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, only the free O-D ~where the D is not H-bonded! stretching mode is initially populated. For MeOD mole fractions <0.025, a 2.15 ps single exponential decay is observed. At mole fractions >0.0375, the signal decays ~2.15 ps decay time! below zero ~increased absorption! and then recovers on time scales of 22 ps and @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. © 2001 American Institute of Physics. @DOI: 10.1063/1.1415447# JOURNAL OF CHEMICAL PHYSICS VOLUME 115, NUMBER 20 22 NOVEMBER 2001