23.05.23 - Florian Brünig "Multiscale memory friction effects in water: reaction kinetics and vibrational dynamics"

 

When described by a one-dimensional reaction coordinate, pair-reaction rates in a solvent depend, in addition to the potential barrier height and the friction coefficient, on the potential shape, the effective mass and the friction relaxation spectrum, but a rate theory that accurately accounts for all these effects does not exist. We show how to extract all parameters of the generalized Langevin equation (GLE) and in particular the friction memory function from molecular dynamics (MD) simulations of two prototypical pair reactions in water, the dissociation of NaCl and of two methane molecules. Simulations of the GLE by Markovian embedding techniques accurately reproduce the pair-reaction kinetics from MD simulations without any fitting parameters, which confirms the accuracy of the approximative form of the GLE and of the parameter extraction techniques. By modification of the GLE parameters, we investigate the relative importance of memory, mass and potential-shape effects. Neglect of memory slows down NaCl and methane dissociation by roughly a factor of 2, neglect of mass accelerates reactions by a similar factor, the harmonic approximation of the potential shape gives rise to slight acceleration. This error cancellation explains why Kramers’ theory, which neglects memory effects and treats the potential shape harmonically, describes reaction rates better than more sophisticated theories.

Secondly, to investigate the OH stretch and HOH bending vibrational dynamics of liquid water, the time-dependent friction functions and time-averaged nonlinear effective bond potentials are extracted for ab initio MD simulations. The obtained friction exhibits not only adiabatic contributions at and below the vibrational time scales but also much slower nonadiabatic contributions, reflecting homogeneous and inhomogeneous line broadening mechanisms, respectively. Intermolecular interactions in liquid water soften both stretch and bend potentials compared to the gas phase, which by itself would lead to a red-shift of the corresponding vibrational bands. In contrast, nonadiabatic friction contributions cause a spectral blue shift. For the stretch mode, the potential effect dominates, and thus, a significant red shift when going from gas to the liquid phase results. For the bend mode, potential and nonadiabatic friction effects are of comparable magnitude, so that a slight blue shift results, in agreement with well-known but puzzling experimental findings. The observed line broadening is shown to be roughly equally caused by adiabatic and nonadiabatic friction contributions for both the stretch and bend modes in liquid water. Thus, the quantitative analysis of the time-dependent friction that acts on vibrational modes in liquids advances the understanding of infrared vibrational frequencies and line shapes.