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

Abstract

The pure rotational spectrum of methyl p-tolyl sulfoxide (MTSO) has been studied using chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy in the frequency range of 18-26 GHz. The only conformer was unambiguously observed in the supersonic jet expansion, which is consistent with the conformational analysis using quantum chemical calculations. The rotational transitions were split into two components ( A and E symmetry) 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. The effective ground state ( r0) geometry was derived upon the rotational constants from the parent species as well as the 34S and eight 13C isotopologues. Compared to two other sulfoxides, methyl phenyl sulfoxide (MPS) and methyl 4-nitrophenyl sulfoxide (MNPSO), 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 the Nudged Elastic Band (NEB-TS) and time-dependent DFT (TD-B3LYP) methods. It was found that the pyramidal inversion in S1 is easier than in S0 and optical excitation to S1 will facilitate an effectively barrier-free inversion.

Ground‐State Photoelectron Circular Dichroism of Methyl p‐Tolyl Sulfoxide by Single‐Photon Ionisation from a Table‐Top Source

“…Through comparison of the RHF/cc‐pVDZ geometry with the DFT geometry shown in Figure 1 , and with high‐resolution microwave spectroscopic measurements, we can exclude the presence of other conformers resulting from methyl rotations or rotations around the CCS=O dihedral angle. [48] Furthermore, a frequency analysis of our optimised structures reveals no imaginary frequencies, additionally confirming that they represent minima of the potential‐energy surface.…”

“…Through comparison of the RHF/cc-pVDZ geometry with the DFT geometry shown in Figure 1, and with high-resolution microwave spectroscopic measurements, we can exclude the presence of other conformers resulting from methyl rotations or rotations around the CCS=O dihedral angle. [48] Furthermore, a frequency analysis of our optimised structures reveals no imaginary frequencies, additionally con-firming that they represent minima of the potential-energy surface.…”