Electronic Wavefunctions for Small Molecules

Citation

Hunt, William James (1972) Electronic Wavefunctions for Small Molecules. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/F5KA-3M29. https://resolver.caltech.edu/CaltechTHESIS:11122009-074130644

Abstract

PART I. A simple variationally-based method for calculating electronic wavefunctions of excited states, the improved virtual orbital (IVO) method, is developed in this work. Calculations are presented for H_2O, O_2, CO, and N_2. While the IVO method gives limited accuracy in the treatment of valence excited states, the description of Rydberg states is very useful. For O_2 the theoretical prediction of 8.70 eV (v' = 2) for the transition from the ^3Σ^-_g ground state to the ^3π_g (1π_g → 3sσ_g) Rydberg state facilitated discovery of this transition in electron impact spectra at 8.65 eV (v' = 2). PART II. The N, T, and V states of ethylene have been studied with the Hartree-Foci (H-F) and configuration interaction (CI) techniques as a function of C-C bond distance and the twist angle between methylene groups. The calculated rotational barrier for the N state is 67.2 Kcal/mole, in good agreement with the experimentally derived activation energy of 65 Kcal/mole for cis-trans isomerization of 1,2di-deutero ethylene. The maximum in the N state curve lies 1.4 Kcal/mole above the minimum of the triplet state (T) curve. Both H-F end CI calculations show that the V state of planar ethylene has a more extended charge distribution than the T state. This charge distribution contracts as the methylene groups are twisted from the -planar geometry. Correlation terms included in the CI calculations contract the charge distribution considerably from its H-F size. A modified Franck-Condon Principle for internal rotation suggests that the maximum absorption observed experimentally does not correspond to vertical excitation for the N → V transition. PART III. A Generalized Valence Bond method combining the computational tractability of the usual MO-SCF approach with the conceptual advantages of a valence bond picture is proposed. The GVB method has been applied to calculation of potential curves for CH_2 in the ^3B_1, ^1A_1, and ^1B_1 states. These calculations predict that the ^3B_1 curve may cross the ^1A_1 curve near the minimum for the^1A_1 state. A study of the ring opening of cycloprooane predicts a barrier height of 61 Kcal/mole for cis-trans isomerization, in good agreement with the experimentally determined activation energy of 65 Kcal/mole for 1,2 di-deutero cyclopropene. An investigation of diatomic hydrides and fluorides in the GVB picture gives a consistent view of the energy levels and one-electron energies of these molecules.

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