ON THE STABILITY OF THE TETRAMERS OF CARBON-MONOXIDE, HYDROGEN ISOCYANIDE, AND VINYLIDENE - A MOLECULAR-ORBITAL THEORETICAL RATIONALIZATION

Semiempirical extended-Huckel and ab initio molecular orbital calculat ions at the correlated second-order Moller-Plesset (MP2) level of theo ry predict that the ground state electronic configuration of tetraoxoc yclobutane (3) is different from those of its congeners [4]radialene ( 5) and tetraiminocyclobutane (4). In the singlet ground state of 3, on ly three out of the four sigma MOs describing the C-C sigma bonds of t he cyclobutane ring are occupied, while in the case of 4 and 5, all fo ur C-C sigma MOs are occupied. By going from 5 to 3, an electron densi ty shift from the cyclobutane carbon atoms to the terminal C, N, and O atoms is observed. The calculated total natural orbital population fo r the cyclobutane carbon atoms diminishes from 4.01 (5) to 3.78 (4) an d 3.64 (3). The MP2/6-31G+ZPVE//MP2/6-31G* (ZPVE: zero-point vibratio nal 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 (1) Sigma(g)(+) CNH or four (1)A(1) CCH2 by 61.6 and 322.3 kcal/mol, respectively. Furthermore, on the C8H8 energy surface, the (1)A(1g), ground state of 5 lies 122.4 k cal/mol below four (1) Sigma(g)(+) C2H2 molecules.