Rotational Spectrum of the Mixed van der Waals Triad Pyridine-Ar-Ne

We report the Fourier transform microwave spectra of four isotopologues of the hetero triad pyridine-Ar-Ne, formed by the combination of two isotopes of the nitrogen atom (N-14 and N-15) in pyridine with two isotopes of the rare gas (RG) atoms (Ne-20 and Ne-22), by using pulsed jet spectroscopy. We detected the conformer denoted [1,1], with the Ne and Ar atoms located one on each side of the ring plane. The [2,0] species, with the two RG atoms on the same side of the ring, was not observed. Ab initio MP2/6-311 ++G** calculations suggest the rotational spectrum of this species to be complicated by the presence of several almost equivalent minima, separated by very low barriers. Four structural van der Waals parameters, R-Ar R-Ne, theta(Ar) and theta(Ne), which localize the RG atoms with respect to pyridine, are determined. The N-14 nuclear quadrupole coupling constants are obtained for the isotopologues containing this nucleus.