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-N-2 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 degrees with the N atom of the NH3 unit closest to the N-2 unit, which is nearly parallel to the NH3 symmetry axis. The intermolecular distance is 7.01 a(0), 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)-N-2, were calculated on this intermolecular potential surface. The computed dissociation energies D-0 are 144.91 cm(-1), 146.50 cm(-1), 152.29 cm(-1), and 154.64 cm(-1) for (o)-NH3-(o)-N-2, (o)-NH3-(p)-N-2, (p)-NH3-(o)-N-2, and (p)-NH3-(p)-N-2, respectively. Guided by these calculations, the pure rotational transitions of the NH3-N-2 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-14 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-N-2 nuclear spin isomers. Published under license by AIP Publishing.