A valence bond description of bond dissociation energy curves

摘要

Bond dissociation energy curves have been calculated ab initio using the valence bond self-consistent field (VBSCF) method for CH_3-Y molecules, with Y = F, OH, NH_2, CH_3, BH_2, CN and NO. Basis sets of at least double-zeta + polarization quality for first row atoms have been used. The VB wavefunction is expanded in terms of one covalent (CH_3: Y) and two ionic (CH_3~+Y~- and CH_3~-Y~+) structures. All results were referenced to the perfect-pairing generalized valence bond method. The VB curves for CH_3-F were calculated using both the same orbitals for the different structures (SODS) and tailored (different) orbitals for the three different structures (TODS). Each of these model levels was examined further using purely localized fragment (CH_3 and Y) orbitals (L-SODS and L-TODS) and allowing delocalization between the CH_3 and Y fragments among the passive orbitals (D-SODS and D-TODS), where only the electron pair bond undergoing homolytic dissociation uses purely localized fragment orbitals. Only the two SODS model levels were used for the other CH_3-Y molecules. The lower energy covalent structure curve for CH_3-F is found to be flat and essentially repulsive at all theory levels. The close but higher energy CH_3~+F~-ionic structure curve has a deep minimum (R_m) at 0.3-0.4 A larger interfragment distance R than the corresponding ground state equilibrium bond length R_e. The resultant normal Morse-type shape ground state dissociation curve results from the steeply rising covalent-ionic off-diagonal (resonance) interactions dominating the widening covalent-ionic energy gap as R decreases from R_m. Within the SODS model, passive electron delocalization contributes a surprisingly large ～ 11 kcal mol~(-1) to the calculated bond dissociation energy, mainly by stabilizing the CH_3:F curve. TODS adds ～ 16kcal mol~(-1) to D_e mainly by stabilizing the CH_3~+F structure energy. In the series, Y = F → OH→NH_2→CH_3, (a) the covalent curve develops an ever deeper energy minimum, approaching its ground state R_e, (b) the contributing weight of the CH_3:Y structure to the ground state wavefunction, and the ionic-covalent energy gap steadily increase, and (c) R_m for the CH_3~+Y~- structure moves out to larger values. CH_3-BH_2 fits into these trends where the lower energy ionic structure is CH_3 ~-BH_2~+. CH_3-CN has a 70 % covalent weight at its R_e. CH_3-NO is ～ 67% CH_3:NO and 24% CH_3~+NQ~-. CH_3-CN has a Morse-type covalent structure energy curve while CH_3:NO is unbound. The last three molecular species have empty low-lying valence orbitals and their explicit effect on the electronic structure description and energy dissociation curves within the VBSCF framework remains to be examined. The results obtained here have direct relevance to the VBSCF study of reaction paths.