Open-shell biradicals

Computational quantum chemistry has emerged in recent years as a viable tool for the elucidation of molecular structure and molecular properties, especially for the prediction of geometrical parameters, kinetics and thermodynamics of highly labile compounds such as nitrosomethanides. However, they are difficult objects for both experimental (high toxicity, redox lability, high reactivity, explosive character etc.) and computational studies, even with today s sophisticated techniques (e.g. NO compounds are often species with open-shell biradical character which requires the application of multi-configuration methods). 

Quantum chemical studies of cyclizations of enediynes and enyneallenes have been reviewed.180 The intermediates are computationally tractable as a result of the unrestricted broken spin symmetry (UBS) approach using GGA functionals for the description of open-shell biradicals. The intermediacy of biradicals in Cope-type rearrangements, to which the Bergman and Myers-Saito reactions belong, are shown to be predictable using a very simple rule biradicals are likely to be intermediates if they are stabilized either by allyl resonance or by aromaticity. 

In view of the failure of the singlet surfaces to account for the production of the final products, and since all the species, except for CHKI, are open-shell biradicals, it was decided to calculate the reaction path way s on a triplet surface as the energy level of the triplets in many cases, when they contain nitrogen atoms, is lower than that of the... 

The next step was the calculation of the MECP from a singlet, open-shell biradical intermediate INTI (Figure 6.42) to the triplet state. It was then followed by the calculation of the singlet triplet crossing probability. The exact procedure has been described in Section 6.1.1. The final situation is such that both pathways— the singlet and the triplet— participate in the process since a considerable part of the species still remains in the singlet state and proceeds on the singlet surface. 

Grafenstein, J., Kraka, E., Cremer, D., 1998, Density Functional Theory for Open-Shell Singlet Biradicals , Chem. Phys. Lett., 288, 593. 

Parasuk and Cramer s calculations indicate that triplet vinylnitrene is 15 kcal/mol more stable than the lowest energy singlet state. The lowest energy singlet state is open shell and resembles a 1,3-biradical. 

If a molecule with no-bond homoaromaticity is investigated, the system in question possesses a non-classical structure with an interaction distance typical of a transition state rather than a closed-shell equilibrium structure. One can consider no-bond homoconjugative interactions as a result of extreme bond stretching and the formation of a singlet biradical, i.e. a low-spin open-shell system. Normally such a situation can only be handled by a multi-determinant description, but in the case of a homoaromatic compound the two single electrons interact with adjacent rc-electrons and form together a delocalized electron system, which can be described by a single determinant ab initio method provided sufficient dynamic electron correlation is covered by the method. 

Luminescence is seldom observed from free radicals and radical ions because of the low energy of the lowest excited states of open-shell species, the benzophenone ketyl radical being however a noteworthy exception. There are few reports of actual photochemical reactions of free radicals, but the situation is different with biradicals such as carbenes. These have two unpaired electrons and can exist in singlet or triplet states and they take part in addition and insertion reactions (Figure 4.90). 

Several anion biradicals have recently been reported, composed of semiquinone and nitroxide functionalities. The ground-state triplet anion biradicals were derived from one-electron reduction of open-shell acceptors, in which p-benzoquinone (BQ) is substituted by a nitronyl (Shultz Farmer 1998) or nitronyl-nitroxide (NN) group (Kumai et al. 1994). Scheme 1-55 gives the BQNN example. 

Figure 4 shows the TS geometry determined by methods applicable to cases with open shells. This state is asymmetrical and of the biradical nature this is evidence for the reaction proceeding by the mechanism described in [15]. This state may be referred to as TS2 its parameters are listed in Table 5. 

Ground-state molecular oxygen (dioxygen), an open-shell triplet (biradical) denoted by spectroscopists 35, is one of the most abundant and important life-supporting species on Earth. It can participate in some photochemically initiated reactions, sometimes called... 

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