Dipolar C[triple-bond]N...C[triple-bond]N interactions in organic crystal structures: database analysis and calculation of interaction energies

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Abstract
The Cambridge Structural Database (CSD) has been used to study nonbonded interactions between dipolar cyano groups. The analysis shows that C[triple-bond]N...C[triple-bond]N interactions form in an analogous manner to those involving carbonyl groups, and with the same interaction motifs: a dominant antiparallel dimer (57.5%) together with smaller populations of perpendicular (19.4%) and sheared parallel (23.0%) motifs. Ab initio calculations using intermolecular perturbation theory (IMPT) show an attractive C[triple-bond]N...C[triple-bond]N interaction in the dominant antiparallel dimer, with E t = −20.0 kJ mol−1 at d(C...N) = 3.30 Å and with the motif having a shear angle close to 102°. The antiparallel C[triple-bond]N...C[triple-bond]N interaction is therefore slightly weaker than the analogous C=O...C=O dimer (−23.5 kJ mol−1), but both interactions have attractive energies similar to that of a medium-strength hydrogen bond and, where sterically favoured, they are important in the stabilization of extended crystal structures.