Knowledge Bank

The microwave spectra of the HF $Cl2$ dimer have shown the molecule to be antihydrogen-bonded, with the three halogen atoms nearly colinear [1]. Two perpendicular bands corresponding to the $HF-35Cl35Cl$ and $HF-35Cl37Cl$ isotopomers were observed with origins of $\sim 33cm-1$ using the Caltech tunable far-infrared laser spectrometer and a slit-jet expansion. Ab initio calculations are used to assign the transitions to the van der Waals $\pi$ bend of the two monomers. Upper state constants were fit to the spectra in which $\ell$-type doubling was observed: $\nu 7=33.04770(2)cm-1,BU=1603.98(1)$ MHz, $DU=2.58(4)kHz,HU=0.50(5)Hz,q=6.64(1)$ MHz for the (35-35) species. The observed nuclear quadrupole coupling constants, rotational constants, and ab initio calculations are used to better determine the equilibrium structure of the dimer. HF undergoes a wide-amplitude bending motion and the potential energy surface contains multiple, non-equivalent minima. The potential surface about the barrier to linearity is modeled as a quartic-quadratic double-minimum well and determined using the fit value of $\nu 7$ and structural parameters.