Dimethylamine (DMA) possesses two equivalent minima. Rotation leads to two tunneling substates 0⁺ and 0^–, the latter being 1323 MHz higher in energy. The rotational transitions obey b-type selection rules, whereas the stronger rotation-tunneling transitions obey c-type selection rules.
The transition frequencies were summarized by
(1) H. S. P. Müller, R. T. Garrod, A. Belloche, V. M. Rivilla, K. M. Menten, I. Jiménez-Serra, J. Martín-Pintado, F. Lewen, and S. Schlemmer, 2023, Mon. Not. R. Astron. Soc. 523, 2887.
Initial microwave data were taken from
(2) J. E. Wollrab and V. W. Laurie, 1968, J. Chem. Phys. 48, 5058;
three assumed typographical errors were corrected. Further, very accurate microwave data were taken from
(3) K. J. Koziol, W. Stahl, and H. V. L. Nguyen, 2021, Virtual ISMS, contribution WH10.
Internal rotation splitting was neglected. It is very small and should not matter for observations of star-forming regions. The calculations were truncated at J = 65 to avoid too large values of g_up. The calculation should be sufficient for most or all observational purposes. Calculated frequencies with uncertainties exceeding 0.1 MHz should be viewed with some caution.
¹⁴N hyperfine splitting, however, may be resolved in astronomical observations. Therefore, a calculation with hyperfine splitting is available up to J = 50 and 275 GHz.
At low temperatures, it may be necessary to discern between ortho-DMA and para-DMA which are described by K_a + K_c being even and odd, respectively in 0⁺, while it is opposite in 0^–. The intensity ratio is 9 : 7 for ortho-DMA : para-DMA.
The partition function takes into account the ground vibrational state only, in addition, the spin-degeneracy of ¹⁴N, and the reduced spin-weight of 9 or 7 (as usual). Vibrational correction factors have been derived in the harmonic approximation for a posteriori corrections of the column densities. See (1) for further details.
The dipole moment was determined in (2).