Classical and non-classical silicon radical cations: HnSiX˙+ species (X = N, O, F, P, S and Cl)

Abstract

A theoretical analysis of the classical and non-classical silicon radical cations, H_nSiX^˙+ with X = N, O, F, P, S and Cl has been carried out. Heats of formation of the ions considered have been determined using ab initio molecular orbital (MO) calculations at the UMP4/6-31 + G(2df, p) level with UMP2/6-31G(d, p) optimized geometries plus vibrational and thermal corrections. No classical halogen radical cations (H₃Si–X^˙+ with X = F and Cl) could be found. In other cases, non-classical ions (H₂Si–XH_n–2^˙+) are consistently more stable than their classical counterparts (H₃Si–XH_n–3^˙+). The difference in stability between both isomers depends on the nature of X: it is large with N and O, but rather small with P and S. Interconversion between both isomeric forms has also been investigated. When X is a second-row atom, the energy of the transition structure for the 1,2-hydrogen shift is close to that of the fragments H_{n– 1}SiH⁺+ H. When X is a third-row atom, the barrier height increases, but remains smaller than the dissociation energy. The potential-energy surface of the H₅SiN^˙+ ions is reported in detail. Overall it has shown that silicon-containing non-classical radical cations do not have a formal separation of both charge and radical centres as is the case in carbon analogues. Therefore they cannot properly be described as ‘distonic radical cations’ under the currently accepted definition.