Characterizing the Dimerizations of Phenalenyl Radicals by ab Initio Calculations and Spectroscopy:  σ-Bond Formation versus Resonance π-Stabilization

Electronic-structure calculations for the self-association of phenalenyl radical (P^•) predict the formation of dimeric species (σ-P₂) in which both moieties are connected by a σ-bond with r_P-P ∼ 1.59 Å and bond dissociation enthalpy of ΔH_D ∼ 16 kcal mol^-1. Such an unusually weak σ-bond is related to the loss of aromatic stabilization energy of ∼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 σ-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 σ-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 σ-dimerization process from interference by π-dimerization. As such, the thermodynamic parameters (determined from the temperature dependence of the ESR signals) with ΔH_D = 14 kcal mol^-1 and ΔS_D = 52 e.u. can be assigned to the formation of the colorless σ-dimer. Similar results are obtained for all phenalenyl derivatives (provided their substitution patterns allow σ-bond formation) to confirm the energetic preference of σ-dimerization over π-dimerization.