Date of Award

3-6-1994

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Abstract

This dissertation in theoretical physical chemistry presents results of calculations on the electronic structures of polyatomic molecules using standard methods (SCF, MCSCF, CI, and UCEPA), as well as the development of new theory (QDCEPA).Using ab initio quantum mechanical techniques, a comparison of bonding involving the first row element nitrogen and the second row element phosphorous was made. The low-lying electronic states of the compounds H$\sb2$N$\sb2$ and H$\sb2$PN were studied, specifically the ground singlet, first triplet and excited singlet. The ground singlet and first triplet of H$\sb2$PCH and H$\sb2$NCH were studied. For each of the molecules, the equilibrium geometries, including bond lengths and bond angles, as well as the energy of each state were determined. These calculations were done using a number of different basis sets, and at different levels of treatment of the active space. Both GVB and CAS were done; variations in the allowed excitations in the CI and UCEPA calculations were also explored. As an extension to the two heavy atom studies, the compound HCPNH was studied and compared with HCNNH. The practical application of these studies is that the compounds are of immediate experimental interest; furthermore, H$\sb2$PN, H$\sb2$N$\sb2$, H$\sb2$PCH, H$\sb2$NCH, HCPNH, and HCNNH are all small, relatively unstable molecules that are not easily studied experimentally. We were interested in P-C and P-N multiple bonds and since HCPNH is the smallest molecule with both, we were particularly curious about this novel compound.This dissertation also includes discussion of theory development work that will permit more accurate calculations in cases where quasidegenerate energy levels are encountered. This new ab initio method (which we refer to as QDCEPA) was developed and tested for the model problem of a coupled anisotropic harmonic oscillator. QDCEPA is shown herein to be a viable method for approximately solving the Schrodinger Equation for anisotropic coupled harmonic oscillators with a very wide range of parameters determining the energy eigenvalue spectrum. It is reasonable to expect that the method will be cost-effective for determining the electronic energy eigenvalues of atoms and molecules.

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