Ab initio study of C—H bond breaking in olefins. II. GVB computations on propene ↔ H + cis- or trans-propen-1-yl

Abstract

The Generalized-Valence-Bond-Perfect-Pairing (GVB-PP) method has been used to investigate the structural behaviour, energy, and dipole moment along the reaction coordinates for propene ↔ H + cis- or trans-propen-1-yl. Geometry optimizations were carried out at the GVB(9)/STO-3G level (complete valence shell) for the minimum energy propene structure (complete optimization) and for numerous structures up to r(C—H) = 10 Å (only the elongated C—H distance kept fixed). The dissociation curves are smooth, without a maximum, and yield predicted dissociation energies of propene to H + cis-propen-1-yl and H + trans-propen-1-yl of 555.8 and 554.8 kJ mol−1, respectively. These values are within several percent of those obtained for C—H bond rupture in ethylene using GVB and MCSCF methods with the same basis set. They are obviously too high but they confirm that removal of a hydrogen atom from the CH2 moiety in propene requires about the same energy as removal of a hydrogen atom from ethylene. GVB(7)/6-31G//GVB(9)/STO-3G computations lower the predicted dissociation energies of propene ↔ H + cis-propen-1-yl and H + trans-propen-1-yl to 448.2 and 448.6 kJ mol−1, respectively.The reduced energy concept (ER = (E∞ − Er)/De) is applied to the reaction coordinates. Linear behaviour for In ER versus bond length is observed at long bond distances. At r(C—H) = 3 Å, the values of the slopes, d(ln ER)/dr(C—H), which are related to the effective Morse constant B are −3.73 and −3.74 (GVB(9)/STO-3G) and −2.75 and −2.81 (GVB(7)/6-31 G//GVB(9)/STO-3G) for the H + cis- and H + tras-propen-1-yl reaction coordinates, respectively.