The hydronium cation undergoes large amplitude tunneling, just as the isoelectronic ammonia, NH₃ molecule. However, the barrier to planarity is much lower so that the two tunneling substates in the ground vibrational state are ∼1.66 THz apart instead of only 23.8 GHz.
The v = 0, v₂ = 1, and ν₂ transition frequencies known at that time have been critically evaluated in
(1) D.-J. Liu, T. Oka, and T. J. Sears, 1986, J. Chem. Phys., 84, 1312.
The data set included frequencies with microwave accuracy reported by
(2) G. M. Plummer, E. Herbst, and F. C. De Lucia, 1985, J. Chem. Phys., 83, 1428;
and by
(3) M. Bogey, C. Demuynck, M. Denis, and J. L. Destombes, 1985, Astron. Astrophys., 148, L11.
Far-infrared laser sideband measurements summarized in
(4) P. Verhoeve, M. Versluis, J. J. ter Meulen, W. L. Meerts, and A. Dymanus, 1989, Chem. Phys. Lett., 161, 195;
as well as ground state combination differences from
(5) D. Uy, E. T. White, and T. Oka, 1997, J. Mol. Spectrosc., 183, 240;
and from
(6) J. Tang and T. Oka, 1999, J. Mol. Spectrosc., 196, 120;
were also used in the fit. In the course of fitting the data all experimental data has been scrutinized. A small number of lines had to be exclude from the line list; reasons for this may be unaccounted ion velocity shifts, incorrect referencing of the far-infrared or infrared data, or difficulties in representing the data by a conventional Hamiltonian in particular at higher quantum numbers. All predictions should be viewed with some caution. Predictions with uncertainties larger than 50 MHz can not be trusted.
Transitions with experimental uncertainties ≥ 100 kHz have not been merged.
The partition function takes into account v₂ = 0 and 1.
The ab initio dipole moment has been reported by (7) P. Botschwina, P. Rosmus, and E. A. Reinsch, 1985, Chem. Phys. Lett., 102, 299.