The Potential Constants of Ethane

Abstract

The infrared and Raman data of light and heavy ethane (C₂H₆ and C₂D₆) have been reexamined for the purpose of determining as accurately as possible the potential constants of the ethane molecule. In order to fill in some of the gaps in the spectroscopic data, additional high resolution measurements have been made on the infrared spectrum of heavy ethane which have given more precise values for the active fundamental frequencies and zeta‐values. Resolution of the fine structure associated with the parallel band ν₅^* has given the value of the large moment of inertia of C₂D₆, thus completing the information required for the spectroscopic determination of the dimensions of ethane. The data yield, C–C distance=1.543A, C–H distance=1.102A, H–C–C angle=109°37′, and H–C–H angle=109°19′. The twenty‐two distinct potential constants compatible with the D_3d symmetry of ethane have been determined through their relationships to the normal frequencies and zeta‐values of C₂H₆ and C₂D₆. The normal frequencies have been obtained by addition of anharmonic corrections to the spectroscopically observed fundamental frequencies. These corrections were estimated by means of the known anharmonic corrections for methane and the conditions imposed by the Teller product rule. The fundamental frequencies and zeta‐values have been taken directly from the observed band centers and rotational spacings wherever possible. In the cases of resonance, the influence of the couplings were either calculated or estimated and the corresponding unperturbed values for the frequencies and zeta‐values selected. The potential function is determined first in terms of a set of simple symmetry coordinates, and then reexpressed in terms of valence coordinates to permit comparison of the valence force constants of ethane and methane.