Knowledge Bank

The rotational spectrum of the K=0 levels of the van der Waals complex ArHCN has been obtained by molecular beam electric resonance spectroscopy in an argon expansion. For ArHCN the $J\to$ J+1, $J=0$ to 4 transitions were observed while for ArDCN the $ J\rightarrow J+1$, $J=0$ to 6 transitions were observed. The spectroscopic constants for the K = 0 levels are: [FIGURE] The spectrum does not fit well to the linear rotor Hamiltonian $ H=BJ(J+1)-D_{J}J^{2}(J+1)^{2}$. Furthermore, the derived centrifugal distortion constant $DJ$, and its change upon deuterium substition are unexpectedly large. The rotational constants are consistent with an average structure in which the hydrogen points toward the argon. The average distance between the argon and the center of mass of the HCN is 4.34\AA. The angle between the HCN axis and the a-axis as derived from both the dipole moment and quadrupole coupling is about $30\circ$. Stretching and bending frequencies of ArHCN derived from the usual treatment are each $10cm-1$ suggesting that excited levels should be populated in the beam and attempts are presently underway to observe transitions from these levels. The small bending and stretching frequencies are not understood since bond strength of ArHCN must be greater than that of $ Ar_{2} (D_{e}=100cm^{-1}, \omega_{e} = 26cm^{-1})$