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As complex materials are widely used in emerging technologies for environmental and energy applications, it is important to be able to quantify their stimuli-response behaviors. Light is a useful stimulus to modulate multifunctional electrochemical, magnetic, optical, and structural properties in metal–organic frameworks (MOFs); however, the underlying mechanisms and kinetics of light-induced structural changes are not well understood. Herein, a double [2 + 2] photocyclization in photoactive [Cd2(stil)2(Py2TTF)2] (stil2– = 4,4′-stilbenedicarboxylic acid, Py2TTF = 2,6-bis(4′-pyridyl)-tetrathiafulvalene) offers a powerful platform to quantitatively probe solid-state photocyclization kinetics. Variable-temperature Raman spectroscopy revealed a nonlinear temperature dependence of these parameters, which could be analyzed using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetic model to yield a maximum rate observed between 0 °C and 20 °C of approximately 0.172 s–1. These results offer the first example of the quantification of the photocyclization kinetics in a MOF. Density functional theory (DFT) calculations support a singlet reaction mechanism for the double [2 + 2] photocyclization, which is facilitated by the cofacial alignment of Py2TTF ligands. Establishing mechanistic and kinetic models that can be applied to multistimuli-responsive materials provides a powerful platform for their future design for applications in sensing, switching, and molecular separations.

We present an approximate approach for the calculation of ionization potential (IP) and electron affinity (EA) by exploiting the complementary energy non-linearity errors for a species M and its one-electron-ionized counterpart (M

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.

In the present study, we introduce the accurate Wn-P34 quantum chemistry composite methods with applicability to heavy p-block elements up to xenon. For a set of thermochemical properties for prototypical third- and fourth-row species and for a diverse set of small light-main-group species, they show accuracies of ∼3 kJ mol-1 or better. Overall, the Wn-P34 methods are comparable in accuracy to Wn, with a widened applicability to heavier elements. We have used Wn-P34 to compile the P34 set of accurate thermochemical values for heavy p-block species, and we have applied this set to assess a wide range of lower-cost methods. The results of our assessment show that the G4(MP2)-XK composite method provides adequate treatments for these species, but several widely used double-hybrid density functional theory (DH-DFT) methods show uncharacteristically large deviations. In contrast, we find it presently surprising that some pure and hybrid DFT methods such as TPSS and SCANh perform quite well. We hope that our findings and new tools would facilitate the application of computational chemistry for heavy elements, of which the properties are yet to be broadly explored.

In the present study, we have investigated the applicability of long-range-corrected (LC) functionals to a Kohn-Sham (KS) Koopmans'-type theorem. Specifically, we have examined the performance of optimally tuned LCgau-core functionals (in combination with BOP and PW86-PW91 exchange-correlation functionals) by calculating the ionization potential (IP) within the context of Koopmans' prediction. In the LC scheme, the electron repulsion operator, 1/

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

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...

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 ...

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 ...

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.

Sodium germanate is a nontransition-metal catalyst that is active in the selective dehydrogenation of formic acid. However, bulk sodium germanate has a very low surface area, limiting the availability of the germanate sites for catalysis. The dispersion of germanate in the zeolite ZSM-5 has been investigated both computationally and experimentally as a method for the provision of greater surface area and, therefore, higher activity per germanate site. Nanodomain islets of germanate dispersed in the germanium ZSM-5 zeolite invert selectivity from dehydration (in ZSM-5) to dehydrogenation of formic acid, potentially making Na-Ge-ZSM-5 a cost-effective catalyst for releasing hydrogen from formic acid.

We have examined hydrogen-atom-abstraction reactions for combinations of electrophilic/nucleophilic radicals with electrophilic/nucleophilic substrates. We find that reaction between an electrophilic radical and a nucleophilic substrate is relatively favorable, and vice versa, but the reactions between a radical and a substrate that are both electrophilic or both nucleophilic are relatively unfavorable, consistent with the literature reports of Roberts. As a result, the regioselectivity for the abstraction from a polar substrate can be reversed by reversing the polarity of the attacking radical. Our calculations support Roberts' polarity-reversal-catalysis concept and suggest that addition of a catalyst of appropriate electrophilicity/nucleophilicity can lead to an enhancement of the reaction rate of approximately 5 orders of magnitude. By exploiting the control over regioselectivity associated with the polar nature of the radical and the substrate, we demonstrate the possibility of directing the regioselectivity of hydrogen abstraction from amino acid derivatives and simultaneously providing a significant rate acceleration.

