Structur increased-valence

Where appropriate, increased-valence structures [2-5] will be used to provide qualitative VB representations of electronic structure. Increased-valence structures involve localised one-electron and fractional electron-pair bonds, as well as "normal" electron-pair bonds [2-5]. These features will be re-described by reference to HCNO. 

Fig. 1. X-ray absorption near-edge structure (XANES) of reference compounds with various As valence states and mine tailings samples. The As K-edge excitation potential for arsenic in the ground state (As0) is at 11868 eV. The As K-edge excitation potential increases with increasing valence state.

However, there are some cases when an unpaired electron is localized not on the n, but on the o orbital of an anion-radical. Of course, in such a case, a simple molecular orbital consideration that is based on the n approach does not coincide with experimental data. Chlorobenzothiadiazole may serve as a representative example (Gul maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the chloride ion from the 5-chlorobenzothiadiazole anion-radical does not take place (Solodovnikov and Todres 1968). At the same time, the anion-radical of 7-chloroquinoline readily loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to 5-chlorobenzothiadiazole in the sense of structure and electrophilicity of the heterocycle. To explain the mentioned difference, calculations are needed to clearly take into account the o electron framework of the molecules compared. It would also be interesting to exploit the concept of an increased valency in the consideration of anion-radical electronic structures, especially of those anion-radicals that contain atoms (fragments) with available d orbitals. This concept is traditionally derived from valence-shell expansion through the use of d orbital, but it is also understandable in terms of simple (and cheaper for calculations) MO theory, without t(-orbital participation. For a comparative analysis refer the paper by ElSolhy et al. (2005). Solvation of intermediary states on the way to a final product should be involved in the calculations as well (Parker 1981). 

Kekule, Dewar and increased-valence structures for HCNO... 

The electronic structures of C2-symmetry N6 and quasi-linear (CNO)2 have attracted some attention recently [8,9]. The results of ab initio MO studies [8,9] indicate that these two species are respectively unstable and bound relative to their N2 and CNO dissociation products. The VB structures that we shall use to represent the primary features of the electronic structures of N6 and (CNO)2 are examples of increased-valence structures [2-5], Such structures may be generated from familiar (Kekul6-type) Lewis structures by delocalising non-bonding electrons into bonding localised MOs (LMOs). 

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]. 

Because 7 = 3<- ll<- 12<- 13, increased-valence structure 7 involves fractional kx(CN) and 7iy(CN) electron-pair bonds [2,4,10-14,16]. Therefore its C-N bond-number, or bond-order is less than 3. A thin bond line is used to represent a fractional electron-pair bond [2-4]. An N-O double bond, which consists of an electron-pair a bond, and one-electron nx and ny bonds, is also present in this VB structure. With these bond properties, the N-0 and C-N bond-lengths that are implied by increased-valence structure 7 are in accord with the following observations with regard to its bond-lengths [171 ... 

The corresponding wavefunction for the 7i-electrons of increased-valence structure 7 is given by Eq.(9),... 

For quasi-linear (CNO)2, as ONC-C N O, the Kekule and increased-valence structures 14 and 15... 

The Kekule-type Lewis structure 24 is a component of the primary increased-valence structure 25. The presence of lone-pair electrons on the Nc and N c atoms of structure 24 correlates with the calculated bending that occurs at these atoms to generate the C2 geometry. For (CN0)2, the Nc and N c atoms of N6 are replaced by the carbon atoms, and the absence of lone-pair electrons for these atoms in the Lewis structure 14 permits the molecule to adopt either a linear or a quasi-linear geometry. 

Increased-valence structure 36 for N20(CooV) is analogous to increased-valence structure 7 for isoelectronic HCNO. It has been used recently to develop a VB representation for the N20(CooV) - N20(cyclic,C2V) —> NON(Dooh) isomerization process [32]. In Ref. [10], comparisons are made between 36 and other VB structures with apparently quinquevalent nitrogen atoms, and in Section 4.3, 36 provides the VB representation for N2O when it is formed in NCO-NO and OCN-NO2 decomposition reactions. 

The increased-valence structures 39 and 45 for CH2N2 and CH3NO2 are generated from the Kekule-type Lewis structures 37 and 42 via the one-electron delocalisations that are indicated in structures 47 and 48. More... 

Alternative VB representations for the cycloaddition process are displayed and discussed in Refs. [2,10,19,34], Some of them also include the less-important increased-valence structures, such as 8-10 here, in the resonance schemes. Recent MO [35] and spin-coupled VB [36] studies for this process do not give consideration to the concerted diradical formulation. The VB studies of Ref. [37] correspond to the concerted diradical mechanism discussed in Ref. [34]. 

Increased-valence structure 79 retains the double-bond character for each of the O2 moieties. This structure also involves fractional intermolecular 0=0 bonds - their a and n bond-numbers are both equal to 0.25 [43] - thereby implying that the latter bonds should be substantially longer than normal 0=0 bonds. These bond properties are in accord with calculated estimates of the 0-0 bond-lengths for O4 (1.21 and 3.2-3.5 A [43]). However the results of ST0-6G VB calculations show that the and n y-n y spin-pairings alone are... 

By analogy with S2O2 and O4, an S = 0 state for Fen02FeH, with an increased-valence structure of type 65, but with opposed spins for the two... 

With S=S replacing 0=0 in VB structure 78, the primary increased-valence structure for C2 symmetry S2O2 is obtained [2,4,51]. 

One-electron and three-electron chemical bonding, and increased-valence structures... 

Valence bond and molecular orbital approaches to descriptions of one-electron bonds and Pauling three-electron bonds are reviewed, and attention is given to the incorporation of Pauling three-electron bonds into the valence bond structures for molecular systems that involve four-electron three-centre, five-electron three-centre and six-electron four-centre bonding units. Many of these valence bond structures are examples of increased-valence structures. Examples are provided for a few of the large number of phenomena that involve these types of bonds and valence bond structures. 

As a tribute to Pauling s contributions, I shall restate and summarize some of the implications for bonding theory that arise when the three-electron bond is incorporated as a mainstream component for VB descriptions of the electronic structures of electron-rich molecules. Attention will be focussed on increased-valence structures for molecular systems that involve four-electron three-centre and six-electron four-centre bonding units. However initially, consideration will be given to the one-electron bond, for which Pauling also provided some attention to both the theory and examples of systems that involve this type of bond in their VB structures. As indicated in ref. [8(a)], experimentally one-electron bonds and three-electron bonds are abundant and well-characterized for odd-electron systems. 

THREE-ELECTRON BONDS AND INCREASED-VALENCE STRUCTURES FOR FOUR-ELECTRON THREE-CENTRE BONDING... 

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