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

Abstract

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 Schrödinger 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 the minima and the hydrogen bond is of the “normal” type. We have also analyzed the effect of temperature on the chemical shift by performing Boltzmann averages along the vibrational states in each case. We have found that for hexabenzyloxymethyl-XDK monoanion the chemical shift decreases as the temperatures increases whereas the reverse trend is observed for the hydrogen oxalate 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 bond in gas phase and possibly in inert solvents.