Diradical definition

In a definitive study of butadiene s reaction with l,l-dichloro-2,2-difluoio-ethylene, Bartlett concluded that [2+4] adducts of acyclic dienes with fluorinated ethylenes are formed through a mixture of concerted and nonconcerted, diradical pathways [67] The degree of observed [2+4] cycloaddition of fluorinated ethylenes IS related to the relative amounts of transoid and cisoid conformers of the diene, with very considerable (i.e., 30%) Diels-Alder adduct being observed in competition with [2+2] reaction, for example, in the reaction of 1,1 -dichloro-2,2-difluoro-ethylene with cyclopentadiene [9, 68]... 

Tbe best conditions for observing S3 are 440 C and lOmmHg when 0-20% of vapour species comprise this deep cbeny-red bent Iriatomic species like ozone, p. 607, it bas a singlet ground slate. Tbe best conditions for Sj are 450 C and 20 mmHg (concentration 20 v ) but tbe structure is still not definitely established and may, in fact be a strained ring, an unbranched diradical chain, or a branched-chain isostructural with SOi(g) (p. 703). 

Definition A distonic ion is a positive radical ion, which would formally arise by ionization of a zwitterion or a diradical, by isomerization or fragmentation of a classical molecular ion, or by ion-molecule reactions. Consequently, distonic ions have charge and radical at separate atoms in a conventional valence bond description. [42,43]... 

Butadiene has also been reported to form a [4 + 2] cycloaddition-like product at the Si(l 11)—7 x 7 surface. Theoretically, the reaction has been predicted to occur between a rest atom-adatom pair via a step-wise, diradical path pathway that is unactivated [19,249]. Recent STM studies of butadiene adsorbed on Si(l 11)—7 x 7 have observed the formation of either a [4+ 2]-like or a [2+ 2]-like product with the Si adatom-rest atom pair, but were not able to definitely assign the product as one or the other [250]. Interestingly, the STM images also reveal that about a third of the cycloaddition product forms between two Si adatoms. Because both adatoms are positively charged, the stepwise reaction does not seem likely, and the authors suggest that this reaction occurs in a concerted fashion [250]. 

A further point of interest is the reaction regiochemistry. For the reaction in Scheme 2 we might consider two possible vinylcyclopropane products. Thus the intermediate cyclo-propyldicarbinyl diradical might open in two ways. However, only one product is formed. In Scheme 3 this is shown more definitively where the two modes of unzipping the cyclopropyl dicarbinyl diradical are depicted. A general rule is that the less stabilized diradical center utilizes its odd-electron density to open the three-membered ring. This leads onwards to the more stabilized Diradical II 3. 

Fig. 4. Definition of the geometries of the three diradicals face-to-face [FF(a)l, edge-to-face lEF(a)l and edge-to-edge EE(oi)J...

In this chapter, we focus on the class of reactive intermediates that bear at least two unpaired electrons diradicals and carbenes. The exact definition of a diradical is somewhat in the eye of the beholder. Salem and Rowland provided perhaps the most general, yet effective, definition—a diradical is a molecule that has two degenerate or nearly degenerate orbitals occupied by two electrons. With this definition, carbenes can be considered as a subcategory of diradicals. In a carbene, the two degenerate molecular orbitals are localized about a single carbon atom. 

In the limited trajectoiy study for Reaction 8.1, Carpenter noted 10 (out of 1(X)) direct trajectories with the AMI surface and 4 (out of 100) direct trajectories on the PM3 surface. While the sample size is too small to say anything definitive about these percentages, he did note that the diradical is in a shallower well in AMI than in PM3. It is reasonable that the shallower well will be more easily (i.e., directly) crossed. In particular, a trajectory is less likely to be trapped within a shallow energy well than a deep energy well. 

As mentioned above, the photooxidation was discovered by exposure of compound 22 to sunlight. The reaction proved to be of great value for angucycline synthesis because the -hydroxy group present in most natural products which is easily eliminated under basic or acidic conditions (see Scheme 2) and the carbonyl group at C-1 can thus be introduced under mild neutral conditions. We assume that the reaction is initiated by Norrish type II y-hydrogen abstraction of the excited carbonyl in 25 to yield a diradical 26 as shown in Scheme 8 with 1-deoxyrabelomycin (25) as the example [39]. The H-abstraction requires a very definite steric environment in which the benzylic protons have to be in proximity of the excited carbonyl group. Subsequent addition of the diradical 26 with... 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

The mechanism of the thermal and photochemical rearrangement is believed to proceed as shown (1 - 2), with the dilemma of concerted vs. diradical nature not definitely resolved for all cases. A comparison of energy parameters for the thermolysis of the parent 1,2-divinyl-cyclopropane and of l-(hex-l-enyl)-2-vinylcyclopropane ( - and Z-isomers, tram- and cis-cyclopropanes) has been reported. When both alkenes bear a Z-positioned substituent, the rearrangement of the divinylcyclopropane system becomes very slow and other processes, such as the [1,5] sigmatropic shift of alkyl(vinyl)cyclopropanes or the vinylcyclopropane to cyclo-pentene rearrangement, may compete (see also Section 2.4.3.). Both the mechanism and the applications of this rearrangement have been reviewed. 

Figure 7. Definition of electronic substituent effects in terms of spin donors (SD) and spin acceptors (SA) and their consequences on the D parameter of the triplet diradicals 10.

The D values in Table 4 display some interesting and remarkable trends in the spin-delocalizing ability of the heteroaryl substituents. The three regio-isomeric pyridyl-substituted triplet diradicals 12g ( — 0.01), 12i ( — 0.05), and 12k (—0.06) delocalize spin worse than the phenyl reference system in the order para > ortho > meta phenyl. Especially in the ortho and para isomers delocalization is definitely less effective than for phenyl. Thus, the pyridyl derivatives act as spin donors by enhancing the spin density at the radical site, although the effects are relatively small. The small effects are mainly due to destabilizing aminyl-type radical structures with unfavorable spin accumulation at the nitrogen atom (Fig. 20) [61]. 

On the other hand, modern nomenclature is based on the chemical structure of the macromolecules. The name of a polymer of unspecified degree of polymerization consists of the prefix poly and the name of the smallest repeating unit. With unbranched polymers, the smallest repeating unit is a diradical. The name of this diradical is the same as that met with in the nomenclature for low-molecular-weight organic diradicals. Thus, the group —CH2— is called methylene and the corresponding polymer is called poly(methylene) (example 1 in Table 1-4). The diradicals —CH2—CH2— and —CH2—CH(CH3)— do not have simple definitive names the trivial names ethylene and propylene are retained. Examples of names for other diradicals are as follows ... 

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