Anomalously Slow Solvent Structural Relaxation Accompanying High-Energy Rotational Relaxation

Ultrafast Rotational and Translational Energy Relaxation in Neat Liquids

The excess energy flow pathways during rotational and translational relaxation induced by rotational or translational excitation of a single molecule of and within each of four different neat liquids (H₂O, MeOH, CCl₄, and CH₄) are studied using classical molecular dynamics simulations and energy flux analysis. For all four liquids, the relaxation processes for both types of excitation are ultrafast, but the energy flow is significantly faster for the polar, hydrogen-bonded (H-bonded) liquids H₂O and MeOH. Whereas the majority of the initial excess energy is transferred into hindered rotations (librations) for rotational excitation in the H-bonded liquids, an almost equal efficiency for transfer to translational and rotational motions is observed in the nonpolar, non-H-bonded liquids CCl₄ and CH₄. For translational excitation, transfer to translational motions dominates for all liquids. In general, the energy flows are quite local; i.e., more than 70% of the energy flows directly to the first solvent shell molecules, reaching almost 100% for CCl₄ and CH₄. Finally, the determined validity of linear response theory for these nonequilibrium relaxation processes is quite solvent-dependent, with the deviation from linear response most marked for rotational excitation and for the nonpolar liquids.

Vibrational energy relaxation of large-amplitude vibrations in liquids

Given the limited intermolecular spaces available in dense liquids, the large amplitudes of highly excited, low frequency vibrational modes pose an interesting dilemma for large molecules in solution. We carry out molecular dynamics calculations of the lowest frequency ("warping") mode of perylene dissolved in liquid argon, and demonstrate that vibrational excitation of this mode should cause identifiable changes in local solvation shell structure. But while the same kinds of solvent structural rearrangements can cause the non-equilibrium relaxation dynamics of highly excited diatomic rotors in liquids to differ substantially from equilibrium dynamics, our simulations also indicate that the non-equilibrium vibrational energy relaxation of large-amplitude vibrational overtones in liquids should show no such deviations from linear response. This observation seems to be a generic feature of large-moment-arm vibrational degrees of freedom and is therefore probably not specific to our choice of model system: The lowest frequency (largest amplitude) cases probably dissipate energy too quickly and the higher frequency (more slowly relaxing) cases most likely have solvent displacements too small to generate significant nonlinearities in simple nonpolar solvents. Vibrational kinetic energy relaxation, in particular, seems to be especially and surprisingly linear.