The singlet-triplet splitting Delta E = E-T - E-S is calculated for the ortho-, meta-, and para-xylylenes and their heteroatomic analogous. It is shown that when the radical centers R-. (R-. = H2C. -, H2O.+ - or H (N) under bar(.)-H2O .+ -, H2N .+ - or H (N) under bar .-) are twisted sufficiently far out of conjugation with the benzene ring, the Delta E tends to zero or is negative, i.e. in some cases the para-isomers have a triplet ground state, while the meta-isomers have a singlet ground state. The concept of indirect exchange is proposed for explanation of the nature of the magnetic interaction between the radicals R-. through benzene ring.