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 rP-P ∼ 1.59 Å and bond
dissociation enthalpy of ΔHD ∼ 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 ΔHD =
14 kcal mol-1 and ΔSD = 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.