Electronegativity controversy

There is considerable controversy regarding the degree of covalent character in a carbon-lithium bond15-1S). An uncritical comparison of electronegativities does indicate a high degree of ionic character as do extended Hiickei molecular orbital... 

In many cases, experimental data have been discussed in terms of variations of the paramagnetic contribution, nuclear shielding constant (Equation (2)). Empirical correlations with parameters related to the electronic properties of other substituents in the molecule (electronegativities, Hammett substituent constants etc.) have been found. This kind of investigation has provided useful information about the electronic structure of the sulphur atom in different bonding situations and most of all about the electronic properties of the S-O bond, which is still a rather controversial matter. 

The contribution of polar structures reduces the barrier and also the intrinsic barrier. This results for non thermoneutral reaction in a reduction of isotope effect. This has been a controversial subject for several years it is extensively covered by Russell29. The variation with substituents in the low isotope effects for the reaction of aryl radical with arene thiols were explained using such an effect. We may possibly further account for the lower intrinsic barrier for the R-H-Cl system (3.8 Kcal) than for the R-H—S system (5 Kcal) in terms of the greater electronegativity of chlorine. 

Chapter 2 presents the principles of the Linear Combination of Atomic Orbitals (LCAO) approximation. This is the dominant approximation of qualitative (and 99 % of quantitative) MO-theory. It has become so established that we very often lose sight of the fact that it is an approximation and there have been many controversial discussions about methods for analysing the properties of individual atoms within molecules over the last 25 years. Chapter 2 also describes the effects of the electronegativities of the individual elements and introduces the Walsh diagram approach to analysing the structures of small molecules and fragments. 

Thiol esters have been studied by Rasmussen [18], and more recently by Nyquist and Potts [82] and Baker and Harris [83]. Dialkyl thiolesters show their carbonyl bands in the narrow range 1698 to 1690cm". The reasons for this substantial fall as compared with normal esters are not fully understood and have been the subject of some controversy [80]. However, it is probably a direct consequence of the low electronegativity of sulphur. a -Con-jugation in aryl alkyl thiolesters lowers the frequency to 1665 to 1670 cm" which is a rather larger shift than is found in esters. On the other hand the rise in frequency when an aromatic ring is attached to the sulphur atom is less than in the phenyl esters, and such compounds absorb near 1710 cm". ... 

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