PHENYLENE COUPLING OF METHYLENE SITES - THE SPIN STATES OF BIS(X-METHYLENE)-P-PHENYLENES AND BIS(CHLOROMETHYLENE)-M-PHENYLENE

We judge the energetic sequence of spin states in substituted methylen es by ab initio multiconfigurational computations and, where feasible, density functional modeling techniques. The best of these calculation s reproduce well-established singlet-triplet gaps in X-C-Y species, in which X can be phenyl and Y can be H, methyl, or chloro. Similar comp utations on p-phenylene-coupled Y-methylenes and meta-coupled Y-methyl enes support the suggestion by Zuev and Sheridan that bis(chloromethyl ene)-p-phenylene has a singlet diradical ground state. However, despit e the density functional computations' support for those authors' sugg estion that bis (chloromethylene)-m-phenylene has a singlet ground sta te, we find that our best MCSCF calculations place the quintet ground state suggested by the simplest theory almost equal in energy to that singlet.