Bis radical

For a more in-depth interpretation of the inertness of dioxygen, the fact that 02 is a triplet state bi-radical, i.e. it has two unpaired electrons in the 2jig orbitals, needs to be considered. It follows that the oxidation of singlet state substrates by the triplet 02 to form singlet products is spin-forbidden and, as a consequence, relatively slow. 

A similar spiro-fused starting material was prepared to study the thermolysis of a 1,3-dioxane analog. As found for the dithia compound (cf. Section 8.11.6.3.1), a carbene-derived dimer was formed as the major detectible product (20%) <2002TL1927>. Other products, such as tricycle 185, have been identified in subsequent studies <2004CJC1769>. However, phenyl substitution at G-4 provided completely different thermolysis products, probably via formation of an open-chain bis-radical. Thus, 3-phenyl-7-butyrolactone and, after CO2 extrusion, phenylcyclo-propane are the major reaction products (Scheme 58) <2002CJC1187>. 

Scheme 2 Bond homolysis in bismuth aryloxides leading to Bi radicals...

Theoretical studies have indicated that m-bcnzync is monocyclic with a C(l)-C(3) distance of 2.0 A whereas in tetrafluoro-w-benzyne the increased eclipsing strain between fluorine atoms stabilizes the bicyclo[3.1.0]hexatriene form with a C(l)-C(3) distance of 1.75 A.56 Computational studies coupled with gas-phase experimental studies show that appropriate substituents can be used to tune the reactivity of 1,3-arynes. Thus the presence of NH+ at C(5) makes (13) mildly carbocationic whereas the addition of OH at C(4) in (14) gives a highly reactive (bi)radical.57... 

Degenerate chain branching may occur between various radicals produced in the autoxidation sequence, and involves bi-radical termination reactions. 

The so-called acyloin condensation consists of the reduction of esters—and the reduction of diesters in particular—with sodium in xylene. The reaction mechanism of this condensation is shown in rows 2-4 of Figure 14.51. Only the first of these intermediates, radical anion C, occurs as an intermediate in the Bouveault-Blanc reduction as well. In xylene, of course, the radical anion C cannot be protonated. As a consequence, it persists until the second ester also has taken up an electron while forming the bis(radical anion) F. The two radical centers of F combine in the next step to give the sodium glycolate G. Compound G, the dianion of a bis(hemiacetal), is converted into the 1,2-diketone J by elimination of two equivalents of sodium alkoxide. This diketone is converted by two successive electron transfer reactions into the enediolate I, which is stable in xylene until it is converted into the enediol H during acidic aqueous workup. This enediol tautomerizes subsequently to furnish the a-hydroxyketone—or... 

The biradical intermediates have in some cases been detected by flash spectroscopy, or trapped by added reagents such as a t-alkyl nitroso compound. The enoi produced by cleavage of the bi radical is relatively inert at low temperatures, and it can be studied spectroscopically after irradiation of the ketone in solutions cooled below —50°C. There have been many mechanistic studies of the Norrish... 

A somewhat modified MO LCAO scheme, without restriction on the identity of spin orbitals (p and

unrestricted Hartree-Fock (UHF) method and is usually used to treat open-shell systems (free radicals, triplet states, etc.). Electron correlation is partially taken into account in this method, and therfore it can be expected to be more efficient than the RHF method when applied to calculate potential energy surfaces of chemical rearrangements whose intermediate or final stages may involve the formation of free- or bi-radical structures. The potentialities of the UHF method are now under active study in organic reaction calculations. Also, it is successfully coming into use in chemisorption computations (6). 

Comparison of the computed profiles with experiment may in principle be used to establish values of some of the unknown rate coefficients. The radical pool in this computation includes molecular oxygen as a bi-radical. The validity of the partial equilibrium assumptions will be discussed in Sect. 5.4.4. 

Molecules containing two trivalent carbon atoms occur in compounds such as the meta derivatives of diphenyl, e.g. XVII, which cannot form a quino-noid structure in the same way as the corresponding para derivative XVIII in which all the carbon atoms are quadrivalent. Magnetic studies of the dimeric form of XVII shows it to Ibe paramagnetic thus confirming the existence of the bi-radical. 

The case of Ghichibabin s hydrocarbon (C0H5)2 G-GgH4—G0H4-G (GeH5)2 is somewhat more complicated. This hydrocarbon is deeply coloured and readily reacts with oxygen and for these reasons the suggestion was made that it may exist in the form of the bi-radical XIX ... 

Chapter 4 introduces the fundamental concepts needed for a discussion of photophysical and photochemical phenomena. Here, the section on bi-radicals and biradicaloids has been particularly expanded relative to the German original. The last three chapters deal with the physical and chemical transformations of excited states. The photophysical processes of radiative and radiationless deactivation, as well as energy and electron transfer, are treated in Chapter 5. A qualitative model for the description of photochemical reactions in condensed media is described in Chapter 6, and then used in Chapter 7 to examine numerous examples of phototransformations of organic molecules. All of these chapters incorporate the recent advances in the understanding of the role of conical intersections ( funnels ) in singlet photochemical reactions. 

When two identical activated alkene functions are included in the same molecule, inter-molecular coupling has to compete with intramolecular hydrocyclization. In most cases the intramolecular reaction, which corresponds to an overall two-electron process, takes precedence. Few mechanistic studies of intramolecular couplings have been reported. The main question is whether the coupling takes place at the mono-radical anion stage in an RS-type reaction (one unit reduced, the other not reduced), or at the bis(radical anion) stage in an RR-type reaction (both units reduced). The last case implies weak electronic interaction between the electrophores. 

However, there are also very important limitations. As we mentioned above, even in oxidation of methane and ethane many elementary reactions are not accessible for direct and detailed investigation. When we shift from ethane to propane, not only the number of carbon atoms in the molecule increases, but also the complexity of the reaction network. In particular, one may assume that even in propane oxidation the formation of complex oxygenate intermediates, including bi-radicals and complex peroxides, may take place. Although such compounds can play a principal role in kinetically relevant steps (such as chain-branching), up to now our knowledge about such compounds is negligible. 

The molecular oxygen 02(3-Tg) found in nature is paramagnetic. This fact indicates that the spins of the two electrons are parallel and thus the molecule shows the characteristics of a bi-radical. This is extremely important in oxidation reactions, since the oxygen molecule very rapidly combines with free radicals forming peroxy type radicals... 

The bimolecular decay of the transient which absorbs at 300 m/x is probably not caused by the interaction of two bi-radical species—e.g.,... 

Ozone is a triangular molecule with a bond distance of 1.27 A, and a bond angle of 117°. This could be compared with the bond distance in the double bonded O2 molecule, 1.21 A, or to the single bond in HOOH, 1.46 A. We conclude that partial double bonds are formed between the end and central oxygen atoms. How can we explain this One would expect the ozone molecule to be a bi-radical because the end atoms can only use one of their free valencies to form a bond with the central atom. But ozone is not a biradical, even if it is quite reactive. The radical structure is quite high in energy and there are ionic structures that may compete. We show the three most important valence structures in Fig. 25.6. 

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