# REDUCTION OF THE LARGE AMPLITUDE MOTIONS IN THE AMMONIA DIMER BY DEUTERATION

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 Title: REDUCTION OF THE LARGE AMPLITUDE MOTIONS IN THE AMMONIA DIMER BY DEUTERATION Creators: Fraser, G. T.; Loeser, J. G.; Saykally, R. J.; Karayakin, E. N.; Suenram, R. D. Issue Date: 1995 Publisher: Ohio State University Abstract: The microwave and submillimeter spectrum of the $(ND_{3})_{2}$ has been investigated using an electric-resonance optothermal spectrometer with backward-wave-oscillator frequency synthesizers. As in the case of the totally protonated $dimer^{1}$, the spectrum is complicated by the large-amplitude motions associated with the internal rotation of the two subunits about their three-fold axis and the interchange tunneling which allows the two monomers to exchange bonding roles in the complex. In addition, the much smaller splittings due to monomer inversion are also observed. Guided by dynamical calculations on a potential surface optimized to fit the $(NH_{3})_{2}$ $spectrum^{2}$ the $(ND_{3})_{2}$ spectrum has been assigned and fit to linear-molecule-type energy-level expressions to obtain values for the various tunneling splittings. For the states correlating to two k = 0 monomer units, the interchange tunneling spitting of 264 GHz is reduced by nearly a factor of two from the protonated value of $483 GHz^{1}$ and by - 25% from the predicted $value^{2}$ of 331 GHz. The analysis of the spectrum for the dimer composed of inequivalent monomer units (k = 0 and k = 1)indicates that there is less of an internal rotation-induced assymetry relative to interchange-induced mixing in $(ND_{3})_{2}$ than in the analogous states in $(NH_{3})_{2}$. This result supports the argument of $Olthof et al.^{2}$ for the origin of the smaller dipole moment in the $(ND_{3})_{2}$ compared to the $(NH_{3})_{2}$. The measured monomer inversion splittings are in good agreement with the predictions of $Olthof et al^{3}$. Description: $^{1}$ J.G. Loeser, C.A. Schmuttenmaer, R.C. Cohen, M.J. Elrod, D.W. Steyert, R.J. Saykally, R.E. Bumgarner, and G.A. Blake, J. Chem. Phys. 97, 4727 (1992). $^{2}$ E.H.T. Olthof, A. van der Avoird, and P.E.S. Wormer, J. Chem. Phys. 101, 8430 (1994). $^{3}$ E.H.T. Olthof, A. van der Avoird, P.E.S. Wormer, J.G. Loeser, and R.J. Saykally, J. Chem. Phys. 101, 8443 (1994). Author Institution: National Institute of Standareds and Technology, Gaithersburg MD 20899; University of California, Berkeley, CA 94720; Applied Physics Institute, Nizhnii Novgorod, Russia URI: http://hdl.handle.net/1811/29604 Other Identifiers: 1995-RD-01