Excitation spectra of the 1H-naphthalene (1-C10H9) and 1D-naphthalene (1-C10H8D) radicals, and their cations, are obtained by laser spectroscopy and mass spectrometry of a skimmed free-jet expansion following an electrical discharge. The spectra are assigned on the basis of density functional theory calculations. Isotopic shifts in origin transitions, vibrational frequencies and ionization energies were found to be well reproduced by (time-dependent) density functional theory. Absolute bond dissociation energies, ionization energies and proton affinities were calculated using high-level quantum chemical methods.

We have examined the performance of a variety of density functional theory procedures for the calculation of complexation energies and proton-exchange barriers, with a focus on the Minnesota-class of functionals that are generally highly robust and generally show good accuracy. A curious observation is that M05-type and M06-type methods show an atypical decrease in calculated barriers with increasing proportion of Hartree-Fock exchange. To obtain a clearer picture of the performance of the underlying components of M05-type and M06-type functionals, we have investigated the combination of MPW-type and PBE-type exchange and B95-type and PBE-type correlation procedures. We find that, for the extensive E3 test set, the general performance of the various hybrid-DFT procedures improves in the following order: PBE1-B95 → PBE1-PBE → MPW1-PBE → PW6-B95. As M05-type and M06-type procedures are related to PBE1-B95, it would be of interest to formulate and examine the general performance of an alternative Minnesota DFT method related to PW6-B95.

The first synthesis of the furan-containing snyderane, (+)-luzofuran, is reported. The key step in this approach was an electrophilic brominative cyclization, which was accomplished using a nucleophilic N-heterocycle-flanked phosphoramidite catalyst in combination with the common laboratory reagent N-bromosuccinimide.

The new one-dimensional coordination framework (Zn(DMF)NO3)2(NDC)(DPMNI), where NDC = 2,6-naphthalenedicarboxylate and DPMNI = N,N'-bis(4-pyridylmethyl)-1,4,5,8-naphthalenetetracarboxydiimide, which has been crystallographically characterized, exhibits two redox-accessible states due to the successive reduction of the naphthalenediimide (NDI) ligand core. Solid-state electrochemical and vis-near-IR spectroelectrochemical measurements coupled with density functional theory (DFT) calculations enabled the origins of the optical transitions in the spectra of the monoradical anion and dianion states of the material to be assigned. Electron paramagnetic resonance (EPR) spectroscopy revealed that the paramagnetic radical anion state of the DPMNI core could be accessed upon broad-spectrum white light irradiation of the material, revealing a long-lived excited state, possibly stabilized by charge delocalization which arises from extensive π-π* stacking interactions between alternating NDC and NDI aromatic cores which are separated by a distance of 3.580(2) Å.

In this study, we address the issues associated with predicting usefully accurate heats of formation for moderately-sized molecules such as corannulene and C(60). We obtain a high-level theoretical heat of formation for corannulene through the use of reaction schemes that conserve increasingly larger molecular fragments between the reactants and products. The reaction enthalpies are obtained by means of the high-level, ab initio W1h thermochemical protocol, while accurate experimental enthalpies of formation for the other molecules involved in the reactions are obtained from the Active Thermochemical Tables (ATcT) network. Our best theoretical heat of formation for corannulene (Δ(f)H°(298)[C(20)H(10)(g)] = 485.2 ± 7.9 kJ mol(-1)) differs significantly from the currently accepted experimental value (Δ(f)H°(298)[C(20)H(10)(g)] = 458.5 ± 9.2 kJ mol(-1)), and this suggests that re-examination of the experimental data may be in order. We have used our theoretical heat of formation for corannulene to obtain a predicted heat of formation of C(60) through reactions that involve only corannulene and planar polyacenes. Current experimental values span a range of ~200 kJ mol(-1). Our reaction enthalpies are obtained by means of double-hybrid density functional theory in conjunction with a large quadruple-ζ basis set, while accurate experimental heats of formation (or our theoretical value in the case of corannulene) are used for the other molecules involved. Our best theoretical heat of formation for C(60) (Δ(f)H°(298)[C(60)(g)] = 2521.6 kJ mol(-1)) suggests that the experimental value adopted by the NIST thermochemical database (Δ(f)H°(298)[C(60)(g)] = 2560 ± 100 kJ mol(-1)) should be revised downward.

