Lee Heesung, Kim So-Yeon, Lim Jean Sun, Kim Junggil, Kim Sang Kyu
Department of Chemistry, KAIST, Daejeon 34141, Republic of Korea.
J Phys Chem A. 2020 Jun 11;124(23):4666-4671. doi: 10.1021/acs.jpca.0c03452. Epub 2020 May 27.
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.
对3-甲硫基苯甲醚的构象异构体进行了光谱研究，以揭示在S(ππ*)-S激发时构象异构体特异性的结构变化。发现在超音速膨胀过程中的构象冷却在Ar载气中非常有效，在分子束中仅产生构象异构体，而当样品在Ne中传输时，两种构象异构体都存在于射流中。使用斯塔克偏转器，明确识别出了两种构象异构体，根据它们分别为1.013或1.670 D的足够不同的偶极矩显示出明显的斯塔克偏转轮廓。对于某构象异构体，在S中处于重叠几何构型的α-位甲基在S中转变为交错几何构型，从而激活了一系列CH扭转模式。使用自由转子基组求解具有一维正弦扭转势的哈密顿量来解释实验，得到S和S的三重扭转势垒分别为34和304 cm⁻¹。另一方面，对于另一种构象异构体，由于在最低能量点S和S均采用交错几何构型，CH扭转在S-S跃迁中几乎未被激活。该构象异构体各谱带的双峰归因于S中非常低的27 cm⁻¹的CH扭转势垒导致的隧穿分裂。发现该构象异构体在S-S跃迁时发生从平面到假平面的结构变化。基于双阱模型势曲线的理论计算能够很好地解释实验结果，表明该构象异构体在S中的SCH部分相对于苯基部分的平面变为面外。这意味着标题分子的两种构象异构体的激发态预解离动力学可能不同。
