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, χcc, 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.