High-level W1w ab initio calculations have been used to investigate the structural and thermochemical changes that result from heteroatomic deprotonation of CH(3)YH molecules and (•)CH(2)YH radicals (YH = BH(2), CH(3), NH(2), OH, AlH(2), SiH(3), PH(2), and SH). The thermochemical quantities considered include gas-phase acidities, various bond dissociation enthalpies, and heats of formation. The high-level ab initio results are compared with available experimental data and generally show excellent agreement. In a small number of cases in which we find discrepancies that persist at even higher theoretical levels (e.g., W4), we suggest that the experimental data should be re-examined. We find that the C-Y bond lengths of (•)CH(2)YH contract upon deprotonation, whereas for CH(3)YH, the predicted effect, in general, is a lengthening of the C-Y bond. These structural changes, for the most part, are reflected in the changes to the C-Y bond dissociation enthalpies. The CH(3)YH molecules are calculated to be 50-200 kJ mol(-1) less acidic in the gas phase than the corresponding (•)CH(2)YH radicals, indicating relative stabilization of the (•)CH(2)Y(-) radical anions. The structural and thermochemical changes are rationalized using a combination of resonance and orbital interaction arguments.

In recent computational studies of hydrogen-atom abstraction from amino acid derivatives, two distinct rationalizations have been put forward for the relative inertness of the α-C-H. Of these, the proposal that the inertness is due to a "kinetic trap" associated with particularly stable complexes is shown to be unlikely because of unfavorable entropies. On the other hand, the proposed existence of deactivating polar effects at the α-position in Cl(•) abstractions is likely also to be applicable to OH(•) abstractions, but to a lesser extent.

We have analyzed the performance of several widely used density functional theory procedures, namely B-P86, B-PW91, B-LYP, B3-P86, B3-PW91, and B3-LYP, for the E3 set of thermochemical properties. Each of these procedures employs a local density approximation (LDA) functional and a gradient correction for the correlation energy. We find that the VWN3 LDA functional in B-P86, B-PW91, B3-P86, and B3-PW91 leads to extremely large deviations from benchmark values for heats of formation (as large as -455.6 kJ mol(-1) for the B-PW91(VWN3) value for azulene!) and that VWN3 also gives significant errors in the calculated ionization energies and electron affinities. The PW91 gradient correction generally performs much better than P86 for heats of formation, and we propose that this is because P86 severely violates a uniform scaling condition that PW91 almost satisfies. Thus, of the procedures that we have examined, we recommend the use of the VWN5 or PW92 forms of LDA, preferably in combination with the PW91 gradient correction. Our results confirm previous findings that VWN3 is a more suitable LDA than VWN5 for B3-LYP, and we attribute this to fortuitous error cancellation between understabilization of molecules by LYP and overstabilization by VWN3.

