Superacids as a Means of Bringing Gas Phase Proton Transfer into the Small-Cluster Regime: Microwave and Computational Characterization of the 1:1 Ion Pair Formed from Trimethylamine and Fluorosulfonic Acid

The 1:1 complex formed from fluorosulfonic acid (FSO 3 H) and trimethylamine (TMA) has been observed by microwave spectroscopy under supersonic jet conditions. The experimental rotational constants are consistent with quantum chemical calculations that predict the system to be best described as a (CH 3 ) 3 NH + –FSO 3 – ion pair, i.e., one in which the acidic proton has been transferred to the trimethylamine moiety. The observed 14 N nuclear quadrupole coupling constants support this picture and simple arguments based on gas-phase ion energetics are employed to rationalize this result. While the formation of ion pairs in 1:1 gas phase complexes containing Brønsted–Lowry acids and bases is not unprecedented, it is unusual. The ability of fluorosulfonic acid to transfer its proton to TMA without the stabilization afforded by near-neighbor interactions is likely due to its superacidity. Examination of the component of the 14 N quadrupole coupling tensor along the N–H bond axis for a series of acids suggests that both the deprotonation energy of the acid and the ability of its conjugate base to delocalize negative charge away from the protonated base influence the ability of a complex to form an isolated contact ion pair. For superacids such as FSO 3 H and CF 3 SO 3 H, which can delocalize charge in the deprotonated form, the low deprotonation energy appears to be the dominant factor. Therefore, superacids may be a useful means of reducing the microsolvation requirements for proton transfer, thus bringing it into the small cluster regime where rotationally resolved spectroscopies are most readily employed.