Evaluation of fundamental organic concepts
Wodrich, Matthew Davisson
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The concept of protobranching, defined as the net stabilization provided by 1,3-alkyl-alkyl interactions in all branched and n-alkanes except methane and ethane, and its implications are highlighted. Protobranching is assessed via Pople’s isodesmic bond separation energy and has an average value of 2.8 kcal/mol for n-alkanes. Protobranching has a significant impact on the quantification of physical organic phenomena, including ring/cage strain, conjugation and hyperconjugation, and aromatic resonance energies. Historically, these quantities have been evaluated using propane and larger alkanes (both linear and branched) as reference compounds. However, the inherent stabilization within these “reference” compounds has not previously been considered. Reevaluated energies for the above mentioned phenomena are discussed. Protobranching has also been utilized to create a new isodesmic additivity scheme, capable of calculating heats of formation of alkanes, alkyl radicals, alkenes, and alkynes to high accuracy. Data fitting schemes based on geminal interactions are also explored. The ability of computational methods (HF, DFT, and post-HF) to compute the magnitude of protobranching stabilization is investigated. Pople’s isodesmic bond separation reactions of n-alkanes (propane to decane) show systematic underestimation for all DFT functionals tested; they are unable to accurately account for the protobranching stabilization. In the final chapter, double aromaticity, the existence of two mutually orthogonal Hückel frameworks within the same molecule, of small carbon, boron, and borocarbon monocycles is analyzed using refined NICS techniques. Double aromaticity is confirmed in C6 and C10, as had been previously hypothesized. C8 and C12 are shown to be doubly antiaromatic. Select boron and borocarbon compounds are also shown to be doubly aromatic.