Reactions between Li Atoms and HX (X = F, Cl) Molecules:  Semiempirical SCF MO and Matrix Isolation ESR Studies

Examination of reaction between the Li atom and the HX (X = F, Cl) molecule by a semiempirical molecular orbital method (AM1) revealed the spontaneous formation of the Li:HCl and Li:HF complexes with the respective heats of complexation of 75 and 84 kJ/mol. The calculations yielded totally different structures for the two complexes, however. In the Li:HCl complex, the unpaired electron is in the antibonding σ orbital of the H−Cl moiety and the elongated H−Cl is sided by the Li atom bearing a substantial positive charge (+0.47). For the Li:HF complex, a linear Li−F−H structure was predicted. The unpaired electron resides in a s−p hybridized orbital of the Li atom directed away from the F atom, and the complex is formed by the dative interaction of the fluorine electrons with vacant Li orbitals. A substantial net negative charge (−0.18) is thus borne by the Li atom of the complex. Examination by ESR of argon matrixes in which Li atoms and HCl (and HF) molecules had been co-condensed indeed revealed the spontaneous formation of complexes exhibiting the ESR spectra which were in total accord with the structures and the SOMOs (singly occupied molecular orbitals) predicted above. In the case of Li/HCl/Ar system, the formation of the second complex, Li2Cl, was observed and was ascribed to the reaction between Li atoms and LiCl molecules inadvertently formed during deposition (Li + HCl → H + LiCl). An examination of the Li:LiCl system by the AM1 method revealed a spontaneous formation of the complex with the heat of complexation of 121 kJ/mol. The complex has an isosceles triangular form, and the unpaired electron renders a bent, one-electron bond between the two Li atoms.