FAR-INFRARED SPECTRA OF THE HF-$Cl_{2}$ DIMER-OBSERVATION OF THE PI BEND

[1] F.A. Baiocchi, T.A. Dixon and W. Klemperer, Journal of Chemical Physics, 74, 1632 (1982).Author Institution: Division of Chemistry and Chemical Engineering, California Institute of Technology; Division of Engineering and Applied Science, California Institute of Technology; Division of Geological and Planetary Sciences, California Institute of TechnologyThe microwave spectra of the HF $Cl_{2}$ dimer have shown the molecule to be antihydrogen-bonded, with the three halogen atoms nearly colinear [1]. Two perpendicular bands corresponding to the $HF-^{35}Cl^{35}Cl$ and $HF-^{35}Cl^{37}Cl$ isotopomers were observed with origins of $\sim 33 cm^{-1}$ 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