AN ABINITIO STUDY OF POTENTIALLY AROMATIC AND ANTIAROMATIC 3-MEMBERED RINGS

Ab initio calculations were performed on 18 three-membered ring systems including 11 closed-shell and seven open-shell systems. The geometries of all systems were completely optimized on the RHF/6-31G** level. Both known and unknown closed-shell molecules were investigated including the cyclopropenyl cation, C3H3+, 1, the azirinyl cation, C2H2N+, 2, the diazirinyl cation, CHN2+, 3, the triazirinyl cation, N3+, 4, and its acyclic isomers, borirene, C2H3B, 5, borazirene, CH2NB, 6, boradiazirine, N2BH, 7, azadiboridine, NB2H3, 8, the borazarinyl cation, CHNB+, 9, azirene, C2H3N, 10, and the diazaridinyl cation, CH3N2+, 11. Open-shell systems studied included the cyclopropenyl radical, C3H3., 12, the azirinyl radical cation, C2H3N.+, 13, the diazirinyl radical, CHN2., 14, the triazirinyl radical cation, N3H.+, 15, the borazirinyl radical, CHNB., 16, the azadiboridinyl radical cation, NB2H3.+, 17, and the diaziridinyl radical CH3N2., 18. This series includes both potentially aromatic and potentially antiaromatic structures. The optimized structures are discussed in terms of their valence bond resonance hybrid representations. Previous theoretical and experimental results are compared with these calculations. The effect of boron and nitrogen in aromatic and nonaromatic systems is considered. Several unforseen results were found. Radicals 14 and 16 and radical cations 13, 15, and 17 were of interest since they might be systems with three-pi-electrons. However, the azirinyl, 13, and triazirinyl, 15, radical cations were each nonplanar with two-pi-electrons. The borazirinyl radical, 16, and azadiboridinyl radical cation, 17, were planar. Borazirinyl radical, 16, is a two-pi-electron system with the unpaired electron in the sigma-system largely located at the boron. 17 has a two-pi-electron aromatic system with a weak BB-sigma-bond (approximately a one-electron bond) with some BB-pi-overlap. Its unpaired electron is also in a sigma-orbital. Only the diazirinyl radical, 14, was found to be a three-pi-electron planar system in spite of any antiaromaticity associated with this structure.