Overcoming lability of extremely long alkane carbon–carbon bonds through dispersion forces

• Published in: Nature (London) Vol. 477; no. 7364; pp. 308 - 311
• Main Authors: Schreiner, Peter R., Chernish, Lesya V., Gunchenko, Pavel A., Tikhonchuk, Evgeniya Yu, Hausmann, Heike, Serafin, Michael, Schlecht, Sabine, Dahl, Jeremy E. P., Carlson, Robert M. K., Fokin, Andrey A.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 15.09.2011; Nature Publishing Group
• Subjects: 639/638/406/910; Aliphatic compounds; Bond strength; Chemical bonds; Chemical reactivity; Chemistry; Crystal structure; Exact sciences and technology; Humanities and Social Sciences; Hydrocarbons; letter; Life sciences; multidisciplinary; Organic chemistry; Preparations and properties; Reactivity and mechanisms; Science; Molecular rigidity; Intramolecular reaction; Stability; Hydrocarbon; Molecular structure; Chemical bond; Theoretical study; Molecular interaction; Diamond; Decomposition; Branched compound; Dissociation energy; Dispersion; Steric effect; Carbon carbon bond; Thermodynamic parameter; Alkane; Van der Waals interaction; Density functional method; Inclusion compound; Electron density; Chemical reactivity; Chemical synthesis
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature10367

Strength at length in carbon–carbon bonds The synthesis of alkane hydrocarbons containing extremely long carbon–carbon (C–C) bonds, the longest observed in alkanes to date, is reported. Long C–C bonds are usually weaker than short ones, but these new alkanes are surprisingly stable, with decomposition occurring only above 200 °C. Quantum mechanical calculations show that, remarkably, the compounds are stabilized by attractive interactions between bulky groups at either end of the long C–C bonds. The stabilizing interactions observed in these compounds could prove useful in the development of new materials that utilize attractive dispersion interactions. Steric effects in chemistry are a consequence of the space required to accommodate the atoms and groups within a molecule, and are often thought to be dominated by repulsive forces arising from overlapping electron densities (Pauli repulsion). An appreciation of attractive interactions such as van der Waals forces (which include London dispersion forces) is necessary to understand chemical bonding and reactivity fully. This is evident from, for example, the strongly debated origin of the higher stability of branched alkanes relative to linear alkanes 1 , 2 and the possibility of constructing hydrocarbons with extraordinarily long C–C single bonds through steric crowding 3 . Although empirical bond distance/bond strength relationships have been established for C–C bonds 4 (longer C–C bonds have smaller bond dissociation energies), these have no present theoretical basis 5 . Nevertheless, these empirical considerations are fundamental to structural and energetic evaluations in chemistry 6 , 7 , as summarized by Pauling 8 as early as 1960 and confirmed more recently 4 . Here we report the preparation of hydrocarbons with extremely long C–C bonds (up to 1.704 Å), the longest such bonds observed so far in alkanes. The prepared compounds are unexpectedly stable—noticeable decomposition occurs only above 200 °C. We prepared the alkanes by coupling nanometre-sized, diamond-like, highly rigid structures known as diamondoids 9 . The extraordinary stability of the coupling products is due to overall attractive dispersion interactions between the intramolecular H•••H contact surfaces, as is evident from density functional theory computations with 10 and without inclusion of dispersion corrections.