The isotopic dependence of axes switching in thiophosgene induced by A˜1A2(nπ∗)←X˜1A1 electronic excitation

A line-by-line rotational analysis of a selected group of bands in the high-resolution A˜1A2(nπ∗)←X˜1A1 fluorescence excitation spectrum has been carried out to derive the structure of thiophosgene in the Ã1A2(nπ∗) or S1 electronic state. The fit of the rotational line structure yields changes of +0.094 Å for the CS bond relative to the S0 value of 1.600 and −0.007 Å for the CCl bond (1.727 Å). The ClCCl bond angle increases by +6.4° (111.2°) while the out-of-plane angle advances to 23.9°. The in-plane a- and b-principal axes are susceptible to axis-switching mediated by chlorine 35-37 isotope substitution and also by excitation to the à state. In the X˜ or S0 ground state, the a-axis lies along the CS bond for the 35Cl2CS isotopomer and switches under chlorine isotope substitution in 37Cl2CS to the in-plane perpendicular direction. Electronic excitation also induces a/b axis-switching giving rise to anomalous rotational line intensities in the centers of the 35Cl2CS bands.