Intramolecular vibrational energy redistribution in the CD3H molecule

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Abstract
We present a study of the intramolecular vibrational energy redistribution (IVR) from a highly excited C–H overtone of the CD3H molecule. The whole vibrational manifold has been explicitly considered in the calculations. Two different approaches have been used and compared. In a direct approach, we have kept all the states located below a given threshold energy, resulting in a basis set of 92 000 states. The second approach consisted in selecting the important states in order to define a dynamically relevant active space (AS) of much lower size (≊2000). The two approaches were first applied to the calculation of the n=6 C–H overtone spectrum, showing that the AS method is quite able to reproduce the exact results. More stringent test concerned the actual time evolution of the C–H stretch local mode ‖6ν1≳0. Explicit time propagation has been carried out in the larger basis set, over a 15 ps interval. Results show that the dynamics is mainly governed by a few resonant states involving the C–H bending and C–D stretching motions. Very slow relaxation out of the C–H chromophore is observed over this 15 ps period. The AS method, using a much smaller basis set, was shown to reproduce the correct behavior of the C–H chromophore dynamics during the first 2 ps.