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Quantitative Determination of Electronic Effects in Free Radicals through Open-Shell Hammett Substituent Constants

Journal Article · · Journal of the American Chemical Society
DOI:https://doi.org/10.1021/jacs.5c04058· OSTI ID:2570127
 [1];  [1];  [1];  [2];  [2];  [2];  [1]
  1. Princeton Univ., NJ (United States)
  2. Brookhaven National Laboratory (BNL), Upton, NY (United States)
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Hammett substituent constants (σ), which quantify the electronic effects of functional groups, are widely used for predicting the properties of organic compounds and investigating reaction mechanisms. While these values have been obtained for a wide range of closed-shell substituents, measurements of analogous values for open-shell substituents are rare due to challenges associated with their short lifetimes. Here, in this report, we developed a combined experimental and computational approach for quantifying the electronic properties of open-shell substituents based on changes in nitrile vibrational frequencies (ν(C≡N)). By coupling pulse radiolysis and time-resolved infrared spectroscopy (PR-TRIR), we measured ν(C≡N) IR bands of 30 para- and meta-substituted benzonitriles bearing C-, N-, and S-centered radicals. A linear scaling relationship was obtained between these experimental values and values obtained from DFT calculations. Using these computed values, two different Hammett constants, σm, and σp+, were determined for a series of C-, N-, O-, S-, Si-, and B-centered radicals. The differences between σm and σp+ values enable the separate evaluation of inductive and resonance effects in these open-shell substituents. The results suggest that there are three classes of radicals: one is electron withdrawing (σm, σp+ > 0), one is electron donating (σm, σp+ < 0), and one is inductively withdrawing but resonance donating (σm > 0, σp+ < 0). Our study represents a general approach to the analysis of the electronic properties of open-shell species and has potential applications in a wide range of molecular processes involving free radical intermediates.
Research Organization:
Brookhaven National Laboratory (BNL), Upton, NY (United States); Princeton Univ., NJ (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
SC0012704; SC0019370
OSTI ID:
2570127
Journal Information:
Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 25 Vol. 147; ISSN 0002-7863; ISSN 1520-5126
Publisher:
American Chemical Society (ACS)Copyright Statement
Country of Publication:
United States
Language:
English

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Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
Anions
Nitrogen compounds
Quantum mechanics
Resonance structures
Substituents