Cyclopropabenzenes and Alkylidenecyclopropabenzenes a Synergistic Relation between Theory and Experiment

Abstract

Ab initio calculations, mostly with the 3-21G basis-set were carried out for cyclopropabenzene (1), the linear- and angularcyclobutacyclopropabenzenes (2 and 3 respectively), the linear- and angular dicyclopropabenzenes 4 and 5 respectively, tricyclopropabenzene (6), alkylidenecyclopropabenzene (7) and two calecene homologues, i.e., 8 - containing an exocyclic cyclopropene moiety and 9 - containing an exocyclic cyclopentadiene moiety. The geometries and the total and relative energies of compounds l-9 are reported. The calculated geometries of 1 and 2 are in excellent agreement with recent low-temperature X-ray structures. In all molecules, except 6, the fused bonds are shorter than the other aromatic bonds, in conflict with the Mills-Nixon “bond fixation” concept. The following strain energies were calculated for compounds 1-6 (in kcal/mol): (70, also measured experimentally), 2 (102); 3 (103); 4 (133); 5 (140); 6 (217). The surprising theoretical prediction that the strain of the cyclopropane ring in 2 is slightly lower than that in has been confirmed by recent experiments. Compounds 7-9 exhibit considerable polarization of the u-electrons, so that the cyclopropabenzene skeleton is positively charged in 7 and 9 and negatively charged in 8. Consequently, the direction of the dipole moment in 8 is opossite to that in 7 and 9.