J dependence of χa(14N) for the Ar–HCN dimer: Coupling of stretching and bending in the potential function

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Abstract
The previously reported rotational spectrum of Ar–HCN [J. Chem. Phys. 8 1, 4922 (1984)] has shown the weakly bound dimer to be highly nonrigid. Superficially linear, the dimer has several anomalies, including large centrifugal distortion and an unexpectedly large bending amplitude of the HCN. We here describe high‐resolution rotational spectra which identify another anomaly. The 14N hyperfine interaction constant of the dimer increases linearly with J(J+1) for Ar–HCN, 36Ar–HCN, and Ar–DCN, indicating a decrease in the average HCN bending amplitude (θ). For Ar–HCN this is from 30.97° for J=0 to 30.17° for J=5. At the same time, there is an increase in the average Ar to HCN c.m. separation R from 4.3433 to 4.3496 A˚. The cause of this behavior and of the other anomalies is found to be the shape of the potential function as calculated with a largely classical electrical model employing low‐order moments and multipole polarizabilities. The calculated potential surface exhibits strong coupling between radial and angular motions, with smaller angles favored at larger R. There is an axial hump in the potential curve generated by rotating the HCN through the linear structure, and the height of the hump is sensitive to R. In contrast, a hump is not found in the equilibrium region of the potential surface for Ar–HF. The difference between the two surfaces arises from the combination of their similar electrical interactions with different hard‐wall effects. Basically, the differences in the shapes of HF (nearly spherical) and HCN (cigar shaped) give interaction surfaces that cause the properties of Ar–HCN to contrast with those of Ar–HF. An important part of the constrast is the fact that the hydrogen halides are unusual in being more spherical than most other highly polar molecules.