Ab initio potential energy surface and microwave spectrum of the NH-N van der Waals complex.

We present a five-dimensional intermolecular potential energy surface (PES) of the NH-N 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 NH symmetry axis and the intermolecular axis is 58.7° with the N atom of the NH unit closest to the N unit, which is nearly parallel to the NH symmetry axis. The intermolecular distance is 7.01 a, and the binding energy D is 250.6 cm. The bound rovibrational levels of the four nuclear spin isomers of the complex, which are formed when ortho/para (o/p)-NH combines with (o/p)-N, were calculated on this intermolecular potential surface. The computed dissociation energies D are 144.91 cm, 146.50 cm, 152.29 cm, and 154.64 cm for (o)-NH-(o)-N, (o)-NH-(p)-N, (p)-NH-(o)-N, and (p)-NH-(p)-N, respectively. Guided by these calculations, the pure rotational transitions of the NH-N 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 N 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 NH-N nuclear spin isomers.