The rotational transition frequencies were summarized in
(1) A. L. Steber, B. J. Harris, J. L. Neill, and B. H. Pate, 2012, J. Mol. Spectrosc. 280, 3.
They included also data from
(2) J. Demaison, D. Boucher, J. Burie, and A. Dubrulle, 1983, Z. Naturforsch. 38a, 447;
and from
(3) B. M. Landsberg and R. D. Suenram, 1983, J. Mol. Spectrosc. 98, 210.
Presently, we did not consider data from
(4) G. Bestmann and H. Dreizler, 1985, Z. Naturforsch. 40a, 263.
The data in (4) report small internal rotation splitting around 100 kHz; center frequencies, uncertainties, and the compatibility with data in (3) would have to be evaluated. Three lines of (2) were omitted in (1); we used one of these and employed a slightly different parameter set. Data from (3) were not merged because of the relatively large uncertainties of 100 kHz.
The predictions should be sufficient for astronomical searches; they should be viewed with some caution if the calculated uncertainties exceed 0.3 MHz.
The methyl internal rotation, resolved in (4), was deemed to be negligible for astronomical observations.
The dipole moment was measured in (3).