Structure, stability and bonding of (BC 2 N) n , n =2,3: an ab initio study

The dimer and trimer of BC2N are studied at the HF and MPn levels using 3-21G, 6-31G∗ and 6-31+G∗ basis sets. It is found that stability is enhanced by maintaining –BN– units together in the cyclic form. Both molecules have cyclic ground state structures with cumulenic-type equal bond lengths in the –BNBN– and polyacetylenic –CCCC– fragments. The linear–cyclic energy difference increases with the size of the cluster. Electron correlation substantially favors the cyclic structures over the linear forms. No general trend of atomic combination has been found in the linear isomers. Diffuse functions have negligible effects on the relative energies and geometries of the dimers.