MCSCF AND CASPT2N CALCULATIONS ON THE EXCITED-STATES OF 1,2,4,5-TETRAMETHYLENEBENZENE - THE UV-VIS SPECTRUM OBSERVED BELONGS TO THE SINGLET-STATE OF THE DIRADICAL

CASSCF and CASPT2N calculations with the 6-31G basis set have been pe rformed in order to understand the UV-vis spectrum that has been obser ved for TMB. The calculations lead to the assignment of the 1(1)A(g) - -> 2(1)A(g) transition as being responsible for the longest wavelength absorption in the spectrum. The nature of the 2(1)A(g) excited state is discussed. The CASPT2N excitation energies calculated for the four excited singlet states of lowest energy are in excellent agreement wit h those observed. This agreement provides evidence that the 1(1)A(g) s tate of TMB is responsible for the UV-vis spectrum. Therefore, our res ults confirm that, as predicted theoretically and as found by the expe riments of Berson and co-workers, this disjoint diradical has a single t ground state and, thus, violates Hund's rule.