# MULTIDIMENSIONAL HYDROGEN TUNNELING DYNAMICS IN THE GROUND VIBRATIONAL STATE OF THE AMMONIA DIMER

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 Title: MULTIDIMENSIONAL HYDROGEN TUNNELING DYNAMICS IN THE GROUND VIBRATIONAL STATE OF THE AMMONIA DIMER Creators: Loeser, J. G.; Elrod, M. J.; Saykally, R. J.; Schmuttenmaer, C. A.; Cohen, R. C.; Steyert, D. W.; Bumgarner, R. E.; Blake, Geoffrey A. Issue Date: 1992 Publisher: Ohio State University Abstract: We have measured and assigned more than 800 new far infrared absorption lines and 12 new microwave absorption lines of the ammonia diner. 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 diner connected by electric dipole allowed transitions are separated into three groups that correspond to different combinations of monomer transitional 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 $K_{4}=O$ and $K_{4}=1$ 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 dinner is $G_{144}$. This, in turn, implies that three kind 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 $C_{3}$ symmetry axes, and quite unexpectedly, umbrella” inversion tunneling. In the $K=O$ 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 MHz 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 $16.1 cm^{-1}$ and $19.3 cm^{-1}$, respectively. In the A+E states, the measured $20.5 cm^{-1}$ 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 frequence out-of-plane intermolecular vibration predicted in several theoretical studies. [C.E. Dykstra and L. Andrews, J. Chem. Phys. 92 , 6043 (1990); M.J. Frisch, J.E. Del Bene, J.S. Binkley, and H.F. Schaefer III, J. Chem. Phys. 84 , 2279 (1986).] In their structure determination, Nelson et al. assumed that monomer umbrella inversion tunneling was completely quenched and that donor-acceptor” interchange tunneling was nearly quenched in the ammonia dimer. [D.D. Nelson, G.T. Fraser, and W. Klemperer, J. Chem. Phys. 83 , 6201 (1985); D.D. Nelson, W. Klemperer, G.T. Fraser, F.J. Lovas, and R.D. Suenram, J. Chem. Phys. 87 , 6364 (1987). Description: Author Institution: Department of Chemistry, University of California; Department of Chemistry, University of Rochester; Department of Chemistry, Harvard University; Department of Chemistry, Colgate University; Division of Biology, California Institute of Technology; Division of Geological and Planetary Sciences, California Institute of Technology URI: http://hdl.handle.net/1811/12963 Other Identifiers: 1992-WG-14