Substituent and charge distribution effects on the redox potentials of radicals. Thermodynamics for homolytic versus heterolytic cleavage in the 1-naphthylmethyl system

1992 ◽  
Vol 70 (1) ◽  
pp. 121-127 ◽  
Author(s):  
Paul H. Milne ◽  
Danial D. M. Wayner ◽  
Dayal P. DeCosta ◽  
James A. Pincock
Keyword(s):  
Charge Density ◽  
Substituent Effect ◽  
Substituent Effects ◽  
Radical Cations ◽  
Relative Magnitude ◽  
Redox Potentials ◽  
Heterolytic Cleavage ◽  
Reduction Potentials ◽  
Oxidation Potentials ◽  
Interesting Conclusion

The electrochemical oxidation and reduction potentials of a number of substituted 1-methylnaphthalenes (1a-l) and 1-naphthylmethyl radicals (2a-l•) as well as 2-methylnaphthalene (3) and the 2-naphthylmethyl radical (4•) have been measured by cyclic voltammetry and photomodulation voltammetry. The oxidation potentials correlate with σ+ (ρ+ = −7.1 and −8.4 for 1 and 2• respectively) while the reduction potentials correlate with σ− (ρ− = 10.1 and 13.0 for 1 and 2• respectively). The relative magnitude of the ρ values can be rationalized when the charge density distribution in these systems is considered. This leads to the interesting conclusion that even though a full charge is placed in the π-system of 1 when it is oxidized or reduced, the fraction of the charge that accumulates at C4 is actually less than in 2+ or 2− where only 50–70% of the charge is delocalized into the ring. A correlation between ρ for the redox reactions of 1, 2•, benzyl, diphenylmethyl, and cumyl and the calculated (AM1) charge density at C4 is established, implying that the sensitivity of the corresponding ions to substituent effects increases as the fraction of charge at that site increases. The redox data have been used in thermochemical cycles in order to estimate the substituent effect on the homolytic, mesolytic, and heterolytic cleavage reactions of 1 and its corresponding radical ions. The implication of these results on the C—C cleavage versus deprotonation of radical cations and on the photochemical homolysis versus heterolysis of naphthylmethyl halides and acetates is discussed. Keywords: electrochemistry, homolysis, heterolysis, naphthylmethyl, substituent effect.

Correlation of the photoelectron and electronic spectra of thiochromones and thiochromanones with their electrochemical data

1979 ◽  
Vol 57 (6) ◽  
pp. 638-644 ◽  
Author(s):  
Rafik O. Loutfy ◽  
Ian W. J. Still ◽  
Michael Thompson ◽  
Toong S. Leong
Keyword(s):  
Gas Phase ◽  
Substituent Effects ◽  
Spectroscopic Properties ◽  
Oxidation Potential ◽  
Molecular Orbitals ◽  
Absolute Difference ◽  
Linear Free Energy Relationship ◽  
Redox Potentials ◽  
Reduction Potentials ◽  
Oxidation Potentials

The gas phase ionization potentials, electrochemical redox potentials and spectroscopic properties of a series of thiochroman-4-one and thiochromone derivatives have been studied. A dramatic shift in the energies of the lowest vacant and highest occupied molecular orbitals of the parent thiochromanone as a function of the addition of a double bond and/or oxidation of the sulphur atom was observed. This shift in energy of the molecular orbitals was reflected in their spectroscopic characteristics. The lowest singlet (and triplet) state of compounds 1–3 in solution is π,π* in nature, while that of compounds 4–6 is n,π*. These results are best explained in terms of substituent effects on the energetics of the acetophenone moiety. The change in the nature of the lowest excited state from π,π* (1–3) to n,π* (4–6) should result in quite different types of photochemistry for the two series.A linear free-energy relationship between the singlet and triplet energies and the absolute difference between the oxidation and reduction potentials of the two series was found. These correlations have been utilized to estimate the half-wave oxidation potentials of compounds 3–6. A correlation was found to exist between the gas phase ionization potential and the solution electrochemical oxidation potential.

Electrostatic effects on ionization equilibria: An MNDO study of proton and hydrogen transfer reactions of 4-fluorobutylamine

1990 ◽  
Vol 55 (1) ◽  
pp. 1-9 ◽  
Author(s):  
Zdeněk Friedl ◽  
Stanislav Böhm
Keyword(s):  
Hydrogen Transfer ◽  
Substituent Effects ◽  
Radical Cations ◽  
Transfer Reactions ◽  
Protonated Forms ◽  
Minor Part ◽  
Bond Strengths ◽  
Ionization Equilibria ◽  
A Minor ◽  
Gas Phase Basicities

The relative enthalpies of proton transfer δ ΔH0and homolytic bond strengths δDH0(B-H+) were calculated by the MNDO method for the sp and ap conformers of 4-flurobutylamine. The data obtained, along with the experimental gas phase basicities, are compared with the values predicted by the electrostatic theory. It is shown that the substituent polar effects FD on the basicities of amines are predominantly due to interactions in their protonated forms (X-B-H+) and/or radical-cations (X-B+.), those in the neutral species (X-B) playing a minor part. A contribution, which is considerably more significant in the sp conformer than in the ap conformer, arises probably also from substituent effects on the homolytic bond strength DH0(B-H+.

