Hybrid Hartee–Fock/density functional (HF/DF) calculations of adiabatic electron affinities (EAad's) of neutral hydroquinone radicals of 1,4-benzoquinone (1) and 1,4-benzoquinone imine (2)

Density Functional Theory Study of Semiquinone Radical Anions of Polychlorinated Biphenyls in the Syn- and Anti-like Conformation

“…This is not unexpected because the addition of one electron to a benzoquinone moiety formally converts one double bond into a single bond (i.e., CO + e – → C–O •– ). Similar changes in the C–O bond lengths have been reported for 1,4-benzoquinone − and other 1,4-benzoquinone derivatives using a variety of computational approaches. , For example, Boesch and Wheeler predicted an increase of the C–O bond lengths of 1,4-benzoquinone and 2,3,5,6-tetrachloro-1,4-benzoquinone ( p -chloranil) by 0.041 and 0.033 Å, respectively, whereas a more pronounced lengthening of one C–O bond (0.047 Å) has been predicted for ubiquinones . An increase of 0.041 Å in the C–O bond length has also been reported for the reduction of 1,2-benzoquinone to the corresponding semiquinone radical anion at the UB3LYP/6-311G** level of theory .…”

General and Theoretical Aspects of Phenols

“…These fundamental vibrations are sketched in Figure 43 along with the experimental frequencies. High level calculation 358,359,498,510,514,518 agreed on the identity and absolute values of most of these modes. There is now a large consensus that the band observed near 1505 cm −1 , characterized by the strongest intensity in the resonance Raman spectra, should be assigned to a primary CO stretching, whereas the band centred at 1398 cm −1 , which was assigned earlier to the CO stretch 31 , corresponds rather to the CC stretch.…”

“…A certain similarity exists between the phenolate and enolate anions regarding the C−O distances. Quantum chemical calculations 115,353,358,360 of vibrational frequencies for free PhO − in the ground state did show some discrepancies with experimental data 365,366 . While IR frequencies determined using DFT methods compare reasonably with the FTIR results in the case of the modes ν 4 and ν 5 , the frequency of the C−O stretching mode is overestimated in all calculations.…”

“…Density functional theory, in particular when using the hybrid B3LYP functionals, could reproduce the EAs of aromatic radicals with an absolute error of 0.03 eV with respect to the experimental estimates 358,359 . As substituents on the ring, the halogen atoms tend to increase this quantity by up to 0.4 eV, in the decreasing order: meta > ortho > para position, relative to the value for the parent radical.…”

“…Indeed, values ranging from 1.22Å to 1.38Å were reported for the CO distance from a variety of wave functions. While both CASSCF (9,8) 510 and UMP2 515 treatments, in conjunction with various basis sets, resulted in a short distance of 1.22-1.23Å, density functional methods yielded a consistently longer distance of 1.25-1.28Å 358,359 (cf. Figure 41).…”