Diradical structure

This result can be rationalized in terms of a cyclopropyl diradical structure by noting that the bond cleavage of the intermediate will occur to give the more stable of the two possible 1,3-diradicals. The cyclopropane ring in the final product will then incorporate this terminus ... 

Figure 7.22. Diradical structures 47 and 48, showing the model based on the X-ray structure of 48. (Adapted from ref. 123.)...

In connection with Chichibabin s hydrocarbon it is appropriate to mention that there is little or no resonance between states of different multiplicity.4 Thus the singlet-triplet transition represented by the covalent and diradical structures of the hydrocarbon is a true equilibrium and not an example of resonance. 

Scheme 73. Diphosphinines Lowest Energy Diradical Structure for Isomer According to Ref 233...

Not all even AHs can be represented by Kekule structures. The w-quino-dimethane 471 can only be represented by diradical structures, and molecules... 

The analogy of these dimerization processes to thermal Diels-Alder type reactions which sometimes also yield cyclobutane structures is worth noting and may be taken as one of the arguments for a diradical structure of the transition in the latter process. Also, it may be pointed out that the photoexcited state involved is presumably the same one involved in the well-known photochemical trans-cis interconversion of such olefins. 

It appears, therefore, that real-time studies of these reactions should allow one to examine the nature of the transformation and the validity of the diradical hypothesis. We recently reported direct studies of the femtosecond dynamics of the transient diradical structures. The aim was at freezing the diradicals in time in the course of the reaction. Various precursors were used to generate the diradicals and to monitor the formation and the decay dynamics of the reaction intermediate(s). The parent (cyclopentanone) or the intermediate species was identified distinctly using time-of-flight mass spectrometry. The concept behind the experiment and some of the results are given in Fig. 16. 

Now we turn to selected examples of interest. One involves a multiplicity dependence of the regioselectivity. This is illustrated in Scheme 4. While one simplistically might write a single intermediate cyclopropyldicarbinyl diradical structure for both singlet and triplet multiplicity, the regioselectivity of unzipping depends on which species is involved37. 

The relatively constant kinetic disadvantage experienced by an average vinylcyclopropane rearrangement, compared with a (kt + k2) stereomutation, amounts to a A A (7 of about 3 kcal moT1. This may well be associated with configurationally distinct sets of diradical structures with those of E stereochemistry favored thermodynamically over those of Z stereochemistry by about 3 kcal mol 1. Only the latter may lead to cyclopentene products212. 

Orgel and Longuet-Higgins analysis can be explained in the following way. Figure 7.8 shows that two n electrons of CB are non-bonding. They make no contribution to molecular stability but still introduce interelectronic repulsions. Thus, they confer thermodynamic instability. Furthermore, Hund s rule requires that T, and both contain a single electron. This adds kinetic instability because diradical structures are extremely reactive.36... 

Express this wave function in terms of VB structures, and show that Cl has the effect of increasing the weight of the diradical structure, and lowering those of the ionic structures, especially the 1,3-dipolar ones. 

Note that this is the result of 2 x 2 CI. A complete Cl in the tt space would further increase the weight of the diradical structure, to 0.60 (see Ref. 2). 

Comparison of Equations 5.1 and 5.2 clearly demonstrates that the diradical structure 1 is more stable than the doubly bonded Lewis structure 2 (or 2%... 

When Heitler-London AO-type wavefunctions (i.e.. ..aabP +. ..baaP in which a and b are AOs) are used to represent electron-pair 7i c(CN) and 7i y(CN) bonds, it can be deduced [2,4,16, cf. also Eq.(ll) below] that VB structure 7 is equivalent to resonance between the Kekule Lewis structure 3 and the Dewar or "long-bond" Lewis structures 11-13. Only nearest-neighbour spin-pairing is indicated in increased-valence structures [2-5,10]. When the "long" or formal bonds are omitted from structures 11-13, these structures are designated as singlet diradical structures [2-4]. 

