Journal of Physical Chemistry p. 3541 - 3547 (1982)
Update date:2022-08-11
Topics:
Holroyd, R. A.
Although p-benzoquinone has a high electron affinity (1.9 eV), its reaction with excess electrons exhibits an unusual solvent and temperature dependence.The reaction is fast and has a positive activation energy in n-pentane but is much slower and has a negative activation energy in Si(CH3)4, neopentane, and 2,2,4,4-tetramethylpentane.The results can be explained in terms of an equilibrium with a short-lived excited state of the anion: e- + benzoquinine <*> benzoquinone(-)*.The attachment rate is fast in all solvents.The reverse, autodetachment from the excited anion, is characterized by an activation energy, which depends on the value of V0, the energy level of the electron in the liquid.For Si(CH3)4 for which V0 is the lowest (-0.55 eV), the activation energy is 5.7 kcal/mol.As V0 increases, the activation energy increases and is 9.4 kcal/mol in 2,2,4,4-tetramethylpentane for which V0=-0.35 eV.When V0 is close to zero, as in n-pentane, the activation energy is so large that detachment from the excited anion state is too slow to compete with deactivation and a fast diffusion-controlled forward rate is observed.The spectrum of benzoquinone anion as observed by the laser photodetachment technique is reported, and the threshold for photodetachment is found to be 2.32 eV in Si(CH3)4 and 2.58 eV in 2,2,4-trimethylpentane.The quantum yield of photodetachment reaches a maximum value at 480 nm in Si(CH3)4.
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