C vs N: which end of the cyanide anion is a better hydrogen bond acceptor?

ABSTRACT

The ability of the C and N ends of the cyanide anion (CN(-)) as acceptors of hydrogen bonds, an experimentally difficult problem, has been computationally examined in this study. Structures obtained in our previous work involving cyanide binding within the cavity of a triazolophane macrocycle (Chem.-Eur. J. 2011, 17, 9123-9129) were used to analyze the problem. Three different approaches involving (a) breakdown of the triazolophane into smaller components, (b) population analyses, and (c) ion-dipole analyses helped demonstrate that the N terminus of cyanide is a slightly better hydrogen bond acceptor than the C terminus even though it is not the site of protonation or covalent bond formation. This outcome reflects a competition between the preference for noncovalent interactions at the nitrogen and covalent bond formation at the carbon.