Temperature dependence of proton NMR chemical shift as a criterion to identify low-barrier hydrogen bonds

The presence of a negative slope of the chemical shift as a function of the temperature can be used to identify hydrogen bonds as low-barrier hydrogen bonds in gas phase and even in inert solvents. This was gleaned from an analysis of the nuclear magnetic resonance chemical shifts of the proton involved in the intramolecular hydrogen bond in hexabenzyloxymethyl-XDK monoanion and hydrogen oxalate anion. It was found that the chemical shift decreases as the temperature rises for hexabenzyloxymethyl-XDK monoanion, while the opposite is true for the hydrogen oxalate anion.