Kinetic Study of Hydrolysis of Benzoates. Part XXVI. Variation of the Substituent Effect with Solvent in Alkaline Hydrolysis of Substituted Alkyl Benzoates

2006 ◽  
Vol 71 (11-12) ◽  
pp. 1557-1570 ◽  
Author(s):  
Vilve Nummert ◽  
Mare Piirsalu ◽  
Ilmar A. Koppel
Keyword(s):  
Kinetic Study ◽  
Alkaline Hydrolysis ◽  
Rate Constants ◽  
Substituent Effect ◽  
Substituent Effects ◽  
Alkyl Substituent ◽  
Solvent Parameters ◽  
Substituent Constants ◽  
Hydrolysis Of ◽  
Main Factor

The second-order rate constants k2 (dm3 mol-1 s-1) for the alkaline hydrolysis of substituted alkyl benzoates C6H5CO2R have been measured spectrophotometrically in aqueous 0.5 M Bu4NBr at 50 and 25 °C (R = CH3, CH2Cl, CH2CN, CH2C≡CH, CH2C6H5, CH2CH2Cl, CH2CH2OCH3, CH2CH3) and in aqueous 5.3 M NaClO4 at 25 °C (R = CH3, CH2Cl, CH2CN, CH2C≡CH). The dependence of the alkyl substituent effects on different solvent parameters was studied using the following equations:      ∆ log k = c0 + c1σI + c2EsB + c3∆E + c4∆Y + c5∆P + c6∆EσI + c7∆YσI + c8∆PσI     ∆ log k = c0 + c1σ* + c2EsB + c3∆E + c4∆Y + c5∆P + c6∆Eσ* + c7∆Yσ* + c8∆Pσ* .  ∆ log k = log kR - log kCH3. σI and σ* are the Taft inductive and polar substituent constants. E, Y and P are the solvent electrophilicity, polarity and polarizability parameters, respectively. In the data treatment ∆E = ES - EH2O , ∆Y = YS - YH2O , ∆P = PS - PH2O were used. The solvent electrophilicity, E, was found to be the main factor responsible for changes in alkyl substituent effects with medium. When σI constants were used, variation of the polar term of alkyl substituents with the solvent electrophilicity E was found to be similar to that observed earlier for meta and para substituents, but twice less when σ* constants were used. The steric term for alkyl substituents was approximately independent of the solvent parameters.

scholarly journals Substituent Effects on EI-MS Fragmentation Patterns of 5-Allyloxy-1-aryl-tetrazoles and 4-Allyl-1-aryl-tetrazole-5-ones; Correlation with UV-Induced Fragmentation Channels

Molecules ◽  
2021 ◽  
Vol 26 (11) ◽  
pp. 3282
Author(s):  
Alina Secrieru ◽  
Rabah Oumeddour ◽  
Maria L. S. Cristiano
Keyword(s):  
Theoretical Calculations ◽  
Substituent Effects ◽  
Radical Cations ◽  
Electronic Effects ◽  
Diverse Range ◽  
The Matrix ◽  
Electron Impact Mass ◽  
Fragmentation Patterns ◽  
Tetrazole Derivatives ◽  
Impact Mass

1,4- and 1,5-disubstituted tetrazoles possess enriched structures and versatile chemistry, representing a challenge for chemists. In the present work, we unravel the fragmentation patterns of a chemically diverse range of 5-allyloxy-1-aryl-tetrazoles and 4-allyl-1-aryl-tetrazolole-5-ones when subjected to electron impact mass spectrometry (EI-MS) and investigate the correlation with the UV-induced fragmentation channels of the matrix-isolated tetrazole derivatives. Our results indicate that the fragmentation pathways of the selected tetrazoles in EI-MS are highly influenced by the electronic effects induced by substitution. Multiple pathways can be envisaged to explain the mechanisms of fragmentation, frequently awarding common final species, namely arylisocyanate, arylazide, arylnitrene, isocyanic acid and hydrogen azide radical cations, as well as allyl/aryl cations. The identified fragments are consistent with those found in previous investigations concerning the photochemical stability of the same class of molecules. This parallelism showcases a similarity in the behaviour of tetrazoles under EI-MS and UV-irradiation in the inert environment of cryogenic matrices of noble gases, providing efficient tools for reactivity predictions, whether for analytical ends or more in-depth studies. Theoretical calculations provide complementary information to articulate predictions of resulting products.

