Evidence by NMR of temperature-dependent solvent electric field effects on proton transfer and hydrogen bond geometries

The H-1 NMR spectra of hydrogen bonded complexes AHB of the acids AH=HCl, acetic acid, and chloroacetic acid with pyridine-N-15 as base B dissolved in 2:1 mixtures of CDClF2 with CDF3 were measured in the temperature range between 100 K and 150 K. Under these conditions, the regime of slow proton and hydrogen bond exchange is reached where the intrinsic concentration independent NMR parameters of AHB are observed. The chemical shifts delta of the hydrogen bonded protons and their scalar coupling constants (1)Ji(H-)15(N) with the N-15 nucleus of B provide information on the hydrogen bond geometry and its temperature dependence. When the temperature is decreased the following phenomena are observed: (i) the average proton positions are shifted away from A towards B; (ii) the molecular complex A-H ... B with AH=acetic acid contracts; (iii) the zwitterionic complex A(-)... H-B+ with AH=HCl lengthens, (iv) the chloroacetic acid-pyridine complex A-H ... B firstly contracts until the quasisymmetric structure A(delta-)... H ... B-delta+ is reached and then the zwitterionic form A(-)... H-B+ in which the heavy atom distance A...B increases again. These findings are interpreted in terms of electric dipole moments induced in AHB by the electric field arising from the solvent dipoles. Due to solvent ordering around AHB, this field increases at low temperature, In A-H ... B, the dipole moment is increased mainly by charge transfer from B to AH because of the large polarizability of the hydrogen bond, and is associated with a contraction of the Tatter. By contrast, in A(-)... H-B+ the dipole moment increase arises from a lengthening of the hydrogen bond. As a result, the electric field created at the solute by the solvent dipoles is the driving force for the hydrogen bond contraction assisted proton transfer. The acidity of the proton donor decreases with increasing strength of the electric field required for the proton transfer to occur.