Conformer Specific Excited-State Structure of 3-Methylthioanisole.

and conformers of 3-methylthioanisole have been spectroscopically investigated to reveal the conformer specific structural changes upon the S(ππ*)-S excitation. The conformational cooling during the supersonic expansion is found to be quite efficient in the Ar carrier gas giving the conformational isomer exclusively in the molecular beam, whereas both and conformers are populated in the jet when the sample is carried in Ne. Using the Stark deflector, and conformers are unambiguously identified, showing the distinct Stark deflection profiles according to their sufficiently different dipole moments of 1.013 or 1.670 D, respectively. For the conformer, the methyl moiety on the -position adopting the eclipsed geometry in S transforms into the staggered geometry in S to activate a series of the CH torsional mode. A Hamiltonian with the one-dimensional sinusoidal torsional potential is solved using the free-rotor basis set to explain the experiment, giving the 3-fold torsional barrier of 34 and 304 cm for S and S, respectively. For the conformer, on the other hand, the CH torsion is little activated in the S-S transition as both S and S adopt the staggered geometry at the minimum energy points. The doublet of each band of the conformer is ascribed to tunneling split due to the very low CH torsional barrier of 27 cm in S. It is found that the conformer undergoes a planar to pseudoplanar structural change upon the S-S transition. Theoretical calculation based on the double-well model potential curve could explain the experiment quite well, suggesting that the SCH moiety of the conformer in S becomes out-of-plane with respect to the plane of the phenyl moiety. This implies that excited-state predissociation dynamics of and conformers of the title molecule might be different.