Unusual vibrational dynamics of the acetic acid dimer

The vibrational relaxation of the C=O stretching mode of the CH3CO2H cyclic dimer, the CH3CO2D cyclic dimer, and CH3CO2CH3 were measured in CCl4 solution at room temperature. The population relaxation of the v=1 state of the C=O mode is nonexponential, modeled with a biexponential decay having a fast time constant in the subpicosecond regime and a slow time constant of a few picoseconds. For the cyclic dimers of the acetic acids, the fast component dominates the population decay, whereas the slow component dominates the decay of the CH3CO2CH3, the model compound for the monomeric acetic acid. Deuteration of the dimer increases the relaxation time constant. The non-hydrogen-bonding monomer methyl acetate also has a subpicosecond decay constant. The pump-probe anisotropy decay reveals that the orientational dynamics of these molecules also occurs on the subpicosecond time scale and is reasonably well described by rotational diffusion in the slip hydrodynamic limit. Stimulated infrared photon echo decay experiments reveal that the correlation function of the frequency fluctuations of the cyclic acid dimer has a motionally narrowed process described by a 4 ps pure dephasing time and process with a 2.1 ps correlation time, comparable to a solvent response time. The dephasing dynamics is dominated by the population relaxation. In analyzing the photon echo data, the contribution from the rotational diffusion is incorporated by approximating the cyclic acid dimer as a symmetric top diffuser with its transition dipole located in the molecular plane but not parallel to any of the principal axes. General formulas, which will be useful in other applications, for incorporation of the diffusive dynamics of the symmetric top into the third order response functions are obtained. Nonexponential fast vibrational relaxation of C-CO2-X moiety is not adequately described by the anharmonic coupling with the nearby combination and overtone bands. In the regime where the rotational, vibrational, and dephasing times are all comparable, the solvent memory effects may play a role in vibrational dynamics, causing unusually rapid nonexponential population decay. (C) 2001 American Institute of Physics.