Sparking Potential and Molecular Structure of Unsaturated Hydrocarbon Gases

Abstract

In earlier experiments, we have shown that it is possible to relate sparking potentials of saturated hydrocarbon gases to certain molecular properties. From measurements of electric breakdown in a series of these gases, it has been found that the effective molecular cross section for electron scattering is proportional only to the number of carbon-hydrogen bonds in the molecule and is independent of their disposition. The lack of a contribution by the carbon-carbon bonds has been attributed to a shielding effect. In the present paper, we describe and discuss the results of a similar investigation of breakdown in unsaturated hydrocarbon gases. It was found that, for a series of olefin gases, the cross section associated with the carbon-carbon double bond is greater than three times that of the carbon-hydrogen bond, while that of the carbon-carbon single bond is essentially zero as it was in the alkane series. An attempt is made to interpret this observation in terms of (1) a decrease in the shielding effect, and (2) the larger spatial configuration of the π-electron orbitals associated with the ethylenic linkage. Variations in the position of the double bond within the molecule has little or no effect upon its contribution to the total cross section. Breakdown measurements in butadiene-1,3 and benzene indicate that conjugation results in an increase in the effective cross section of each double bond, as might be expected. Evidence of a small but measurable amount of double-bond character was also found in cyclopropane from sparking potential measurements. This is in agreement with the predictions of other investigators.