FIXED PARTIAL CHARGE CALCULATIONS OF VIBRATIONAL CIRCULAR DICHROISM IN ALANINE AND DEUTERATED ISOTOPOMERS

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1981

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Ohio State University

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Vibrational circular dichroism (VCD) and infrared intensities have been calculated for alanine and various deuterated isotopomers using the harmonic fixed partial charge (FPC) model and a Urey-Bradley force field refined to reproduce Raman frequencies of five isotopomers. The aim of these theoretical studies is to reproduce and properly interpret the VCD patterns observed in the CH stretching region of alanine-Nd3, alanine-Cd1Nd3 and alanine-Cd3Nd3. The interpretation of the VCD first involves a detailed vibrational assignment of the CH stretching spectra of aqueous solutions of these molecules in both infrared and Raman. Since these spectra contain overlapping bands due to four fundamentals and at least two overtones, curve resolutions of both the infrared and Raman data were performed. A calculation of Fermi resonance coupling between the overtones of the degenerate methyl deformation modes and the symmetric methyl stretching mode then allowed further refinement of the vibrational force field and calculation of the VCD for the observed bands. Agreement in the pattern of the observed VCD is obtained but VCD intensities are too low by one to several orders of magnitude indicating a serious short-coming of the FPC model for alanine in the CH stretching region.

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