Proton and Electron Transfer. Biradicals

Steenken, S.
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Surprisingly, all triradicals produce very abundant DMDS radical cations.

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A single-step mechanism involving electron transfer from DMDS to the triradicals is highly unlikely because the experimental adiabatic ionization energy of DMDS is almost 3 eV greater than the calculated adiabatic electron affinities of the triradicals. The unexpected reactivity can be explained based on an unprecedented two-step mechanism wherein the protonated triradical first transfers a proton to DMDS, which is then followed by hydrogen atom abstraction from the protonated sulfur atom in DMDS by the radical site in the benzene ring of the deprotonated triradical to generate the conventional DMDS radical cation and a neutral biradical.

A single-step mechanism involving electron transfer from DMDS to the triradicals is highly unlikely because the experimental adiabatic ionization energy of DMDS is almost 3 eV greater than the calculated adiabatic electron affinities of the triradicals. The unexpected reactivity can be explained based on an unprecedented two-step mechanism wherein the protonated triradical first transfers a proton to DMDS, which is then followed by hydrogen atom abstraction from the protonated sulfur atom in DMDS by the radical site in the benzene ring of the deprotonated triradical to generate the conventional DMDS radical cation and a neutral biradical.

Quantum chemical calculations as well as examination of deuterated and methylated triradicals provide support for this mechanism.

On the basis of the MS spectra, products 4 , 5 and 6 were identified as 1,7-dimethyldibenzofuran, 3,7-dimethyldibenzofuran, and 1,9-dimethyldibenzofuran, respectively. The proposed formation pathways of these products are illustrated in Figure 7. In the first step of these pathways, two types of quinonemethide radicals 5-methyl-2,4-cyclohexadieneone radical and 5-methyl-3,5-cyclohexadieneone radical are formed by proton radical donation from the hydroxyl group of m-cresol the first half of route A.

These radicals are considered to be important intermediates. Radical coupling with each other yields three kinds of dimers of 5-methyl-cyclohexadieneone. These dimers are then subjected to intra molecular coupling route C , as shown in Figure 5, to form 3,7-dimethyldibenzofuran, 1,7-dimethyldibenzofuran and 1,9-dimethyldibenzofuran.

Conference Description

Paramagnetic relaxation of liquid solutions for perpendicular fields. Owing to its potentially large energetic driving force, SET to or from singlet and triplet excited states often plays a key role in guiding the nature of photochemical processes. The proposed formation pathways of these products are illustrated in Figure 5. It is found that protonation of His in the presence of the hydroquinone-anion electron donor causes spontaneous, as opposed to photoinduced, coupled proton and electron transfer to the photoproduct. Chemistry World. Scheme Indirect approach to the preparation of lariat-type crown ethers. Aufnahmeverfahren und Beispiele.

Figure 7: Proposed pathway for the formation of products 3 - 5. The proposed formation pathway of product 7 is illustrated in Figure 9. Guaiacol, as a starting material, is subject to rearrangement of the methoxyl group to epoxide, which is initiated by proton radical donation from the methoxyl group. Then, the epoxide is opened by cleavage of the O—C aromatic bond to form the hydroxyl methyl group followed by elimination to form the o-cresol radical route D.

These reaction mechanisms are proposed by Asmadi et al. A proton radical donation at the hydroxyl group followed by addition of a methyl radical produces the biradical of 2-methylanisol. The radical undergoes intra molecular coupling to form benzofuran route E. This compound was detected as the minor product.

GERHARD LUDWIG CLOSS

Proton and Electron Transfer. Biradicals. Editors. H. Fischer. Book. 1 Citations · 1 Readers · 78 Downloads. Part of the Landolt-Börnstein - Group II Molecules. Proton and Electron Transfer. Biradicals / Protonen- und Elektronenaustauschreaktionen. Biradikale. Authors: Dohrmann, J.K., Scaiano, J.C., Steenken, S.

