ANOMALOUS STABILIZING AND DESTABILIZING EFFECTS IN SOME CYCLIC PI-ELECTRON SYSTEMS

Ab initio computational studies have been carried out for three molecu les that are commonly classed as antiaromatic:cyclobutadiene (1), 1,3- diazacyclobutadiene (7), and 1,4-dihydropyrazine (6). Their dinitro an d diamino derivatives were also investigated. Stabilizing or destabili zing energetic effects were quantified by means of the isodesmic react ion procedure at the MP2/6-31G//HF/3-21G level, and calculated molecu lar electrostatic potentials (HF/STO-5G//HF/3-21G) were used as a prob e of electron delocalization. Our results do not show extensive deloca lization in the pi systems of any one of the three parent molecules. T he destabilization found for 1 and 7 is attributed primarily to strain and to repulsion between the localized pi electrons in the C=C and C= N bonds, respectively. However, 6 is significantly stabilized, presuma bly due to limited delocalization of the nitrogen lone pairs. NH2 grou ps are highly stabilizing, apparently because of lone pair delocalizat ion. NO2 is neither uniformly stabilizing nor destabilizing.