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| - Estimation of Electron Transfer Parameters from AM1Calculations
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| - The utility of AM1 calculations for estimation of the electron-transfer parameters λ‘v (the enthalpypart of the Marcus internal reorganization energy) and Hab (the electronic coupling between thecharge-bearing units) is considered for some charge-localized intervalence bis(hydrazine) radicalcations, for which these parameters have been experimentally determined from optical measurements. The Koopmans estimate of λ‘v that employs the orbital separation for the neutral compoundat the radical cation geometry is far from that calculated from the enthalpies of the species involved(eq 1) and is not correct. The eq 1 λ‘v enthalpies estimated by AM1 are reasonably good for compoundswith only alkyl substituents but are overestimated by 33−59% for aryl-substituted hydrazines.The Koopmans estimate of Hab as half the orbital separation for the neutral species at the transitionstate geometry requires adjustment for the twist angles to those of the relaxed ground state toproduce useful Hab values. Symmetry breaking occurs for the electron-transfer transition states ofthe compounds with saturated bridges, and the Koopmans estimate predicts Hab values that areslightly less than half as large as the optical measurements.
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