scholarly journals Spectral Probe for Electron Transfer and Addition Reactions of Azide Radicals with Substituted Quinoxalin-2-Ones in Aqueous Solutions

2021 ◽  
Vol 22 (2) ◽  
pp. 633
Author(s):  
Konrad Skotnicki ◽  
Slawomir Ostrowski ◽  
Jan Cz. Dobrowolski ◽  
Julio R. De la Fuente ◽  
Alvaro Cañete ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Aqueous Solutions ◽  
Absorption Spectra ◽  
Density Functional ◽  
Reduction Potential ◽  
Biological Significance ◽  
Radical Cations ◽  
Basis Sets ◽  
Double Bonds ◽  
Reduction Potentials

The azide radical (N3●) is one of the most important one-electron oxidants used extensively in radiation chemistry studies involving molecules of biological significance. Generally, it was assumed that N3● reacts in aqueous solutions only by electron transfer. However, there were several reports indicating the possibility of N3● addition in aqueous solutions to organic compounds containing double bonds. The main purpose of this study was to find an experimental approach that allows a clear assignment of the nature of obtained products either to its one-electron oxidation or its addition products. Radiolysis of water provides a convenient source of one-electron oxidizing radicals characterized by a very broad range of reduction potentials. Two inorganic radicals (SO4●−, CO3●−) and Tl2+ ions with the reduction potentials higher, and one radical (SCN)2●− with the reduction potential slightly lower than the reduction potential of N3● were selected as dominant electron-acceptors. Transient absorption spectra formed in their reactions with a series of quinoxalin-2-one derivatives were confronted with absorption spectra formed from reactions of N3● with the same series of compounds. Cases, in which the absorption spectra formed in reactions involving N3● differ from the absorption spectra formed in the reactions involving other one-electron oxidants, strongly indicate that N3● is involved in the other reaction channel such as addition to double bonds. Moreover, it was shown that high-rate constants of reactions of N3● with quinoxalin-2-ones do not ultimately prove that they are electron transfer reactions. The optimized structures of the radical cations (7-R-3-MeQ)●+, radicals (7-R-3-MeQ)● and N3● adducts at the C2 carbon atom in pyrazine moiety and their absorption spectra are reasonably well reproduced by density functional theory quantum mechanics calculations employing the ωB97XD functional combined with the Dunning’s aug-cc-pVTZ correlation-consistent polarized basis sets augmented with diffuse functions.

scholarly journals Unveiling Extreme Photoreduction Potentials of Donor-Acceptor Cyanoarenes to Access Aryl Radicals from Aryl Chlorides

2021 ◽  
Author(s):  
Jinhui Xu ◽  
Jilei Cao ◽  
Xiangyang Wu ◽  
Han Wang ◽  
Xiaona Yang ◽  
...  
Keyword(s):  
Limited Range ◽  
Redox Potentials ◽  
Aryl Chlorides ◽  
Aryl Radicals ◽  
Reduction Potentials ◽  
Anion Form ◽  
Donor Acceptor ◽  
High Reduction ◽  
Very High ◽  
Excited Radical

Since the seminal work of Zhang in 2016, donor-acceptor cyanoarene-based fluorophores, such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), have been widely applied in photoredox catalysis, and used as excellent metal-free alternatives to noble metal Ir- and Ru-based photocatalysts. However, all the reported photoredox reactions involving this chromophore family are based on harnessing the energy from a single visible light photon, with a limited range of redox potentials from -1.92 V to +1.79 V. Here, we document the unprecedented discovery that this family of fluorophores can undergo consecutive photoinduced electron transfer (ConPET) to achieve very high reduction potentials. One of the newly synthesized catalysts, 2,4,5-tri(9H-carbazol-9-yl)-6-(ethyl(phenyl)amino)isophthalonitrile (3CzEPAIPN), possesses a long-lived (12.95 ns) excited radical anion form, 3CzEPAIPN<sup>•</sup><sup>−</sup>*, which can be used to activate reductively recalcitrant aryl chlorides (E<sub>red </sub>≈ -1.9 to -2.9 V) under mild conditions. The resultant aryl radicals can be engaged in synthetically valuable aromatic C-B, C-P, and C-C bond formation to furnish arylboronates, arylphosphonium salts, arylphosphonates, and spirocyclic cyclohexadienes, respectively.

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