Concentration-dependent dynamics of hydrogen bonding between acetonitrile and methanol as determined by 1D vibrational spectroscopy

Solutions of acetonitrile (MeCN) in methanol (MeOH) at various concentrations have been investigated by variable temperature Raman spectroscopy. In the η(CN) region of the spectrum, the variable temperature spectra at each concentration show two overlapping bands from hydrogen bound and free MeCN. These two species undergo dynamic exchange that gives rise to increasing coalescence of the two bands with increasing temperature. By simulation of the band shape, the rate of exchange was determined at each temperature. Arrhenius plots yielded values for the activation energy, E _a, and the natural log of the pre-exponential factor, ln[A/s ^-1], for the hydrogen bond formation/cleavage. Both of these dynamic parameters were found to depend on the relative amounts of MeCN and MeOH in the solutions. In particular, two different concentration regimes of dynamic hydrogen bonding were observed. First, at low MeCN concentration, the dynamics are largely independent of changes in MeCN concentration. Second, at higher MeCN concentration (above ∼0.2 MeCN mole fraction) the dynamics are strongly dependent on further increases of MeCN content. Over the range of MeCN mole fractions that we studied (0.03-0.5), the ln[A/s^-1] changes from 32.5 ± 0.1 to 30.1 ± 0.2 and E_a changes from 3.73 ± 0.08 to 2.7 ± 0.1 kcal/mol. We suggest the observed changes in dynamics arise from changes in the local solvent microstructure that occur above a critical mole fraction of MeCN.