Therefore, this formation pathway may be an inconsequential reaction in guaiacol pyrolysis. Figure 9: Proposed pathway for the formation of products 3 and 6.

Marcus Theory of Electron Transfer

On the basis of their MS spectra, these products are assumed to have an indanone structure and products 8 , 9 and 10 were identified as 2-indanone, 1-indanone and 1-dehydroindanone, respectively. The proposed formation pathways of these products are illustrated in Figure The pyrolitic reaction is initiated by the formation of o-quinone from pyrocatechol, and CO elimination occurs to form a pentadienone biradical.

Intra molecular radical coupling of the biradical produces cyclopentadienone as an important intermediate. The cyclopentadienone is subjected to two different electron transfers, forming two biradical structures: 3-cyclopenteneone a and 2-cyclopenteneone b. Further reactions occur as two proton additions and an electron transfer occur in biradical structure a to form a new biradical structure c.

Proton-coupled electron transfer

These biradical intermediates couple with each other and produce three indanone derivatives route B. Coupling a with c and an electron transfer produces 2-indanone product 8 , route F. Coupling a with b and an electron transfer produces 1-dihydroindanone product 10, route G.

Figure Effect of pyrolysis temperature on products yields of 7 - 9. Figure Proposed pathway for the formation of products 7 - 9 from pyrocatechol.

GERHARD LUDWIG CLOSS

Product 6 was identified as benzofuran and the only polycyclic compound obtained from syringol. Peaks were identified as 1,3-dimethoxybenzene, guaiacol, o-cresol, 1,3-dimethyphenol and homovanillin, respectively. These compounds are formed by elimination of the methoxyl or hydroxyl groups and rearrangement of the methoxyl group to methyl or aldehyde groups.

Benzofuran product 6 is formed via a guaiacol intermediate in route E as shown in the pyrolysis of guaiacol section.

Dr. P.K. Das - Assistant Professor, Physical Sciences, Cameron University

Products 3 , 16 , 20 and 21 are not polycyclic compounds and are identified as phenol, 4-ethylanisol, p-cresol and 4-vinylanisol, respectively. These four polycyclic products were detected as benzofuran derivatives. Product 7 was identified as benzofuran, and products 17 , 18 and 19 were identified as5-metylbenzofuran, 5-ethylbenzofuran and 5-ethenylbenzofuran, respectively. These benzofuran derivatives are formed in the same reaction route E, as shown in Figure 9 , except for ethyl radical addition.

These radicals are induced from phenol, ethyl phenol and ethenyl phenol as well as o-quinonmetide.

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Addition of the ethyl radical tocyclohexadieneone radicals followed by intra molecular radical coupling produced the dihydrobenzofuran radicals. Elimination of the two proton radicals resulted in products 7 and route I. Figure Proposed pathway for the formation of products 16 - Six phenols with different pendant groups were used in this study for the investigation into the reaction mechanism leading to polycyclic compounds such as polycyclic aromatic hydrocarbons and heterocyclic compounds.

In this mechanism, a cyclohexadieneone radical was considered an important intermediate for the formation of these products. Cyclohexadieneone radical was also found to be an important intermediate for the dimerization reaction. Guaiacol pyrolysis yielded phenol as the major product along with a small amount of benzofuran.

In this formation mechanism, rearrangement of a methoxyl group to a methyl group and addition of a methyl radical on the phenoxy radical was a crucial route. From syringol pyrolysis, only benzofuran was produced, and from ethyl phenol, three types of benzofuran were detected containing different pendant groups such as methyl, ethyl and ethenyl group. From the results, it was determined that the ortho position of the phenol is considered an important reaction site for the formation of polycyclic compounds in pyrolysis.

The polycyclic compounds detected in this study are formed easily from pyrolysis of phenols, and those phenols are abundant in the bio-oil. Therefore, incomplete combustion of bio-oil may lead to the emission of various polycyclic compounds. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.