The microwave spectrum of the K=0 states of Ar–NH3

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Abstract
The microwave spectrum of Ar–NH3 has been obtained using molecular beam electric resonance spectroscopy and pulsed nozzle Fourier transform microwave spectroscopy. The spectrum is complicated by nonrigidity and most of the transitions are not yet assigned. A ΔJ=1, K=0 progression is assigned, however, and from it the following spectroscopic constants are obtained for Ar–14NH3: (B+C)/2=2876.849(2) MHz, DJ =0.0887(2) MHz, eqQaa =0.350(8) MHz, and μa =0.2803(3) D. For Ar–15NH3 we obtain (B+C)/2 =2768.701(1) MHz and DJ =0.0822(1) MHz. The distance between the Ar atom and the 14NH3 center of mass RCM is calculated in the free internal rotor limit and obtained as 3.8358 Å. In the pseudodiatomic approximation, the weak bond stretching force constant is 0.0084 mdyn/Å which corresponds to a weak bond stretching frequency of 35 cm−1. The NH3 orientation in the complex is discussed primarily on the basis of the measured dipole moment projection and the quadrupole coupling constant. It is concluded that the Ar–NH3 intermolecular potential is nearly isotropic and that the NH3 subunit undergoes practically free internal rotation in each of its angular degrees of freedom. Spectroscopic evidence is presented which indicates that the NH3 subunit also inverts within the complex. These conclusions concerning the internal dynamics in the Ar–NH3 complex support the model initially proposed in our previous study of the microwave and infrared spectra of this species.