Anomalous stabilizing and destabilizing effects in some cyclic π-electron systems

Abstract

Ab initio computational studies have been carried out for three molecules that are commonly classed as antiaromatic: cyclobutadiene (1), 1,3-diazacyclobutadiene (7), and 1,4-dihydropyrazine (6). Their dinitro and diamino derivatives were also investigated. Stabilizing or destabilizing energetic effects were quantified by means of the isodesmic reaction procedure at the MP2/6-31G*//HF/3-21G level, and calculated molecular electrostatic potentials (HF/STO-5G//HF/3-21G) were used as a probe of electron delocalization. Our results do not show extensive delocalization in the π systems of any one of the three parent molecules. The destabilization found for 1 and 7 is attributed primarily to strain and to repulsion between the localized π electrons in the C=C and C=N bonds, respectively. However, 6 is significantly stabilized, presumably due to limited delocalization of the nitrogen lone pairs. NH2 groups are highly stabilizing, apparently because of lone pair delocalization. NO2 is neither uniformly stabilizing nor destabilizing.

Antiaromaticity in relation to 1,3,5,7‐cyclooctatetraene structures

“…(3) is applied to 2 and 3, the resulting isodesmic energies are -2.6 and -6.6 kca]/mole, respectively.) Thus, our results for 3 show again [9] that an antiaromatic system need not necessarily show a net destabilization. …”

“…Antiaromaticity is commonly viewed as being associated with a destabilizing effect; for cyclobutadiene, for instance, this has been reported as being at least 12 -16 kcal/mole [6]. However in a recent computational study of 1,4-dihydropyrazine, 4, which is normally described as antiaromatic [7,8], we found evidence of significant stabilization, rather than the reverse [9].…”

Cyclobutadiene: The Antiaromatic Paradigm?

“…An additional complication, pointed out originally by Politzer et al, [21] concerns the observation that in CBD two p bonds are forced into a much more proximate position than in any other polyene. The fact that the CÀC bond Scheme 1.…”

Cyclobutadien: der Inbegriff eines antiaromatischen Moleküls?

“…Eine zusätzliche Komplikation, auf die Politzer et al hingewiesen haben,21 entsteht dadurch, dass die beiden π‐Bindungen in CBD viel näher beieinander liegen als in irgendeiner anderen Verbindung mit mehreren Doppelbindungen. Die Tatsache, dass die C‐C‐Bindung in CBD (Länge 1.57 Å nach den besten Rechnungen22) um 0.1 Å länger ist als in Butadien, wogegen die CC‐Bindungen in den beiden Verbindungen fast gleich lang sind, ist damit vielleicht ein Ausdruck der Abstoßung zwischen den π‐Elektronen.…”