Ring basic mechanism

Nonadiabatic transitions definitely play crucial roles for molecules to manifest various functions. The theory of nonadiabatic transition is very helpful not only to comprehend the mechanisms, but also to design new molecular functions and enhance their efficiencies. The photochromism that is expected to be applicable to molecular switches and memories is a good example [130]. Photoisomerization of retinal is well known to be a basic mechanism of vision. In these processes, the NT type of nonadiabatic transitions play essential roles. There must be many other similar examples. Utilization of the complete reflection phenomenon can also be another candidate, as discussed in Section V.C. In this section, the following two examples are cosidered (1) photochromism due to photoisomerization between cyclohexadiene (CHD) and hexatriene (HT) as an example of photoswitching molecular functions, and (2) hydrogen transmission through a five-membered carbon ring. 

The propenyl group can then supply electrons to the cobalt atom by a resonance effect. This basic mechanism may also be applied to explain the marked accelerating effect of the aroyl group. Except in cases where ortho substitution sterically hinders formation of a cyclic transition state substitution on the aromatic ring has relatively little effect on the rate of decomposition. 

Depending on the particular jet initial conditions, several or no axis rotations can occur in the first few diameters of jet development (e.g., [3, 16]). Results of experiments, stability analysis, and simulations support the concept that the basic mechanism for the first axis rotation of the jet cross-section is the selfdeformation of the vortex rings due to nonuniform azimuthal curvature at the... 

Now you have three basic mechanisms for aromatic rings — electrophilic aromatic substitution, Sj Ar, and elimination/addition. How do you choose among these The first consideration is what types of other reagents are present. If the reagents include an electrophile, then the reaction will be electrophilic aromatic substitution. The presence of a nucleophile may lead to either S Ar or elimination/addition. If the system meets the three requirements for SfjAr, then the reaction will follow that mechanism. If not, it will be an elimination/addition. 

The basic mechanism of fragmentation (equation 1) is a concerted one, with a contribution from each of the five centers to the transition state. Two-step carbonium ion or, very rarely, carbanion mechanisms mean in all cases that substitution, elimination or ring closure can take place rather than fragmentation. However, the one-step synchronous fragmentation is allowed only when the compound can adopt a conformation where the leaving group X as well as the electron pair of the electrofuge Y are antiperiplanar to the C(2)—C(3) bond (Scheme 5). ... 

This basic mechanism was originally proposed for oxidation of alkylaromatics but was thought to require the /r-system of the aromatic ring e.g. according to eq. (26) ... 

Under basic conditions // a 5-2-hydroxymethyl-l-acylcyclopropanes can be transformed into c/i-isomers, and this occurs by an intramolecular nucleophilic ring-opening and subsequent ring-reclosure mechanism. ... 

EMSORB Sorbitan Fatty Acid Esters, or polyol esters, are created by the two basic mechanisms of dehydration and esterification. Sorbitol, with six hydroxyl groups, is dehydrated, undergoing ring closure with the loss of two hydroxyl groups. The cyclic sorbitan moiety is produced. Through the esterification reaction, hydrophobic fatty acids are combined with the sorbitan, thereby decreasing its hydrophilic attributes. The exact degree of hydrophllicity depends on the size and number (mono-, di-, tri-) of acid molecules added to one sorbitan molecule. 

As a function of the catalytic system used in the polymerization of cycloolefins, the reaction may proceed by a cationic, Ziegler-Natta, or ring-opening metathesis mechanism. The three basic mechanisms for each type of polymerization reaction are briefly outlined. 

The basic building blocks of polymers are called monomers , which are typically polymerised by radical-forming or ring-opening mechanisms to form a linear polymer. Polymers are macromolecular chains, and entanglement of such chains gives the... 

The basic mechanism for the polymeric or cyclic carbonate formation involves initiation of the reaction by the opening of the epoxide to give an alkoxo linkage. This is shown in Figure 4.14 by the conversion of 4.58 to 4.59 where an attack by the nucleophile, e.g., a halide, opens the epoxide ring. The source of the nucleophile could be the metal complex itself or a cocatalyst such as a Lewis acid salt A X". 

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