Revised assignment of the vibrational spectra of methylpyrazines based on scaled DFT force fields

In this work the vibrational spectra of the monomethyl- and all possible dimethyl-derivatives of pyrazine are analyzed. The infrared and Raman spectra of the compounds have been recorded in condensed state, including Raman depolarization measurements in case of liquid materials. The optimized geometrical parameters and corresponding force fields of the investigated molecules have been calculated using the density functional theory with the Becke3P86 functional and the 6-311G (d,p) basis set. The resulting force fields were transformed to internal coordinates and the calculated vibrational frequencies and normal modes were utilized in the assignment of the observed vibrational fundamentals. The measured spectral data were used to refine the vibrational force constants by means of a small number of scale factors. After the refinement, the relative mean deviations between the measured and calculated frequencies were about 1% or less for every investigated molecule. Substituent effects on the vibrational modes are discussed in some detail. The analysis of the spectra led to a revised assignment for a number of bands.