Nucleophilic additions to alkylidene bis(sulfoxides): stereoelectronic effects in vinyl sulfoxides

Conjugate additions of nucleophiles (e.g. enolates, amines and malonate anions) to bis(p-tolylsulfinyl)alkenes, alkylidene-1,3-dithiane-1,3-dioxides and alkylidene-1,3-dithiolane-1,3-dioxides have recently been published. Reasons for different selectivities and reaction rates will be discussed by consideration of steric and electronic effects. The preferred mode of attack can be explained by stereoelectronic effects (hyperconjugation) in the primarily carbanion, which is stabilized by n-->S-O-sigma* interaction with an antiperiplanar S=O group. Calculation of the transition states [BP86/aug-TZVP] for the addition of acetone enolate to the dithiane-derived alkylidene bis(sulfoxide) revealed that 6.6-7.3 kJ mol(-1) more energy is needed for an attack leading to a less-stabilized carbanion. Two axial S=O groups in dithiolane-derived alkylidene bis(sulfoxides) lead to a higher reactivity towards nucleophiles.     

The 1deltag dioxygen ene reaction with propene: a density functional and multireference perturbation theory mechanistic study

This study aims to determine whether a balance between concerted and non-concerted pathways exists, and in particular to ascertain the possible role of diradical/zwitterion or peroxirane intermediates. Three non-concerted pathways, via 1) diradical or 2) peroxirane intermediates, and 3) by means of hydrogen-abstraction/radical recoupling, plus one concerted pathway (4), are explored. The intermediates and transition structures (TS) are optimized at the DFT(MPW1K), DFT(B3LYP) and CASSCF levels of theory. The latter optimizations are followed by multireference perturbative CASPT2 energy calculations. (1) The polar diradical forms from the separate reactants by surmounting a barrier (deltaE(++)(MPW1K)=12, deltaE++(B3LYP)=14, and deltaE(++)(CASPT2)=16 kcal mol(-1) and can back-dissociate through the same TS, with barriers of 11 (MPW1K) and 8 kcal mol(-1) (B3LYP and CASPT2). The diradical to hydroperoxide transformation is easy at all levels (deltaE(++)(MPW1K)<4, deltaE(++)(B3LYP)=1 and deltaE(++)(CASPT2)=1 kcal mol(-1)). (2) Peroxirane is attainable only by passing through the diradical intermediate, and not directly, due to the nature of the critical points involved. It is located higher in energy than the diradical by 12 kcal mol(-1), at all theory levels. The energy barrier for the diradical to cis-peroxirane transformation (deltaE(++)=14-16 kcal mol(-1)) is much higher than that for the diradical transformation to the hydroperoxide. In addition, peroxirane can very easily back-transform to the diradical (deltaE(++)<3 kcal mol(-1)). Not only the energetics, but also the qualitative features of the energy hypersurface, prevent a pathway connecting the peroxirane to the hydroperoxide at all levels of theory. (3) The last two-step pathway (hydrogen-abstraction by (1)O(2), followed by HOO-allyl radical coupling) is not competitive with the diradical mechanism. (4) A concerted pathway is carefully investigated, and deemed an artifact of restricted DFT calculations. Finally, the possible ene/[pi2+pi2] competition is discussed.     

Density functional theory for molecular and periodic systems using density fitting and continuous fast multipole method: Stress tensor

A full implementation of the analytical stress tensor for periodic systems is reported in the TURBOMOLE program package within the framework of Kohn–Sham density functional theory using Gaussian-type orbitals as basis functions. It is the extension of the implementation of analytical energy gradients (Lazarski et al., Journal of Computational Chemistry 2016, 37, 2518–2526) to the stress tensor for the purpose of optimization of lattice vectors. Its key component is the efficient calculation of the Coulomb contribution by combining density fitting approximation and continuous fast multipole method. For the exchange-correlation (XC) part the hierarchical numerical integration scheme (Burow and Sierka, Journal of Chemical Theory and Computation 2011, 7, 3097–3104) is extended to XC weight derivatives and stress tensor. The computational efficiency and favorable scaling behavior of the stress tensor implementation are demonstrated for various model systems. The overall computational effort for energy gradient and stress tensor for the largest systems investigated is shown to be at most two and a half times the computational effort for the Kohn–Sham matrix formation. © 2019 Wiley Periodicals, Inc.     

