Water-induced relaxation of a degenerate vibration of guanidinium using 2D IR echo spectroscopy.

The nearly degenerate asymmetric stretch vibrations near 1600 cm(-1) of the guanidinium cation in D-glycerol/D(2)O mixtures having different viscosity were studied by 2D IR photon echo spectroscopy. The polarization-dependent photon echo signal shows two separate frequency distributions in the 2D spectrum in D(2)O, even though only one band is evident from inspection of the linear FTIR spectrum. The split components are more clearly seen at higher viscosity where the distortion of the molecule from 3-fold symmetry is even more evident. The interactions with solvent induce energy transfer between the degenerate component modes on the time scale of 0.5 ps. The energy transfer between modes is directly observed in 2D IR and distinguished by the waiting time dependence of the cross peaks from the transfers between configurations of the distorted ion and solvent. The 2D IR analysis carried out for various polarization conditions gave frequency-frequency auto- and cross-correlation functions for the degenerate components which derive from the solvent induced wagging of the -ND(2) groups of the guanidinium ion.