High pressure investigations on hydrous magnesium silicate-phase A using first principles calculations: H---H repulsion and changes in hydrogen bond geometry with compression

We have carried out first principles structural relaxation calculations on the hydrous magnesium silicate phase A (Mg₇Si₂O₈(OH)₆) under high pressures. Our results show that phase A does not undergo any phase transition up to ～45 GPa. We find that non-bonded H---H distance reaches a limiting value of 1.85 Å at about 45 GPa. The hydrogen bond bending mechanism for countering the H---H repulsive strain that was proposed by Hofmeister et al. for phase B is not the only one operating in phase A. It also has contributions based on the reduction of one of the O–H bond distances and opening up of the H---O---H angle with compression. The contraction of the O–H distance with pressure, perhaps the first one by density function theory calculations, will have implications for the spectroscopic measurements.     

Dispersion interactions between neighboring Bi atoms in (BiH3)2 and Te(BiR2)2

Triggered by the observation of a short Bi⋯Bi distance and a Bi Te Bi bond angle of only 86.6° in the crystal structure of bis(diethylbismuthanyl)tellurane quantum chemical computations on interactions between neighboring Bi atoms in Te(BiR₂)₂ molecules (R = H, Me, Et) and in (BiH₃)₂ were undertaken. Bi⋯Bi distances atoms were found to significantly shorten upon inclusion of the d shells of the heavy metal atoms into the electron correlation treatment, and it was confirmed that interaction energies from spin component‐scaled second‐order Møller–Plesset theory (SCS‐MP2) agree well with coupled‐cluster singles and doubles theory including perturbative triples (CCSD(T)). Density functional theory‐based symmetry‐adapted perturbation theory (DFT‐SAPT) was used to study the anisotropy of the interplay of dispersion attraction and steric repulsion between the Bi atoms. Finally, geometries and relative stabilities of syn–syn and syn–anti conformers of Te(BiR₂)₂ (R = H, Me, Et) and interconversion barriers between them were computed. © 2018 Wiley Periodicals, Inc.     

Acceleration of the GAMESS‐UK electronic structure package on graphical processing units

The approach used to calculate the two‐electron integral by many electronic structure packages including generalized atomic and molecular electronic structure system‐UK has been designed for CPU‐based compute units. We redesigned the two‐electron compute algorithm for acceleration on a graphical processing unit (GPU). We report the acceleration strategy and illustrate it on the (ss|ss) type integrals. This strategy is general for Fortran‐based codes and uses the Accelerator compiler from Portland Group International and GPU‐based accelerators from Nvidia. The evaluation of (ss|ss) type integrals within calculations using Hartree Fock ab initio methods and density functional theory are accelerated by single and quad GPU hardware systems by factors of 43 and 153, respectively. The overall speedup for a single self consistent field cycle is at least a factor of eight times faster on a single GPU compared with that of a single CPU. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011     

How Lewis Acids Catalyze Diels–Alder Reactions

The Lewis acid(LA)‐catalyzed Diels–Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled‐cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I₂<SnCl₄<TiCl₄<ZnCl₂<BF₃<AlCl₃. Emerging from our activation strain and Kohn–Sham molecular orbital bonding analysis was an unprecedented finding, namely that the LAs accelerate the Diels–Alder reaction by a diminished Pauli repulsion between the π‐electron systems of the diene and dienophile. Our results oppose the widely accepted view that LAs catalyze the Diels–Alder reaction by enhancing the donor–acceptor [HOMO_diene–LUMO_dienophile] interaction and constitute a novel physical mechanism for this indispensable textbook organic reaction.     

Explicit and implicit multi‐center integrations

We present truncated expansions of multicenter one‐electron nuclear attraction and two‐electron repulsion integrals over localized basis functions in terms of one‐ and two‐center integrals of “Coulomb,” “exchange,” and “hybrid” type. Two variants are discussed: the “Explicit Multi‐center Integrations” and the “Implicit Multi‐Center Integrations” (abbreviated as “EMCI” and “IMCI”, respectively). While EMCI also deals with individual integrals, the IMCI option is the more appealing one: it enables us to evaluate the entire matrix elements of “Restricted Hartree–Fock”‐type in a very effective and chemically meaningful way. Due to the diatomic nature of our expansions, integrations over “Slater‐Type Orbitals” become well‐feasible, too. © 2012 Wiley Periodicals, Inc.     

Reactivity of C-H and O-H bonds in reactions with aminyl and nitroxyl radicals and cyclic mechanisms of chain termination in oxidizable alcohols and olefins

The parabolic model of transition state has been used to analyze the problem of why aromatic amines and nitroxyl radicals cause the cyclic mechanism of chain termination in oxidizable alcohols and olefins (where HO₂· and >C(OH)0₂· radicals participate in chain propagation) and not in oxidizable hydrocarbons. The differences are caused by the existence of a weak triplet repulsion in transition states with the N...H...0 and 0...H...0 reaction centers, while the triplet repulsion is strong in transition states with reaction centers of the C...H...0 and C...H...N type in oxidizable hydrocarbons.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1972–1976, August, 1996.     

Superelectrophiles: charge-charge repulsive effects

Over the years, a significant number of di- and tricationic superelectrophiles have been studied. Many of these superelectrophiles exhibit charge-charge repulsive effects due to the interaction of the positive charges. Charge-charge repulsion can lead to novel molecular rearrangements, profoundly influence reactivities, and may significantly effect molecular structure.