Hydrogen Bonding

When a proton is directly bonded to a strongly electronegative atom such as oxygen or nitrogen its chemical shift is critically dependent on the nature of the solvent, temperature, concentration, and whether acidic or basic impurities are present. The usual variations in chemical shift for such protons are so large (up to 5 ppm for alcohols) that no very useful correlations exist.

Hydrogen bonding is the major reason for the variable chemical shifts of OH and NH protons. In general, hydrogen bonding results in deshielding, which causes the resonances to move downfield. The extent of hydrogen bonding varies with concentration, temperature, and solvent, and changes in the degree of hydrogen bonding can cause substantial shift changes. This is very evident in the nmr spectrum of ethanol taken at different concentrations in CCl4 (Figure 9-29). The hydroxyl resonance will be seen to move upfield by hydrogen bonding through equilibria such as

Section 9-10 I. There is no significant change in the relative shifts of the CH2 and CH3 lines as the concentration is changed.