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Origin of Stability in Branched Alkanes
Author(s) -
Kemnitz Carl R.,
Mackey Joel L.,
Loewen Mark J.,
Hargrove Julie L.,
Lewis Joshua L.,
Hawkins Whitney E.,
Nielsen Adam F.
Publication year - 2010
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200902550
Subject(s) - stability (learning theory) , computer science , machine learning
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 σ→σ* delocalization, particularly of the type that involves adjacent CC 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.