Temperature Dependence of Chemical Shifts of Protons in Hydrogen Bonds

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Abstract
The observed variations with temperature of the NMR chemical shifts of protons involved in hydrogen bonds have hitherto been attributed to shifts in association equilibria favoring a larger fraction of broken hydrogen bonds with increasing temperature. We now suggest that another mechanism may account for a large part of these variations. The chemical shift for the associated species depends quite strongly on the degree of excitation of the hydrogen‐bond‐stretching vibrational mode. Because this is an unusually low‐frequency motion, several excited states are significantly populated even at temperatures as low as 200°K. When an appropriate average over vibrational levels is used to evaluate the chemical shift, the results show a temperature dependence similar to those found experimentally even though it is assumed in the calculations that no dissociation of the hydrogen‐bonded species occurs.