On the Stability of the Tetramers of Carbon Monoxide, Hydrogen Isocyanide, and Vinylidene. A Molecular Orbital Theoretical Rationalization

Abstract

Semiempirical extended-Hückel and ab initio molecular orbital calculations at the correlated second-order M0ller-Plesset (MP2) level of theory predict that the ground state electronic configuration of tetraoxocyclobutane (3) is different from those of its congeners [4]radialene (5) and tetraiminocy-clobutane (4). In the singlet ground state of 3, only three out of the four σ MOs describing the C-C σ bonds of the cyclobutane ring are occupied, while in the case of 4 and 5, all four C-C σ MOs are occupied. By going from 5 to 3, an electron density shift from the cyclobutane carbon atoms to the terminal C, N, and O atoms is observed. The calculated total natural orbital population for the cyclobutane carbon atoms diminishes from 4.01 (5) to 3.78 (4) and 3.64 (3). The MP2/6-31G+ZPVE//MP2/6-31G (ZPVE: zero-point vibrational energy) ab initio calculations predict that four CO molecules are more stable than the tetramer 3 by 32.8 kcal/mol. For 4 and 5, however, the tetramers are more stable than four [formula omited] CNH or four 1A1 CCH2 by 61.6 and 322.3 kcal/mol, respectively. Furthermore, on the C8H8 energy surface, the 1A1g ground state of 5 lies 122.4 kcal/ mol below four [formula omited] C2H2 molecules. © 1995, American Chemical Society. All rights reserved.