Promoting and inhibiting tunneling via nuclear motions

Abstract

Accurate, experimental rotational-vibrational energy levels determined via the MARVEL (Measured Active Rotational-Vibrational Energy Levels) algorithm and published recently for the symmetric-top (14)NH3 molecule in J. Quant. Spectrosc. Radiat. Transfer, 2015, 116, 117-130 are analyzed to unravel the promoting and inhibiting effects of vibrations and rotations on the tunneling splittings of the corresponding symmetric (s) and antisymmetric (a) rovibrational energy level pairs. The experimental transition data useful from the point of view of the present analysis cover the range 0.7-7000 cm(-1), sufficiently detailed rovibrational energy sets worth analyzing are available for 20 vibrational bands. The highest J value, where J stands for the rotational quantum number, within the experimental dataset employed is 30. Coupling of the "umbrella" motion of (14)NH3 with other vibrational degrees of freedom has only a minor effect on the a-s tunneling splitting characterizing the ground vibrational state, 0.79436(70) cm(-1). In the majority of the cases rotation around the C3 axis increases, while rotation around the two perpendicular axes decreases the tunneling splittings. For example, for the pair of vibrational ground states, 0(+) and 0(-), the tunneling splitting basically disappears at around J = 25 for the (J,K) = (J,1) states, where K = |k| is the usual quantum number characterizing the projection of the rotational angular momentum on the principal axis. The tunneling splittings, defined as energy differences E(a) - E(s) of corresponding energy level pairs, as a function of J and K show a very regular behavior for the ground state (GS) and the nν2 bands. For the other bands investigated exceptions from a regular behavior do occur, especially for bands characterized by degenerate vibrations, and occasionally the data available are not sufficient to arrive at definitive conclusions. The most irregular behavior is observed for rotational states characterized by the k - l = 3n rule (l is the vibrational angular momentum quantum number), with n = 0, 1, 2,… High-quality, variationally computed rovibrational data support all the conclusions of this study based on experimental energy levels.

Improved potential energy surface and spectral assignments for ammonia in the near-infrared region

“…(1) 12 and f (1) 14 changed by 17%, 21%, 11% and 2% respectively. This level of change in the low-order parameters is acceptable, and the apparent instability of higher-order parameters is not too worrying; in truth there are a large number of possible combinations that can produce similarly shaped potentials.…”

“…The result is a huge increase in computational demand, but we found it necessary in order to converge all rotationally excited states belonging to the stretching overtones at 18 0 0 0 cm −1 . Our complete list of energies extends to 23 0 0 0 cm −1 , and is included in the supplementary material along with associated quantum labels ( ν 1 …”

“…Ammonia is the classic example of a system undergoing tunneling which remains a subject of ongoing studies [1] . Ammonia production has more than doubled during the period of industrialisation [2] and is probably the most abundant manmade gas in the world.…”

Communication: Tunnelling splitting in the phosphine molecule

“…3 The subtle effects of this tunnelling on the energy levels of ammonia are now well-studied. 4 As a direct analogue of ammonia, phosphine can also be expected to display splitting of its energy levels due to the tunnelling effect. However, splitting in PH 3 is yet to be observed, despite multiple attempts spread over more than 80 years.…”

Trendbericht Physikalische Chemie 2017: Atomare und molekulare Tunnelprozesse

“…22,23) Exakte Verfahren sind in Programmpaketen wie Geniush implementiert. [24][25][26] Sie eignen sich dazu, Näherungsverfahren zu überprüfen, etwa das quasiadiabatische Kanal-RPH-Verfahren 22,23,27) (Reaction Path Hamiltonian), 28) das ältere Konzepte wie das quasiadiabatische Kanalmodell 29-31) oder ähnliches 32,33) nutzt. Verwandte Näherungskonzepte finden sich in der Instanton-Theorie für Tunnelprozesse.…”