American Chemical Society
jp054244n_si_005.pdb (3.02 kB)

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

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posted on 2005-12-15, 00:00 authored by David Small, Sergiy V. Rosokha, Jay K. Kochi, Martin Head-Gordon
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.