On the large σ-hyperconjugation in alkanes and alkenes

  • Judy I-Chia Wu
  • Changwei Wang
  • William Chadwick McKee
  • Paul von Ragué Schleyer
  • Wei Wu
  • Yirong Mo
Original Paper

DOI: 10.1007/s00894-014-2228-2

Cite this article as:
Wu, J.IC., Wang, C., McKee, W.C. et al. J Mol Model (2014) 20: 2228. doi:10.1007/s00894-014-2228-2
Part of the following topical collections:
  1. Topical Collection on the occasion of Prof. Tim Clark’s 65th birthday

Abstract

The conventional view that the σCC and σCH bonds in alkanes and unsaturated hydrocarbons are so highly localized that their non-steric interactions are negligible is scrutinized by the block-localized wavefunction (BLW) method. Even molecules considered conventionally to be “strain free” and “unperturbed” have surprisingly large and quite significant total σ-BLW-delocalization energies (DEs) due to their geminal and vicinal hyperconjugative interactions. Thus, the computed BLW-DEs (in kcal mol−1) for the antiperiplanar conformations of the n-alkanes (CNH2N+2, N = 1-10) range from 11.6 for ethane to 82.2 for n-decane and are 50.9 for cyclohexane and 91.0 for adamantane. Although σ-electron delocalization in unsaturated hydrocarbons usually is ignored, the σ-BLW-DEs (in kcal mol−1) are substantial, as exemplified by D2h ethylene (9.0), triplet D2d ethylene (16.4), allene (19.3), butadiene (19.0), hexatriene (28.3), benzene (28.1), and cyclobutadiene (21.1). While each individual geminal and vicinal hyperconjugative interaction between hydrocarbon σ-bonding and σ-antibonding orbitals tends to be smaller than an individual π conjugative interaction (e.g., 10.2 kcal mol−1 in anti-1,3-butadiene, the presence of many σ-hyperconjugative interactions (e.g., a total of 12 in anti-1,3-butadiene, see text), result in substantial total σ-stabilization energies (e.g., 19.0 kcal mol−1 for butadiene), which may surpass those from the π interactions. Although large in magnitude, σ-electron delocalization energies often are obscured by cancellation when two hydrocarbons are compared. Rather than being strain-free, cyclohexane, adamantane, and diamantane suffer from their increasing number of intramolecular 1,4-C…C repulsions resulting in elongated C–C bond lengths and reduced σ-hyperconjugation, compared to the (skew-free) antiperiplanar n-alkane conformers. Instead of being inconsequential, σ-bond interactions are important and merit consideration.

Keywords

Block localized wavefunction Conjugation Electron delocalization Hauche interactions Hyperconjugation 

Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  • Judy I-Chia Wu
    • 1
  • Changwei Wang
    • 2
  • William Chadwick McKee
    • 1
  • Paul von Ragué Schleyer
    • 1
  • Wei Wu
    • 2
  • Yirong Mo
    • 3
  1. 1.Department of ChemistryUniversity of GeorgiaAthensUSA
  2. 2.Department of Chemistry, College of Chemistry and Chemical Engineering, the State Key Laboratory for Physical Chemistry of Solid States, Center for Theoretical ChemistryXiamen UniversityXiamenPeople’s Republic of China
  3. 3.Department of ChemistryWestern Michigan UniversityKalamazooUSA

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