Vibrational relaxation of normal and deuterated liquid nitromethane.

Anti-Stokes Raman scattering is used to monitor vibrational energy redistribution in the ambient temperature liquids nitromethane (NM-h3) and perdeuterated nitromethane (NM-d3) after ultrafast IR excitation of either the symmetric or asymmetric CH- or CD-stretch transitions. The instantaneous populations of most of the fifteen NM vibrations are determined with good accuracy, and a global fitting procedure with a master equation is used to fit all the data. The pump pulses excite not only CH- or CD-stretches but also certain combinations of bending and nitro stretching fundamentals. The coupled vibrations that comprise the initial state are revealed via the instantaneous rise of the anti-Stokes transients associated with each vibrational fundamental. In contrast to many other polyatomic liquids studied previously, there is little energy exchange among the CH-stretch (or CD-stretch) excitations, which is attributed to the nearly free rotation of the methyl group in NM. The vibrational cooling process, which is the multistep return to a thermalized state, occurs in three stages in both NM-h3 and NM-d3. In the first stage, the parent CH- or CD-stretch decays in a few picoseconds, exciting all lower-energy vibrations. In the second stage, the midrange vibrations decay in 10-15 ps, exciting the lower-energy vibrations. In the third stage, these lower-energy vibrations decay into the bath in tens of picoseconds. The initial excitations are thermalized in approximately 150 ps in NM-h3 and there is little dependence on which CH-stretch is excited. VC is somewhat faster in NM-d3 with more dependence on the initial CD-stretch, taking approximately 100 ps with symmetric CD-stretch excitation and approximately 120 ps with asymmetric CD-stretch excitation. Comparison is made with earlier nonequilibrium molecular dynamics simulations of VC [Kabadi, V. N.; Rice, B. M. Molecular dynamics simulations of normal mode vibrational energy transfer in liquid nitromethane. J. Phys. Chem. A 2004, 108, 532-540]. The simulations do a good job of reproducing the observed VC process and in addition they predicted the slow interconversion among CH-stretch excitations and the slower relaxation of the asymmetric CH-stretch now observed here.