Pietro Cremaschi

C-H Group as Proton Donor by Formation of a Weak Hydrogen Bond

A systematic CNDO/2 study has been carried out on the weak C-H...F, C-H...O, and C-H...N hydrogen bonds. An H-bond formation has been revealed for proton donors with negative excess charge on the hydrogen. The concept of the positive character of the hydrogen as well as some essential features of the hydrogen bond have been discussed. Some useful correlations have been found and the importance of the charge shifts has been emphasized.

The effect of hydrogen chemisorption on titanium surface bonding

The effect of hydrogen chemisorption on the strength of Ti-Ti bonds is studied byab initio configuration interaction techniques using an embedding theory to describe the electronic structure. A Ti adatom on Ti(0001) is modelled by a Ti20H cluster with boundary potentials determined from the embedding treatment. Hydrogen atom chemisorption is highly exothermic for adsorption atop the adatom, a three-fold site formed by the adatom and in the interstitial site below the adatom. Compared to the planar Ti(0001) surface the adatom region binds hydrogen much more strongly. Removal of Ti from the surface is energetically much more favorable if H remains on the surface as opposed to the removal of TiH. The exchange reaction Ti20+H→Ti19H+Ti is endothermic by 0.3 eV. These results suggest high reactivity of the adatom region on Ti(0001) but not such that the surface is more easily fragmented by removal of Ti or TiH.

Ab initio chemisorption studies of H on Fe(110)

Ab initio configuration interaction calculations are performed to study the chemisorption of atomic H on a Fe(110) surface. The lattice is modeled as an embedded three-layer, 40-atom cluster with the Fe atoms fixed at the bulk position. Fe 3d orbitals are explicitly included on five Fe atoms on the surface. Hydrogen strongly binds to the Fe(110) surface at the long-bridge, short-bridge, and quasi three-fold sites. The calculated adsorption energies are 2.76, 2.73, and 2.71 eV, respectively. H-surface bonding at the on-top Fe site is more than 0.4 eV weaker. The calculated H-surface distances are 0.89, 1.03, and 0.87Afor H at the long-bridge, short-bridge, and quasi three-fold sites, respectively, which agrees well with the LEED value of 0.9 ± 0.1A. The H-surface stretching vibrational frequencies are calculated to be 1070, 1066, and 1073 cm−1, at the long-bridge, short-bridge, and quasi three-fold sites, respectively. The work function of Fe(110) decreases on H adsorption. The present calculations indicate that H diffusion into the bulk through the short-bridge site will have a much higher activation barrier than via the long-bridge and quasi three-fold sites.

Influence of spin contamination and basis set on electrostatic potential and Hfs coupling constants of organic radicals

Ab initio UHF wavefunctions for organic radicals have been obtained with different basis sets, before and after spin annihilation. It has been verified that the shapes of the electrostatic potential surfaces of these radicals are basis and spin contamination independent, at difference from hfs coupling constants.

Modified Hückel molecular orbital calculation of nuclear spin coupling constants in simple hydrocarbons and aldehydes

Abstract An iterative modified Huckel method is used to calculate contact contribution to HH, 13 CH and 13 C 13 C coupling constants in simple hydrocarbons and aldehydes.

Ab initio calculations including solvent effects of the structure of pyrazine, 4-nitropyridine and dicyanobenzenes ion pairs

The potential surfaces of pyrazine, 4-nitropyridine, 1,2-, 1,3-, 1,4- dicyanobenzene radical anions have been calculated within theab initio and electrostatic molecular potential formalism, including the effect of the solvent. These potential surfaces suggest models of ion pairs with alkali cations, which are in agreement with those deduced from related ESR spectra.

Antisymmetry in quantum Monte Carlo methods

Abstract Various solutions to the antisymmetry problem in fermion Monte Carlo methods are reviewed and the implementation of a method for building flexible nodal surfaces, self-adapting to the signed walkers distribution, is analyzed.

The HeHe Coulomb and exchange interaction energy

Abstract The Coulomb and exchange interaction energy between two helium atoms is calculated, increasing systematically the basis set, by a supermolecular method which excludes the superposition error by using non-orthogonal orbitals. At 5.6 bohr an energy limit of 9.69 K is found at the Hartree-Fock level; for correlated wavefunctions the corresponding value is 11.86 K using only s orbitals, 10.70 K including p orbitals and 10.58 K adding d orbitals.

Barmer to internal rotation in the methylammonium ion

Abstract As experimentally available data only give indirect information about geometrical and electronic properties of the methylammonium ion, ab initio MO-LCAO-SCF calculations, with different minimal and extended basis sets, have been performed on this ion to determine its optimized geometry and internal rotation barrier. Results suggest the existence of a hindered internal rotation with a barrier of about 2 kcal mole−1.

Ion pairs formed by alkali metals. Part 1.—Complexes of lithium, sodium and potassium with 4-nitropyridine

The e.s.r. spectra in dimethoxyethane and tetrahydrofuran of the ion pairs formed by 4-nitropyridine anion radical with lithium, sodium and potassium cations have been studied at various temperatures. The temperature dependence of the alkali metal splitting is interpreted on the basis of the Atherton–Weissman model. To discuss the structure of the ion pair, different quantum mechanical calculations were performed.