AB-INITIO STUDY OF C-H BOND BREAKING IN OLEFINS .2. GVB COMPUTATIONS ON PROPENE[--]H PLUS CIS-PROPEN-1-YL OR TRANS-PROPEN-1-YL

The Generalized-Valence-Bond-Perfect-Pairing (GVB-PP) method has been used to investigate the structural behaviour, energy, and dipole momen t along the reaction coordinates for propene <-> H + cis- or trans-pro pen-1-yl. Geometry optimizations were carried out at the GVB(9)/STO-3G level (complete valence shell) for the minimum energy propene structu re (complete optimization) and for numerous structures up to r(C-H) = 10 Angstrom (only the elongated C-H distance kept fixed). The dissocia tion curves are smooth, without a maximum, and yield predicted dissoci ation 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 wit hin several percent of those obtained for C-H bond rupture in ethylene using GVB and MCSCF methods with the same basis set. They are obvious ly 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 h ydrogen atom from ethylene. GVB(7)/6-31G//GVB(9)/STO-3G computations l ower 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), respect ively. The reduced energy concept (E(R) = (E(infinity) - E(r))/D-e) is applied to the reaction coordinates. Linear behaviour for In E(R) ver sus bond length is observed at long bond distances. At r(C-H) = 3 Angs trom, the values of the slopes, d(ln E(R))/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)/6-31G//GVB (9)/STO-3G) for the H + cis- an d H + trans-propen-1-yl reaction coordinates, respectively.