On models of bonding and structural stability in hydrocarbons

Abstract

Aromatic rings, and aliphatic rings and chains comprise the backbone of organic chemistry. But while advances in quantum computational chemistry have facilitated accurate estimates of the observable properties of simple hydrocarbon species, qualitative and semi-quantitative models for understanding the predictions of theory are incomplete. The celebrated theoretical chemist Charles Coulson, after attending a scientific lecture, is rumored to have once remarked “give us insight, not numbers!” We strive here to heed Coulson’s advice, and provide insight into the relative stabilities and bonding capacities of simple hydrocarbon systems of fundamental importance to organic chemistry. Branched alkanes have long been known to be more stable than their linear n-alkane isomers. This “alkane branching effect” is due to electron correlation effects arising from the greater number of 1,3 alkyl-alkyl interactions, called “protobranches,” present in branched alkanes. Such protobranching interactions exist also in most linear and cyclic alkanes (e.g., the 1,3 methyl-methyl interaction in propane), and stabilize these species accordingly. In 1964 Heilbronner predicted that 4n π-electron annulenes might achieve closed shell stability, with no consequent loss in resonance energy, by adopting “Möbius-type” conformations which enforce a 180 degree twist in their carbon p atomic orbitals. However despite being potentially stabilized by “Möbius aromaticity”, neutral medium sized Möbius annulenes are less stable than their untwisted Hückel counterparts. This is due in part to uneven p orbital twisting in Möbius isomers, which significantly reduces their resonance energies. Despite being Hückel aromatic, the 2π-electron cyclobutadiene dication and related isoelectronic derivative are all non-planar. Their puckering is caused by stabilizing cross-ring σ→π* hyperconjugation, which is possible only in non-planar geometries. Elementary Lewis bonding theory holds that carbon forms four 2-center 2-electron bonds. However the actual bonding capacity of carbon is not so confined, and neutral molecules exhibiting hexa and octavalent carbon atoms bound only to other carbons are possible. The hypervalent C-C interactions in these species are the result of electron deficient bonding, which ensures that the hypervalent carbon atoms obey the octet rule. The concepts developed in this thesis are general, and are expected to be transferable to a host of hydrocarbon species not considered herein.