Density functional theory for molecular and periodic systems using density fitting and continuous fast multipole method: Analytical gradients

A full implementation of analytical energy gradients for molecular and periodic systems is reported in the TURBOMOLE program package within the framework of Kohn–Sham density functional theory using Gaussian‐type orbitals as basis functions. Its key component is a combination of density fitting (DF) approximation and continuous fast multipole method (CFMM) that allows for an efficient calculation of the Coulomb energy gradient. For exchange‐correlation part the hierarchical numerical integration scheme (Burow and Sierka, Journal of Chemical Theory and Computation 2011, 7, 3097) is extended to energy gradients. Computational efficiency and asymptotic O(N) scaling behavior of the implementation is demonstrated for various molecular and periodic model systems, with the largest unit cell of hematite containing 640 atoms and 19,072 basis functions. The overall computational effort of energy gradient is comparable to that of the Kohn–Sham matrix formation. © 2016 Wiley Periodicals, Inc.     

Experimental and theoretical multiple kinetic isotope effects for an SN2 reaction. An attempt to determine transition-state structure and the ability of theoretical methods to predict experimental kinetic isotope effects

The secondary alpha-deuterium, the secondary beta-deuterium, the chlorine leaving-group, the nucleophile secondary nitrogen, the nucleophile (12)C/(13)C carbon, and the (11)C/(14)C alpha-carbon kinetic isotope effects (KIEs) and activation parameters have been measured for the S(N)2 reaction between tetrabutylammonium cyanide and ethyl chloride in DMSO at 30 degrees C. Then, thirty-nine readily available different theoretical methods, both including and excluding solvent, were used to calculate the structure of the transition state, the activation energy, and the kinetic isotope effects for the reaction. A comparison of the experimental and theoretical results by using semiempirical, ab initio, and density functional theory methods has shown that the density functional methods are most successful in calculating the experimental isotope effects. With two exceptions, including solvent in the calculation does not improve the fit with the experimental KIEs. Finally, none of the transition states and force constants obtained from the theoretical methods was able to predict all six of the KIEs found by experiment. Moreover, none of the calculated transition structures, which are all early and loose, agree with the late (product-like) transition-state structure suggested by interpreting the experimental KIEs.     

Ab initio static and molecular dynamics study of 4-styrylpyridine.

We report an in‐depth theoretical study of 4‐styrylpyridine in its singlet S₀ ground state. The geometries and the relative stabilities of the trans and cis isomers were investigated within density functional theory (DFT) as well as within Hartree–Fock (HF), second‐order Møller–Plesset (MP2), and coupled cluster (CC) theories. The DFT calculations were performed using the B3LYP and PBE functionals, with basis sets of different qualities, and gave results that are very consistent with each other. The molecular structure is thus predicted to be planar at the energy minimum, which is associated with the trans conformation, and to become markedly twisted at the minimum of higher energy, which is associated with the cis conformation. The results of the calculations performed with the post‐HF methods approach those obtained with the DFT methods, provided that the level of treatment of the electronic correlation is high enough and that sufficiently flexible basis sets are used. Calculations carried out within DFT also allowed the determination of the geometry and the energy of the molecule at the biradicaloid transition state associated with the thermal cis?trans isomerization and at the transition states associated with the enantiomerization of the cis isomer and with the rotations of the pyridinyl and phenyl groups in the trans and cis isomers. Car–Parrinello molecular dynamics simulations were also performed at 50, 150, and 300 K using the PBE functional. The studies allowed us to evidence the highly flexible nature of the molecule in both conformations. In particular, the trans isomer was found to exist mainly in a nonplanar form at finite temperatures, while the rotation of the pyridinyl ring in the cis isomer was incidentally observed to take place within ≈1 ps during the simulation carried out at 150 K on this isomer.     

Ab initio calculations on halogen-bonded complexes and comparison with density functional methods

A systematic theoretical investigation on a series of dimeric complexes formed between some halocarbon molecules and electron donors has been carried out by employing both ab initio and density functional methods. Full geometry optimizations are performed at the Moller-Plesset second-order perturbation (MP2) level of theory with the Dunning's correlation-consistent basis set, aug-cc-pVDZ. Binding energies are extrapolated to the complete basis set (CBS) limit by means of two most commonly used extrapolation methods and the aug-cc-pVXZ (X = D, T, Q) basis sets series. The coupled cluster with single, double, and noniterative triple excitations [CCSD(T)] correction term, determined as a difference between CCSD(T) and MP2 binding energies, is estimated with the aug-cc-pVDZ basis set. In general, the inclusion of higher-order electron correlation effects leads to a repulsive correction with respect to those predicted at the MP2 level. The calculations described herein have shown that the CCSD(T) CBS limits yield binding energies with a range of -0.89 to -4.38 kcal/mol for the halogen-bonded complexes under study. The performance of several density functional theory (DFT) methods has been evaluated comparing the results with those obtained from MP2 and CCSD(T). It is shown that PBEKCIS, B97-1, and MPWLYP functionals provide accuracies close to the computationally very expensive ab initio methods.     

