Ultrafast relaxation and 2D IR of the aqueous trifluorocarboxylate ion

The asymmetric stretching vibration of the amphiphilic trifluoroacetate ion and its (13)C=(16)O isotopologue in D(2)O were investigated with infrared spectroscopy (FTIR), ultrafast infrared pump probe, and two dimensional vibrational photon echo techniques and simulations. Trifluoroacetate ions have a nonexponential depopulation of the first vibrational excited state, which is well described by a kinetic mechanism involving a temperature dependent solvent assisted relaxation to the symmetric stretch mode. The vibrational spectrum of the asymmetric stretch of the (13)C=(16)O isotopologue presents an unusual spectral shape. The frequency-frequency autocorrelation function shows a static term not present in the (13)C=(16)O form, which is caused by an accidental degeneracy with a combinational mode. A newly developed frequency map for carboxylate is used to characterize the processes and dynamics observed in the frequency fluctuations of the carboxylate asymmetric stretch mode in aqueous solution. An assignment of the molecular processes that govern the frequency fluctuations is suggested from an analysis of the solvation shell configurations obtained from molecular dynamics simulations.