The Role of Quantum Intramolecular Dynamics in Unimolecular Reactions

The dynamics of unimolecular reactions can be modelled by classical mechanics for the motion of nuclei on Born-Oppenheimer or other effective potential surfaces, by the corresponding quantum mechanical equations of motion and, perhaps, by quantum statistical treatments. In this paper I provide a synopsis of fundamental, qualitatively important effects arising from the quantum nature of intramolecular dynamics, as opposed to classical mechanics, and illustrate these with theoretical predictions and experimental examples from the work of my group in Zurich. These include quantum nonlinearity in infrared (IR) multiphoton excitation and reaction, non-classical wavepacket spreading in the Fermi resonance coupled modes in CHX₃ molecules, effects of zero point energy and angular momentum in unimolecular reactions, nuclear spin symmetry conservation and interconversion and the hypothetical effects arising from the violation of parity and time reversal symmetry in unimolecular reactions. Specific applications to experiments include IR laser chemistry of CF₃I and CF₃Br, IR spectroscopy and dynamics of CHF₃ and predissociation spectra and dynamics of H₃⁺. Hamiltonian systems with a finite number of degrees of freedom have traditionally been divided into two types: those with few degrees of freedom, which were supposed to exhibit some kind of regular ordered motion, approximately soluble by hamiltonian perturbation theory, and those with large numbers of degrees of freedom for which the methods of statistical mechanics should be used. The past few decades have seen a complete change of view, affecting almost all practical applications of classical mechanics. The motion of a hamiltonian system is usually neither completely regular nor properly described by statistical mechanics. It exhibits both regular and chaotic motion for different initial conditions, and the transition between the two types of motion as the initial conditions are varied is subtle and complicated. Variational principles, cantori, and their role in determining the transport properties of chaotic motion in hamiltonian systems and modular smoothing, a method for the rapid calculation of critical functions, which form the fractal boundary between regular and chaotic motion, have appeared in Percival (1987, 1990).