Strongly bonded bimolecular complexes between HCN and HNC

The equilibrium structures, vibrational properties, and interaction energie s for four bimolecular complexes of HCN and HNC (HCN...HCN, HNC...HNC, HCN. ..HNC, and HNC...HCN) were studied with ab initio methods. Different electr on correlation levels and basis sets up to CCSD(T)/6-311++G(2d,2p) were use d in the geometry optimizations and interaction energy calculations, and th e vibrational frequencies were calculated at the MP2 and MP3 levels of theo ry. To study the nature of the intermolecular interactions, an energy decom position analysis was carried out. For the HNC...HCN complex, which possess es a hydrogen bridge between two carbons, the counterpoise-corrected intera ction energy and its decomposition has been calculated as a function of the intermolecular distance. The effect of subunit deformation on the interact ion energy has also been considered, The results indicate that the HNC dime r has considerably stronger intermolecular interaction than the HCN dimer, and the HNC...HCN complex is bonded as strongly as the HCN dimer. The compa rison of the various properties of the complexes and the energy decompositi on analysis in particular indicate that carbon can act as both a hydrogen a cceptor and donor, and the resulting hydrogen bonds in these complexes are relatively strong.