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

Abstract

Raman measurements on the v ₁ band of pyridine in binary mixtures with ethanol have been interpreted in terms of the Strauss-Bratos model for rapid chemical exchange between (multiple) sites in a liquid. Although certain assumptions about the behaviour of the dephasing rates (with temperature) are needed, we report rate constants which appear to be internally consistent and of the correct order of magnitude. For the equilibrium pyr ⇌ pyr-eth the association rate constant k ₁₂ varies in the range 1–6 × 10¹⁰ dm³ mol^-1 s^-1 with an activation energy which is sensitive to the proportion of ethanol in the mixture (36 kJ mol at x _eth = 0·32 but very small for x _eth = 0·58). For the dissociation rate constant (k ₃₂) of the equilibrium pyr-eth ⇌ pyr-(eth)₂ we find values in the range 1–6 × 10¹¹ s^-1 with an activation energy of about 10–15 kJ mol^-1. Arrhenius behaviour is observed over the (restricted) temperature range investigated. It is emphasized that an alternative interpretation, in terms of varying T ^-1 ₂ dephasing contributions, cannot be completely ruled out, but that consistent results within a very large number of data, point to the viability of an exchange dynamics model for the system.