Hybrid character and rotational structure of the [formula omitted] transitions of thionaphthene and benzofuran

Abstract

Rotational analyses of the 0-0 bands of thionaphthene and benzofuran are reported. The electronic transitions are at 2936 and 2781 Å, respectively. Using computer simulation techniques it is possible to obtain changes in rotational constants upon excitation (Δ A = −0.002797 cm −1; Δ B = −0.000450 cm −1; Δ C = −0.000465 cm −1 for thionaphthene; and Δ A = −0.005888 cm −1; Δ B = −0.000650 cm −1; Δ C = −0.000841 cm −1 for benzofuran), to assign numerous rotational features, and to demonstrate the hybrid character of the transition moment. For both molecules the latter is shown to lie in-plane. However, in benzofuran the transition moment is predominantly A-type (90% A-type-10% B-type ± 5%), while in thionaphthene B-type character predominants (10% A-type-90% B-type ± 10%). These facts confirm the assignment of the transition as π ∗«π . The 0-0 band origins of thionaphthene and benzofuran are found to be at 34 055.73 ± 0.05 cm −1 and 35 920.02 ± 0.05 cm −1, respectively.