Charge Transfer and Blue Shifting of Vibrational Frequencies in a Hydrogen Bond Acceptor

A comprehensive Raman spectroscopic/electronic structure study of hydrogen bonding by pyrimidine with eight different polar solvents is presented. Raman spectra of binary mixtures of pyrimidine with methanol and ethylene glycol are reported, and shifts in ν₁, ν₃, ν_6a, ν_6b, ν_8a, ν_8b, ν_9a, ν₁₅, ν_16a, and ν_16b are compared to earlier results obtained for water. Large shifts to higher vibrational energy, often referred to as blue shifts, are observed for ν₁, ν_6b, and ν_8b (by as much as 14 cm^–1). While gradual blue shifts with increasing hydrogen bond donor concentration are observed for ν_6b and ν_8b, ν₁ exhibits three distinct spectral components whose relative intensities vary with concentration. The blue shift of ν₁ is further examined in binary mixtures of pyrimidine with acetic acid, thioglycol, phenylmethanol, hexylamine, and acetonitrile. Electronic structure computations for more than 100 microsolvated structures reveal a significant dependence of the magnitude of the ν₁ blue shift on the local microsolvation geometry. Results from natural bond orbital (NBO) calculations also reveal a strong correlation between charge transfer and blue shifting of pyrimidine’s normal modes. Although charge transfer has previously been linked to blue shifting of the X–H stretching frequency in hydrogen bond donors, here, a similar trend in a hydrogen bond acceptor is demonstrated.