Torsion-Vibration-Rotation Interactions in Methanol. III. Barrier Height in an Excited Vibrational State of CH3OH

A weak Q branch starting with a J =1 line at 26 256.80 MHz has been observed in the microwave spectrum of normal methanol. We have assigned this Q branch to the $\text{K=0← 1}$ E₂ transition in an excited vibrational state, on the basis of the Stark effects and the observation of the $1_1{\text{← 0}}_0$ and $1_0{\text{← 0}}_0$ E₂ lines at 21 900.08 and 48 156.93 MHz, respectively. The excited vibrational state has an energy of $\text{933± 150\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ , and is most probably the CH₃ in‐plane rocking mode. A value for the excited state barrier to internal rotation of $\text{557± 35\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ is required to reproduce the Q‐branch origin frequency, if reasonable limits on geometry changes are assumed. This is an increase of 50% over the ground state value of 376 cm⁻¹.