Natural coordinates for polyatomic reactions

Abstract

The curvilinear natural coordinates (NRC) are considered to describe the polyatomic reaction dynamics. An invariant equation is found for the reaction path curve (RP), allowing its calculation in an arbitrary initial coordinate system. Apart from common properties, well known for triatomic reactions, the polyatomic NRC take explicit account of a smooth change of normal vibrational modes of a chemical system during its evolution along RP. This generates special terms in the kinetic energy operator that induce nonadiabatic vibrational transitions and do not vanish in a classical limit, no matter how small the curvature of the RP is. For the case when the amplitudes of vibrations perpendicular to the RP are small it is shown that, as a first approximation, the vibrational transitions can be treated independently of the rotational degrees of freedom. This conclusion is obtained as a generalization of Eckart theory stating the possibility of vibration—rotational separation for stable molecules.