Diradicals ground state

Fig. 22A-E. The C2H4 + CH2 (triplet) system A) coordinates B) activated complex for the addition C) biradical at the end of addition D) diradical ground state E) activated complex for interconversion (Interatomic distances in A)...

Tliis is Huckel s famous Am+ 2 rule, which ascribes extra stability to monocyclic conjugated systems that satisfy (16.52). Mth Am + lot Am — tt electrons, the compound is a free radical. With Am ti electrons, there are two electrons in a sheU that can hold four electrons, and Hund s rule predicts a triplet (diradical) ground state. 

The dianion of dihydroacepentalene is of considerable interest as a potential precursor for the elusive hydrocarbon acepentalene, CioHe, which has been predicted to have a triplet diradical ground state. The corresponding dianion, CioHe ", is significantly more stable and can be generated in solution by treating triquinacene with a superbasic mixture of n-butyllithium and potassium t-pentoxide in the presence of tmeda [27]. More recently a crystalline sample of the dimethoxyethane solvate [ Li(DME) 2CioH6]2, 33, has been successfully prepared according to Eq.(l) by means of a transmetallation reaction between 4,7-bis(trimethylstannyl)-dihydroacepentalene and methyllithium in DME solution at -60 °C [28]. 

Trimethylenemethane (1) is a cross-conjugated it system with a diradical ground state. Its chemistry has been reviewed as early as 1972 by Dowd [8]. The Hiickel molecular orbital (MO) scheme resembles that of cyclobutadiene, and accordingly trimethylenemethane can be stabilized as a ligand in transition metal complexes. Trimethylenemethane complexes can generally be obtained by treatment of a trimethylenemethane dianion with a metal halide or by treatment of organic halides with low-valent metal complexes. In addition to these general methods, there are some more specialized procedures. 

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) ... 

For conjugated carbonyl compounds, such as a,) -enones, the orbital diagram would be similar, except for the recognition that the HOMO of the ground state is ij/2 of the enone system, rather than the oxygen lone-pair orbital. The excited states can sometimes be usefully represented as dipolar or diradical intermediates ... 

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 irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

In the case of 60, the ion has been prepared and has been shown to be a diradical in the ground state, as predicted by the discussion on page 58. Evidence that 60 is not only nonaromatic but also antiaromatic comes from studies on 64 and 66. When 64 is treated with silver perchlorate in propionic acid, the molecule is rapidly solvolyzed (a reaction in which the intermediate 65 is formed see Chapter 5). Under the same conditions, 66 undergoes no solvolysis at all that is, 60 does not form. If 60 were merely nonaromatic, it should be about as stable as 65 (which of course has no resonance stabilization at all). The fact that it is so much more reluctant to form indicates that 60 is much less stable than 65. It is noted that under certain conditions, 65 can be generated solvolytically. ... 

It is possible that some of these photochemical cycloadditions take place by a lA + A] mechanism, which is of course allowed by orbital symmetry when and if they do, one of the molecules must be in the excited singlet state (5i) and the other in the ground state.The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,making it likely that the [n2s + n2s] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain... 

Stability of diradicals is important for photochemical reactions. Spin multiplicity of the ground states is critical for the molecular magnetic materials. The relative stability of singlet (triplet) isomers and the spin multiplicity of the ground states (spin preference) [48] has been described to introduce the orbital phase theory in Sects. 2.1.5 and 2.1.6. Applications for the design of diradicals are reviewed by Ma and Inagaki elsewhere in this volume. Here, we briefly summarize the applications. 

Table 1 Spin preference of ground state and the calculated singlet-triplet energy separation of some selected Jt-conjugated diradicals...

The preceding orbital phase predictions of some topological units (like 1, 4-6) can be easily extended to more complex cychc diradicals [29], as shown in Fig. 12. On the basis of TMM sub-structure (1), diradicals 8-11 are predicted to be phase continuous in their triplet states. Such a triplet preference in their ground states is in agreement with calculation results and available experiments, as listed in Table... 

Among the non-Kekule diradicals, tetramethyleneethane (TME, 7) has evoked lasting attention during the last two decades due to the controversy over its spin preference in the ground state between experiments and theoretical predictions [48-59], Now TME is known to be a slightly favored singlet diradical with a negligible... 

Like TME, the diradical 15 was shown to have nearly degenerate singlet and triplet states by magnetic susceptibility [60, 61], although the early works by Dowd identified a triplet ground state on the basis of ESR spectrum [62, 63], The UCCSD(T) calculations predicted a singlet ground state with a small S-T gap of... 

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