Reactions between Li atoms and HX (X = F, Cl) molecules: Semiempirical SCFMO 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:MF complexes with the respective heats of c omplexation of 75 and 84 kJ/mol. The calculations yielded totally different structures for the two complexes, however. In the Li:HCl complex, the unpa ired electron is in die antibonding sigma orbital of the H-CI moiety and th e elongated H-CI is sided by the Li atom bearing a substantial positive cha rge (+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 inte raction of the fluorine electrons with vacant Li orbitals. A substantial ne t negative charge (-0.18) is thus borne by the Li atom of the complex. Exam ination by ESR of argon matrixes in which Li atoms and HCl (and HF) molecul es had been co-condensed indeed revealed the spontaneous formation of compl exes exhibiting the ESR spectra which were in total accord with the structu res and the SOMOs (singly occupied molecular orbitals) predicted above. In the case of Li/HCl/Ar system, the formation of the second complex, Li2Cl, w as observed and was ascribed to the reaction between Li atoms and LiCl mole cules inadvertently formed during deposition (Li + HCl --> H + LiCl). An ex amination of the Li:LiCl system by the AMI method revealed a spontaneous fo rmation of the complex with the heat of complexation of 121 kJ/mol. The com plex has an isosceles triangular form, and the unpaired electron renders a bent, one-electron bond between the two Li atoms.