Sigma- and Pi-Electron Delocalization:  Focus on Substituent Effects

Krygowski, Tadeusz M.; Stępień, Beata T.

doi:10.1021/cr030081s

Cited by 324 publications

(314 citation statements)

References 287 publications

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“…The observed rates of O−H bond scission gave a good linear correlation with Hammett σ + values for these substituted phenols (48). Plots of log k 2 vs. σ + afforded ρ values of −0.67 and −3.3 for APO-I and APO-II, respectively (Fig.…”

Section: Kinetic Characterization Of Apo-ii Toward C−h and O−h Substrmentioning

confidence: 70%

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Wang

Ullrich

Hofrichter

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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A kinetic and spectroscopic characterization of the ferryl intermediate (APO-II) from APO, the heme-thiolate peroxygenase from Agrocybe aegerita, is described. APO-II was generated by reaction of the ferric enzyme with metachloroperoxybenzoic acid in the presence of nitroxyl radicals and detected with the use of rapidmixing stopped-flow UV-visible (UV-vis) spectroscopy. The nitroxyl radicals served as selective reductants of APO-I, reacting only slowly with APO-II. APO-II displayed a split Soret UV-vis spectrum (370 nm and 428 nm) characteristic of thiolate ligation. Rapid-mixing, pHjump spectrophotometry revealed a basic pK a of 10.0 for the Fe IV −O−H of APO-II, indicating that APO-II is protonated under typical turnover conditions. Kinetic characterization showed that APO-II is unusually reactive toward a panel of benzylic C−H and phenolic substrates, with second-order rate constants for C−H and O−H bond scission in the range of 10-10 7 M −1 ·s −1 . Our results demonstrate the important role of the axial cysteine ligand in increasing the proton affinity of the ferryl oxygen of APO intermediates, thus providing additional driving force for C−H and O−H bond scission.

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Section: Kinetic Characterization Of Apo-ii Toward C−h and O−h Substrmentioning

confidence: 70%

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Wang

Ullrich

Hofrichter

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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“…[4][5][6][7][8] In so doing, the physical-organic community in particular has been able to develop tremendous physical and mechanistic insights into the cause-and-effect relationship between charge polarization and chemical reactivity. [9][10][11][12][13] A comparable level of understanding of the effect of spin on chemical reactivity suffers in part from the lack of a similarly intuitive counterpart to electronegativity for spin polarization. Experimental techniques for measuring the distribution of unpaired spin density, most notably EPR and ENDOR spectroscopies, have contributed greatly toward describing spin polarization in a wide range of chemical systems.…”

Section: Introductionmentioning

confidence: 99%

Differential Polarization of Spin and Charge Density in Substituted Phenoxy Radicals

Fehir

McCusker

2009

J. Phys. Chem. A

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We have carried out a theoretical study of a series of para-substituted phenoxy radicals in an effort to understand the factors influencing spin and charge density distribution in open-shell systems. The calculations reveal that the distribution of spin and charge are not correlated: cases were found for which spin and charge move together, whereas for other substituents the two quantities exhibit spatially distinct intramolecular polarizations. Charge density variations across the series were found to correlate well with both the Hammett (σ p ) and Hammett-Brown (σ p + ) constants for each substituent, indicating that inductive and/or resonance effects are primarily responsible for the polarization of charge within the molecule. In contrast, the distribution of unpaired spin density could not be adequately accounted for using any of the typical Hammett-type spin delocalization constants cited in the literature. We uncovered an empirical correlation between the polarization of spin density and the R-HOMO-R-LUMO gap of the substituted phenoxy radicals: this led to the development of a simple model based on a three-electron, two-orbital bonding scheme in which mixing between the HOMO of the substituent and the SOMO of the phenoxy moiety serves to define the nature and extent of unpaired spin polarization throughout the molecule. This analysis yielded a correlation coefficient of r > 0.97 for the 15 substituents examined in the study; spin polarization effects in compounds that exhibited the greatest deviation from this correlation could also be readily explained within the context of the model. The underlying reason for the ability to differentially polarize spin and charge likely stems from the fact that net unpaired spin density is completely carried by the unpaired electron (and thus tracks the spatial characteristics of the SOMO), whereas charge density reflects the behavior of all of the electrons of the system. These results could have implications in the field of molecular magnetism, suggesting a means for synthetically tuning the magnitude of intramolecular exchange interactions, as well as providing guidance for the design of catalysts for radical-radical coupling reactions.

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“…The Hammett equation (and its extended forms) represents one of the most widely used means for the study and interpretation of organic reactions and their mechanisms. Hammett constants, s p (for substituent in the para position) and s m (for substituent in the meta position), are able to successfully predict the equilibrium and rate constants for a variety of reactions (Hansch et al, 1991;Krygowski and Stępień, 2005). Fig.…”

Section: Computational Detailsmentioning

confidence: 98%

Theoretical study of the energetics of carboxylic O–H bond cleavage in the para- and meta-substituted benzoic acid derivatives

Škorňa

Vagánek

Poliak

et al. 2015

Acta Chimica Slovaca

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Gas phase bond dissociation enthalpies (BDE) of para- and meta-substituted benzoic acids and proton affinities (PA) of their carboxylate anions were investigated using the B3LYP/6-311++G** method for 15 substituents with various electron-donating and electron-withdrawing effects. The employed computational method provided BDE and PA values in fair agreement with experimental data. The substituent effect on BDEs and PAs was analyzed in terms of Hammett constants. Found dependences exhibit satisfactory linearity and enable quick estimation of BDEs and PAs from the Hammett constants.

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Sigma- and Pi-Electron Delocalization: Focus on Substituent Effects

Cited by 324 publications

References 287 publications

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Differential Polarization of Spin and Charge Density in Substituted Phenoxy Radicals

Theoretical study of the energetics of carboxylic O–H bond cleavage in the para- and meta-substituted benzoic acid derivatives

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