MICROWAVE SPECTRUM OF A SUPERACID AND ITS MONOHYDRATE: TUNNELING AND LARGE AMPLITUDE MOTION IN CF3SO2OH AND CF3SO2OH-H2O

The triflic acid monomer (CF$_{3}$SO$_{2}$OH) and its 1:1 complex with water (CF$_{3}$SO$_{2}$OH-H$_{2}$O) have been observed by chirped-pulse and cavity Fourier transform microwave spectroscopy. A pair of tunneling states was identified in the rotational spectra for both species. For CF$_{3}$SO$_{2}$OH, the assignment of \textit{b}-type spectra led to the direct measurement and experimental determination of the tunneling energy ($\Delta$E = 52.96704(90) MHz). M06-2X/6-311++G(3df,3pd) calculations predict a 2.8 kcal/mol barrier for the tunneling motion of the hydroxyl proton rotating around the S-O bond through a transition state in which the O-H is oriented \textit{anti} with respect to the CF$_{3}$ group. A complete $360^{\circ}$ scan of the hydroxyl proton around the S-O bond shows an additional transition state in the \textit{syn} orientation resulting in a 6.2 kcal/mol barrier. In the CF$_{3}$SO$_{2}$OH-H$_{2}$O complex, spectral doublets were observed with an approximate 3:1 intensity ratio indicative of an equivalent exchange of the protons in H$_{2}$O resulting from rotation about its C$_{2}$ axis. The proposed water motion is further supported by the quenching of the spectral pairs for the asymmetrically deuterated CF$_{3}$SO$_{2}$OH-DOH complex. Despite the strong acidity of triflic acid, no evidence of protonation of the water was observed