Sun Wenhao, Kleiner Isabelle, Senftleben Arne, Schnell Melanie
Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, 22607 Hamburg, Germany.
Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), CNRS UMR 7583, Université de Paris, Université Paris-Est Créteil, Institut Pierre Simon Laplace, 61 Avenue du Général de Gaulle, 94010 Créteil, France.
J Chem Phys. 2022 Apr 21;156(15):154304. doi: 10.1063/5.0083534.
A pure rotational spectrum of methyl p-tolyl sulfoxide (MTSO) was studied using chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 18-26 GHz. A single conformer was unambiguously observed in the supersonic jet expansion, which is consistent with the conformational analysis performed using quantum-chemical calculations. Rotational transitions were split into two components of A and E symmetries due to the low-barrier internal rotation of the ring methyl group [V = 11.0178(23) cm]. The low energy barrier for the methyl top internal rotation implies an electron-withdrawing effect of the group at the opposite side of the phenyl ring, in comparison with other para-substituted toluenes. The effective ground state (r) geometry was derived using the rotational constants from the parent species and the S and eight C singly substituted isotopologues. Compared to two other sulfoxides, methyl phenyl sulfoxide and methyl 4-nitrophenyl sulfoxide, the sulfoxide group in MTSO is slightly more twisted with respect to the plane of the phenyl ring, which could be attributed to the moderate electron-donating effect of the p-methyl group. Furthermore, the pyramidal inversion that interconverts the handedness at the sulfur stereogenic center was explored in the electronic ground (S) and excited (S) states with nudged elastic band and time-dependent density functional theory methods. It was found that the pyramidal inversion in S is easier than in S, showing that optical excitation to S will facilitate an effectively barrier-free inversion.
使用啁啾脉冲傅里叶变换微波光谱技术，在18 - 26 GHz频率范围内研究了对甲苯基亚砜（MTSO）的纯转动光谱。在超声速射流膨胀中明确观察到单一构象异构体，这与使用量子化学计算进行的构象分析一致。由于环甲基的低势垒内旋转[V = 11.0178(23) cm]，转动跃迁分裂为A和E对称性的两个分量。与其他对取代甲苯相比，甲基顶部内旋转的低能垒意味着苯环另一侧基团的吸电子效应。利用母体物种以及S和八个C单取代同位素异构体的转动常数推导出有效基态(r)几何结构。与另外两种亚砜，即苯基亚砜和4 - 硝基苯基亚砜相比，MTSO中的亚砜基团相对于苯环平面稍微更扭曲一些，这可归因于对甲基的适度供电子效应。此外，使用推挤弹性带和含时密度泛函理论方法，在电子基态(S)和激发态(S)中研究了在硫立体中心处使手性相互转换的锥形反转。发现S中的锥形反转比S中更容易，表明光激发到S将促进有效无势垒反转。
