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

The previously reported rotational spectrum of Ar–HCN [J. Chem. Phys. 81, 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 Ar. 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 radi...