ELECTRONICALLY EXCITED-STATES OF 1,4 5,8-BISMETHANO-1,4,4A,5,8,8A-HEXAHYDRONAPHTHALENE, A NONCONJUGATED DIENE - COMPARISON OF THEORY AND EXPERIMENT/

Electron energy-loss spectroscopy (EELS) is used to determine the ener gies of low-lying singlet and triplet states of the title compound. Th e EELS measurements give a splitting of 0.45 eV between the two lowest pi --> pi singlet states, as confirmed by optical absorption measure ments, but do not give a discernable splitting between the correspondi ng triplet states. The experimental results are interpreted with the a id of ab initio electronic structure calculations using the CIS and CA SPTZ methods. The calculations are consistent with the experiments, gi ving a much larger splitting between the pi --> pi singlet as compare d to the pi --> pi triplet states. They also reveal that the splittin g between the two lowest pi --> pi triplet states is dominated by thr ough-bond (TB) interactions, whereas that between the corresponding si nglet states includes also a significant contribution from the dipole- dipole coupling mechanism.