Ab initio study of C-H bond breaking in olefins: Part I. General valence bond and multiconfigurational self-consistent field computations on 2H 4 → H 2C = CH + H

Abstract

The structural behaviour, energies and dipole moment of the ethylene molecule on elongation of one of the C-H bonds has been studied using the general-valence-bond (GVB) perfect pairing and multi-configurational SCF (MCSCF) methods. The active space included the HOMO-1 (σ 2), HOMO (π 2), LUMO (π ∗0) and LUMO+1 (σ ∗2) orbitals. All properties show that the C-H bond is essentially broken for r(C-H) greater than 3.5 Å. For the same molecular geometry the MCSCF computations give a total dissociation energy about one percent lower that found with the GVB computations. The inexpensive treatments used in this work yield total dissociation energies which are within 20% of the best experimental value. These results also indicate that GVB or MCSCF computations using a selected, restricted active space and a minimal basis set can give dissociation energies which can be calibrated by a multiplication factor. For C-H bond dissociation in ethylene this factor is 0.8. Application of the reduced energy concept ( E r = ( E r - E ∞)/ D e) to bond breaking in molecules is discussed. Linear behaviour for In .E R versus bond length is observed at long bond distances for C-H bond rupture in ethylene. The values of the slope, d(lnE R)/dr(C-H), which is related to the effective Morse constant B, were determined for ethylene at r(C-H) = 3Å. For each series of GVB and MCSCF calculations it was found to be 3.7Å −1, independent of the molecular geometry used.