Parent, ³⁴S, and deuterated triflic acid: Microwave spectra and tunneling splittings due to hydroxyl torsion

Spectra for the parent, ³⁴S, and deuterated forms of triflic acid (CF₃SO₃H) have been observed by chirped-pulse and cavity Fourier transform microwave spectroscopy. The observation of a-, b-, and c-type transitions definitively establishes that the molecule does not have C_s symmetry, and M06-2X/6-311++G(3df,3pd) calculations concur, yielding a structure in which the OH bond is nearly perpendicular to the C-S-O(H) plane. The rotational spectrum for each isotopologue exhibits a pair of tunneling states resulting from large amplitude motion of the hydroxyl hydrogen between two equivalent structures with opposite directions of the OH bond. The experimentally determined tunneling energies, ΔE, for the parent and ³⁴S species are 52.96784(65) MHz and 52.8761(16) MHz, respectively. For the -OD isotopologue, the tunneling energy decreases significantly, with a value of only ΔE = 0.2460(20) MHz. Curiously, we observe that b-type transitions cross between tunneling states in the parent and ³⁴S spectra, while c-type transitions cross in the spectra of the deuterated species. This likely arises because the molecule is close to a symmetric top, with only the location of the hydrogen defining the orientation of the b- and c-inertial axes, enabling slight structural changes to switch the axis orientations at the transition state. Calculations at the M06-2X/6-311++G(3df,3pd) level of theory predict a 2.8 kcal/mol barrier for the large amplitude motion of the hydroxyl hydrogen rotating around the S-O bond through a C_s symmetric transition state in which the O-H is oriented anti with respect to the S-C bond. A complete scan of the hydroxyl proton around the S-O bond shows an additional transition state of C_s symmetry in the syn orientation with a 6.2 kcal/mol barrier relative to the equilibrium configuration.