In the present study, we use composite methods, denoted CM(5)Λ and CM5, with post-CCSD(T) terms up to CCSDTQ(5)Λ and CCSDTQ5, respectively, to evaluate the atomization energies for CrO, CrO2, and CrO3. The heats of formation (ΔHf,298) based on our best estimated atomization energies are 198.3 ± 5 kJ mol(-1) (CrO), -81.3 ± 5 kJ mol(-1) (CrO2), and -286.8 ± 20 kJ mol(-1) (CrO3). Standard G4-type composite methods yield atomization energies that are adequate for CrO, less good for CrO2, and least good for CrO3. CrO3 is highly multireference in character, and therefore, a "black box" approach of using a single-reference RHF wave function is inadequate, even for "high-level" G4-type methods. We find that, for CrO3, there is a very large difference in the G4 atomization energies depending on whether an RHF or a UHF reference is used, which is mainly associated with large differences in the MP4 components. In general, we propose that a large R-versus-U difference is likely to be an indication of potential problems in the theoretical treatment. Going beyond G4 to a more rigorous UCCSD(T)-based composite scheme [termed U-CM(3:[DZ,TZ]) in the present study], we again find a large difference (but significantly smaller than that for G4) between the CrO3 atomization energies based on RHF or UHF references. Intriguingly, the use of Brueckner orbitals as reference orbitals in all components, as in the corresponding Brueckner Doubles (BD) procedure [B-CM(3:[DZ,TZ])], produces results for CrO3 that are independent of whether RHF or UHF orbitals are used as the starting point.

Calculation of accurate water-water interaction energies is of fundamental importance in computational modeling of many biological and chemical phenomena. We have obtained benchmark barrier heights for proton-exchange reactions and complexation energies in water clusters (H2O)n (n = 1-6) by means of the high-level W1-F12 procedure. We find that lower-cost composite procedures (e.g., G4(MP2) and G4(MP2)-6X), as well as MP2 and SCS-MP2, exhibit surprisingly poor performance for the barrier heights of reactions involving multiple proton exchanges. Moreover, the performance significantly deteriorates with increasing size of the clusters. Similar observations apply to complexation energies in water clusters, and to barrier heights for proton exchange in ammonia and hydrogen fluoride clusters. We propose a modified version of G4(MP2)-6X (denoted G4(MP2)-6X+) that includes sp- and d-diffuse functions in the CCSD(T) term, which gives excellent proton-exchange barrier heights at a computational cost only slightly greater than that of standard G4(MP2). G4(MP2)-6X+ also leads to a substantial improvement over G4(MP2) and G4(MP2)-6X for the calculation of electron affinities.

A variety of combinations of B-LYP-based double-hybrid density functional theory (DHDFT) procedures and basis sets have been examined. A general observation is that the optimal combination of exchange contributions is in the proximity of 30% Becke 1988 (B88) exchange and 70% Hartree-Fock (HF) exchange, while for the correlation contributions, the use of independently optimized spin-component-scaled Møller-Plesset second-order perturbation theory (SCS-MP2) parameters (MP2OS and MP2SS) is beneficial. The triple-ζ Dunning aug'-cc-pVTZ+d and Pople 6-311+G(3df,2p)+d basis sets are found to be cost-effective for DHDFT methods. As a result, we have formulated the DuT-D3 DHDFT procedure, which employs the aug'-cc-pVTZ+d basis set and includes 30% B88 and 70% HF exchange energies, 59% LYP, 47% MP2OS, and 36% MP2SS correlation energies, and a D3 dispersion correction with the parameters s6 = 0.5, sr,6 = 1.569, and s8 = 0.35. Likewise, the PoT-D3 DHDFT procedure was formulated with the 6-311+G(3df,2p)+d basis set and has 32% B88 and 68% HF exchange energies, 63% LYP, 46% MP2OS, and 27% MP2SS correlation energies, and the D3 parameters s6 = 0.5, sr,6 = 1.569, and s8 = 0.30. Testing using the large E3 set of 740 energies demonstrates the robustness of these methods. Further comparisons show that the performance of these methods, particularly DuT-D3, compares favorably with the previously reported DSD-B-LYP and DSD-B-LYP-D3 methods used in conjunction with quadruple-ζ aug'-pc3+d and aug'-def2-QZVP basis sets but at lower computational expense. The previously reported ωB97X-(LP)/6-311++G(3df,3pd) procedure also performs very well. Our findings highlight the cost-effectiveness of appropriate- and moderate-sized triple-ζ basis sets in the application of DHDFT procedures.

