Electron Impact Spectra of Ethane and Other Saturated Hydrocarbons

Abstract

Electron impact spectra for ethane are reported at accelerating voltages from 50 to 180 V and scattering angles to 9°. Spectra of methane at 0° and 6° and for propane and butane at 0° are reported at an accelerating voltage of 50. Spectra of perdeutero ethane are reported at 0° and accelerating voltages of 50 and 100. A normal coordinate analysis of an excited electronic state of ethane is carried out using the frequencies observed in C₂H₆ and C₂D₆. A model, suggested by semiempirical molecular orbital theory, leads to the conclusion that the C–C internuclear distance decreases on excitation. The model is tested by means of an application of the Franck–Condon principle based on the normal coordinate analysis and the model proves to be self consistent if (a) the C–C internuclear distance decreases to 1.39 Å and (b) the bond angle C–C–H is increased 10° above the tetrahedral. The significance of this result in the theory of chemical bonding is discussed.