Ab initio study of C-H bond breaking in olefins

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

“…As expected the single configuration Self-ConsistentField (SCF) computations predicted an energy plateau at large C-H separations but this plateau is much too high energetically. The dipole moment increased with increasing C-H separation, indicating that the separated species constitute an ionic pair (25). This behaviour was expected: it is the result of the methodology employed in generating the wave function and is independent of the composition of the basis.…”

“…The seven valence -virtual orbital pairs chosen for the correlations represent the three a(C-H) bonds, the two u(C-C) bonds, the r(C1-C2) bond, and the T-bond involving the methyl group and localized on the C3, H5, and H6 atoms. The two valence orbitals excluded in the excitations were essentially localized on the methyl group and this should not influence the relative energetics to any appreciable extent (25,41,42).…”

“…43. e f~h e sum of the energies from a SCFISTO-3G computation on an isolation H atom (E = -0.466582 hartree) and for the ROHF-GVB (8) In our previous work it was shown that inexpensive GVB and MCSCF computations could yield reasonable potential energy curves and total dissociation energies that are within 20% of the best experimental values (25). This paper is the first of several that will be devoted to the chemistry of 1, including new experimental results on its direct photolysisl and theoretical studies on decomposition/isomerization processes in its ground and first triplet electronic states.…”

“…Recently we reported an ab initio study of the reaction coordinate for ethylene t , H + vinyl(5), corresponding to C-H bond-breaking in ethylene or to hydrogen atom capture by 5 (25). As expected the single configuration Self-ConsistentField (SCF) computations predicted an energy plateau at large C-H separations but this plateau is much too high energetically.…”

“…Both GVB-PP and Multi-Configuration-SCF (MCSCF) methods were used: the single configuration SCF method is inadequate except (a) at equilibrium geometries or (b) at long internuclear distances with an open-shell methodology. The minimal levels necessary for the GVB-PP and MCSCF calculations to obtain satisfactory results were determined, not only for the C-H reaction coordinate in ethylene, but also for H-X reaction coordinates where X = CH3, OH, and F (25). 1 have not yet been determined experimentally but the participation of H e C H -C H 3 radicals of undefined geometry in its shock tube pyrolysis has been proposed by Hidaka et al (1).…”