A unifying hypothesis for the observed organic chemistry was advanced by Huisgen [132], who suggested that all tetramethylenes lie on a continuous scale between zwitterionic and diradical structures and may be regarded as resonance hybrids of the two extreme forms. The predominant nature of the tetramethylene intermediate is determined by the terminal substituents, and the termini can interact with each other by through-bond interaction [132, 134]. 

The common intermediate has appreciable diradical structure, and the products result from the alternative modes of its collapse through different transition states. 

Fine structures. Di- and tetradehydropyrazines (as their derivatives) are sometimes implicated as transient intermediates in proposed reaction mechanisms. Some theoretical studies have suggested that didehydropyrazine would exist as the diradical structure (3),454 whereas others seem to suggest more normal formulations for di- (4) and tetradehydropyrazine (5).235... 

In a later article, complexes of Ni(II), Cu(II), Pd(II), and V02+ ions with the same tetra-substituted porphyrin were reported. Stepwise oxidation of these complexes gave products for which the authors proposed quinonoid, monoradical, and diradical structures. The most prolonged oxidations yielded the diradical products, which were isolated as dark purple crystals, relatively stable in air (40). The monoradical vanadyl complex was observed to be diamagnetic, suggesting antiferromagnetic coupling between the phenoxyl radical and unpaired electron on vanadium, whereas in the copper complex no such coupling was observed. More detailed studies of these systems seem warranted. 

Examples of 1,3-dipoles include diazoalkanes, nitrones, carbonyl ylides and fulminic acid. Organic chemists typically describe 1,3-dipolar cycloaddition reactions [15] in terms of four out-of-plane 71 electrons from the dipole and two from the dipolarophile. Consequently, most of the interest in the electronic structure of 1,3-dipoles has been concentrated on the distribution of the four Jt electrons over the three heavy atom centres. Of course, a characteristic feature of this class of molecules is that it presents awkward problems for classical valence theories a conventional fashion of representing such systems invokes resonance between a number of zwitterionic and diradical structures [16-19]. Much has been written on the amount of diradical character, with widely differing estimates of the relative weights of the different bonding schemes. 

Enough substituted cyclopropanes have now been subjected to careful kinetic studies so that a characteristic pattern of reactivity and stereochemical preferences has emerged. Substituents facilitate stereomutations in proportion to their ability to stabilize 1,3-trimethylene diradical structures. The values for both k 2 and (k + k2) stereomutation rate constants relate linearly with consistent measures of substituent radical stabilization energies with equal sensitivities. Experimentally determined (A , + / 2)- i2 ratios do not vary widely they range from 1.4 to 2.5 over a fair diversity of substituents. Neither do kf.kj ratios vary widely. The majority fall between 1 1 and 2.5 1 the largest yet reported gives 2(CHD) a symmetry corrected kinetic advantage over A i(CDPh) in 1-phenyl-1,2,3-d3-cyclo-propanes of 5 1. 

The low activation energy of the thermal addition polymerization reaction confirms the necessity of a (librational) motion of the molecules in the initiation process. The first addition process differs from all the following addition proccesses by the metastable monomer diradical structure, which — in contrast to the DR , AC , and DC structures with n > 2 — has a limited life-time given by the phosphorescence decay of the monomer triplet state. Therefore, the librational excitation must be performed during the life-time of the monomer reaction centre. In the case of the low temperature photopolymerization reaction the librational excitation has to be prepared optically via the decay of the electronic excitation. This is in contrast to the photopolymerization reaction at high temperatures, where numerous molecular motions are thermally and stationary present in the crystals. Due to this difference two photons (2hv) are required in every dimer initiation process at low temperatures and only one photon (hv -i- kT) is required at high temperatures. The two paths of the photoinitiation reaction are illustrated below by the arrows in Fig. 26. The respective pair states are characterized by M M and M M as discussed below. 

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