Theoretical studies on the diazacyclopentadienylidenes: An analysis of aromatic versus nonaromatic systems and singlet versus triplet reactivity

✍ Scribed by Daniel J. Pasto

• Publisher: John Wiley and Sons
• Year: 1992
• Tongue: English
• Weight: 660 KB
• Volume: 3
• Category: Article
• ISSN: 1042-7163
• DOI: 10.1002/hc.520030103

✦ Synopsis

Ab initio moleculur orbital calculations have been carried out on the various electronic states of2,3-(6), 2,4-(7), 2,s-(8), mid 3,4-diazacyclopentadienylidene (9) at the /idly geometry optimized 6-31 G?' level, with single point calculations being carried out at the MP216-31 G* lever. The calculated geometries are interpreted in terms of the degree of occupancy and the nature ofthe rr and u-nonbonded MO's. At the 6-31 G" level the five welectron, rr, cr-triplet states were calculated to be considerably lower in energy. At the MP2/6-31GX level, howevev, with 7 the six .rr-electron singlet state is calculated to lie only slightly above the five rr-electron triplet (0.4 kcal mole-'), whereas with 8 and 9 the aromatic six rr-electron singlet states are calculated to he lower in energy (9.0 and 8.1 kcal mole-'). With 3 and 9 the aromatic six .rr-electron utriplet states lie only 3.6 and 5.5 kcal mole-' above the lowest energy states. It is concluded that, in general, the eiiergy gained by having an electron in a lower energy cr-t,vpe MO instead o f a higher energy rr MO effectively offsets the energy gained by having an aromatic rr system. The results are discussed in terms o f t h e observed chemistry of 6-9 and their substituted systems.