Ammonia dimer: Further structural studies

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Abstract
New experimental results on the structural and dynamical properties of NH3 dimer are reported in this work. J=1–0, K=0 transitions of 14NH3–15NH3, 15NH3–14NH3, ND3 dimer, and ND3–ND2H have been measured at high resolution and 14N electric quadrupole coupling constants are reported for each of these species. The NH3 subunits comprising the dimer are inequivalent. The quadrupole coupling constant associated with the first ammonia subunit eqQ1aa, is measured in 14NH3–15NH3 [−627(8)kHz], in ND3 dimer [−531(15) kHz], and in ND3–ND2H [−991(18) kHz]. For the other subunit, eqQ2aa is reported in 15NH3–14NH3 [892(8)kHz], in ND3 dimer [745(13) kHz], and in NH3–ND2H [1013(18) kHz]. These numbers can be used to estimate the vibrationally averaged polar angles of these isotopomers of NH3 dimer. The result is (including the primary isotopomer) θ1 for 14NH3–14NH3 is 48.6°, for 14NH3–15NH3 is 48.7°, for ND3 dimer is 49.6° and for ND3–ND2H is 45.3°; while θ2 for 14NH3–14NH3 is 64.5°, for 15NH3–14NH3 is 64.3°, for ND3 dimer is 62.6°, and for ND3–ND2H is 65.8°. The remarkable invariance of these values rules out the possibility of large vibrational averaging or tunneling averaging in this system and establishes that the angles θ1=49° and θ2=65° are near equilibrium. The isotope effect in the component of the electric dipole moment along the a inertial axis μa, is shown to correlate well with the trend in polar angles given by the quadrupole coupling constants. The absence of interchange tunneling effects in the observed states of NH3 dimer implies that these states are asymmetrically excited internal rotor states of the complex. These experimental structural results are in disagreement with all previous theoretically determined structures for NH3 dimer except one. A recent electronic structure calculation which incorporates correlation through the coupled pair functional approach (while systematically varying geometry) obtains a compact, asymmetric structure for the dimer in close accord to observations.