THE EVOLUTION BETWEEN SLOW AND RAPID CHEMICAL-EXCHANGE PROCESSES IN A LIQUID BINARY MIXTURE

Binary mixtures of pyridine and ethanol display Raman spectra (nu1 of pyridine) with significantly different band shapes at different temperatures (between 98 and 368 K). At the lowest temperature these spectra indicate the formation of a hydrogen-bonded complex in the slow modulation limit (giving a Gaussian profile). At intermediate temperatures there is good evidence for two complexes, C1 and C2 (with one or two ethanol molecules), between which chemical exchange is "slow" on the vibrational time scale. At highest temperatures exchange between C1 and C2 complexes becomes "rapid" and "collapse" of the complexed pyridine spectrum occurs to give a single Lorentzian profile. It is noted that "free" pyridine exists at all concentrations in ethanol over times long compared with the inverse observation frequency.