Fourier transform microwave rotational spectra of the Ne2–N2O and Ar2–N2O van der Waals trimers

Abstract

A pulsed molecular beam cavity Fourier transform microwave spectrometer was used to measure pure rotational spectra of nine isotopomers of Ne2–N2O, and of three isotopomers of the Ar2–N2O van der Waals trimer. For Ne2–N2O, these are 20Ne20Ne–14N14N16O, 20Ne22Ne–14N14N16O, 22Ne22Ne–14N14N16O, 20Ne20Ne–15N14N16O, 20Ne22Ne–15N14N16O, 22Ne22Ne–15N14N16O, 20Ne20Ne–14N15N16O, 20Ne22Ne–14N15N16O, and 22Ne22Ne–14N15N16O. Those for Ar2–N2O are 40Ar40Ar–14N14N16O, 40Ar40Ar–15N14N16O, and 40Ar40Ar–14N15N16O. The spectra were measured in the frequency range between 3 and 18 GHz. Both a- and c-type transitions were measured for all Ne2–N2O isotopomers. In the case of the mixed, 20Ne22Ne containing, isotopomers a small b-dipole moment occurs and two b-type transitions were measured. In the spectra of Ar2–N2O only b- and c-type transitions were measured. Rotational and centrifugal distortion constants were determined for all the isotopomers of each complex. The spectral analyses show that Ne2–N2O is a highly asymmetric prolate rotor (κ=−0.158 for 20Ne20Ne–14N14N16O) while Ar2–N2O is a highly asymmetric oblate rotor (κ=0.285 for 40Ar40Ar–14N14N16O). Both trimers were found to have distorted tetrahedral structures with the rare gases tilted towards the O atom of the N2O subunit. Nuclear quadrupole hyperfine structures due to both terminal and central 14N nuclei were observed and analyzed to give the nuclear quadrupole coupling constants, χaa(1), χbb(1) and χaa(2), χbb(2). The resulting spectroscopic constants were utilized to derive ground state effective structures, ground state average structures, and partial substitution structures. Harmonic force field analyses were performed for each complex using the obtained quartic centrifugal distortion constants. The results of the spectroscopic analyses are discussed in the light of possible three-body nonadditive interactions.