Real-time observation of intra- and intermolecular vibrational energy flow of selectively excited alkyl iodides in solution: The effect of chemical substitution

Intramolecular vibrational energy redistribution (IVR) and intermolecular vibrational energy transfer (VET) of two alkyl iodides (CF3CH2I and CH3CH2-I) selectively excited in the two quanta overtone region of the CH stretch vibration (upsilon(CH) = 2) were measured in real time in solution. In this study, we have focused on the effect of chemical substitution on the mechanisms and time scales of IVR and VET of this family of molecules. With a simple model. we have obtained global IVR and VET rate coefficients for both molecules. The magnitude and the variation of the relaxation rates upon chemical substitution provide evidence for a survival of hierarchical IVR in these solvated molecules. which is governed by specific low-order intramolecular interactions and which can be rationalized with a simple low-order coupling model. At the same time, the general assumption that VET is simply dominated by the lowest-frequency modes in a molecule is not supported.