Molecular structure effects on electron ranges and mobilities in liquid hydrocarbons: chain branching and olefin conjugation: mobility model

Abstract

The effect of molecular structure on electron behavior in liquids was studied by measuring secondary electron penetration ranges bGP and thermal electron mobilities ue in substituted methanes and ethylenes. The penetration ranges are smaller (energy transfer cross sections are larger) when the alkane molecules are less rigid. It was confirmed that the epithermal electron energy transfer interaction radius in a liquid phase alkane molecule is limited to two C—C bonds in series. This modifies the earlier noted correlation between bGP and the degree of sphericity of the molecules. For example, the density normalized range bGPd in the relatively sphere-like tetraethylmethane (54 × 10−8 g/cm2) is more similar to that in the distinctly nonspherical diethylmethane (n-pentane, 43 × 10−8 g/cm2) than to that in the sphere-like tetramethylmethane (126 × 10−8 g/cm2). Tetraethylmethane is too large for the entire molecule to interact with an electron in the liquid phase, and the possibility of rotations about the C—C bonds in the ethyl groups makes the molecule less rigid. Electrons sense these relatively sphere-like molecules to be similar to those of a n-alkane. Connecting tert-butyl groups to olefinic or acetylenic carbons creates sphere-like quasi neopentyl groups which greatly enhance electron ranges in the unsaturated compounds. In conjugated olefins cis–trans effects are largely overshadowed by the general efficiency of these compounds as electron energy sinks. The earlier noted correlation between bGP and ue contains fine structure. For a given value of bGP, ue increases in the order n-alkane < cyclo or branched alkane < olefin. Electron mobilities are interpreted in terms of a model that contains a Gaussian distribution of solvated electron state energies, a conduction band, and thermally activated transitions between them. The model is a combination of our treatment of electrons in ethers and Schiller's treatment of electrons in hydrocarbons. The percolation model does not provide a sufficiently complete interpretation of electron migration in hydrocarbons.