C-C BOND IN NEO-PENTANE, BONDING RADIUS OF TERTIARY-BUTYL GROUP, AND THEIR BEARING ON BONDING HYPOTHESES

The molecular structure of neo-pentane, (CH3)4C, has been studied in the gas phase by electron diffraction. The C-C bond length, 1.539 ± 0.002 Å, is intermediate between the lengths in ethane and diamond. The observed length is readily understood in terms of separate bonding radii, rp = 0.767 and rt = 0.772 Å, for primary and tertiary carbon atoms, respectively. The origin of the significant difference between these radii is discussed. Two semi-empirical theoretical approaches are tested, (i) the hypothesis that all variations in C-C bond lengths are due to changes in the degree of hybridisation of the two carbon atoms, (ii) the hypothesis that hybridisation, conjugation and hyperconjugation each have a role in determining C-C bond lengths. Hybridisation differences are considered to be the origin of the difference between rp and rt. Quantitatively, however, the observed difference suggests that the second of the approaches is the more realistic, i.e. changes in hybridisation have only a limited effect on C-C bond lengths; the predominant effects are conjugation and hyperconjugation. The anomalous effect of adjacent hetero-atoms on C-C bond lengths is pointed out. © 1969.