Stability atomic

The sulfur atom stabilizes a-carbanions this has been used in the thiazole series to give a new synthetic pathway for various hydrocarbons (113) (Scheme 57) (281). 

Section 20 13 Thioesters undergo reactions analogous to those of esters but at faster rates A sulfur atom stabilizes a carbonyl group less effectively than an oxygen... 

A sulfur atom stabilizes a carbonyl group less effectively than an oxygen. 

Other interstitial atoms stabilizing such clusters are B, C, N and Examples... 

Figure 5-4. Hydrogen bonds (dotted lines) formed between H and 0 atoms stabilize a polypeptide in an a-helical conformation. (Reprinted, with permission,...

The leuco quinoneimine dyes are unstable to isolate but the substitution of an electron-withdrawing group such as acyl,13 carbamoyl,13 carboxy,13 or arylsulfonyl 12 group at the amino nitrogen atom stabilizes the... 

The concept of dipole hardness permit to explore the relation between polarizability and reactivity from first principles. The physical idea is that an atom is more reactive if it is less stable relative to a perturbation (here the external electric field). The atomic stability is measured by the amount of energy we need to induce a dipole. For very small dipoles, this energy is quadratic (first term in Equation 24.19). There is no linear term in Equation 24.19 because the energy is minimum relative to the dipole in the ground state (variational principle). The curvature hi of E(p) is a first measure of the stability and is equal exactly to the inverse of the polarizability. Within the quadratic approximation of E(p), one deduces that a low polarizable atom is expected to be more stable or less reactive as it does in practice. But if the dipole is larger, it might be useful to consider the next perturbation order ... 

The core ionization of an atom stabilizes all the valence electrons in the atom. Depending on whether the electronic transition shifts electron density to or from an atom, the energy separation for a shake-up peak of that atom will be less than or greater than the energy of the neutral molecule ionization81. As an illustration of these effects, let us consider the shake-up spectra of formamide, H2NCH082. The principal transitions involved are the vl - n3 and 7r2 - 7r3 transitions. The tTj... 

Scheme 1. a-Acceptors and jr-donors vicinal to the carbene-caibon atom stabilize carbenes. 

The carbodicarbenes C(NHC)2 constitute a new class of compounds, first theoretically predicted [8, 9] and shortly after synthesized and characterized by X-ray analyses [10, 11]. Carbon(O) atoms stabilized by two NHC ligands behave similar to carbodiphosphoranes and are also equipped with HOMO and HOMO-1 orbitals of a and n symmetry. The first carbodicarbene was prepared in a two-step reaction as depicted in Fig. 14. 

A-Nitroimides derived from tertiary amines contain a quaternary nitrogen atom which has a zwitterionic structure with the negative charge on one nitrogen atom stabilized by the electron-withdrawing effect of the adjacent nitrogen atom. A-Nitroimides derived from secondary... 

The proposed mechanism is depicted in Scheme 38. The aldehyde is activated due to coordination on the silicon atom. A hydrogen bond between the aldehyde function and the benzyl substituted tertiary nitrogen atom stabilizes the transition state, and the benzyl group ensures that the cyclopentadiene attacks the dienophile only from one side. 

Umpolung The reversal of polarity of the carbonyl carbon atom is termed umpolung (German for polarity reversal). Normally the carbonyl carbon atom of an aldehyde (or a ketone) is partially positive i.e., electrophilic and therefore it reacts with nucleophiles. When the aldehyde is converted to a dithiane by reaction with 1,3-propanedithiol and reacted with butyl lithium the same carbon now becomes negatively charged to react with electrophiles. This reversed polarity of the carbonyl carbon is termed umpolung which increases the versatility of the carbonyl group in synthesis. The sulphur atoms stabilize... 

The surface modification by O, S, and C atoms and its effects on the methanol chemistry has been investigated on several metal surfaces. It has been reported that these modifier atoms stabilize methoxyl species on Fe(lOO) 18,19), Ni(l 10) 28), Mo(lOO) 4) and W(112) 38 The stabilization of methoxyl species has been ascribed to the blocking of the sites necessary for the methoxyl decomposition. 

The greater stability of sterically hindered siloxanes indicates that oxidation occurs at the silicon atom. Stability toward oxidative cleavage is dependent on both the nature of the organic groups and the backbone structure. 

Polythiophene is a highly crystalline polymer with the chain analog to cis-polyacetylene. The sulfur atoms stabilize the structure and interacts poorly with... 

Let us now consider the formation of three-electron bonds between different atoms. Stabilization of an oxidized sulfur atom can, in principle, be achieved in cases of its interaction with other heteroatoms if they provide free (preferably p-) electron pairs. Nitrogen, oxygen, and halogens (except fluorine) can be mentioned as such heteroatoms (Anklam et al. 1988 Carmichael 1997). The stability of these bonds is generally not as high as that of a symmetric S.. S system. An important reference for the enhanced stability of symmetrical three-electron bonds is Clark s (1988) calculations. 

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