Microwave spectra and nuclear quadrupole structure of the NH₃–N₂ van der Waals complex and its deuterated isotopologues

The microwave spectrum of the NH₃–N₂ van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Two pure rotational R(0) transitions (J = 1 − 0) with different hyperfine structure patterns were detected. One transition belongs to the (ortho)-NH₃–(ortho)-N₂ nuclear spin isomer in the ground K = 0 state reported earlier [G. T. Fraser et al., J. Chem. Phys. 84, 2472 (1986)], while another one is assigned to the (para)-NH₃–(para)-N₂ spin isomer in the K = 0 state not reported before (K is the projection of the total angular momentum J on the intermolecular axis). The complicated hyperfine structure arising from three quadrupole ¹⁴N nuclei of NH₃–N₂ was resolved for both transitions, and the quadrupole coupling constants associated with the NH₃ and N₂ subunits were precisely determined for the first time. These constants provided the dynamical information about the angular orientation of ammonia and nitrogen indicating that the average angle between the C₃ axis of NH₃ and the N₂ axis is about 66°. The average van der Waals bond lengths are slightly different for (ortho)-NH₃–(ortho)-N₂ and (para)-NH₃–(para)-N₂ and amount to 3.678 Å and 3.732 Å, respectively. Similar results for the deuterated isotopologues, ND₃–N₂, NHD₂–N₂, and NH₂D–N₂, and their nuclear spin isomers were also obtained thus confirming and extending the analysis for the parent NH₃–N₂ complex.