Partial Proton Transfer in the Gas Phase: A Spectroscopic and Computational Analysis of the Trifluoroacetic Acid - Trimethylamine Complex

The 1:1 complex formed from trifluoroacetic acid (TFA) and trimethylamine (TMA) has been observed in the gas phase by rotational spectroscopy and further investigated by DFT and MP2 methods. Spectra of both the parent form and the -OD isotopologue have been obtained. The complex is structurally similar to a hydrogen bonded system, with the O-H bond directed toward the nitrogen of the TMA. However, both the spectroscopic and computational results indicate that it is intermediate between a hydrogen bonded complex and a proton-transferred ion pair. Two metrics are used to assess the degree of proton transfer from the acid to the base. The first is based on experimental ¹⁴N nuclear quadrupole coupling constants. Specifically, the component of the ¹⁴N nuclear quadrupole coupling tensor along the c-inertial axis of the complex, χ_cc, is 31% of the way between that of free TMA (no proton transfer) and that of TMAH⁺ (complete proton transfer). A second metric, adapted from that of Kurnig and Scheiner [Int. J. Quantum Chem. Quantum Biol. Symp. 1987, 14, 47-56], is based on calculated O-H and H-N distances and corroborates this description. These results indicate that the degree of proton transfer in TFA-TMA is very similar to that in the TMA complex of HNO₃, which has been previously studied and for which the proton affinity of the conjugate anion (NO₃^-) is almost identical to that of CF₃COO^-. While the solid salt, TMAH⁺·CF₃COO^-, is an ionic plastic above 307 K and exhibits free rotation of the ions, no such motion is observed in the cold 1:1 gas phase adduct.