| Abstract
| - The vibrational relaxation dynamics of pseudo-halide anions XCN- (X=O, S, Se) in polar solvents werestudied to understand the effect of charge on solute-to-solvent intermolecular energy transfer (IET) and solventassisted intramolecular vibrational relaxation (IVR) pathways. The T1 relaxation times of the CN stretch inthese anions were measured by IR pump/IR probe spectroscopy, in which the 0−1 transition was excited,and the 0−1 and 1−2 transitions were monitored to follow the recovery of the ground state and decay of theexcited state. For these anions in five solvents, H2O, D2O, CH3OH, CH3CN, and (CH3)2SO, relaxation ratesfollowed the trend of OCN-> SCN-> SeCN-. For these anions and isotopes of SCN-, the relaxation ratewas a factor of a few (2.5−10) higher in H2O than in D2O. To further probe the solvent isotope effect, therelaxation rates of S12C14N-, S13C14N-, and S12C15N- in deuterated methanols (CH3OH, CH3OD, CH3OH,CD3OD) were compared. Relaxation rate was found to be affected by the change of solvent vibrational bandat the CN- stretching mode (CD3 symmetric stretch) and lower frequency regions, suggesting the presenceof both direct IET and solvent assisted IVR relaxation pathways. The possible relaxation pathways andmechanisms for the observed trends in solute and solvent dependence were discussed.
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