6-PI AROMATICITY IN 4-MEMBERED RINGS

6ir aromaticity has been investigated for a number of four-membered rings by ab initio MO methods. The rings were fully optimized within the symmetry constraints. The order of the stationary points was established by calculating the harmonic frequencies of the normal modes. It is shown that carbocycles show no 6x aromaticity, in contrast with the nitrocycles. The stability of the planar D^, rings is determined by two factors: the bond strength in the ring and the stability of the planar orientation of the ring substituent. The latter is strongly influenced by 1,3 π repulsions, which result from the fact that in 6t four-membered rings the two highest occupied τ molecular Orbitals are 1,2-nonbonding and 1,3-antibonding. The role of the hydrogen substituents is investigated by the inclusion of polarization functions on the ring atoms. It is found that the Pj-d hybridization reduces the electron repulsion by “bending” the π frame outward, thereby increasing the electron density above the hydrogen bond. This stimulates the out-of-plane vibrations of the hydrogen atoms as shown by smaller force constants. © 1990, American Chemical Society. All rights reserved.