Amino and cyano N atoms in competitive situations: which is the best hydrogen-bond acceptor? A crystallographic database investigation

The relative hydrogen-bond acceptor abilities of amino and cyano N atoms ha ve been investigated using data retrieved from the Cambridge Structural Dat abase and via ab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors, N-T-C=N (push-pull nitrile s) and N-(Csp(3))(n)-C=N molecular fragments show that the hydrogen bonds a re more abundant on the nitrile than on the amino nitrogen. In the push-pul l family, in which T is a transmitter of resonance effects, the hydrogen-bo nding ability of the cyano nitrogen is increased by conjugative interaction s between the lone pair of the amino substituent and the C=N group: a clear example of resonance-assisted hydrogen bonding. The strength of the hydrog en-bonds on the cyano nitrogen in this family follows the experimental orde r of hydrogen-bond basicity, as observed in solution through the pK(HB) sca le. The number of hydrogen bonds established on the amino nitrogen is great er for aliphatic aminonitriles N-(Csp(3))(n)-C=N, but remains low. This beh aviour reflects the greater sensitivity of the amino nitrogen to steric hin drance and the electron-withdrawing inductive effect compared with the cyan o nitrogen. Ab initio molecular orbital calculations (B3LYP/6-31+G** level) of electrostatic potentials on the molecular surface around each nitrogen confirm the experimental observations.