The dimer of cyanodiacetylene:: Stacking vs. hydrogen bonding

The intermolecular energy surface of the cyanodiacetylene dimer was investigated at the MP2 level applying medium to large basis sets. Extensive 2D scans of selected sections of the energy surface were performed. The most stable structure turns out to be an antiparallel stacked dimer. The fully linear structure with a conventional C-H ... N=C hydrogen bond is less stable than the antiparallel stacked arrangement by at least 8 kJ. mol(-1). The intramolecular geometry relaxations relative to the monomer and the vibrational frequency shifts induced by intermolecular interaction are reported. Moreover, the structural and energetic trends in the series (HCN)(2), (HC3N)(2), and (HC5N)(2) are discussed.