The molecule displays internal rotation of the methyl group. The present analysis is based on the transition frequencies summarized in
(1) B. P. van Eijck, A. Dubrulle, J. Demaison, and J.-L. Ripoll, 1985, J. Mol. Spectrosc. 112, 95.
Besides these millimeter wave data, microwave transition frequencies were used from
(2) D. den Engelsen, 1969, J. Mol. Spectrosc. 30, 466.
The lines were fit with erham. The calculation was truncated at J = 40 ans 400 GHz because of the limitated quantum number range. The calculation are sufficient for observations in colder sources and possibbly also for searches 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 by
(3) D. den Engelsen, H. A. Dijkerman, and J. Kerssen, 1965, Rec. Trav. Chim. 84, 1357.