Raman spectroscopic studies of vibrational relaxation and chemical exchange broadening in hydrogen bonded systems

The v 1 Raman band of pyridine in binary mixtures with ethanol has been studied across the whole concentration range and over a temperature range between 98 K and 370 K. The band intensities are readily explained in terms of a multiple equilibrium model which includes ethanol trimers as a measure of the alcohol network. It is this aggregation phenomenon which leads to the detection of free (i.e., non-hydrogen bonded) pyridine even in solutions containing a very large proportion of ethanol. The bandwidth data have been interpreted in terms of the molecular dephasing caused by hydrogen bonding interactions. At low temperatures, the Gaussian band shapes are formed in the slow modulation limit but, as the temperature is raised, motional narrowing occurs and the band shape changes from Gaussian to Lorentzian. The dephasing rate therefore slows up due to more rapid thermal fluctuations. At temperatures above room temperature the bandwidths increase again, and this is considered to be an indicator of the importance of chemical exchange dynamics for this system.