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 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 identical with N (push-pull nitriles) and N-(Csp(3))(n)-C identical with N 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 identical with 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-(Csp(3))(n)-C identical with 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.
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Laboratoire de Spectrochimie, Faculté des Sciences et des Techniques de Nantes, 2 rue de la Houssinière, BP 92208 44322 Nantes CEDEX 3, France.
Graton
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Laurence
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Le Questel
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