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=H Cl, acetic acid, and chloroacetic acid with pyridine-N-15 as base B di ssolved in 2:1 mixtures of CDClF2 with CDF3 were measured in the tempe rature range between 100 K and 150 K. Under these conditions, the regi me of slow proton and hydrogen bond exchange is reached where the intr insic concentration independent NMR parameters of AHB are observed. Th e chemical shifts delta of the hydrogen bonded protons and their scala r coupling constants (1)Ji(H-)15(N) with the N-15 nucleus of B provide information on the hydrogen bond geometry and its temperature depende nce. When the temperature is decreased the following phenomena are obs erved: (i) the average proton positions are shifted away from A toward s B; (ii) the molecular complex A-H ... B with AH=acetic acid contract s; (iii) the zwitterionic complex A(-)... H-B+ with AH=HCl lengthens, (iv) the chloroacetic acid-pyridine complex A-H ... B firstly contract s until the quasisymmetric structure A(delta-)... H ... B-delta+ is re ached and then the zwitterionic form A(-)... H-B+ in which the heavy a tom 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 mome nt is increased mainly by charge transfer from B to AH because of the large polarizability of the hydrogen bond, and is associated with a co ntraction of the Tatter. By contrast, in A(-)... H-B+ the dipole momen t increase arises from a lengthening of the hydrogen bond. As a result , the electric field created at the solute by the solvent dipoles is t he driving force for the hydrogen bond contraction assisted proton tra nsfer. The acidity of the proton donor decreases with increasing stren gth of the electric field required for the proton transfer to occur.