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 solut ion 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 fas t time constant in the subpicosecond regime and a slow time constant of a f ew picoseconds. For the cyclic dimers of the acetic acids, the fast compone nt 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-hy drogen-bonding monomer methyl acetate also has a subpicosecond decay consta nt. The pump-probe anisotropy decay reveals that the orientational dynamics of these molecules also occurs on the subpicosecond time scale and is reas onably well described by rotational diffusion in the slip hydrodynamic limi t. Stimulated infrared photon echo decay experiments reveal that the correl ation function of the frequency fluctuations of the cyclic acid dimer has a motionally narrowed process described by a 4 ps pure dephasing time and pr ocess with a 2.1 ps correlation time, comparable to a solvent response time . The dephasing dynamics is dominated by the population relaxation. In anal yzing the photon echo data, the contribution from the rotational diffusion is incorporated by approximating the cyclic acid dimer as a symmetric top d iffuser with its transition dipole located in the molecular plane but not p arallel to any of the principal axes. General formulas, which will be usefu l in other applications, for incorporation of the diffusive dynamics of the symmetric top into the third order response functions are obtained. Nonexp onential fast vibrational relaxation of C-CO2-X moiety is not adequately de scribed by the anharmonic coupling with the nearby combination and overtone bands. In the regime where the rotational, vibrational, and dephasing time s are all comparable, the solvent memory effects may play a role in vibrati onal dynamics, causing unusually rapid nonexponential population decay. (C) 2001 American Institute of Physics.