An ab initio study of the electronic structure of SiH4, SiH3F and SiH2F2

Changes in molecular properties and in the electronic charge distribution of the molecules SiH₄, SiH₃F and SiH₂F₂ are studied within the framework of the ab initio Hartree—Fock SCF—LCAO—MO method. The ionisation potentials, calculated with the use of Koopmans' theorem, correlate well with the experimental vertical ionisation potentials. The combined s and p electronic populations of the Si atom are not substantially altered when adding Si 3d functions to the basis set.     

Ab initio SCF MO study of hydrogen bonds between H2S and H2O molecules

The hydrogen bonds between H₂S and H₂O molecules are calculated through anab initio, LCAO MO SCF method using a Gaussian type orbital double-zeta basis set. The capacity of the H₂S molecule to act as an electron acceptor is confirmed. Consultant of the Instituto Mexicano del Petróleo.     

Nonadditivity of interaction in water trimers

The energy of (H₂O)₃ has been calculated for 29 geometrical configurations of the trimer using the SCF LCAO MO method and extended as well as minimal basis sets of Gaussian functions. For two configurations two intermediate basis sets have also been tested. The results show the nonadditive component of the interaction energy to be small. They also indicate that fairly reliable results for the trimer can be obtained using minimal basis sets and the counterpoise method to eliminate the basis set superposition error. The nonadditive contribution to the interaction energy is shown to be mainly due to the long-range induction interaction.     

Monohydration selectivity mechanism of alkali cations in the potassium channel of excitable biomembranes

Nonempirical quantum chemical method Hartree–Fock–Roothan LCAO SCF MO in a two-exponent Dunning basis with the use of an extended set of Gaussian functions by Huzinaga–Dunning with consideration of electron correlation according to the Meller–Plesset theory of excitations of the second order was used to study monohydrates of Li⁺, Na⁺, K⁺, and HCOO^? ions. The indicated basis was supplemented with polarization functions of d-type on the O atom and of p-type on the hydrogen atom as well as with diffusion functions of p-type on the oxygen atom. It has been found that binding energies of the water molecule with Li⁺, Na⁺ appeared to be higher and with K⁺ lower than with HCOO^? · H₂O. Potential curve shapes of K⁺ + H₂O and HCOO^? + H₂O reactions are shown to be similar. The molecular mechanism of K⁺ channel selectivity of an excitable membrane is explained on the basis of the obtained calculations.     

Physical nature of the chemical bond II. Valence atomic orbital and energy partitioning studies of linear nitriles

The valence atomic orbitals (VAO 's) of several linear nitriles are determined using non-empirical SCF –LCAO –MO wave functions expanded in a minimal (CN^?, HCN, FCN, C₂N₂), double-zeta (CN^?, HCN), or double-zeta + polarization (HCN) basis of Slater atomic orbitals (AO 's). The molecular energy of each system (except the double-zeta + polarization HCN system) is partitioned according to the procedure of Ruedenberg to obtain numerical values of nitrile C and N atomic and C?N bond components of the energy. In addition, the nitrile results are compared with minimal AO basis results obtained previously by other authors for homonuclear diatomics, diatomic hydrides and H₂O. The numerical data are used to test the internal self-consistency of the various definitions entering the partitioning method, i.e. whether or not analogous quantities assume similar values in chemically similar situations. The analysis of nitrile SCF –MO wave functions in terms of the set of VAO 's characteristic of the system under consideration is shown to be a promising approach to the problem of extracting useful information from the wave functions. In general, numerical results for the nitrile systems studied are fairly consistent with the concepts on which the partitioning method is based: promotion, quasi-classical interaction, sharing penetration, sharing interference and charge transfer. However, the VAO expansions for several energy components need to be investigated further and possibly revised.     

