Formation of dimers in quadrupolar gases

The equilibrium constant for dimerization K for quadrupolar gases has been computed under two limiting approximations: (i) K is calculated on the fixed relative orientations with a 12-6-5 potential model and then these values are averaged by giving equal weight to all orientations; (ii) the potential is `pre-averaged' over all orientations and then K is calculated. The former method is more appropriate for slow rotating molecules, and the latter one for fast rotating molecules. The percentage of dimers has been calculated from the contribution of dimers to the second virial coefficient for N2, CO2 and C6H6 on the 12-6-5 and Lennard-Jones 12-6 potentials. It is found that the presence of an appreciable quadrupole moment in the molecules greatly increases the tendency of molecules to form dimers.