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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2005-12, Vol.109 (49), p.11261-11267
• Main Authors: Small, David, Rosokha, Sergiy V, Kochi, Jay K, Head-Gordon, Martin
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp054244n

Electronic-structure calculations for the self-association of phenalenyl radical (P •) predict the formation of dimeric species (σ-P 2) 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-P 2 +• 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.