Internal Rotation in (CH3)2X Molecules of C2υ Symmetry–Barrier to Internal Rotation in Dimethyl Ether

A procedure is described for calculating the periodic barrier to internal rotation of $C_{\text{2υ}}$ molecules with two methyl groups based on one observed torsional frequency for each of the isotopic species (CH₃)₂X and (CD₃)₂X. The method is applied to dimethyl ether using the observed gas‐phase $B_1$ torsional frequencies of (CH₃)₂O and (CD₃)₂)O. The calculated coefficients of the potential function are $V_{\mathrm{effective}}\text{\hspace{0.17em}=\hspace{0.17em}946\hspace{0.17em}±\hspace{0.17em}15}{\mathrm{cm}}^{\text{−1}}$ and $V_{12}\text{′\hspace{0.17em}=\hspace{0.17em}30\hspace{0.17em}±\hspace{0.17em}9}{\mathrm{cm}}^{\text{−1}}$ . The evaluation of the threefold barrier $\text{(V\hspace{0.17em}=\hspace{0.17em}907}{\mathrm{cm}}^{\text{−1}})$ for the isotopic species CH₃OCD₃ suggests that the CH₃ and CD₃ torsional levels are in resonance. Comparison of the results for the three isotopic species allows the value of $V_{12}$ to be estimated as − 20 cm⁻¹.