Effect of configuration and conformation on the spin multiplicity in xylylene type biradicals

The singlet-triplet splitting energy gap △ES-T = ES- ET is calculated for the ortho-, meta-, and para-xylylenes and their heteroatomic analogous by means of AM1-CI approach. It is shown that when the radical centers R.(R.=H2C°-,H2N°+- or HN°- ) are twisted sufficiently far out of conjugation with the benzene ring, △S-T tends to zero or is negative, i.e. ortho-, meta-, and para-phenylenes turn into weak ferromagnetic or antiferromagnetic coupling unit, while they are strong ferromagnetic (meta-isomers) or antiferromagnetic (ortho-, para-isomers) coupling units under planar conformation. It is suggested that serious twisted conformation is not recommended candidate for the design of novel high-spin molecules with stable high-spin ground states by ortho- or para-phenylene coupling unit.