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

摘要

Intrinsic reaction coordinate (IRC) calculations of the torsional potentials of C_2H_6,CH_3CD_3,and C_2D_6 have been carried out at the MP2/6-31++G~** level.The C_2H_6 potential was corrected at the coupled-cluster single double (triple) [CCSD(T)] level with extrapolation to the complete basis set limit (CBS).For CH_3CD_3 and C_2D_6 and C_2D_6,the MP2 potentials were scaled by 0.862 to approximate CCSD(T)/CBS results.The IRC potential for the D_3h->D_3d relaxation in C_2H_6 was fit to a two-term Fourier series containing V_3 and V_6 coefficients for which the barrier height,V_3,was set to the CCSD(T)/CBS value (941 cm~(-1)),and V_6 was optimized at 6.7 cm~(-1).Sixfold ftorsional potentials were constructed from the CCSD(T)/CBS profiles and the resulting eigenvalues were used to calculate the DELTA_n(v_4)=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 V_3 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 experinental results.