Temperature Dependence of Proton NMR Chemical Shift As a Criterion To Identify Low-Barrier Hydrogen Bonds

The NMR chemical shifts of the proton participating in the intramolecular hydrogen bond in a realistic model of hexabenzyloxymethyl-XDK [m-xylidenediamine-bis(Kemp's triacid)-imide] monoanion and hydrogen oxalate anion have been theoretically analyzed. Ab initio and density functional theory (DFT) calculations are fitted to a monodimensional potential energy surface where the nuclear Schrodinger equation can be solved to obtain the vibrational levels and their corresponding wave functions. Our results indicate that for hexabenzyloxymethyl-XDK monoanion, the first vibrational level appears above the transition state, and the ground vibrational state wave function has a maximum value just at the transition state region so that, as observed experimentally, the hexabenzyloxymethyl-XDK monoanion has a low-barrier hydrogen bond. Conversely, for the hydrogen oxalate anion, the ground vibrational level is well below the energy barrier separating the two minima so that the proton is most probably found at or near ...