Hydrogen bonding: The structure of HF–HCl

Abstract

The structure of the hydrogen bonded complex formed between HF and HCl has been determined by molecular beam electric resonance spectroscopy. The molecule HF–HC1 is a slightly asymmetric prolate top. The spectroscopic constants determined from K=0 spectra of several isotopic species are 〈eqQ^D_a〉 for HF D³⁵Cl is 146(15) kHz. The equilibrium atomic arrangement is HF–HCl with the internal proton colinear with the two heavy atoms spaced 2.12 Å from the fluorine atom. The exterior proton is off axis by 50°. The bonding of this complex is discussed with respect to other gas phase complexes and hydrogen halide crystal structures. It appears that a model which involves electron donation from the highest occupied molecular orbital (HOMO), of one submolecule to the lowest unoccupied molecular orbital (LUMO) of the other gives a useful qualitative description of complex formation between closed shell species.