Closed-shell compounds systems

The present contribution considers general electronic states of solvated molecules and is not limited to closed shell molecular compounds. For closed shell molecular systems, methods utilizing closed shell coupled-cluster electronic structure and closed shell density density functional theory for the electronic structure of the solvated system have appeared in the literature [54-67],... 

Historically, the era of electroweak quantum chemistry began with the calculation of parity violating energy differences between enantiomers of chiral compounds within one-component frameworks. Traditionally the emphasis was on closed shell chiral systems, although systems in electronically excited states have received attention recently, both in one- and four-component approaches [47,100]. 

It should be remembered that it is a radical species that results from the addition of muonium to the close-shell compound that is studied in xSR. In studies of molecular, whole body dynamics where the muon appears to be an almost passive observer, it is the very light mass of the muon that makes possible for the observation of properties which are almost identical to those of the parent molecule. There are a number of such studies of organic systems [1]. jlSR studies of the dynamics of fullerenes Ceo and C70 are particularly interesting examples of such whole body motions where the change in the moment of inertia because of the added muonium is neglegible [27,28]. However the use of jlSR to study intramolecular dynamics presents at least two distinct scenarios. 

Frequently we are dealing with the special but common situation that the system has an even number of electrons which are all paired to give an overall singlet, so-called closed-shell systems. The vast majority of all normal compounds, such as water, methane or most other ground state species in organic or inorganic chemistry, belongs to this class. In these... 

Dications 222+ and 232+ were synthesized by hydride abstraction reaction of the corresponding hydro derivatives as stable dark-brown powder. The p/CR+ values for these dications are also extremely high for doubly-charged systems (222+ 11.7 and 232+ 11.7). The electrochemical reduction of 222+ and 232+ exhibited a reduction wave at less negative potentials than that of dication 212+. This wave corresponds to the reduction of two cation units by a one-step, two-electron reduction to form thienoquinoid products. Chemical reduction of 222+ and 232+ afforded the closed-shell thienoquinoid compounds (22 and 23), which exhibited high electron-donating ability. The formation of the closed-shell molecules is in contrast with the result from reduction of dication 212+connected via a / -phenylenediyl spacer. 

For quantum chemistry, first-row transition metal complexes are perhaps the most difficult systems to treat. First, complex open-shell states and spin couplings are much more difficult to deal with than closed-shell main group compounds. Second, the Hartree—Fock method, which underlies all accurate treatments in wavefunction-based theories, is a very poor starting point and is plagued by multiple instabilities that all represent different chemical resonance structures. On the other hand, density functional theory (DFT) often provides reasonably good structures and energies at an affordable computational cost. Properties, in particular magnetic properties, derived from DFT are often of somewhat more limited accuracy but are still useful for the interpretation of experimental data. 

HMO-treatment reveals a closed-shell system for the dianion of 6, thus indicating stabilization for isoelectronic cycl[2,2,2]azines with an additional nitrogen atom in a peripheral position, e.g., 7. Compounds of this type can be prepared by cycloaddition reactions. 

Cyclazinylium salts (233), which following MO considerations should be unstable antiaromatic compounds, have not been described so far. Topologically equivalent -systems containing two additional electrons, however, appear to be closed shell systems and some compounds corroborating this expectation have been prepared. 

Unlike [3.3.3]cyclazine (230), the cyclopentacyclazines (285) are unequivocally diatropic compounds (see Table 11). The five-membered ring double bonds have been shown to be delocalized on the basis of the vicinal coupling constants of the corresponding protons. These observations suggest that the cyclazines (285) may be considered as [13]annulene anions weakly coupled to a localized azomethinium cross-link represented as in (286). The mutual interaction between the two closed shell systems may be small since the bonding MOs of the peripheral [13]annulene can interact only weakly with the antibonding MO of the azomethine group by reason of symmetry. 

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. 

Although the resonance structures of benzene show it as a cyclo-hexatriene, because of its fully delocalized n system and the closed shell nature of this n system, benzene does not undergo addition reactions like ordinary unsaturated compounds. The destruction of the n electron system during addition reactions would make the products less stable than the starting benzene molecule. However, benzene does undergo substitution reactions in which the fully delocalized closed n electron system remains intact. For example, benzene may be reacted with a halogen in the presence of a Lewis acid (a compound capable of accepting an electron pair) to form a molecule of halobenzene. 

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