Proton magnetic resonance studies of hydrogen bonding of formamide with bromide ion in solution. The non-equivalence of the amide hydrogens

Abstract

The proton magnetic resonance spectrum of a dilute solution of formamide —15N in CH3CN at 15 °C undergoes definite changes as bromide ion in the form of (C4H9)4NBr is added. The two amide resonances shift downfield, but at different rates. It is found that each amide hydrogen forms hydrogen bonds with bromide ions, apparently by way of independent equilibrium processes. The hydrogen bond at the trans position to the carbonyl oxygen is favored by about 0.7 kcal mole−1 over that at the cis position. It is postulated that electrostatic repulsion between any base and the carbonyl oxygen of an amide group, tends to favor formation of hydrogen bonds at the more remote, trans, hydrogen. This may account for the formation of hydrogen-bonded chains by N-monosubstituted amides preferentially with the amide hydrogen trans to oxygen.