Reply to the "Comment on 'The Concept of Protobranching and Its Many Paradigm Shifting Implications for Energy Evaluations'"

Center for Computational Chemistry and Department of Chemistry, University of Georgia, Athens, Georgia, 30602.
The Journal of Physical Chemistry A (Impact Factor: 2.69). 02/2010; 114(10). DOI: 10.1021/jp909910f
Source: PubMed
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    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.
    Chemistry - A European Journal 06/2010; 16(23):6942-9. DOI:10.1002/chem.200902550 · 5.73 Impact Factor
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    ABSTRACT: The ring strain energies of carbomeric-cycloalkanes (molecules with one or more acetylene spacer units placed into carbon single bonds) are assessed using a series of isodesmic, homodesmotic, and hyperhomodesmotic chemical equations. Isodesmic bond separation reactions and other equations derived from the explicitly defined hierarchy of homodesmotic equations are insufficient for accurately determining these values, since not all perturbing effects (i.e., conjugation and hyperconjugation) are fully balanced. A set of homodesmotic reactions is proposed, which succeeds in balancing all stereoelectronic effects present within the carbomeric rings, allowing for a direct assessment of the strain energies. Values calculated from chemical equations are validated using an increment/additivity approach. The ring strain energy decreases as acetylene units are added, manifesting from the net stabilization gained by opening the C-CH(2)-C angle around the methylene groups and the destabilization arising from bending the C-C identical withC angles of the spacer groups. This destabilization vanishes with increasing parent ring size (i.e., the angle distortion is less in the carbomeric-cyclobutanes than in the carbomeric-cyclopropanes), leading to strain energies near zero for carbo(n)-cyclopentanes and carbo(n)-cyclohexanes.
    The Journal of Physical Chemistry A 06/2010; 114(24):6705-12. DOI:10.1021/jp1029322 · 2.69 Impact Factor
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    ABSTRACT: Bond separation reactions of highly branched alkanes are used to assess (de)stabilizing interactions associated with various 1,3-nonbonded substituent patterns. While n- and singly methylated alkanes show positive bond separation energies (BSEs), which increase systematically along the series, permethylated alkanes are characterized by decreasing BSEs. Analysis shows that singly methylated alkanes are more stabilized than linear alkane chains and that the unique destabilizing feature of permethylated alkanes arises from the close proximity of bulky methyl groups causing highly distorted geometries along the carbon backbone.
    Organic Letters 07/2010; 12(13):3070-3. DOI:10.1021/ol1010642 · 6.36 Impact Factor
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