Comparison of ab initio and density functional methods for vibrational analysis of TeCl4

Vibrational analysis of tellurium tetrachloride, TeCl₄, was performed with Hartree–Fock (HF), MP2, and generalized gradient approximation density functional theory (DFT) methods supplemented with polarized double-zeta split valence (DZVP) basis sets and relativistic effective core potentials (RECP) of Hay and Wadt. The molecular geometry is best reproduced at the HF and MP2/RECP+DZVP [polarized Hay and Wadt RECP for Te and 6–31G(d) basis set for Cl] levels of theory. The DFT methods gave rise to poorer results, especially those using Becke's 1988 exchange functional. Generally, the vibrational frequencies calculated by the MP2 and B3-type DFT methods with the all electron and RECP+DZVP basis sets as well as at the HF/RECP level were in satisfactory accord with the experimental data. The agreement was good enough to assist the assignment of the measured vibrational spectra. The best agreement with the experimental vibrational frequencies was achieved with the scaled HF/RECP force field. Consistent results were obtained for the unobserved A₂ (ν₄) fundamental, where the results of the best methods were within 4 cm⁻¹. The best force fields were obtained with the following methods: Becke3–Lee–Yang–Parr and Becke3–Perdew/all electron basis, MP2 and Becke3-Perdew/RECP+DZVP, and HF/RECP. The methods using RECPs are advantageous for large-scale computations. The RECP basis set effectively compensates the errors of the HF method for TeCl₄; however, it provides poor results with correlated methods. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 308–318, 1998     

Stereoelectronic effects on type I 1,2-dyotropic rearrangements in vicinal dibromides

The effects of different substituents on type I-dyotropic rearrangements of open-chain and cyclic 1,2-dibromo hydrocarbons have been studied by means of DFT calculations. The activation energy (E(a)) of this transformation decreases with the pi-donor ability of the substituent attached to the reacting ethylenic system. This is due to donation of electronic density by conjugation or hyperconjugation. This donation leads to longer C--C and C--Br bond lengths in the corresponding four-membered transition states (TSs). Linear relationships between the E(a) and either the sigma(p) Hammett substituent constants and the C--C bond length of the TSs were also found. In all cases, the processes have a high value of synchronicity, which is mainly independent on the substituents. A model based on the second-order perturbational analysis for one ethylene unit with two apical bromine radicals accounts for all the computed results.     

Ionic dissociations of chlorosulfonic acid in microsolvated clusters: A density functional theory and ab initio MO study

Ionic dissociation of chlorosulfonic acid (HSO3Cl) in the molecular clusters HSO3Cl-(H2O)n (n = 1-4) and HSO3Cl-NH3-(H2O)n (n = 0-3) was investigated by density functional theory and ab initio molecular orbital theory. The equilibrium structures, binding energies, and thermodynamic properties, such as relative enthalpy and relative Gibbs free energy, and were calculated using the hybrid density func- tional (B3LYP) method and the second order M?ller-Plesset approximation (MP2) method with the 6-311 G** basis set. Chlorosulfonic acid was found to require a minimum of three water molecules for ionization to occur and at least one water molecule to protonate ammonia. The corresponding clusters with fewer water molecules were found to be strongly hydrogen-bonded. The related properties and acid strength of chlorosulfonic acid were discussed and compared to the acid strengths of perchloric acid and sulfuric acid in the context of clusters with ammonia and water. The relative stabilities of these clusters were also investigated.     

Performance of parallel TURBOMOLE for density functional calculations

The parallelization of density functional treatments of molecular electronic energy and first-order gradients is described, and the performance is documented. The quadrature required for exchange correlation terms and the treatment of exact Coulomb interaction scales virtually linearly up to 100 nodes. The RI-J technique to approximate Coulomb interactions (by means of an auxiliary basis set approximation for the electron density) even shows superlinear speedup on distributed memory architectures. The bottleneck is then linear algebra. Demonstrative application examples include molecules with up to 300 atoms and 3000 basis functions that can now be treated in a few hours per geometry optimization cycle in C₁ symmetry. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1746–1757, 1998     

Scope and limitations of Baird's theory on triplet state aromaticity: application to the tuning of singlet-triplet energy gaps in fulvenes

Utilizing Baird's theory on triplet state aromaticity, we show that the singlet-triplet energy gaps (DeltaE(ST)) of pentafulvenes are easily varied through substitution by as much as 36 kcal mol(-1). This exploits the fact that fulvenes act as aromatic chameleons in which the dipoles reverse on going from the singlet ground state (S(0)) to the lowest pipi* triplet state (T1); thus, their electron distributions are adapted so as to achieve some aromaticity in both states. The results are based on quantum chemical calculations with the OLYP density functional theory method and the CASPT2 ab initio method, as well as spectroscopic determination of DeltaE(ST) by triplet sensitization. The findings can also be generalized to fulvenes other than the pentafulvenes, even though the effect is attenuated as the size of the fulvene increases. Our studies thus reveal that triplet-state aromaticity can greatly influence the properties of conjugated compounds in the T1 state.     

