RAMAN-SPECTROSCOPIC STUDIES ON THE INTERACTION AND DYNAMICS OF N,N-DIMETHYLFORMAMIDE IN THE LIQUID-PHASE - THE EFFECT OF ISOTOPIC DILUTION

Raman isotropic, nu(C = O) band profiles studied across the whole concentration range for mixtures of DMF-h7 and DMF-d7 show that band broadening occurs with isotopic dilution. This increase in vibrational relaxation rate and the accompanying reduction in non-coincidence splitting, (DELTA), must be related to details of intermolecular coupling in the dense fluid. Two models for resonant coupling have been examined. The data are quite well represented by the Logan model which predicts the observed linear dependence of DELTA on concentration for an isotopic mixture. In addition, our band shape and width behaviour fit quite well the model of Knapp. We see a distinct band asymmetry to the high frequency side as predicted by this model for strong dipolar coupling. Furthermore, the broadening observed on isotopic dilution may also be accommodated within the model in terms of an exciton 'hopping' mechanism where the original band profile was motionally narrowed by a rapid intersite exchange process (but keeping the total amount of intermolecular coupling at a particular site fixed). The extent of band asymmetry and the band narrowing process due to resonant coupling strongly imply that the dephasing of this band in the pure liquid is dominated by inhomogeneous processes, due to slowly varying, long ranged interactions.