A Initio Molecular Orbital Theory Studies of Compounds and Reactions of Trivalent Aluminum Reagents, and, Exploration of the Polarization Correction to the Molecular Electrostatic Potential.
Abstract
Ab initio molecular orbital methods were used in the study of the interaction of aluminum with carbon -carbon multiple bonds and carbonyl compounds. Additionally, methods for efficiently determining the correction to the molecular electrostatic potential due to the adiabatic polarization potential were explored. The utility of the gradient of the polarization potential, the reaction field, in predicting the preferred orientation of polar reactants with respect to a substrate was also examined. First, pi-complexes of trivalent aluminum reagents with alkenes, believed to be involved in aluminum -mediated polymerization, were examined. Characterization of these complexes by normal mode analysis indicates that they are stable intermediates on the potential energy surface, not transition states as previously believed. Second, symmetric and bent isomers of a bridged alkynylaluminum dimer, [ R_2Al(mu -CCR)) _2 were studied. In accordance with experiment, the bent dimer is found to be more stable than the symmetric dimer. This added stability is attributed to a pi-type interaction in the bent isomer that is not present in the symmetric isomer. The alkyne-aluminum pi-interaction in the bent alkynylaluminum dimer is here shown to be a good experimental model for the alkyne-aluminum pi-complexes discussed above. Next, the reaction mechanisms of the reduction of carbonyls by two classes of trivalent aluminum reagents, aluminum hydrides and aluminum alkyls with beta-hydrides, were examined. Both experimentally observed pathways for the hydride reactions were examined (Al:C=O ratios of 1:1 and 2:1) and involve a nearly-planar four-centered transition state (Al, H being transferred, carbonyl C=O). The 2:1 pathway is found to have a lower activation energy than the 1:1 pathway, in agreement with experimental results. The reduction of carbonyls by trivalent aluminum reagents containing beta-hydrides was found to proceed by a six -centered cyclohexane-like transition state. Finally, an efficient method for approximating the polarization correction to the molecular electrostatic potential, polarization potential, was developed using transferable atom-in-molecule polarizabilities. Approximate potentials obtained from these polarizabilities compare favorably to the exact potentials generated from the ab initio wavefunction. The gradient of the polarization potential, the reaction field, was calculated for a small cavity and the preferred position of a dipole in the cavity was determined. The plot of the reaction field may prove useful for predicting the orientation of a dipole in a cavity, however, further work in this area is required.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- Bibcode:
- 1994PhDT........90S
- Keywords:
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- Chemistry: Physical; Chemistry: Inorganic; Physics: Molecular