CIO自由基与CN自由基反应的密度泛函理论研究

应用量子化学从头算和密度泛函理论（DFT）对CIO与CN的双自由基反应进行了研究．结果表明,CIO自由基的O原子进攻CN自由基的C原子是主要的进攻方式,并形成了中间体1 CIOCN．随后,中间体1发生异构化和分解反应得到热力学上可行的3种产物P4（CINCO）,P1（CO＋CIN）和P3（NO＋CCl）．其中P4是主要产物,P1和P3是次要产物．与单态势能面上相比,三态势能面对整个反应的贡献可以忽略．     

Exploring odd-even effects of simple oligomer-like DRCNnT series: a study based on density functional theory/time-dependent density functional theory calculations

Odd-even effects of short-circuit current density and power conversion efficiency (PCE) are an interesting phenomenon in some organic solar cells. Although some explanations have been given, why they behave in such a way is still an open question. In the present work, we investigate a set of acceptor-donor-acceptor simple oligomer-like small molecules, named the DRCNnT (n = 5-9) series, to give an insight into this phenomenon because the solar cells based on them have high PCE (up to 10.08%) and show strong odd-even effects in experiments. By modeling the DRCNnT series and using density functional theory, we have studied the ground-state electronic structures of the DRCNnT (n = 5-9) series in condensed phase. The calculated results reproduce the experimental trends well. Furthermore, we find that the exciton-binding energies of the DRCNnT series may be one of the key parameters to explain this phenomenon because they also show odd-even effects. In addition, by studying the effects of alkyl branch and terminal group on odd-even effects of dipole moment, we find that eliminating one or two alkyl branches does not break the odd-even effects of dipole moments, but eliminating one or two terminal groups does. Finally, we conclude that removing one alkyl branch close to the terminal group of DRCN5T can decrease highest occupied molecular orbital (HOMO) energy (thus increasing open circuit voltage) and increase dipole moment (thus enhancing charge separation and short-circuit current). This could be a new and simple method to increase the PCE of DRCN5T-based solar cells.     

Molecular structure and vibrational spectra of phenobaraitone by density functional theory and ab initio hartree-Fock calculations

Quantum chemistry calculations have been performed using Gaussian03 program to compute optimized geometry, harmonic vibrational frequency along with intensities in IR and Raman spectra at RHF/6-31++G** and B3LYP/6-31++G** levels for phenobarbitone (C₁₂H₁₂N₂O₃) in the ground state. The scaled harmonic vibrational frequencies have been compared with experimental FT-IR and FT-Raman spectra. Theoretical vibrational spectra of the title compound were interpreted by means of potential energy distributions (PEDs) using MOLVIB program. A detailed interpretation of the infrared spectra of the title compound is reported. On the basis of the agreement between the calculated and observed results, the assignments of fundamental vibrational modes of phenobarbitone were examined and some assignments were proposed. The theoretical spectrograms for FT-IR and FT-Raman spectra of the title compound have been constructed.     

Ab initio and density functional theory studies of the structure,gas-phase acidity and aromaticity of tetraselenosquaric acid

Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure,acidity (free energy of deprotonation,G0) and aro-maticity of tetraselenosquaric acid (3,4-diselenyl-3-cy-dobutene-1,2-diselenone,H2C4Se4) are reported.The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation.The ZZ iso-mer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy.The optimized geometric parameters exhibit a bond length equalization relative to reference compounds,cyclobu-tanediselenone,and cydobutenediselenol.The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6 - 311 G //RHF/6 - 311 G) and - 54.8 kJ/mol (B3LYP/6 - 311 G //B3LYP/6 -311 G).The aromaticity of tetraselenosquaric add is indicated by the calculated diamagnetic susceptibility exaltation (A) - 19.13 (CSGT(IGAEM) - RHF/6 - 311 G// RHF/6-     

Why do Cationic Hydridoiridium(III) Complexes with β‐Aminophosphane Ligands Favour the Transfer Hydrogenation of Ketones over the Direct “H2‐Hydrogenation”?—A Computational Approach

Density functional theory and ab initio molecular orbital calculations show that the observed inability of cationic hydridoiridium(III) complexes with beta-aminophosphane ligands to catalyse the direct hydrogenation of carbonyl compounds with dihydrogen ("H2-hydrogenation") in contrast to their ruthenium(II) equivalents is due to the inability of H2 to displace a coordinated solvent molecule from an intermediate hydrido complex.