Hydrogen bond dissociation and reformation in methanol oligomers following hydroxyl stretch relaxation

Vibrational relaxation and hydrogen bond dynamics in methanol-d dissolved in CCl4 have been measured with ultrafast infrared pump-probe spectroscopy. We excited the subensemble of methanol-d molecules both accepting and donating hydrogen bonds at similar to2500 cm(-1). Following vibrational relaxation with a similar to500 fs lifetime, the signal does not decay to zero. Rather, the signal increases to a second maximum at similar to4 ps. The decay from the second maximum occurs on two time scales. We propose a model in which hydrogen bond dissociation, following vibrational relaxation, decreases the concentration of methanol-d molecules that accept and donate hydrogen bonds and produce the observed long-lived bleach of the absorption signal. Using a set of coupled kinetic equations, the time constants for hydrogen bond dissociation and reformation have been determined. Hydrogen bond breaking occurs with similar to200 fs and similar to2 ps time constants. We attribute the fast rate to a direct breaking mechanism wherein the excited hydroxyl stretch decays into modes that directly lead to the hydrogen bond dissociation. The slower rate of breaking is attributed to an indirect mechanism wherein the dissociation of hydrogen bonds follows vibrational energy flow from the initially excited molecule to other components of the same oligomer. The final stage of relaxation, after the second maximum, involves reformation of transiently broken hydrogen bonds. The bonds that break directly recover with similar to7 ps and much greater than10 ns time constants, while the bonds that break indirectly recover with similar to20 ps and much greater than10 ns time constants. Experiments conducted on ethanol-d solutions in CCl4 demonstrate that the same vibrational relaxation and hydrogen bond dynamic events occur with very similar amplitudes and rate constants. Measurements of the rates of spectral diffusion and polarization anisotropy decay via vibrational excitation transfer and orientational relaxation verify that the initial fast decay of the signal is dominated by vibrational relaxation.