Characterizing the dimerizations of phenalenyl radicals by ab initio calculations and spectroscopy: sigma-bond formation versus resonance pi-stabilization.
Electronic-structure calculations for the self-association of phenalenyl radical (P*) predict the formation of dimeric species (sigma-P2) in which both moieties are connected by a sigma-bond with rP-P approximately 1.59 A and bond dissociation enthalpy of DeltaH(D) approximately 16 kcal mol(-1). Such an unusually weak sigma-bond is related to the loss of aromatic stabilization energy of approximately 34 kcal mol(-1) per phenalenyl moiety, largely owing to rehybridization. Ab initio calculations also reveal that the corresponding (one-electron) bond between phenalenyl radical and its closed-shell cation in sigma-P2+* is unstable relative to dissociation. Time-dependent DFT computations indicate the absence of any (strongly allowed) electronic transition in the visible region of the absorption spectrum of phenalenyl sigma-dimer. Such theoretical predictions are supported by experimental (ESR and UV-NIR) spectroscopic studies, in which the availability of a series of sterically hindered phenalenyl radicals allows definitive separations of the sigma-dimerization process from interference by pi-dimerization. As such, the thermodynamic parameters (determined from the temperature dependence of the ESR signals) with DeltaH(D) = 14 kcal mol(-1) and DeltaS(D) = 52 e.u. can be assigned to the formation of the colorless sigma-dimer. Similar results are obtained for all phenalenyl derivatives (provided their substitution patterns allow sigma-bond formation) to confirm the energetic preference of sigma-dimerization over pi-dimerization.