Effect of methylation on wavenumber shift of ring breathing mode of pyrimidine in the solution of H2O and D2O by employing Raman difference spectroscopic technique and DFT approach

The relative structural and dynamic properties of hydrogen-bonding between Pyrimidine (Pmd) + H2O and Pmd + D2O, and 4-Methylpyrimidine (Mpmd) + H2O and Mpmd + D2O are investigated experimentally by linear Raman spectroscopy using Raman difference spectroscopic (RDS) technique. The focus has been given to the ring breathing mode (ν1). The effect of methylation on the Pmd ring has been studied in terms of wavenumber shift (Δν), peak-position and linewidth variation with mole fraction of the solvent. The wavenumber shift has been calculated by assuming the Voigt profile of the Raman band. In order to explain our experimental results, we have optimized single Pmd and 4Mpmd molecules and their various complexes with H2O and D2O in the stoichiometric ratio of 1:1, 1:2, 1:3 and 1:4 by employing DFT/B3LYP functional with 6-311+G(d,p) basis set using Gaussian software. There is a good correspondence between experimental and theoretical results. Our result reveals that with RDS technique, Δν of a band up to 1/100th of the FWHM can be measured precisely.