The G4-SP and G4(MP2)-SP procedures are introduced, as alternatives to G4 and G4(MP2), to overcome shortcomings associated with the treatment of spin polarization (SP) in large open-shell systems. The new methods employ a converging SP term, to replace the diverging A' treatment used in the G4 and G4(MP2) formulations. The G4-SP and G4(MP2)-SP procedures have mean absolute deviations (MADs) from experimental energies of 3.49 and 4.37 kJ mol(-1), respectively, for the G3/05 test set, which are comparable to the MAD values for G4 and G4(MP2) but eliminate the problem of a diverging A' term. For energies involving larger radicals, G4(MP2)-SP performs better than standard G4(MP2). Alternative methods, including G4-5H, G4(MP2)-5H, G4-sc, and G4(MP2)-sc, are also introduced to avoid the problem of an indefinitely increasing SP correction in standard G4 or G4(MP2) for reactions involving larger open-shell systems.

Quantum chemistry calculations have been used to study the uncatalyzed transfer hydrogenation between a range of hydrogen donors and acceptors, in the gas phase and in solution. Our study shows in the first place that in order to obtain reliable condensed-phase transition structures, it is necessary to perform geometry optimization in the presence of a continuum. In addition, the use of a free energy of solvation obtained with the UB3-LYP/6-31+G(d,p)/IEF-PCM/UA0 combination, in conjunction with UMPWB1K/6-311+G(3df,2p)//B3-LYP/6-31+G(d,p) gas-phase energies, gives the best agreement with experimental barriers. In condensed phases, the geometries and energies of the transition structures are found to relate to one another in a manner consistent with the Hammond postulate. There is also a correlation between the barriers and the energies of the radical intermediates in accord with the Bell-Evans-Polanyi principle. We find that in the gas phase, all the transfer-hydrogenation reactions examined proceed via a radical pathway. In condensed phases, some of the reactions follow a radical mechanism regardless of the solvent. However, for some reactions there is a change from a radical mechanism to an ionic mechanism as the solvent becomes more polar. Our calculations indicate that the detection of radical adducts by EPR does not necessarily indicate a predominant radical mechanism, because of the possibility of a concurrent ionic reaction. We also find that the transition structures for these reactions do not necessarily have a strong resemblance to the intermediates, and therefore one should be cautious in utilizing the influence of polar effects on the rate of reaction as a means of determining the mechanism.

Ab initio molecular orbital calculations have been employed to investigate the interactions between a set of basic substrates (B) with H+ and HF, and the interaction between acids of varying strength (AH+) with two bases, vinylamine and furan. The preferred site for protonation of the substrates appears to be determined primarily by the ability of the protonated species (BH+) to delocalize the acquired positive charge. On the other hand, localization of a pair of electrons at a proton-acceptor site of B tends to be more important in determining the preferred site for hydrogen bonding with HF. The behavior of acids stronger than HF lies between these extremes. Consistent with a previously proposed Hammond postulate for complexes, when a substrate (B) interacts with a range of acids (AH+), proton transfer is generally found to occur when the proton affinity of A is significantly less than that of B. When the proton affinity of A is greater than that of B, a hydrogen-bonded complex is generally formed without proton transfer. Strongest binding (relative to the lowest energy components) occurs when the proton affinities of A and B are comparable. Proton transfer from AH+ is found to take place in some cases when this would not be predicted on the basis of protonation energies alone, because of specific interactions in the resulting complexes. © 2005 American Chemical Society.

Quantum chemistry calculations show that the barriers for HAlF(4)-catalyzed hydrogenolysis reactions of the carbon-heteroatom bond are reduced substantially compared with those for the uncatalyzed reactions. For example, the condensed-phase free-energy barrier for the HAlF(4)-catalyzed hydrogenolysis of CH(3)-F is about 130 kJ mol(-1), compared with 373 kJ mol(-1) for the uncatalyzed reaction. The reactions are facilitated in the case of substrates that can give rise to a stable carbocation and for strongly acidic catalysts.