Multidimensional hydrogen tunneling dynamics in the ground vibrational state of the ammonia dimer

Abstract

We have measured and assigned more than 800 new far-infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far-infrared and microwave spectra for this cluster. The vibration-rotation-tunneling (VRT) states of the ammonia dimer connected by electricdipole-allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A +A states (states formed from the combination of two ammonia monomers in A states), A +E states, and E+ E states. We present complete experimentally determined energy-level diagrams for the K,=O and K,= 1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G,# This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the "donor" and "acceptor" roles of the monomers, internal rotation of the monomers about their C, symmetry axes, and quite unexpectedly, "umbrella" inversion tunneling. In the K,= 0 A + E and E+ E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In K,= 1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm-', respectively. In the A + E states, the measured 20.5 cm-' splitting can result from a difference in "donor" and "acceptor" internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far-infrared subbands is the very-low-frequency out-of-plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990) Chem. Phys. 97, 4750 (1992>], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.

Ab initio search for the equilibrium structure of the ammonia dimer

“…Loeser et al 4 have shown that (NH 3 )z is nonrigid in many coordinates, including the umbrella mode V2 of NH 3 , and that a potential surface in all the soft degrees of freedom is probably required in order to contemplate executing the coupled large amplitUde vibrations that occur in the dimer. In this work only a highly restricted portion of configuration space is examined.…”

Rotation-tunneling spectrum of the deuterated ammonia dimer

“…A subsequent analysis by Loeser et al 5 of the farinfrared spectrum of the NH 3 dimer 5-7 revealed that the spectrum of the dimer is significantly more complex than initially suggested by the microwave study. Loeser et al 5 found, in fact, that the monomer inversion motion is not quenched in the dimer, thus requiring the use of the G 144 molecular symmetry group in the analysis.…”

Terahertz Vibration−Rotation−Tunneling Spectroscopy of the Ammonia Dimer. II. A−E States of an Out-of-Plane Vibration and an In-Plane Vibration

“…VRT spectra of the A + E states, Loeser et al 4 observed transitions involving the K ) 2 state and included them in their analysis to achieve a good fit, so it is likely that the missing manifold here is likewise a K ) 2 manifold. Several approaches were attempted to treat these transitions.…”