Atomic properties and the reactivity of carbenes

Abstract

The properties of the atoms in a number of substituted carbenes and silylene provide an understanding of the relative stabilities of their lowest lying singlet and triplet states and of their differing chemical reactivities. The theory of atoms in molecules defines all of an atom's properties, including its energy. In this research one finds that the methylenic carbon or silicon atom is energetically most stable in the triplet state of each system, while the ligands are most stable in the corresponding singlet state. It is also found that the average electron populations of the carbon and silicon atoms are largest in the triplet states. In systems where the carbon or silicon atom bears a net positive charge, as found in CHNH2, CF2, and SiH2, the excess in the transfer of charge to the more electronegative ligands in the singlet states stabilizes the ligands more than it destabilizes the central atom. These systems have singlet ground states. The relative susceptibility of singlet and triplet carbenes to electrophilic and nucleophilic attack is determined by the properties of the Laplacian of the charge distribution. This quantity assimilates the model of localized electrons in terms of local concentrations of electronic charge. It also determines regions from which electronic charge is locally depleted. These regions are found to coincide with those where the lowest-lying vacant orbital is concentrated.