Campus Units

Chemistry

Document Type

Article

Publication Version

Submitted Manuscript

Publication Date

2-22-2020

Journal or Book Title

Journal of the American Chemical Society

DOI

10.1021/jacs.0c00190

Abstract

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 variable-temperature 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.

Comments

This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in Journal of the American Chemical Society, copyright © American Chemical Society after peer review. To access the final edited and published work see DOI: 10.1021/jacs.0c00190. Posted with permission.

Copyright Owner

American Chemical Society

Language

en

File Format

application/pdf

Published Version

Included in

Chemistry Commons

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