Quantum-chemical modeling of endohedral derivatives of buckminsterfullerenes Gd@C<Subscript>60</Subscript>(CHR)<Subscript>2</Subscript> and Gd@C<Subscript>80</Subscript>(CHR)<Subscript>2</Subscript>

The structural energies of the endohedral derivatives of buckminsterfullerenes Gd@C₆₀(CHR)₂ and Gd@C₈₀(CHR)₂ were calculated by quantum-chemical methods – semiempirical PM3 and nonempirical RHF SCF MO LCAO with the S. Huzinaga MINI minimum basis set and GAMESS software.     

Reactivity of molecular oxygen on the surface of ionic crystals

A possibility of multiplicity change for ground‐state molecular oxygen adsorbed on the surface of regular and doped broad‐gap ionic crystals was considered in the framework of cluster approximation by using SCF MO LCAO quantum chemical methods [semiempirical INDO approximation and ab initio calculation with the 6‐311G** basis set taking into account the correlation effects on the level of second‐order Meller–Plesset perturbation theory (MP2)]. The formation energetics of cyclic products of addition reactions of dioxygen in different multiplet states to furan and cis‐butadiene in the gas phase and on the surface of ionic crystals was considered. (These reactions are typical for the O₂ singlet state in the gas phase.) It is shown that the presence of sites with high effective charge on the crystal surface can result in a situation not requiring, as in the gas phase, multiplicity change in the transition of a system from an initial to the final state, which can significantly affect the kinetic parameters of the reactions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Quantum chemical studies of propene,ethylene, acetylene and dihydrogen reactivity in the insertion reaction into the titanium-alkyl bond

Using the ab initio method of SCF MO LCAO

1 SCF MO LCAO: Self-consistent field molecular orbital linear combination of atomic orbitals.

in a valency-splitted basis of the Gaussian functions we have studied the addition of various monomers (C₃H₈, C₂H₄, C₂H₂) and dihydrogen to the titanium-alkyl bond in the complex H₂TiCH₃. The structure of transition states in the insertion reaction, heats of π-complex formation and activation energies for the insertion of the coordinated monomers have been calculated. The calculation results show that the reactivity decreases in the order C₂H₂ > C₂H₄ > C₃H₈ > H₂. According to the results obtained, the energy of the π^*-antibonding orbital of monomers can serve as an index of relative reactivity in the insertion reaction into the metal-alkyl bond.     

An ab initio study of the hydrated positron

Ab initio calculations are presented for the hydration energy of the positron. Tetrahedral molecular-dipole-oriented clusters e⁺(H₂O)₄ are considered. In performing these calculations, the Hartree—Fock MO LCAO SCF approximation with the 4-31G split-valence basis set is used. The method was modified to treat the positron problem. It is shown that e⁺ in liquid water, like an electron, can be strongly solvated, with the hydration energy 0.2–0.3 eV greater than that of e⁺.     

SCF MO LCGO studies on the hydration of ions: The system Li+·2H2O

The energy of the dihydrated lithium cation Li⁺·2H₂O is studied in several different points within the SCF MO LCAO framework, using a gaussian basis set to approximate the wavefunction. The computed binding energies (hydration energies) and bond distances are compared to the values found for the monohydrate. The results are discussed in view of ion-solvent interaction, and especially of the effect of ions on adjacent hydrogen bonds, in aqueous solutions.It is a pleasure to thank our technical staff for the careful preparation of the input for the programs and for its enthusiastic and skilful assistance in running the computer.     

Perturbation expansion theory corrected from basis set superposition error II. Charge transfer,pair correlationand dispersion terms

The second-order perturbation theory based on the locally projected molecular orbitals is developed. A few test calculations with cc-pVDZ and aug-cc-pVDZ basis sets are carried out for the dimers, (H₂O)₂ and (HF)₂. The charge transfer terms remove the deficiency of the locally projected self-consistent field method for molecular interaction (LP SCF MO MI), and the potential energy curves calculated with aug-cc-pVDZ are very close to the corresponding curves of the counterpoise-corrected SCF energy. Only after adding the spin-exchanged dispersion type to the dispersion and intra-molecular pair correlation terms, the calculated potential energy curves become close to those of the counterpoise-corrected second-order Møller–Plesset (MP2). Pragmatic approaches for reducing the influence of the basis set superposition error are proposed.     

