Knowledge Bank

The rotational spectrum of $Ar2$-HCN has been observed between 2.5 and 11.5 GHz with a pulsed nozzle. Fourier transform, Balle/Flygare microwave spectrometer. The ground-state rotational constants were determined to be 1769.366, 1743.854, and 857.600 MHz, with b-dipole transitions. The centrifugal distortion, fitted by a Watson Hamiltonian, is extremely large with values for $\tau 1,\tau 2,\tau aaaa\tau bbbb$, and $\tau cccc$ of -6.54, -1.199, -0.68, -0.360, and -0.052 MHz and for $Hj$ and $Hjk$ or 0.077 and -1.7 KHz. Even with the two 6th order terms the fit has a large ras devlation or 160 KHz. The equilibrium geometry is found to be T-shaped with $C2v$ symmetry and ostensibly planar; however, the inertial defect is $13.9\mu A2$. extremely large for a planar structure. The N hyperfine interaction depends somewhat on j and K and shows that bending vibrations of the large and highly anisotropic, averaging $\mathrm{<mrow\; data-mjx-texclass="ORD">34</mrow>}N$ in the plane of the cluster $\theta ab$ and $25\circ$ out-of-plane $\theta bc$. The Ar to HCN center of mass distance is significantly shorter than in Ar-HCN dimer, 4.16 versus A. The unusual properties of the trimer are a more extreme version of those found for the Ar-HCN dimer. As in the case of the latter, they are attributed to the shape of the potential function as calculated with a largely cassical electrical model using low-order moments and multipole $polarizabllitles.2$ The potential surface for $Ar2-HCN$, as a function of $\theta ab$ and $\theta bc$, has a broad elliptical minimum while that for $Ar2-HF$ is steep and more circular. There are similar differences in the dependence on the $Ar2c.m$. to HX o.m. distance.