ROTATIONAL SPECTRUM AND POTENTIAL SURFACE FOR AR2-HCN - A T-SHAPED CLUSTER WITH INTERNAL-ROTATION

The rotational spectrum of Ar2-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 their 14N 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 T-shaped with C2v symmetry 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 Å shorter than in the Ar-HCN dimer, and the average angular displacement of the HCN from the C2 axis is both large (39°) and highly anisotropic (10°). In contrast, the Ar2 subunit exhibits an in-plane, average angular displacement of only 6°. These anomalies led us to calculate potential surfaces for Ar2-HCN and Ar2-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 Ar2-HCN the HCN axis rotates about the C2 axis maintaining an angle of ∼40° between them for the m = 0 internal rotation state. Such internal rotation accounts at least qualitatively for the otherwise anomalous rotational behavior of the Ar2-HCN cluster. © 1990 American Institute of Physics.