SPIN-ORBIT-COUPLING IN ORGANIC BIRADICALS - ZERO-FIELD SPLITTING IN TRIPLET TETRAMETHYLENEETHANE

Zero-field splitting in the triplet tetramethyleneethane biradical has been calculated as a function of angle of twist about the central C-C bond, using both the spin dipole-spin dipole and spin-orbit coupling contributions. The former contribution was evaluated at two approximat ions to the full CI-SD/ROHF/6-31G level and was found to dominate the D'/hc and E'/hc values totally. The latter, calculated at the CASSCF( 6,6)/6-31G level, is negligible. The calculated value of \D'/hc\ is i nsensitive to the twist angle and agrees well with the observed value. The agreement is best for twist angles near the similar to 50 degrees twist angle calculated previously for the equilibrium geometry of the triplet. The values calculated for the \E'/hc\ parameter depend on th e twist angle and only agree with the observed value at angles close t o 90 degrees. A comparison of calculated and observed zero-field split ting parameters of 1 constrained to a small twist by cyclization (2) s uggests that only the computed value of \D'/hc\ is reliable, while \E' /hc\ is grossly overestimated. It is therefore not possible to conclud e that 1 is twisted to a geometry with orthoganal allyl groups. (C) 19 97 Elsevier Science B.V.