The mechanism of direct spin-orbit coupling arising from electrostatic interaction of the electrons on the perturbed molecule with the nuclei of the perturber is investigated theoretically for the case of benzene in the presence of external heavy atoms. Matrix elements are evaluated using Slater-type orbitals and the von Neumann expansion, and by using gaussian-type orbitals. Results of the present calculation confirm the experimental evidence (from polarization studies by Giachino and Kearns and phosphorescence measurements by Hofeldt, Sahai and Lin) that mixing by the perturber of the triplet state of the molecule with its singlet states is not the mechanism for introducing intensity into the radiativeT1-S0transition.