On the ESR spectra and bonding of lithium complexes of acetylene, ethylene and benzene: a matrix isolation study

Abstract

Electron spin resonance (ESR) and optical absorption spectra of 6LiC 2H 2, 7LiC 2H 2, 7LiC 2H 4, 7LiC 2D 4, 6LiC 2D 4, LiC 6H 6 and LiC 6D 6 complexes embedded in solid argon have been measured at low temperature. Lithium—acetylene and lithium—ethylene complexes exhibit hyperfine structures due to both lithium and equivalent hydrogen nuclear spins. g tensors, alkali-metal atom and proton hyperfine coupling tensors were determined with help of computer simulation of the powder spectra. It is shown that both these complexes have a 2B 2 ground state thus correlating with the Li 2P atomic state. The polyligand Li(C 2H 4) 2 and Li(C 2H 4) 3 complexes have also been observed and their ESR spectra are consistent with D 2h and D 3h symmetry structures. In contrast, LiC 6H 6 presents no lithium hyperfine structure but a hyperfine splitting of unequivalent hydrogen spins. This confirms that LiC 6H 6 has lost the six-fold symmetry of the benzene ring resulting from the complete or near complete transfer of the lithium valence electron into one of the formerly degenerate unoccupied molecular orbitals of benzene, and has C 2v symmetry.