Cyclic bonding

As described in the preceding paragraphs, oxidation products of carotenoids can be formed in vitro as a result of their antioxidant or prooxidant actions or after their autoxidation by molecular oxygen. They can also be found in nature, possibly as metabolites of carotenoids. Frequently encountered products are the monoepoxide in 5,6- or 5, 6 -positions and the diepoxide in 5,6 5, 6 positions or rearrangement products creating furanoid cycles in the 5,8 or 5, 8 positions and 5,8 5, 8 positions, respectively. Products like apo-carotenals and apo-carotenones issued from oxidative cleavages are also common oxidation products of carotenoids also found in nature. When the fission occurs on a cyclic bond, the C-40 carbon skeleton is retained and the products are called seco-carotenoids. 

The energy of the homodesmotic reaction does not exclusively reflect the effect of cyclic (bond) delocalization. The reference structure is hypothetical and one cannot write the equation of a reaction, where a cyclic and an acyclic structure participate, for which the difference between the energies of products and reactants was determined by a single factor, namely, aromatic stabilization (antiaromatic destabilization) (75TCA121). 

The HSE values estimate the contribution by cyclic (bond) delocalization, whereas the AE values for the isodesmic reaction (ISE) [76JCS(P2)1222] refer to the stabilization energy associated with conjugation as a whole clearly, the latter values turn out appreciably larger, cf. HSE (4), (6) and ISE (3), (5). 

The aromatic stabilization of a molecule is the energy contribution due to the cyclic bond delocalization. This contribution is defined as the resonance energy (RE)... 

The problem in determining resonance energies is to single out of the total energy of the molecule the contribution from the cyclic bond delocalization. 

Limbach et 1 performed DFT calculations on. I(N, 11) and 2hJ(N,N) couplings in the anion [C = N-L-N = C] (L = H, D) and in the cyclic bonded formamide dimer (HCNHNH2)2 to study how such couplings depend on the geometry of the hydrogen bond. To describe such a relationship they employed the valence bond order model and compared the NHN and FHF- hydrogen bonded systems. 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

Cyclic Bonding and Stability Chugaev s Rule of Rings 14... 

The energetic advantage of homodromic cyclic bonding was predicted. The... 

The complete synthetic pathway is called the Wittig reaction, and it is a very useful synthetic tool, because the position of the carbon/carbon double bond is known. This method may also be used to produce a carbon/carbon double bond in a position that would be very difficult to achieve by any other means, e.g. exo-cyclic bonds, or 1,4-dienes, i.e. unconjugated. 

D(inter) represents the extent of reaction of intermolecular reaction alone at the gel point, and During) that of cyclization alone. Note that only two FUs (one bond) are wasted every cyclization independently of size of rings [9]. Thus, it is convenient to define that cyclic bonds are equal to excess bonds which, when broken, do not disconnect a polymer molecule. Let [r] be the number concentration (mol/1) of total rings and C(= M0/V) the initial number concentration of monomer units, and one has the equality ... 

Then let us proceed to the formulation of D(inter). Suppose an equilibrium branching process where some fraction,pR, of all the FUs, fM0, is already occupied by cyclic bonds. The remaining FUs then form the equilibrium distribution, fiMj, where Mt denotes the number of monomer units having FUs. Let us express the gel point of this mixing system in the form ... 

Assumption I cyclic bonds distribute randomly over all monomer units (random distribution assumption of cyclic bonds). 

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