Ab initio potential energy surface and microwave spectrum of the NH₃-N₂ van der Waals complex

We present a five-dimensional intermolecular potential energy surface (PES) of the NH₃-N₂ complex, bound state calculations, and new microwave (MW) measurements that provide information on the structure of this complex and a critical test of the potential. Ab initio calculations were carried out using the explicitly correlated coupled cluster [CCSD(T)-F12a] approach with the augmented correlation-consistent aug-cc-pVTZ basis set. The global minimum of the PES corresponds to a configuration in which the angle between the NH₃ symmetry axis and the intermolecular axis is 58.7° with the N atom of the NH₃ unit closest to the N₂ unit, which is nearly parallel to the NH₃ symmetry axis. The intermolecular distance is 7.01 a₀, and the binding energy D_e is 250.6 cm^-1. The bound rovibrational levels of the four nuclear spin isomers of the complex, which are formed when ortho/para (o/p)-NH₃ combines with (o/p)-N₂, were calculated on this intermolecular potential surface. The computed dissociation energies D₀ are 144.91 cm^-1, 146.50 cm^-1, 152.29 cm^-1, and 154.64 cm^-1 for (o)-NH₃-(o)-N₂, (o)-NH₃-(p)-N₂, (p)-NH₃-(o)-N₂, and (p)-NH₃-(p)-N₂, respectively. Guided by these calculations, the pure rotational transitions of the NH₃-N₂ van der Waals complex were observed in the frequency range of 13-27 GHz using the chirped-pulse Fourier-transform MW technique. A complicated hyperfine structure due to three quadrupole ¹⁴N nuclei was partly resolved and examined for all four nuclear spin isomers of the complex. Newly obtained data definitively established the K values (the projection of the angular momentum J on the intermolecular axis) for the lowest states of the different NH₃-N₂ nuclear spin isomers.