Hybridization and electronegativity

Electronegativity and hybridization effects combine to make the carbon of a carbonyl group especially deshielded Normally the carbon of C=0 is the least shielded one m... 

The introduction of heteroatoms into cyclic systems produces significant variations in the molecular geometry that reflect the changes in covalent radii, relative electronegativity and effective hybridization. Consequently, there are changes in the bonding and the physico-chemical characteristics of these heterocyclic systems—particularly in small ring systems. 

In p-block elements, a central-atom lone pair is visualized as the limit of bonding to a ligand of lowest possible electronegativity, and thus acquires high s character. In d-block elements, a central-atom lone pair is of essentially pure d character (except for the special n(h) hybrid of ML2 coordination) and s character is essentially reserved for the covalent sigma skeletal bonds. 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

From what you know of the relationship between ionization energies, electron affinities, and electronegativities, would you expect the addition of some d character to a hybrid to raise or lower the electronegativity for example, will sulfur be more electronegative when hybridized spi or jp [Pg.649]

The length of the covalent radius is a function of several factors, among which are (I) bond order (mulliplicily). (2) electronegativity. (3) hybridization. (4) orbital overlap, (S) steric factors, and (61 special electronic effects. 

Table 11 gives standard covalent radii for most of the elements of the periodic table. Most of these have been calculated from the best data available in the literature using the considerations of paragraphs 1 -3 above. Thus when radii or the appropriate multiplicity and hybridization are added and corrected for difference in electronegativity by the Stevenson and Schomaker relationship, the observed bond length will be obtained. For example, the O-F bond in OF would have the value 0.745 + 0.709 — 0.910.5) = 1.409 A. The experimental value is 1.41 A. 

De Rider, M. L., Wilkens, S. J., Waddell, M. J., Bretcher, L. E., Weinhold, F., Raines, R. T., and Markely, J. L. (2002). Collagen stability Insights from NMR spectroscopic and hybrid density functional computational investigations of the effect of electronegative substituents on prolyl ring conformations. J. Am. Chem. Soc. 124, 2497-2505. 

As a first reassuring statement, we can say that trends in chemical shifts of 13C are somewhat parallel to those of H, so that some of the feeling for H spectra may carry over to l3C spectra. Furthermore, the concept of additivity of substituent effects (see Sections 4.7.1 and 4.7.6) is useful for both spectra. The 13C shifts are related mainly to hybridization, substituent electronegativity, and diamagnetic anisotropy (to a lesser extent) solvent effects are important in both spectra. Chemical shifts for I3C are affected by... 

Important criteria for heterocycles include hybridization, electronegativity, and stereochemistry. For thiophene, the matter is further complicated by the question whether the 3d-orbital of sulfur also participates in bond formation. If so, then the two resonance structures 119 and 120 in Scheme 7 should also be considered in determining the degree of aromaticity. 

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