Vibrational relaxation of the free terminal hydroxyl stretch in methanol oligomers: Indirect pathway to hydrogen bond breaking

Vibrational relaxation of methanol-d (MeOD) in carbon tetrachloride has been investigated via ultrafast infrared pump-probe experiments. Exciting at 2690 cm(-1), only the free O-D (where the D is not H-bonded) stretching mode is initially populated. For MeOD mole fractions less than or equal to0.025, a 2.15 ps single exponential decay is observed. At mole fractions greater than or equal to0.0375, the signal decays (2.15 ps decay time) below zero (increased absorption) and then recovers on time scales of 22 ps and much greater than 300 ps. The increased absorption indicates the formation of additional free ODs caused by the breaking of H-bonds that are not directly coupled to the initially excited vibration. The two-time scale recovery of this signal arises from geminate and nongeminate recombination. The data are fit with a set of kinetic equations that accurately reproduce the data. The results suggest that vibrational relaxation of the initially excited free OD stretch into intramolecular modes of the methanol leads to H-bond breaking. This contrasts studies that suggest direct relaxation of a vibrationally excited OH stretch into an H-bond stretch is responsible for H-bond breaking. (C) 2001 American Institute of Physics.