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Estimation of marcus λ for p ‐phenylenediamines from the optical spectrum of a dimeric derivative
Author(s) -
Nelsen Stephen F.,
Yunta Maria J. R.
Publication year - 1994
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610070202
Subject(s) - chemistry , intermolecular force , acetonitrile , electron transfer , marcus theory , pyridazine , enthalpy , computational chemistry , electron , derivative (finance) , thermodynamics , molecule , reaction rate constant , medicinal chemistry , organic chemistry , quantum mechanics , physics , kinetics , financial economics , economics
The radical cation of N , N ′‐bis‐(4‐dimethylaminophenyl)pyridazine (3 ˙+ ) has a near‐IR band with a transition energy of 29·7 ± 0·5 kcal mol −1 (1 kcal = 4·184 kJ) in acetonitrile. This band is assigned to a Hush‐type charge‐transfer band, and the transition energy to the Marcus λ value for electron transfer. Such a large λ is inconsistent with previous estimates of λ for intermolecular electron transfer between mono‐ p ‐phenylenediamines and their radical cations. It agrees well with estimates of λ based on AM1 semi‐empirical MO calculations, which gives the enthalpy contribution to λ in at about 21 kcal mol −1 for 3· + . It is suggested that the traditional method of estimating λ in by summing bond displacements weighted by force constants produces values which are significantly too low for systems such as phenylenediamines, where pyramidalization changes are important. The results suggest that λ for tetramethyl‐ p ‐phenylenediamine self‐electron transfer (1 0/·+ ) has been significantly underestimated, and that this prototype organic electron transfer reaction is not as strongly diabatic as was previously concluded.