The molecule displays internal rotation of the methyl group. The entry refers to the lower energy Z isomer; the E isomer is about 500 cm^–1 higher in energy. The present analysis is based on the data from
(1) K. Luková, L. Kolesniková, J. Koucký, K. Vávra, P. Kania, J.-C. Guillemin, and Š. Urban, 2022, Astron. Astrophys. 665, Art. No. A9.
The analysis takes into account lower frequency data up to 51 GHz from
(2) P. Groner, R. Gunde, T.-K. Ha, and A. Bauder, 1987, Chem. Phys. Lett. 139, 479.
The calculations should be sufficiently accurate for all observational purposes. Transitions with calculated uncertainties exceeding 0.1 MHz should be viewed with some caution.
¹⁴N hyperfine splitting is probably small in most cases, but may be non-negligible in particular at low values of J. 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. Some lines were not merged because of weakness and some because K_c quantum numbers were evaluated incorrectly by the erham program.
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).