Motional dynamics in liquid 1,2,3,4-tetrahydro-5,6-dimethyl-1,4-methanonaphthalene

Abstract

The rotational motion of molecules about the three space axes in the liquid state can be described by the rotational diffusion tensor. Until recently most investigations concerning rotational motions of molecules in liquids employing dipolar spin-lattice relaxation data have proceeded on the assumption that the orientation of the rotational diffusion tensor principal axis system coincides with that of the intertial tensor. But this is not necessarily the case for liquids which consist of molecules rotating as asymmetrictop rotors with less than two mutually perpendicular mirror planes. Usually it is assumed that, in hydrocarbons, a rotation of the two principal axis systems with respect to each other does not occur because of their weak intermolecular interactions. In order to examine this statement the rotational behaviour of the molecules of the hydrocarbon 1,2,3,4-tetrahydro-5,6-dimethyl-1,4-methanonaphthalene (1) was investigated using ¹³C dipolar spin-lattice relaxation time measurements. The orientation of the rotational diffusion principal axis system was determined by successive variation of the eulerian angles between the two principal axis systems, and, two favoured orientations were obtained which were quite different from the orientation of the inertial tensor principal axis system. For both orientations the rotational diffusion constants and the methyl group jumping rates were evaluated via an extended Woessner-formalism developed by Bluhm (1982).