The rotation-translation coupling in diatomic molecules

Abstract

The coupling between the translational and rotational motions of single diatomic molecules in the liquid state is investigated for both the homo- and heteronuclear cases. ‘Experimental’ data are provided by computer simulations using the method of molecular dynamics. Their analysis is made in terms of low order time correlation functions of the translational velocity V of the centre of mass and the rotational velocity ω; these correlation functions are defined in two different ways: (a) with respect to the fixed laboratory axis and (b) with respect to a moving reference frame attached to the molecule itself. It is observed, at the level of the correlation matrix {V, ω}, that the rotation-translation coupling appears only indirectly in presentation (a) while direct evidence is provided in presentation (b). Further, this second presentation displays qualitative differences between homo- and heteronuclear molecules. These results should help in clarifying the validity of various stochastic equations modelling the rigid body dynamics within a fluid.