Semiempirical Investigation of Stilbene-Linked Diradicals and Magnetic Study of Their Bis(N-tert-butylnitroxide) Variants

Article abstract of DOI:10.1021/jo00094a046

The electronic states of open-shell stilbene diradicals were studied.Previously used semiempirical MO-CI methods were applied to stilbenes having dimethylene, dioxyl, and dinitroxide centers in o,o'-, o,m'-, o,p'-, m,m'-, m,p'-, and p,p'-substitution patterns, and the various singlet-triplet energy gaps were computed to predict qualitative ground state spin multiplicities.Singlet and triplet spin states were essentially degenerate in the m,m'-isomers of disjoint connectivity, while nondisjoint o,m'- and m,p'-isomers had triplet computed ground states.Nitroxide-based diradicals showed considerably smaller computational exchange coupling energies than did other diradicals models studied.As a tests of these computations, stilbenes with two N-tert-butyl nitroxide groups at the o,m'-, m,m'-, and m,p'-positions were prepared, and their magnetic properties were studied by ESR and magnetic susceptibility measurements.The m,m'- and m,p'-isomers were found experimentally to have singlet and triplet ground states, respectively, in accord with the qualitative computed predictions.