TEMPERATURE-DEPENDENCE OF PROTON NMR CHEMICAL-SHIFT AS A CRITERION TOIDENTIFY LOW-BARRIER HYDROGEN-BONDS

The NMR chemical shifts of the proton participating in the intramolecu lar hydrogen bond in a realistic model of hexabenzyloxymethyl-XDK [m-x ylidenediamine-bis(Kemp's triacid)-imide] monoanion and hydrogen oxala te anion have been theoretically analyzed. Ab initio and density funct ional theory (DFT) calculations are fitted to a monodimensional potent ial energy surface where the nuclear Schrodinger equation can be solve d to obtain the vibrational levels and their corresponding wave functi ons. Our results indicate that for hexabenzyloxymethyl-XDK monoanion, the first vibrational level appears above the transition state, and th e ground vibrational state wave function has a maximum value just at t he transition state region so that, as observed experimentally, the he xabenzyloxymethyl-XDK monoanion has a low-barrier hydrogen bond. Conve rsely, for the hydrogen oxalate anion, the ground vibrational level is well below the energy barrier separating the two minima so that the p roton is most probably found at or near the minima and the hydrogen bo nd is of the ''normal'' type. We have also analyzed the effect of temp erature on the chemical shift by performing Boltzmann averages along t he vibrational states in each case. We have found that for hexabenzylo xymethyl-XDK monoanion the chemical shift decreases as the temperature s increases whereas the reverse trend is observed for the hydrogen oxa late anion. Therefore the presence of a negative slope of the chemical shift as a function of the temperature could be used to characterize a hydrogen bond in a symmetric potential as a low-barrier hydrogen bon d in gas phase and possibly in inert solvents.