Microwave and Computational Study of Methanesulfonic Acid and Its Complex with Water.

Spectra of methanesulfonic acid (CH 3 SO 3 H, MSA) and its complex with water have been studied by microwave spectroscopy and density functional theory calculations. For the monomer, spectra were obtained for both the parent and -OD isotopologues and, in each case, revealed a pair of tunneling states that are attributed to large amplitude motion of the hydroxyl hydrogen about the S-O(H) bond. Transitions crossing between tunneling states were not found in the parent spectrum and are estimated to be outside the range of the spectrometer, thus precluding the direct determination of the tunneling energy. For the -OD form, however, the tunneling energy was determined to be Δ E = 6471.9274(18) MHz from direct measurement of the cross-state c -type transitions. In its complex with water, the acidic hydrogen of the MSA forms a hydrogen bond with the water oxygen. A secondary hydrogen bond involving the water hydrogen and an SO 3 oxygen completes a six-membered ring, forming a cyclic structure typical of hydrated oxyacids. No evidence of internal motion was observed. Rotational spectra of the CH 3 SO 3 H···D 2 O and CH 3 SO 3 D···D 2 O isotopologues were also obtained and analyzed. Comparison with theoretical calculations confirms the cyclic structure, though the orientation of the unbound water hydrogen is ambiguous.