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 performed in order to understand the UV-vis spectrum that has been observed 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 with those observed. This agreement provides evidence that the 1(1)A(g) state of TMB is responsible for the UV-vis spectrum. Therefore, our results confirm that, as predicted theoretically and as found by the experiments of Berson and co-workers, this disjoint diradical has a singlet ground state and, thus, violates Hund's rule.