Ammonia dimer: A surprising structure

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Abstract
High resolution rotational spectra for (14NH3)2, (14ND3)2, 15NH3–14NH3, and 14NH3–15NH3 have been obtained using the molecular beam electric resonance technique. For 14NH3 dimer the spectra are simple and consist only of the J=0–1 and J=1–2 transitions with K=0 for two vibrational states (hereafter designated α and β) of the complex. The spectroscopic constants for (14NH3)2 are: 1/2 (B+C)α=5110.412(2), 1/2 (B+C)β=5110.564(2), DJ=0.0529(5), eqQ1aa =−0.639(13), eqQ2aa =0.904(8) (MHz); μαa =0.7497(10), μ βa =0.7376(9) (D). For (ND3)2 the vibrational states are unsplit 1/2 (B+C)=4190.300(50) MHz and μa=0.567(4) D. These spectroscopic constants are inconsistent with the theoretically predicted structure for NH3 dimer. In the predicted structure one NH3 unit hydrogen bonds to the other with a nearly linear N–H‐ –N arrangement so that the C3 axis of the basic NH3 is collinear with the hydrogen bond. If θi is the angle between the C3 axis of the ith NH3 unit and the a‐inertial axis of the complex, θ1∼0° and θ2∼68° for the theoretically predicted structure. In contrast, for 14NH3 dimer we measure the average values of θ1 and θ2 as 48.6(1)° and 115.5(1)°, respectively, using the quadrupole hyperfine structure. Furthermore, the component of the dipole moment along the a‐inertial axis, μa, is found to be much smaller (0.75 D) than expected for the theoretical structure (∼2 D). The distance between the centers of mass of the NH3 subunits, RCM, is also determined and is found to be 3.3374(1) Å. No evidence exists for nonrigidity in the ground vibrational state of this complex, i.e., no high frequency tunneling splittings were observed in either the microwave or the infrared spectrum of the complex. The relationship between these structural results and the concept of hydrogen bonding in NH3 systems is discussed. The apparent inability of measured values of heats of formation for NH3 dimer to correctly predict dissociation energies of the complex is also commented upon. The dissociation energy of NH3 dimer has been previously determined as less than 2.8 kcal/mol.