An intrinsic reaction coordinate calculation of the torsional potential in ethane: Comparison of the computationally and experimentally derived torsional transitions and the rotational barrier

Abstract

Intrinsic reaction coordinate (IRC) calculations of the torsional potentials of C2H6, CH3CD3, and C2D6 have been carried out at the MP2/6-31++G** level. The C2H6 potential was corrected at the coupled-cluster single double (triple) [CCSD(T)] level with extrapolation to the complete basis set limit (CBS). For CH3CD3 and C2D6, the MP2 potentials were scaled by 0.862 to approximate CCSD(T)/CBS results. The IRC potential for the D3h→D3d relaxation in C2H6 was fit to a two-term Fourier series containing V3 and V6 coefficients for which the barrier height, V3, was set to the CCSD(T)/CBS value (941 cm−1), and V6 was optimized at 6.7 cm−1. Sixfold torsional potentials were constructed from the CCSD(T)/CBS profiles and the resulting eigenvalues were used to calculate the Δn(ν4)=2 transitions, which are compared with experimental assignments. Comparisons are also made with observed IR transitions. Our best estimate of the rotational barrier is 941 cm−1. This value, as well as other high-level ab initio results, is about 50 cm−1 smaller than the V3 parameter obtained from the analysis of microwave and Raman data. The one-dimensional IRC potential neglects subtle coupling of ethane’s torsional motion to other modes and is likely responsible for the small, but systematic, differences between the calculated and experimental results.