Of biradicals

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

Closs G L and Redwine O D 1985 Direct measurements of rate differences among nuclear spin sublevels in reactions of biradicals J. Am. Chem. Soc. 107 6131-3... 

The concept of biradicals and biradicaloids was often used in attempts to account for the mechanism of photochemical reactions [2,20,129-131]. A biradical (or diradical) may be defined as [132] an even-electron molecule that has one bond less than the number permitted by the standard rules of valence. 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

It should be noted that CASSCF methods inherently tend to give an unbalanced description, since all the electron correlation recovered is in die active space, but none in the inactive space, or between the active and inactive electrons. This is not a problem if all the valence electrons are included in the active space, but this is only possible for small systems. If only part of die valence electrons are included in the active space, the CASSCF methods tend to overestimate the importance of biradical structures. Consider for example acetylene where the hydrogens have been bent 60° away from hnearity (this may be considered a model for ort/zo-benzyne). The in-plane jt-orbital now acquires significant biradical character. The true structure may be described as a hnear combination of the three configurations shown in Figure 4.11. 

A biradical is a molecule with two unpaired electrons. The unpaired electrons are usually on different atoms, as depicted in (20). In that biradical, one unpaired electron is on one carbon atom of the chain and the second is on another carbon atom several bonds away. In some cases, though, both electrons are on the same atom. One of the most important examples is the oxygen atom itself. Its electron configuration is He]2s22/ x.22py12pzl and its Lewis symbol is -O. The O atom has two unpaired electrons, and so it can be regarded as a special type of biradical. 

Hehre and co-workers have used this approach for the investigation of biradicals and other reactive neutral molecules. For example, by using the bracketing approach, they were able to determine the proton affinities of o- and p-xylylene (o- and p-quinodimethane (lo and Ip) Figure 5.3), from which they were able to determine the enthalpies of formation of the reactive, Kekule molecules. They found the proton affinity of the meta isomer to be too high to be measured directly by bracketing, but were able to assign a lower limit, and subsequently a lower limit to the enthalpy of formation of the m-xylylene diradicals. 

From the parameters highlighted, it can be seen that the p value for 78s is smaller or closer to 0°, than that in 79s and thus preference for cyclization prevails over cleavage. On the contrary, for 79s the values that would favor cleavage are also closer to 0° than in 78s. Not only do these values indicate that 79s is prone to cleavage, but comparing the % and 4 values to those in parenthesis, it is C2-C3 bond that cleaves preferentially over the C2-C3.. These parameters suggest that very small conformational changes may have drastic effects in the chemo-selectivity of biradicals. 

The above biradicals, with the exception of the oxygen molecule, are all highly unstable there are, however, a number of much more stable species that show evidence of biradical character. Thus the hydrocarbon (135) exists, in part, in solution as a biradical ... 

Two types of biradicals can be produced in the copolymers, depending on whether the reaction occurs at a PVK or a TVK site. The change is reflected in the quantum yields, as well as in the lifetime for both, the triplet and the biradical, see Table II. 

The Norrish-Yang reaction [20] is based on the photochemical excitation of ketones followed by an intramolecular hydrogen transfer with the formation of biradicals. Wessig and coworkers used this procedure to prepare functionalized cyclopropyl ketones as 5-75 from 5-72 (Scheme 5.15) [21]. The substrate employed con-... 

Figure 7 (a) 2D spectral-spatial perspective plot of nitroxide biradicals derived from HAS doped in HPEC on one side of the plaque (left), and in a transparent film attached to the plaque on the opposite side (right), (b) Spectral slices determined nondestructively from 2D plot ESR spectra of biradicals located in two amorphous phases in the polymer (upper), and dispersed in a film (bottom). 

ESR Spectra of Biradicals, Triplet States, and other S>lf2 Systems... 

ESR Spectra of Biradicals, Triplet States, and other S>l/2 Systems At constant frequency, v0 = AE/h, the resonant fields are ... 

Eq. 17 is meant to represent the possibility for a concerted formation of oxetane product. A problem that always exist in cycloadditions is the question of whether the reaction takes place by a two-step biradical reaction pathway or through a concerted mechanism. Such questions have not even been resolved for purely thermal reactions. 4> A recent speculation on this point proposes almost universal concertedness for all cycloaddition reactions. 79> In that work, mixed stereochemistry in the products of [2+2] cycloaddition reactions is generally attributed to a mixture of two concerted reactions, suprafacial-suprafacial, and supra-facial-antarafacial. It will be seen later that the PMO calculations generally do not support this idea. A mixture of biradical and concerted reactions is in better agreement with experimental facts. 

Qualitatively, the interaction diagram would closely resemble that in Fig. 3, since electron-donating substituents in both addends would raise the molecular levels of both the carbonyl compound and the olefin. Only the energy gap, E(n)-> F(n), would increase, the net result being that the calculated ratio of concerted to biradical reaction, Eqs. 40 and 41, should be even closer to unity than in the formaldehyde-ethylene case. Detailed calculations 38> support this conclusion, so PMO theory predicts that the overall stereochemical results are due to a combination of concerted (singlet) and biradical (triplet) mechanisms. This explanation agrees with the experimental facts, and it bypasses the necessity to postulate differential rates of rotation and closure for different kinds of biradical intermediates. 

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