Analytical MO study of the singlet–triplet coupling in xylylenes

The singlet–triplet energy difference in para ‐, meta ‐, and ortho ‐xylylenes is studied as the interaction of two radical centers through the benzene ring. An SCF perturbative procedure adapted to open‐shell systems leads to two benzyl‐like nonbonding molecular orbitals ( NBMOS ) and to benzene‐like occupied and vacant MOS whatever the xylylene isomer. The superposition of these NBMOS in para ‐, meta ‐, and ortho ‐positions and their interaction with the benzene‐like MOS lead, at the configuration interaction level, to the following results: The exchange energy (which favors the triplet state) and the charge transfer energy (which favors the singlet state) are important only in the meta ‐xylylene; the dynamic (or double) spin polarization favors the triplet in meta and the singlet in para and ortho ‐isomers; the super‐exchange energy (which favors the singlet) is important only in para ‐ and ortho ‐isomers. The above results are independent of the chosen geometry.