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

摘要

Electronic-structure calculations for the self-association of phenalenyl radical (P) predict the formation of dimeric species (sigma-P_2) in which both moieties are connected by a sigma-bond with r_(P-P) approx1.59 A and bond dissociation enthalpy of DELTA H_D approx 16 kcal mol~(-1).Such an unusually weak sigma-bond is related to the loss of aromatic stabilization energy of approx 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-P_2~(+centre dot) 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 DELTA H_D= 14 kcal mol~(-1) and DELTA S_D - 52 e.u.can be assigned to the formation of the colorless cr-dimer.Similar results are obtained for all phenalenyl derivatives (provided their substitution patterns allow a-bond formation) to confirm the energetic preference of sigma-dimerization over pi-dimerization.