Unraveling the role of microenvironment and hydrodynamic forces on the vibrational relaxation rates of pyridine-water complexes

Polarization dependent Raman spectra of pyridine dissolved in water have been measured in a wide concentration range from dense to extreme dilute solutions. The isotropic band frequencies and line widths of the ν₁₂(Α₁) mode have been analyzed by using various models of line broadening mechanisms. The vibrational dephasing and vibrational frequency modulation have been analyzed by calculating time correlation functions of vibrational relaxation by fits in the frequency domain. Spectral features such as bandwidths, band frequencies and characteristic dephasing times have been estimated and interpreted in the context of concentration induced variations and effects due to solute-solvent interactions. The time-correlation functions of vibrational dephasing were obtained for the ν ₁₂(Α₁) mode and it was found that there is a general agreement in the whole concentration range with the modeling proposed by the Rothschild approach, which applies to complex liquids as is the case of pyridine-water solutions. The evolution of several parameters, such as the characteristic times and the vibrational second moments is indicative of drastic variations at extreme dilution revealing changes in the vibrational relaxation of the pyridine complexes in the aqueous environment. The microenvironment prevailing in the neighborhood of the ν₁₂(Α₁) mode seems to be the key factor in obtaining information concerning vibrational dynamics. Significant correlation is observed between vibrational relaxation rate and solvent parameters namely viscosity, density, refractive index and molecular radius. Microviscosity, involving the size of solute and solvent molecules, is found to be crucial in determining the relaxation rate. The microenvironment appears to play an important role in the vibrational relaxation process.