STUDIES ON THE HARMONIC FORCE-FIELD AND VIBRATIONAL-SPECTRA OF GAMMA-PICONILIC ACID AND ITS ISOTOPIC DERIVATIVES WITH AB-INITIO CALCULATIONS

The harmonic vibrational force field and the vibrational spectrum of gamma-picolinic acid have been calculated by the ab initio gradient program TEXAS using a STO-4-21G basis set. The directly computed theoretical harmonic force field was scaled with a set of scaling factors transferred from related molecules to obtain a prediction of fundamental frequencies. The mean deviation between prediction and experimental IR spectrum of gamma-picolinic acid in solid phase was 20cm(-1) (23cm(-1) for in-plane modes, 11cm(-1) for out-of-plane ones). The vibrational spectra of two isotopic derivatives of gamma-picolinic acid (-(CO2D)-O-16, -(CO2H)-O-18) have been predicted with the SQM force field resulting from the transferred scaling factor. The calculated isotopic shifts are in good agreement with experimental data. A few scaling factors of in-plane modes were optimized to fit theoretical predicted frequencies to the experimental spectrum, reducing the mean deviation of in-plane modes to 15cm(-1), and the total one to 14cm(-1). The deviations and the assignments of some fundamentals were discussed.