Is there a lower limit to the CC bonding distances in neutral radical pi-dimers? The case of phenalenyl derivatives.

Two-electron multicenter (2e/mc) bonding of phenalenyl (PHYL) pi-dimers was found to be significantly affected by the electron density on the bonding active sites. The computational analysis shows that, upon appropriate beta-substitutions, the newly introduced dimers have the shortest and strongest covalent bonding interactions seen in any neutral pi-dimer. The unusual strengthening of the bonding was attributed to the reduced lone pair bond weakening effect, LPBWE, upon substitutions with electron-withdrawing groups.