Electron Diffraction Study of the Structures of Ethane and Deuteroethane

An electron diffraction structure determination for ethane and deuteroethane has been carried out to study the magnitudes of primary and secondary deuterium isotope effects on bond lengths. Mean parameters and estimated standard errors for C2H6 were rg(C–H) = 1.1122±0.0012 Å, lg(C–H) = 0.0760±0.0010 Å, rg(C–C) = 1.5340±0.0011 Å, lg(C–C) = 0.0496±0.0010 Å, and ∠CCH = 111.0±0.2°. Corresponding parameters for C2D6 were rg(C—D) = 1.1071±0.0012 Å, lg(C—D) = 0.0668±0.0010 Å, rg(C–C) = 1.5323±0.0011 Å, lg(C–C) = 0.0517±0.0010 Å and ∠CCD = 111.0±0.2°. Shrinkage corrections have not been made for bond angles. A primary deuterium isotope effect of 0.0050±0.0006 Å was found for the carbon hydrogen bond. A possibly significant secondary effect was observed in which the C–C length in ethane exceeded that in deuteroethane by Δrg(1) = 0.0016±0.0007 Å. Some aspects of secondary isotope effects are discussed briefly.