Fourier Transform Raman and DFT Study of Blue Shift C-H Stretching Vibration of Diazines on Hydrogen Bond Formation

Diazines form various types of hydrogen bonds with water, wherein C–H groups are sometimes involved directly and sometimes indirectly. On hydrogen bond formation, the wavenumber of C–H stretching vibration are usually red shifted (normal hydrogen bond) but blue shift of C–H modes (anomalous hydrogen bond) is also possible in some cases. The Raman spectra of the C–H stretching bands of three diazines; pyrimidine, pyridazine, and pyrazine in pure form and at many concentrations in mole fractions of diazines in the mixture of diazines + water have been measured to analyze the wavenumber shifts experimentally. Theoretical wavenumber shifts have been calculated and NBO analysis has been performed using DFT methods to understand the cause of the shifts. All the four C–H stretching bands of diazines are blue shifted on dilution with water both experimentally and theoretically. The NBO calculations reveal that the cause of the shift is decrease in the charge density in the antibonding orbital of C–H bond on complex formation.