Hydrogen Bond Breaking and Reformation in Alcohol Oligomers Following Vibrational Relaxation of a Non-Hydrogen-Bond Donating Hydroxyl Stretch

Vibrational relaxation in methanol-d, ethanol-d, and 1-propanol-d dissolved in CCl4 has been measured with ultrafast infrared pump−probe experiments. Non-hydrogen-bond donating OD stretches (∼2690 cm-1) are excited. For concentrations ≤2.5 mol %, alcohol monomers dominate the pump−probe signals, and all three alcohols yield monoexponential decays with decay times of ∼2 ps (varying somewhat for the different alcohols). In the 5 mol % samples studied, molecules associated with oligomers dominate the signals. The signals decay to negative values (increased absorption), but the rates of the vibrational excited state decays are unchanged from those observed in the 2.5 mol % samples. The negative signals recover on two time scales. We propose a model in which hydrogen bond dissociation, following vibrational relaxation, increases the concentration of non-hydrogen-bond donating hydroxyl groups and produces the observed negative signal. The observation of the same decay rates with and without hydrogen bond dissoc...