Conformer Specific Excited-State Structure of 3-Methylthioanisole

Trans and cis conformers of 3-methylthioanisole have been spectroscopically investigated to reveal the conformer specific structural changes upon the S₁(ππ*)-S₀ excitation. The conformational cooling during the supersonic expansion is found to be quite efficient in the Ar carrier gas giving the trans conformational isomer exclusively in the molecular beam, whereas both trans and cis conformers are populated in the jet when the sample is carried in Ne. Using the Stark deflector, trans and cis 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 trans conformer, the methyl moiety on the meta-position adopting the eclipsed geometry in S₀ transforms into the staggered geometry in S₁ to activate a series of the CH₃ 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^-1 for S₀ and S₁, respectively. For the cis conformer, on the other hand, the CH₃ torsion is little activated in the S₁-S₀ transition as both S₀ and S₁ adopt the staggered geometry at the minimum energy points. The doublet of each band of the cis conformer is ascribed to tunneling split due to the very low CH₃ torsional barrier of 27 cm^-1 in S₀. It is found that the cis conformer undergoes a planar to pseudoplanar structural change upon the S₁-S₀ transition. Theoretical calculation based on the double-well model potential curve could explain the experiment quite well, suggesting that the SCH₃ moiety of the cis conformer in S₁ becomes out-of-plane with respect to the plane of the phenyl moiety. This implies that excited-state predissociation dynamics of trans and cis conformers of the title molecule might be different.