Methyl internal rotation is frequently resolved in the rotational spectrum of the molecule. Fairly extensive transition frequencies mostly up to 40 GHz were reported by
(1) H. Dreizler and A. M. Mirri, 1968, Z. Naturforsch. A 23, 1313.
Additional transition frequencies mostly in the upper millimeter region were reported by
(2) H. Maes, B. P. van Eijck, A. Dubrulle, and J. Demaison, 1986, Z. Naturforsch. A 41, 743.
The data were fit with erham. Sextic distortion parameters were kept fixed to values of CH₃CCH except for H_KJ. Some transition frequencies reported as belonging to the A symmetry transitions only were occasionally assigned to both, whereas some transition frequencies assigned to both were assigned here only to the A or E species. Please note that erham sometimes swaps the K_c labels for nominally prolate paired E symmetry levels. The calculated spectrum was truncated at J = 70 and at 500 GHz. Caution is advised for transition frequencies with calculated uncertainties exceeding 0.1 MHz. The state number 0 signals the A symmetry component and 1 the E symmetry component.
¹⁴N hyperfine structure splitting may be resolvable for some lower-J transitions, however, erham is not capable of dealing with HFS structure.
The dipole moment components were determined in (1).