Structure and Rotational Isomerization of Free Hydrocarbon Chains

Abstract

An electron diffraction investigation has been carried out for the molecules n‐butane, n‐pentane, n‐hexane, and n‐heptane in the gaseous phase. Mean bond lengths and bond angles were reported in earlier papers. The present paper is concerned with information derivable from the observed spectrum of nonbonded distances. A scheme is presented for determining both trans and gauche bond angles despite the fact that the 2.5‐Å nonbonded C···C trans and gauche radial distribution peaks are too badly overlapped to be resolved. Formulas are given for calculating torsional ``shrinkage effect'' corrections for nonbonded distances. It was found that a given gauche conformation has a higher free energy than its corresponding trans counterpart by about 610 cal/mole. If the source of this free‐energy difference is steric, arising from repulsions between hydrogens on gauchecarbons, there should be structural as well as thermodynamic manifestations. The positions of a total of 21 independent nonbonded C···C radial distribution peaks were interpreted in terms of a model of steric deformations in gauche structures. The results, including estimates of standard errors, were φ(gauche)=61.0±3° dihedral angle, α(trans)=112.65±0.3° C–C–C angle, α(gauche)—α(trans)=0.1°±1°, and the distortion from planarity of trans conformations adjacent to gauche was η=7±4°. The C–C bond length and C–C–C bond angle, averaged over all conformations, were 1.5328±0.002 Å and 112.71±0.15°, respectively.