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

Abstract

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 ...

Bimolecular Hydrogen Abstraction from Phenols by Aromatic Ketone Triplets†

“…The triplet energies are derived from phosphorescence emission spectra recorded in polar glasses (methyltetrahydrofuran [MTHF]), 4:1 ethanol:methanol, or 5:5:2 ether:isopentane:ethanol (EPA) at 77K, while reported reduction potentials have been corrected when necessary to the SCE reference electrode in MeCN solution. (Reversible) half‐wave reduction potentials for benzophenones 1, 3, 10 and 11 , which have evidently not been reported, were estimated from a plot of – E (A ‐ /A) vs πσ for the other compounds in the series (30).…”

“…Most ketones were obtained from Sigma‐Aldrich (Milwaukee, WI) or Lancaster Synthesis (Windham, NH), and were each recrystallized from an appropriate solvent or vacuum‐distilled. 4‐Methoxy‐4′‐methylbenzophenone ( 3 ) (30), 4‐thiomethoxybenzophenone ( 9 ) (31), and 4‐dimethylaminoacetophenone ( 15 ) (32) were prepared and purified according to literature procedures, whereas 4‐methoxy‐α,α,α‐trifluoroacetophenone ( 34 ) was a generous gift from Professor P. J. Wagner. 4MP (Caledon) and phenol (Sigma‐Aldrich) were vacuum‐distilled, 4‐cyanophenol (Sigma‐Aldrich) was recrystallized from water and dried in air and 4‐methoxyphenol (Sigma‐Aldrich) was recrystallized from benzene.…”

Simultaneous quantitative analysis of letrozole, its carbinol metabolite, and carbinol glucuronide in human plasma by LC-MS/MS

“…Reference compounds were synthesized by modifications to methods previously described [17][18][19][20][21]. Detailed information on synthesis is provided as Electronic Supplementary Material (see Electronic Supplementary Material and Fig.…”

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent–Solute Interactions

“…[a] E T values were obtained from the 0-0 band of the phosphorescence spectra at 77 K in ethanol-methanol (4:1), [39,40] E red : half-wave reduction potentials in ACN vs a saturated calomel electrode (SCE). [37,39] [b] The small value of the k q is attributed to the change of character of the lowest triplet state.…”

“…[37,39] [b] The small value of the k q is attributed to the change of character of the lowest triplet state.…”