I. The generalized valence bond theory of electronic structure. II. An orbital interpretation of superexchange in antiferromagnetic insulators

Citation

Hay, Philip Jeffrey (1972) I. The generalized valence bond theory of electronic structure. II. An orbital interpretation of superexchange in antiferromagnetic insulators. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/D2P6-BA46. https://resolver.caltech.edu/CaltechTHESIS:11042009-080816954

Abstract

Part I: A discussion is given of the generalized valence bond (GVB) method--a multi-configuration approach to electronic structure that combines a valence bond interpretation with the self-consistent techniques of Hartree-Fock theory. Ab initio calculations on simple hydrocarbons give improved descriptions of bonding in terms of localized C-C and C-H bonds. The nine lowest states of the ozone molecule are treated by GVB and configuration interaction techniques and an assignment of the spectrum of O_3 is made. A metastable excited singlet state with an equilateral geometry and an energy 1.5 eV above the ground state is discovered. The calculated energy barrier of 60.5 kcal for the cis-trans isomerization of cyclopropane is in good agreement with the experimental value of 64.2 kcal. No barrier to ring closure is found in the trimethylene biradical in contrast to commonly accepted biradical mechanisms. The ^1A_1 state of CH_2 is calculated to be 0.50 eV (11. 5 kcal) above the ground ^3B_1 state. The ^1B_1 ← ^1A_1, transition—calculated to be 1. 40 eV--agrees with the lowest observed ^1B_1 ← ^1A_1 band and suggests a reinterpretation of this as a 0-0 band. A new ^1A_1 state at 3.2 eV is also discussed. Good values of the barrier to internal rotation in ethane and of the dissociation energy of O_2 are obtained. Part II: An orbital interpretation of superexchange suggests that anti-ferromagnetism arises from increased metal-metal overlap due to the ligand orbitals. A theoretical value of the exchange parameter from ab initio calculations on the Ni^(++)-F^- -Ni^(++) "molecule" is 10% of the experimental value in KNiF_3.

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