Electronic structure of small copper clusters

An all-electron ab initio LCAO -MO SCF calculation has been carried out for the electronic structure of small copper clusters (Cu_n, n = 2–6). The basis set superposition error occurring in the calculation, the equilibrium configuration of Cu₃, the bond energy in the clusters, and the localized d-hole in excited and ionized states of Cu₂ are closely examined.     

Electronic structure of friedel crafts catalysts BF3+HF

SCF MO LCAO calculations using two different gaussian basis sets have been performed for the species HF, F^?, BF₃, BF ₄ ^? and HBF₄ with geometry optimization. Differences in electrophilicity and proton donating capability of HF due to the formation of the adduct with BF₃ are evidenced and discussed.     

An ab initio MO calculation of the electronic structure and geometry of protonated methanol

Protonated methanol, CH₃ OH₂⁺, has been studied using the LCAO—MO—SCF method with a 7, 3 and 9, 5, 1 Gaussian orbital basis set on the heavy atoms and 4s on hydrogen. It is found that the ground state is non-planar around oxygen, in contrast with previous calculations, with an inversion barrier of 3 kcal mol^?1. The changes in electron distribution in the reacting systemCH₃⁺ + H₂O → CF₃OH₂⁺is also examined.     

Average dipole polarizabilities from the unsold approximation and ab initio data

An approximate static dipole polarizability equation is developed on the basis of the Unsold approximation, and is cast in closed shell LCAO MO SCF formalism. A brief study is made of the use of Koopmans' theorem as a means of obtaining an average ionization energy, necessary in the polarizability equation presented. Average polarizabilities for the systems He, Ne, H₂, HF, H₂O, NH₂, CH₄, HCN, N₂, C₂H₂, C₂H₄, C₂H₆, C₆H₆, and C₆H₅F are calculated from the dipole polarizability expression developed using a small 4-31G basis set level. Results show good agreement with experimental data.     

Comparison of valence-only model potential and all-electron ab initio LCAO SCF MO studies of Li2 and LiH

A previously developed gaussian-based model potential theory for a single valence electron outside a core has been extended to the simple two-valence electron systems Li₂ and LiH within the LCAO SCF MO formulation, using an extended valence basis set. Comparisons of the results with corresponding ab initio calculations show excellent agreement of the total valence energy and the orbital energy in both systems, and for the dipole moment in LiH.     

Ground electronic state and geometry of SF3

Ab initio LCAO MO SCF calculations with DZ + 3d(S) basis functions show that the sulphur trifluoride radical is a planar π-radical having a ²B₁ ground state. Like ClF₃, it has an umbrella-structure. However, it becomes Y-shaped in its first ²A₁ excited state which has been calculated to lie only ≈ 2.4 eV above the ²B₁ ground state.     

Nonempirical parameters of the nonlocal core pseudopotential for the second and third row elements

For the core pseudopotential (CP) model constructed in terms of Bonifacic-Huzinaga nonlocal CP theory, parameters of the local component of CP are calculated for the second-and third-row elements. The resulting CP are associated with the Coulomb, exchange, and correlation potentials created by the nuclear charge and electron density of the core electrons. The electronic structure and potential energy surface are calculated for the hydrides of the second-row elements (LiH, CH₄, NH₃, H₂O, HF); the calculations are performed by the nonempirical nonlocal CP method. The results of these calculations agree well with those of SCF MO LCAO ab initio calculations and with experimental data.     

An ab initio study of the unimolecular decomposition mechanism of formamidine. 4-31G Characterization of potential energy hypersurface

Ab initio MO calculations have been carried out for the unimolecular decomposition of formamidine. The Hartree–Fock method in LCAO approximation with the 4-31G basis set was used. The 4-31G potential hypersurface has been further studied. The stationary points (R, TS, and P) were localized. A reaction analysis by correlation of bond-order indices and localized molecular orbitals demonstrated that the decomposition is an asynchronous process. The TS can be described as four-membered ring.