Knowledge Bank

A weak Q branch strating with a J=1 line at 26 256.80 MHz has been observed in the microwave spectrum of normal methanol. The Q branch has been assigned to the K = 0 $\leftarrow 1E2$ transition in an excited vibrational state, on the basis of the Stark effects and the observation of the $11$ $\leftarrow O0$ and $10$ $\leftarrow O0E2$ lines at 21 900.08 and 48 156.93 MHz respectively. The excited vibrational state has anergy of $933\pm 150$ $cm-1$, and is most probably the $CH3$ in-plane rocking mode. A value for the excited state barrier to internal rotation of $557\pm 35$ $cm-1$ is required to reproduce the Q-branch origin frequency, with reasonable limits on geometry changes assumed. This is an increase of 50% over the ground state value of $376cm-1$.