Abstract

Ab initio calculations were performed on a model complex of N-methylacetamide (NMA) hydrogen bonded at the NH and CO groups to two formamide molecules. The 3-21G basis set was used and full geometry optimization was performed on the complex. Dipole moment derivatives and force constants were computed and, after scaling the force constants, the vibrational frequencies and IR intensities of the complex were obtained. Basis set superposition effects on the geometry, force constants and dipole derivatives were examined, using the counterpoise method; these effects were found to be negligible except for the CO stretch force constant and dipole derivative. The calculated intensities of the amide modes in NMA agree very well with experiment. The dipole derivatives for NMA were transferred to poly (glycine I) and the intensity predictions were also found to be good. Thus, it should now be possible to calculate reliable IR intensities of polypeptides to help in conformational analysis.

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