The molecule displays internal rotation of the methyl group. The present analysis is based on millimeter transition frequencies from
(1) J. Demaison, H. Maes, B. P. van Eijck, G. Wlodarczak, and M. C. Lasne, 1987, J. Mol. Spectrosc. 125, 214.
Besides these millimeter wave data, microwave transition frequencies were used from
(2) D. den Engelsen, 1967, J. Mol. Spectrosc. 22, 426.
The lines were fit with erham. The calculation was truncated at J = 60 and 720 GHz because of the limitated quantum number range. The calculation should be sufficient for observations in warmer sources. Transitions with calculated uncertainties exceeding 0.1 MHz should be viewed with caution.
Please note that state number 0 and 1 refer to the A and E symmetry components, respectively. This differs slightly from the standard erham notation.
The partition function refers to the complete ground vibrational state which was evaluated with g_I = 1.
The dipole moment components were determined in (2).