Theoretical treatment of hydrogen abstraction by hot tritium atoms

Abstract

Extended Huckel calculations using Cusachs's modification were performed for the methane-tritium system with the objective of determining how the overlap populations (and energies) varied as a function of C-H-T distances. The model chosen was axial attack of the hot tritium atom along the C-H,,, bond direction which leads to abstraction of the H(l) atom.A position of maximum H(, , --T overlap population was found at about the same place as the position of minimum C-H(,) overlap population, thus setting certain limits on geometric configurations which would be expected to undergo the H(,) abstraction reaction. Attention is drawn to the pertinence of overlap population results to the bond energy-bond order method of Johnston for obtaining potential energy surfaces. The overlap populations also indicate why in compounds containing very electronegative substituents, the abstraction reaction occurs at a lower energy than would be predicted by the simple bond dissociation energy dependence found experimentally by Rowland for all other systems. A potential energy surface was also traced out for the CH4-T reaction and a basin was found indicating a stable CH4T (or CH,) molecule. Examination of activation energies calculated previously by Eying and Polyani for axial attack compared to direct attack shows clearly why at low kinetic energies of T the abstraction reaction is favored over the substitution reaction; and why only the abstraction reaction occurs at T kinetic energies lower than -1.8 eV.

Computational Chemistry of Polyatomic Reaction Kinetics and Dynamics: The Quest for an Accurate CH5Potential Energy Surface

“…From a historical perspective, a few very early semiempirical theoretical calculations are worth mentioning. In 1960, Hartmann et al 394 obtained the energy of CH 5 by treating it as a pseudo-sodium atom with hydrogen-like orbitals; in 1967 Kaufman et al 395 performed extended Hu ¨ckel calculations for a limited number of configurations of the rebound-attack abstraction model; and in 1971, Weston and Ehrenson 16 calculated the energies of several configurations of the CH 5 radical (including those of D 3h , C 4V , and C s symmetry) using a modified complete-neglect-of-differentialoverlap (CNDO) semiempirical molecular orbital method. In 1971, Lathan et al 396 performed ab initio calculations with small basis sets for XH n molecules.…”