The rotational spectrum of Ar2ndash;HCN has been observed between 2.5 and 11.5 GHz with the pulsed nozzle, Fourier transform, Balle/Flygare Mark II microwave spectrometer. Eighteen transitions were found and their14N quadrupole hyperfine structure analyzed. The line centers were fitted with the Watson Hamiltonian giving ground state rotational constants of 1769.366, 1743.854, and 857.600 MHz. The effective geometry of the cluster is found to be Thyphen;shaped with C2vsymmetry and the H end of the HCN closest to the Ar2. However, the rms deviation of the fit is poor (300 kHz), the centrifugal distortion and inertial defect are huge, the Ar to HCN c.m. distance is nearly 0.2 Aring; shorter than in the Arndash;HCN dimer, and the average angular displacement of the HCN from the C2axis is both large (39deg;) and highly anisotropic (10deg;). In contrast, the Ar2subunit exhibits an inhyphen;plane, average angular displacement of only 6deg;. These anomalies led us to calculate potential surfaces for Ar2ndash;HCN and Ar2ndash;HF using the molecular mechanics for clusters scheme. A comparison of the surfaces and the rotational properties of the two species prompts us to propose that in Ar2ndash;HCN the HCN axis rotates about the C2axis maintaining an angle of sim;40deg; between them for them=0 internal rotation state. Such internal rotation accounts at least qualitatively for the otherwise anomalous rotational behavior of the Ar2ndash;HCN cluster.
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