Influence of Proton Transfer Degree on the Potential Energy Surface for Two Very Short Hydrogen Bonds

The influence of temperature on the proton location in hydrogen bonds has been systematically studied by neutron diffraction in only a few crystal structures. Two of these are the 1:1 complex of urea – phosphoric acid with an OHO hydrogen bond and 4-methylpyridine –pentachlorophenol with an OHN hydrogen bond. Based on these earlier determined crystal structures the potential energy surface (PES) at different temperatures has now been determined by DFT calculations at the B3LYP/6-31++G** level of theory using the Gaussian03 system. In general PES is practically unchanged as the proton moves from the donor to the acceptor. This is not surprising as the crystal structure does not undergo significant changes as the proton successively moves along the hydrogen bond. For both complexes PES is characterized by only one minimum, which is not located at the centre where the distances of the proton to the bridge atoms are the same. The experimental proton positions are located close to the calculated energy minima; the slight deviations are probably an effect of the crystalline environment which has not been taken into account in the calculations.