Acta Crystallographica Section B

Structural Science

Volume 57, Part 6 (December 2001)

research papers

Acta Cryst. (2001). B57, 850-858 [ doi:10.1107/S0108768101015403 ]

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

N. Ziao, J. Graton, C. Laurence and J.-Y. Le Questel

Abstract: The relative hydrogen-bond acceptor abilities of amino and cyano N atoms have been investigated using data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors, N-T-C [equivalent to] N (push-pull nitriles) and N-(Csp³)_n-CN molecular fragments show that the hydrogen bonds are more abundant on the nitrile than on the amino nitrogen. In the push-pull family, in which T is a transmitter of resonance effects, the hydrogen-bonding ability of the cyano nitrogen is increased by conjugative interactions between the lone pair of the amino substituent and the C [equivalent to] N group: a clear example of resonance-assisted hydrogen bonding. The strength of the hydrogen-bonds on the cyano nitrogen in this family follows the experimental order of hydrogen-bond basicity, as observed in solution through the pK_HB scale. The number of hydrogen bonds established on the amino nitrogen is greater for aliphatic aminonitriles N-(Csp³)_n-C [equivalent to] N, but remains low. This behaviour reflects the greater sensitivity of the amino nitrogen to steric hindrance and the electron-withdrawing inductive effect compared with the cyano 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.

Keywords: competitive situations; hydrogen-bond acceptor; database investigation; amino and cyano nitrogens; molecular orbital calculations.