Both H₃⁺ and D₃⁺ do not have a permanent dipole moment because of symmetry. In contrast, both H₂D⁺ and HD₂⁺ have sizable dipole momenta.
The first entry from April 2004, modified Aug. 2005, and the second entry from Feb. 2012 have been modified considerably. Details are given in
(1) P. Jusko, M. Töpfer, Holger S.P. Müller, P. N. Ghosh, S. Schlemmer, and O. Asvany, 2017, J. Mol. Spectrosc., 332, 33.
Please note that the parameter set is slightly different in the present compilation because of one addition line from (4); X₂₁ was omitted and Y₂₀ was used instead. Multiple transition frequencies were retained as long as they were compatible with each other.
The J = 1 – 0 transition of ortho-HD₂⁺ was taken from
(2) O. Asvany, O. Ricken, H. S. P. Müller, M. C. Wiedner, T. F. Giesen, and S. Schlemmer, 2008, Phys. Rev. Lett., 100, Art. No. 233004.
The J = 1 – 1 transition of para-HD₂⁺ was taken from
(3) T. Hirao and T. Amano, 2003, Astrophys. J., 597, L85.
Five additional frequencies with microwave accuracy were reported by
(4) S. Yu, J. C. Pearson, T. Amano, and F. Matsushima, 2017, J. Mol. Spectrosc., 332, 33.
All except one were used in (1). Three transition frequencies from (2), (3), and one from (4) were remeasured in (1) with much greater accuracy. One additional transition frequency of ortho-HD₂⁺ was reported by
(5) D. A. Jennings, C. Demuynck, M. Banek, and K. M. Evenson, private communication in
O. L. Polyansky and A. R. W. McKellar, 1990, J. Chem. Phys., 92, 4039.
Two infrared ground state combination differences (GSCDs) with nearly microwave accuracy were taken from
(6) P. Jusko, C. Konietzko, S. Schlemmer, O. and Asvany, 2016, J. Mol. Spectrosc., 319, 55.
Additional less accurate GSCDs reported in (3) were also used in the fit.
Note: uncertainties of 999.9999 MHz or 0.03336 cm^–1 are the largest ones displayed ! All predictions should be viewed with caution because of the small data set.
The ab initio dipole moment was mentioned by
(7) S. C. Foster, A. R. W. McKellar, and J. K. G. Watson, 1986, J. Chem. Phys., 85, 664.
At low temperatures, it may be necessary to discern between ortho-HD₂⁺ and para-HD₂⁺. The ortho and para states are described by K_a + K_c even and odd, respectively. The nuclear spin-weight ratio is 2 : 1 for ortho-HD₂⁺ : para-HD₂⁺. The J_KaKc = 1₀₁ is the lowest para state. It is 34.9184 cm^–1 above ground.
Separate para and ortho predictions are available up to J = 3 and K_a = 3 along with separate para and ortho partition function values.