Ab initio potential energy surface and microwave spectrum of the NH3-N-2 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 De 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.