Charge Shift Bonding Concept in Radical π-Dimers

We show that pancake bonding in radical pi-dimers display features of charge shift (CS) bonding. While the CS bonding concept has been developed to interpret the unusual aspects of a-bonds around centers with a large number of lone pairs, such as F-2 and HOOH, we find a similar role played by the nonbonding or slightly bonding pi-electron pairs in pi-stacking radical dimers. Arguments and computational evidence indicate that the CS bonding concept developed by Shaik and Hiberty et al. captures essential features of the intermolecular bonding in radical pi-dimers in which the overlap of the two radical centered singly occupied molecular orbitals (SOMOs) play a crucial role. By using the tetracyanoethylene anion dimer, [TCNE](2)(2-), as a model, we show that compared to CAS (2,2) calculations, significant binding contributions are recovered in the calculations simply by including selected intrapair excitations of the SOMO-SOMO bonding orbitals and the nonbonding pi-orbitals. This observation is the basis for the analogy of chemical bonding between pancake bonded radical pi-dimers and other charge shift bonded molecules, such as F-2. By extending the CS bonding concept to a new class of molecules, we find a novel application of the lone pair bond weakening effect (LPBWE) in which the doubly occupied pi-orbitals play the role of lone pairs.