Practical Valence-Bond Calculations

Publisher Summary This chapter describes that the valence-bond method for calculating molecular structures and energies has had an enduring popularity among chemists as a conceptual tool for interpreting chemical reactivities and processes at the qualitative level. It discusses procedures that treat problems involving a number of systems to the level of accuracy expected with other computational methods and with no greater level of difficulty. It is suggested that using these procedures, valence-bond calculations are not too difficult to be practical. It is suggested that using these procedures, valence-bond calculations are not too difficult to be practical. Many specific calculations are used to illustrate characteristics of the valence-bond approach. One of these is a π-only (frozen σ core) calculation of planar butadiene. It describes the minimal basis calculation and shows how a double-zeta treatment may be used to obtain better energies without relinquishing the easy interpretability of the simple wave function. Other system consists of an ethylene molecule and a methylene radical. Some pathways are considered for the insertion of methylene into C 2 H 4 , to form cyclopropane, with particular emphasis on the symmetries of the interacting states and the single-triplet crossing points in the process.

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