Rotational spectra and structures of the Ar2–H/DF trimers

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Abstract
The feasibility of studying weakly bound, neutral van der Waals clusters through their microwave rotational spectra has been shown by identifying and characterizing Ar2–H/DF. Detection of the trimeric complexes was accomplished with the Flygare Mark II Fourier transform spectrometer, modified to better utilize its inherent sensitivity. Twenty‐six b‐dipole transitions (all ee↔oo) were observed in the 3–18 GHz region for Ar2–HF, and 19 for Ar2–DF, and their hyperfine structure analyzed. Fitting of the line centers with Kirchhoff’s NBS program gave values (all in MHz) for Watson’s determinable parameters of 3576.508(1), 1739.139(1), and 1161.054(1) for the rotational constants A‘, B‘, and C‘ in Ar2–HF, and of 3506.791(1), 1744.056(1), and 1155.636(1) in Ar2–DF. The centrifugal distortion constants τ1, τ2, τaaaa, τbbbb, and τcccc were found to be −0.2039(3), −0.0522(1), −0.5353(5), −0.1159(1), and −0.02021(1) in Ar2–HF, and −0.2116(7), −0.0528(2), −0.4941(7), −0.1123(1), and −0.0191(1) in Ar2–DF. Three small sixth order terms were necessary to fit the data within experimental error. The trimer is found to be a planar T‐shaped, highly asymmetric top (κ=−0.521) with the equilibrium position of the H/DF on the C2v figure axis (the b axis), the H/D end pointing toward the two argon atoms. The structure can be viewed as a composite of three dimers, the Ar2 dimer and two Ar–H/DF’s. In the trimer the Ar–Ar and the two Ar–H/DF distances are a bit longer (0.04 and 0.01 Å) than in the dimers, suggesting the presence of second‐order effects. This picture is supported by an approximate, pseudotriatomic analysis of the force field, based on the four independent planar centrifugal distortion constants. The cluster is quite ‘‘floppy.’’ The hyperfine structure shows that torsional oscillations of the H/DF are comparable with those in the Ar–H/DF dimers, but anisotropic, being 3° larger in‐plane than out‐of‐plane. The in‐plane torsional oscillation of the Ar2 is considered.