Substituent Effects on the Properties of Stable Aromatic Free Radicals. The Criterion for Non-Hammett Behavior<sup>1</sup>

Walter, Robert I.

doi:10.1021/ja00961a014

Cited by 138 publications

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“…The pronounced deviation from Hammett behaviour observed for 4,4'-dicyanobenzophenone (6g) may be an indication of the varying contribution from structure 2 to the resonance hybrid. It has been suggested that when X and Y are strong electronwithdrawing groups, the contribution from 2 should increase since delocalization of an electron pair will occur at the expense of unpaired electron delocalization (12). Furthermore, Exner and co-workers (4b) observed acceptable correlations with o-in their investigation of substituent effects on benzophenone half-wave reduction potentials.…”

Section: Discussionmentioning

confidence: 99%

“…It is known from esr studies that the spin density in the benzophenone radical anion is largely on the carbonyl carbon (9); further, the spin distribution in the radical anion is very similar to that in the diphenylmethyl r a d i~a l .~ The reduction potentials of substituted benzophenones should then correlate with Hammett substituent constants if, and only if, the substituent effect on radical stability is comparatively small, since the negatively charged oxygen is in a position analogous to that in the benzoate anion. Valence bond structures of type 2 (in which the unpaired electron is localized on the oxygen and the negative charge is delocalized into the aromatic n-system) may be more important contributors to the hybrid structure when X and Y are strong electron-withdrawing groups (12), and their stability should of course be subject to an even larger polar substituent effect.…”

Section: Introductionmentioning

confidence: 99%

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Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

Leigh¹,

Arnold²,

Humphreys³

et al. 1980

Can. J. Chem.

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CHEONG WONG. Can. J. Chem. 58, 2537 (1980). The half-wave reduction potentials, measured by cyclic voltammetry, and n,x' triplet energies, measured by phosphorescence spectroscopy, were determined for a series of eighteen symmetrically and unsymmetrically substituted benzophenones. Attempts are made to correlate the results with Hammett substituent constants. Better correlations are observed when the data are correlated with a two-parameter function consisting of Hammett substituent constants and a set of substituent parameters describing variations in free radical stability. Significant deviations from "normal" behaviour are observed for benzophenones substituted by both electron-donating and electron-withdrawing substituents. These deviations are attributed to merostabilization of the radical-like species, and an empirical approach designed to evaluate the importance of this effect is developed. Ab initio calculations of molecular orbital energies in meta-and para-substituted benzaldehydes are used to evaluate the substituent effects on E,12red and ETn"* in terms of the effect on the energies of the n-and n:*-orbitals.WILLIAM J. LEIGH, DONALD R. ARNOLD, ROBERT W. R. HUMPHREYS et P o CHEONG WONG. Can. J. Chem. 58, 2537 (1980). On a determine les potentiels de reduction de demi-vague mesurts par voltametrie cyclique et les Cnergies de l'etat triplet, n,n' mesurees par spectroscopic de phosphorescence, pour une sOrie de 18 benzophenones substituees symttriquement et asymktriquement. On a essaye d'bablir une correlation entre les resultats et les constantes de substituant de Hammett. On a observe les meilleures correlations quand les donnees sont relikes a une fonction adeux paramktres se composant des constantes de substituant de Hammett et a un ensemble de paramktres de substituant decrivant les variations de stabilitk du radical libre. On a observe des deviations importantes par rapport au comportement "normal" dans le cas des benzophenones substitutes a la fois avec des substituants donneurs et des substituants accepteurs d'electrons. Ces deviations sont attribuees a une merostabilisation de I'espltce radicalaire et on a developpk une approche empirique pour tvaluer I'importance de cet effet. On a utilise des calculs a b initio d'energies d'orbitales moltculaires dans des benzaldehydes substitues en me'ta et enpara pour &valuer les effects de substituants sur E112rrdetE~nprr* en fonction de l'effet sur les energies des orbitales n et x'.[Traduit par le journal] Introduction stituents were, in fact, influencing the energy of the One of the early successes of molecular orbital 7~'-0rbita1(3). theory was the recognition that the half-wave reIt was also found that there is a correlation beduction of polynuclear aromatic hy-tween the n,nf triplet energy of a series of substidrocarbons correlates with the energy calculated tuted aromatic carbony1 compounds and their for the lowest unoccupied molecular orbital half-wave reduction potentials (3). This was attri-(LUMO; that is, the n*-orbital) of the aromatic buted to ...

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

Leigh¹,

Arnold²,

Humphreys³

et al. 1980

Can. J. Chem.

