ABSTRACT
The potential origins of stability in branched alkanes are investigated, paying close attention to two recent hypotheses: geminal steric repulsion and protobranching. All alkane isomers through C(6)H(14) along with heptane and octane were investigated at the MPW1B95/6-311++G(d,p) level. Their geminal steric repulsion, total steric repulsion, and orbital interactions were evaluated by using natural bond orbital analysis. All measures of steric repulsion fail to explain the stability of branched alkanes. The extra stability of branched alkanes and protobranching, in general, is tied to stabilizing geminal sigma-->sigma* delocalization, particularly of the type that involves adjacent C-C bonds and, thus, preferentially stabilizes branched alkanes. This picture is corroborated by valence bond calculations that attribute the effect to additional ionic structures (e.g., CH(3) (+) :CH(2) :CH(3) (-) and CH(3):(-) CH(2): CH(3) (+) for propane) that are not possible without protobranching.
ABSTRACT
Complete basis set calculations (CBS-QB3) were used to compute the CN rotation barriers for acetamide and eight related compounds, including acetamide enolate and O-protonated acetamide. Natural resonance theory analysis was employed to quantify the "amide resonance" contribution to ground-state electronic structures. A range of rotation barriers, spanning nearly 50 kcal/mol, correlates well to the ground-state resonance weights without the need to account for transition-state effects. Use of appropriate model compounds is crucial to gain an understanding of the structural and electronic changes taking place during rotation of the CN bond in acetamide. The disparate changes in bond length (DeltarCO << DeltarCN) are found to be consonant with the resonance model. Similarly, charge differences are consistent with donation from the nitrogen lone pair electrons into the carbonyl pi* orbital. Despite recent attacks on the resonance model, these findings demonstrate it to be a sophisticated and highly predictive tool in the chemist's arsenal.
