Correlating NMR H/D Exchange Kinetics with IR Red Shifts: Towards a Comprehensive Scale for Intermolecular Hydrogen Bond Strength

Abstract

In Infrared (IR) spectroscopy, the red shift in the stretching frequencies of hydrogen bond donors serves as a reliable marker for hydrogen bond strength. However, a similar yardstick based on NMR spectroscopy to assess the strength of the hydrogen bonds has remained elusive. While downfield NMR chemical shifts qualitatively indicate hydrogen bond formation, they fail to provide a consistent measure of the strength due to their sensitivity to concentration and local environmental effects. Herein, we have established a direct and systematic correlation between the rate constants for NMR H/D exchange (k_ex^(H/D)) and IR red shifts (Δν) in intermolecular hydrogen bonded complexes, and hence, devised a comprehensive scale for intermolecular hydrogen bond strength. Stronger hydrogen bonds exhibit slower exchange rates with D2O, whereas weaker interactions exchange more rapidly. This scale has been developed by measuring k_ex^(H/D) and Δν values of a diverse set of 29 intermolecular complexes in CDCl3/CHCl3, encompassing six distinct hydrogen bond types, namely, N–H···O=C, N-H···O, N-H···N, O–H···O=C, O-H···O, and O-H···N. The correlation between k_ex^(H/D) and Δν follows an exponential relationship with an excellent goodness of fit (R2 = 0.993). The newly established scale offers an independent reference for probing the strength of intermolecular hydrogen bonds in diverse molecular systems.