Partial proton transfer in a molecular complex: Assessments from both the donor and acceptor points of view

Microwave spectra have been observed for the gas phase complexes (CH ₃) ₃ ¹⁴N-H ¹⁴NO ₃ and (CH ₃) ₃ ¹⁵N-H ¹⁴NO ₃ and rotational and nuclear quadrupole coupling constants are reported. The structure and binding energy have also been calculated at the MP2 level of theory using the 6-311++G(d,p) and 6-311++G(2df,2pd) basis sets both with and without corrections for basis set superposition error. The HNO ₃ forms a near-linear hydrogen bond to the amine nitrogen with a rather short hydrogen bond distance of about 1.5-1.6 Å (depending on the basis set and method of computation). The C ₃ axis of the trimethylamine lies in the plane of the nitric acid. For both the H ¹⁴NO ₃ and the (CH ₃) ₃ ¹⁴N moieties of the parent species, the component of the nuclear quadrupole coupling tensor perpendicular to the molecular symmetry plane, π _cc, is sensitive to the electronic structure at the corresponding nitrogen but independent of relative orientation within the plane. Its value, therefore, provides a convenient experimental measure of the degree of proton transfer within the complex. For the HNO ₃, π _cc lies 62% of the way between those of free HNO ₃ and aqueous NO ₃ ^-, indicating a substantial degree of proton transfer. A similar comparison of the quadrupole coupling constant of (CH ₃) ₃N in the (CH ₃) ₃N-HNO ₃ complex with those of free (CH ₃) ₃N and (CH ₃) ₃NH ⁺ indicates only about 31% proton transfer, about half that determined from the HNO ₃ coupling constant. Though surprising at first, this disparity is to be expected if the quadrupole coupling constants vary nonlinearly with the position of the proton relative to the donor and acceptor atoms. Calculations of the ¹⁴N nuclear quadrupole coupling constants as a function of proton position using density functional theory are reported and confirm that this is the case. We suggest that when proton transfer is assessed according to changes in individual monomer molecular properties, the overall process may be best described in terms of a dual picture involving proton release by the acid and proton acquisition by the base.