- Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers
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Some free radicals are stable enough to be isolated, but most are either unstable transient species or exist as metastable species in equilibrium with a dimeric form, usually a spin-paired sigma dimer or a pi dimer (pimer). To gain insight into the different modes of dimerization, we synthesized and evaluated a library of 15 aryl dicyanomethyl radicals in order to probe what structural and molecular parameters lead to σ- versus π-dimerization. We evaluated the divergent dimerization behavior by measuring the strength of each radical association by variableerature electron paramagnetic resonance spectroscopy, determining the mode of dimerization (σ- or π-dimer) by UV-vis spectroscopy and X-ray crystallography, and performing computational analysis. We evaluated three different hypotheses to explain the difference in the dimerization behavior: (1) that the dimerization behavior is dictated by radical spin densities; (2) that it is dictated by radical polarizability; (3) that it is dictated by London dispersion stabilization of the pimer. However, no single parameter model in itself was predictive. Two-parameter models incorporating either the computed degree of spin delocalization or the radical polarizability as well as computed estimates for the attractive London dispersion forces in the π-dimers lead to improved forecasts of σ- vs π-dimerization mode, and suggest that a balance of spin delocalization of the isolated radical as well as attractive forces between the stacked radicals, govern the formation of diradical pimers.
- Ellern, Arkady,Peterson, Joshua P.,Winter, Arthur H.
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p. 5304 - 5313
(2020/04/08)
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- Effect of Substituents on the Bond Strength of Air-Stable Dicyanomethyl Radical Thermochromes
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A series of substituted aryl dicyanomethyl radicals were synthesized, and the bonding thermodynamic parameters for self-dimerization were determined from van't Hoff plots obtained from variable-temperature electron paramagnetic resonance and ultraviolet-v
- Peterson, Joshua P.,Geraskina, Margarita R.,Zhang, Rui,Winter, Arthur H.
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p. 6497 - 6501
(2017/06/23)
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- The Benzene Ring as a Dipolarophile: Reactions of Monosubstituted Benzenes with Tetracyanoethylene Oxide
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The reaction of tetracyanoethylene oxide on one hand and toluene, chlorobenzene, fluorobenzene, anisole, and nitrobenzene, on the other, has been studied.Toluene and the halogenobenzenes yield mixtures of monoadducts, anisole yield p-methoxyphenylmalononitrile and its dimer and nitrobenzene does not react.These results are discussed with the help of a qualitative orbital model. 1H-NMR analysis of some monoadducts has been carried out.Keywords: Benzene derivatives; Tetracyanoethylene oxide; 1,3-Dipolar cycloaddition.
- Hoz, A. de la,Pardo, C.,Elguero, J.,Jimeno, M. L.
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