Assignment of the vibrational spectrum of trimethylpyrazine

The infrared and Raman spectra of the pure liquid trimethylpyrazine have been recorded and assigned on the basis of ab initio calculations at two different levels of theory. The RHF/3-21G force field was refined by the scaled quantum mechanical force field (SQMFF) method by directly transferring scale factors from pyrazine and 2-methylpyrazine. These a priori estimated vibrational frequencies were also used for scaling the B3-LYP/6-31+G∗ force field. A complete vibrational assignment is proposed by comparing the experimental data with both sets of theoretical frequencies. Additionally, the effect of the methyl substitution on pyrazine ring is discussed. The calculated B3-LYP/6-31+G∗ Infrared and Raman intensities of the isolated molecule fail to reproduce the experimental behaviour.