Substitution-induced Nonplanarity of 3-Fluorothioanisole in the First Electronically Excited State.

Vibronic spectra of 3-fluorothioanisole (3FTA) in the first electronic excited state (S 1 ) and the cationic ground state (D 0 ) have been obtained by one-color resonant two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopy. Spectroscopic measurements and theoretical calculations indicate that both cis- and trans- rotamers of the 3FTA molecule are stable and coexist in the S 0 (the electronic ground state) and D 0 states, and the cis- rotamer is shown to be slightly more stable than the trans- rotamer. In the S 1 state, theoretical calculations predict a stable gauche -structure of 3FTA, manifested by the observation of strong activation of the vibrational modes involving the motion of the -SCH 3 group in the low-frequency regions of the 1C-R2PI and MATI spectra. The electronic excitation energy from the S 0 state to the S 1 state (E 1 ) and the adiabatic ionization energy (IE) are respectively determined to be 34 820 ± 3 and 65 468 ± 5 cm -1 for cis- 3FTA, and those of the trans- rotamer are respectively determined to be 35 047 ± 3 and 65 644 ± 5 cm -1 . The structural properties of the stable rotamers of 3FTA and their comparison with other F- and Cl-substituted thioanisole derivatives are discussed as well.