Radicals diradicals

Spin Hamiltonian Valence Bond Theory and its Applications to Organic Radicals, Diradicals, and Polyradicals... 

Understanding the chemistry revolving around reactive intermediates, including radicals, diradicals, carbenes, and nitrenes, is necessary because they play a major... 

Electron paramagnetic resonance (EPR) spectroscopy (also called electron spin resonance (ESR) spectroscopy), is used to study paramagnetic species with one or more unpaired electrons, e.g. free radicals, diradicals, metal complexes containing paramagnetic metal centres, defects in semiconductors and irradiation effects in solids. While diamagnetic materials are EPR silent, paramagnetic species always exhibit an EPR spectrum. This consists of one or more lines, depending on the interactions between the unpaired electron (which acts as a probe ) and the molecular framework in which it is located. Analysis of the shape of the EPR spectrum (the number and positions of EPR lines, their intensities and line widths) provides information... 

The basic NRMS method and the auxiliary techniques outlined have enabled the discovery and characterization of a large variety of reactive neutrals that had eluded experimental studies due to their unusual structures and reactivities. The species studied so far are summarized in Table 1 and include radicals, diradicals, carbenes, nitrenes, cumulenes, ylides, hypervalent species, and weak intermolecular complexes. 

Keywords DFT B3LYP Transition state Openshell Diketopiper-azine Dioxirane Hydroxylation PES Radical Diradical... 

Upon exposure to uv light, ground-state benzophenone is excited to the ttiplet state (a diradical) which abstracts an alpha H atom from the alcohol, resulting in the formation of two separate initiating radicals. With amine H atom donors, an electron transfer may precede the H-transfer, as in ttiplet exciplex formation between benzophenone and amine (eq. 43) ... 

Only a small fraction of reactant is iavolved ia this step. When naphthenes are iavolved, diradicals are produced. Eor aromatics with side chains, H radicals are produced. 

The electrolysis of adamantylideneadamantane solutions affords the radical cation, which can add molecular (triplet) oxygen to give the peroxide radical anion, which can react with adamantylideneadamantane to give the 1,4-diradical and another molecule of adamantylideneadamantane radical cation. The latter reacts with oxygen, to continue the chain of the reaction, while the former cyclizes to the corresponding 1,2-dioxetane (Scheme 18) (81JA2098). 

The transition state involves six partially delocalized electrons being transformed from one 1,5-diene system to another. The transition state could range in character from a 1,4-diradical to two nearly independent allyl radicals, depending on whether bond making or bond breaking is more advanced. The general framework for understanding the substituent effects is that the reactions are concerted with a relatively late transition state with well-developed C(l)—C(6) bonds. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

Although these reactions are thus closely related to the acyl-alkyl diradical disproportionation to ketenes, the stereospecificity of (55) -> (56) and (57) -> (58) shows that these hydroxyketones cannot proceed through free radicals capable of rotating about single bonds prior to the intramolecular hydrogen... 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

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