Stereomutation dynamics in hydrogen peroxide

Abstract

Hydrogen peroxide (H–O–O–H) is among the simplest prototype molecules showing a chiral equilibrium geometry with the possibility of fast quantum stereomutation in the low barrier limit. We report full dimensional quantum dynamical tunneling calculations on a semi-global fully six-dimensional empirically adjusted potential hypersurface for H 2O 2, which is realistically close to spectroscopic and thermochemical accuracy (B. Kuhn, T.R. Rizzo, D. Luckhaus, M. Quack, M.A. Suhm, J. Chem. Phys. 111 (1999) 2565). Solutions of the time independent Schrödinger equation lead to levels of well defined parity (but undefined chirality), which compare well with available spectroscopic results and provide numerous predictions. Solutions of the time dependent Schrödinger equation with initial conditions of well defined chirality for P and M enantiomers show the time dependent wavepacket motion and periodic change of chirality for time scales between picoseconds and hundreds of picoseconds. Complete six-dimensional dynamics and adiabatic separation of the torsional mode from the high-frequency modes leads to essentially identical results for the stereomutation dynamics in terms of the relevant time dependence. Mode selective inhibition and catalysis of stereomutation by exciting various vibrationally excited levels are reported and discussed in relation to the concept of quasiadiabatic above barrier tunneling. The transition to relaxation behaviour and racemisation is demonstrated with quasithermal wavepackets and analysed in terms of a simple statistical model. At 3000 K the racemisation relaxation time is calculated to be 30 fs. We also discuss the results in the context of recent results on hydrogen peroxide as a prototype system for parity violation in chiral molecules.