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 Aug. 2005 has been revised considerably, and so has the second one from Feb. 2012. The fitting method, including the derivation of initial spectroscopic parameters from quantum chemical calculations, has been described by
(1) P. Jusko, M. Töpfer, H. S. P. Müller, P. N. Ghosh, S. Schlemmer, and O. Asvany, 2017, J. Mol. Spectrosc., 332, 33.
In addition to extensive remeasurements and one new transition frequency, earlier data were scritinized. Additional data were 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;
from
(3) P. Jusko, C. Konietzko, S. Schlemmer, and O. Asvany, 2016, J. Mol. Spectrosc., 319, 55;
from
(4) T. Amano, 2006, Phil. Trans. R. Soc. A, 364, 2943;
from
(5) T. Yonezu, F. Matsushima, Y. Moriwaki, K. Takagi, and T. Amano, 2009, J. Mol. Spectrosc., 256, 238;
from
(6) S. Saito, K. Kawaguchi, and E. Hirota, 1985, J. Chem. Phys., 82, 45;
and from
(7) T. Amano and T. Hirao, 2005, J. Mol. Spectrosc., 233, 7.
Infrared ground state combination differences reported in (7) were also used in the fit.
Predictions should be reliable if J ≤ 4 and K_a ≤ 2, possibly higher. The largest uncertainties shown are 999.9999 MHz.
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 = odd and even, respectively. The nuclear spin-weight ratio is 3 : 1 for ortho-HD₂⁺ : para-HD₂⁺. The J_KaKc = 1₁₁ is the lowest ortho state. It is 60.0292 cm^–1 above ground.
Separate para and ortho predictions are available up to J = 4 and K_a = 3 along with separate para and ortho partition function values.
The dipole moment was estimated in
(8) A. Dalgarno, E. Herbst, S. Novick, and W. Klemperer, 1973, Astrophys. J., 183, L131.