrid density functional procedures and high-level wave-function-based procedures.

We have assessed the performance of a variety of low-cost computational quantum chemistry procedures (semiempirical, pure-DFT, and screened-exchange DFT methods) for computing molecular geometries and thermochemical quantities associated with the vibrational frequencies. Frequency scale factors for zero-point vibrational energies and thermal corrections for 298 K enthalpies and 298 K entropies have been determined. In absolute terms, for small to medium-sized molecules, all procedures perform reasonably well. Semiempirical methods have mean absolute deviations (MADs) of ∼15 kJ mol–1 for total enthalpies and free energies. For DFT procedures, hybrid DFT generally performs better than pure DFT. Remarkably, the N12 pure functional shows very good performances (MADs ∼ 3 kJ mol–1) that are comparable to those for hybrid functionals. An examination of the basis set effect indicates N12/3-21G* and N12/6-31G(d) to be cost-effective for geometry optimization and vibrational frequency calculations, but the use of m...

We have devised the composite procedures WG and WGh to unify the W1X and the (computationally more economical) G4(MP2)-6X protocols. The WG procedure employs a combination of MP2, MP2-F12, CCSD-F12b, and CCSD(T) to approximate the all-electron scalar-relativistic CCSD(T)/CBS energy. In addition, it incorporates features such as the scaling of the energy components and an empirical “higher-level-correction” term. The WGh protocol represents a somewhat more economical variant of WG with partial removal of diffuse functions. Our benchmark shows that, in general, both WG and WGh have similar performance to that for W1X-2, with WGh (predictably) performing somewhat less well for electron affinities. In terms of computational efficiency, WG is approximately an order of magnitude less costly than W1X-2, while WGh gives not only a further slight savings in computer time but also a notably reduced disk requirement.