Internal dynamics of methyl p-tolyl sulfoxide in the gas phase: Rotational spectroscopy and theoretical studies

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 [V₃ = 11.0178(23) cm^-1]. 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 (r₀) geometry was derived using the rotational constants from the parent species and the ³⁴S and eight ¹³C 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 (S₀) and excited (S₁) states with nudged elastic band and time-dependent density functional theory methods. It was found that the pyramidal inversion in S₁ is easier than in S₀, showing that optical excitation to S₁ will facilitate an effectively barrier-free inversion.