sigma*-aromaticity in three-membered rings
The structures and energies of unsaturated three-membered rings of the general formula (CH)(2)XYnm, with charge = m and n substituents Y at X (Y = fluorine, chlorine, bromine, hydrogen, X = phosphorus, silicon, nitrogen, carbon) are compared to their saturated analogs. The structures were optimized with B3LYP/6-311+G(2d,p) and at MP2/6-31+G(d), with single point energy calculations on the latter geometries at MP4SDTQ/6-31+G(d). The geometrical changes in bond lengths and angles, which correlate with substituent electronegativities, are discussed for the different ring systems. The relative stabilities of unsaturated and saturated rings are compared using isodesmic ring-opening reactions and homodesmic substituent-exchange reactions. sigma*-Aromaticity, a hyperconjugative effect found in the disubstituted rings, causes lowering of ring strain energies for the unsaturated rings and preference of unsaturated rings over saturated ones for the more electronegative substituents. For the mono-substituted pi-aromatic silacyclopropenes and cyclopropenes, a destabilization by more electronegative ligands is found. For the neutral rings monosubstituted at main group V atoms like the 1H-phosphirenes and also the isoelectronic negatively charged rings with main group IV atoms like the silacyclopropenium anions, no correlation of stabilization energies or geometrical changes with ligand electronegativity is found.
sigma*-aromaticity hyperconjugation ab initio calculations phosphorus rings weak interactions