Reactions between Li atoms and RCN (R = H or CH₃): Semiempirical SCF MO and matrix isolation ESR studies

Reaction passage between the Li atom and the HCN molecule and that between the Li atom and the CH3CN molecule were examined by a semiempirical molecular orbital method (MNDO). For both systems two distinct passages were revealed; one leads to the formation of an end-on complex (I), HCN···Li, and the other to the formation of a side-on complex (II), Li+(HCN)-. The end-on complex is a weak, van der Waals type, while the side-on complex is a strong, charge-transferred type as indicated. An ESR study of argon matrices in which Li atoms and HCN (or CH3CN) had been co-condensed revealed the presence of the two complexes in accord with the theoretical prediction. Irradiation of the matrix with near-IR light (λ = 850 ± 50 nm) resulted in conversion of complex I to complex II. The g tensors and the hyperfine coupling tensors of the linear and side-on complexes observed in both the Li/HCN and Li/CH3CN systems were determined. In complex I the unpaired electron is in an sp hybridized orbital of Li projected away from the ligand. In complex II, the unpaired electron is essentially in the in-plane antibonding π orbital of the C-N moiety.