Experimental data are available only for the non-radioactive isotopologs. Rotational transition frequencies were reported mostly by
(1) F. C. Wyse, and W. Gordy, 1972, J. Chem. Phys. 56, 2130.
Very accurate ground vibrational and ground rotational transition frequencies were published by
(2) K. D. Hensel, C. Styger, W. Jäger, A. J. Merer, and M. C. L. Gerry, 1993, J. Chem. Phys. 99, 3320.
Also used in the model were rovibrational transition frequencies from
(3) H. G. Hedderich, M. Dulick, and P. F. Bernath, 1993, J. Chem. Phys. 99, 8363.
The quality of the data from (3) differs very much. Unfortunately, no individual uncertainties were given. Therefore, lines with large residuals were omitted, and uncertainties were estimated for the remainder of the lines in each state and for each isotopolog. The present analysis follows largely the constrained one from that work.
The calculated transitions should be accurate enough for all observational purposes. Some caution may be advised for those at higher J. Al or Cl hyperfine splitting may be resolvable at very low J, but these transitions are likely too weak to be observable.
The first 20 vibrational states were included in the calculation of partition function values.
The dipole moments were scaled to those from a quantum-chemical calculation, reported by
(4) M. Yousefi and P. F. Bernath, 2018, Astrophys. J. Suppl. Ser. 273, Art. No. 8.