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“…We found that eq. (1) correlates parameters of the spectra for class S radicals, namely, the frequencies of long-wave maxima of absorption spectra for (p-XC6H4)3N-+ [48] (p* = 0.078 f 0.011, r* = 0.083 f 0.006,s = 0.028, r = 0.990); the coupling constants AN of electron spin resonance spectra for p-XC6H4T(JOBut [51] and p-XC6H&But [52]; and the coupling con- As one can expect, substituted benzyl radicals belong to class S. Therefore all substituents reduce the spin density of the CH2 group (owing to the delocalization of unpaired electrons to substituents) and correspondingly decrease the BDE of XC6H4CH2-H. The delocalization of unpaired electrons to substituents is confirmed by a molecular orbital study of the spin density for substituted benzyl radicals [58].…”

Section: Dependence Of Substituent Effects On the Mechanism Of Free-rmentioning

confidence: 99%

Use of two‐parameter correlations for studying the radical reactivity of aromatic compounds

Afanas’ev¹

1981

Int J of Chemical Kinetics

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Two-parameter equation correlations are reported for radical reactions of aromatic compounds. In these correlations polar and resonance substituent constants identical with the substituent constants of aliphatic compounds were used. The equations correlate the rate constants for H abstraction reactions and for the addition of a variety of free radicals to the ortho-, meta-, and para -substituted aromatic compounds. Besides, they correlate parameters of the spectra for substituted aromatic radicals. The correlations show that the effects of para substituents on the reactions studied are nearly entirely resonance effects, whereas for the meta-and ortho-substituted compounds polar (inductive) effects become essential. Application of the two-parameter correlations permits also to determine the structure of transition states (u or r-complex) in free-radical reactions.

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“…Many reactions were correlated by the Hammett equation, such as electrophilic (4, 31, 32, 33), nucleophilic (4, 34) and radical (35,36,37). Spectroscopic properties, such as infrared and ultraviolet absorption frequencies and intensities (38)• nuclear magnetic resonance shifts of various nuclei, e.g., *H, 19 F, 13 C (39), and mass spectroscopic ionization potentials, fragmentations and distributions (40) were also correlated.…”

Section: The Electronic Substituent Constantmentioning

confidence: 99%

Parameters and Methods in Quantitative Structure-Activity Relationships

Osman

Weinstein

Green

1979

ACS Symposium Series

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In studies of quantitative structure activity relationships (QSAR), the relative potencies of a series of drugs are subjected to analysis with the hope that biological potency will be described by a mathematical equation. QSAR is an actuarial or statistical method in which only objective data are used with no intrusion of models or mechanistic hypotheses. The equation that is obtained not only accounts for the relative potencies of the compounds, but from it are deduced predictions of the potencies of untested compounds; if the equation is valid, the predictions are ineluctable. The method thus has the capacity of yielding new (structurally related) drugs with desired potency, perhaps drugs with enhanced selectivity or fewer side effects.Further, in the method that is most widely used, the equation contains a parameter(s) that reflects a property related to the structural characteristics of the molecules. Therefore, the equation has often been assumed to provide insight into mechanisms that determine the action of drugs. In fact, the parameters that account for the biological potency are usually not truly empirical, observed entities, but rather abstractions. These parameters are frequently reified, burdened with a concrete meaning they were not designed to bear, and in this process the true molecular mechanisms may become obfuscated. Further, to infer a mechanism from a correlation is to assume that the correlation implies a causal relationship between the dependent and independent variables, when in fact the two may be conjoint but not causally related. Causality can be inferred only if a testable mechanistic hypothesis is offered that relates the correlation parameters to the observed biological activity.The attempt to correlate biological activity with chemical structure in quantitative terms assumes that a functional dependence exists between the observed biological response and certain physicochemical properties of molecules. Without a mechanistic model one obtains it by an empirical correlation. A rational way to define such empirical relationships is within the extrathermodynamic approach (1). Although extrathermodynamic relationships 0-8412-0521-3/79/47-112-021$13.55/0

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Substituent Effects on the Properties of Stable Aromatic Free Radicals. The Criterion for Non-Hammett Behavior¹

Cited by 138 publications

References 0 publications

Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

Use of two‐parameter correlations for studying the radical reactivity of aromatic compounds

Parameters and Methods in Quantitative Structure-Activity Relationships

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Substituent Effects on the Properties of Stable Aromatic Free Radicals. The Criterion for Non-Hammett Behavior1

Cited by 138 publications

References 0 publications

Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

Merostabilization in radical ions, triplets, and biradicals. 4. Substituent effects on the half-wave reduction potentials and n,π* triplet energies of aromatic ketones

Use of two‐parameter correlations for studying the radical reactivity of aromatic compounds

Parameters and Methods in Quantitative Structure-Activity Relationships

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Substituent Effects on the Properties of Stable Aromatic Free Radicals. The Criterion for Non-Hammett Behavior¹