Inherent atom electronegativity

Infrared (IR) spectroscopy, 65-71. 235, 238 of carbonyl complexes, 428-431. 633-634 Inherent atom electronegativity. 186... 

Such symbiosis is also observed on the inherent atomic hardnesses that are not included in Table 5. When considered. separately, CF3 carbon (f) = 38.04) is much harder than CHj carbon (fj = 17.91) the latter being more electronegative. Both CFj and CHj seem to become much softer, when bonded to an aromatic ring (fj = 8.8). Value fj = 8.43 is the mean average value for all types of C(sp ). The phenomenon of symbiosis has been observed experimentally and described in many situations [18]. 

Since chlorine is less electronegative than fluorine, the role of the inherent relative acidities of the positions on the pyridine ring becomes more important in chloropyridine lithiations. Not surprisingly, 197 is lithiated cleanly in the 4-position rather than the less acidic 3-position (Scheme 98). When 198 is lithiated with BuLi, the preference for 4-lithiation wins out even though the intermediate lithium lacks an adjacent acidifying Cl atom. By contrast, LDA lithiation of this compound under thermodynamic control gives the expected regioselectivity. A similar effect is observed in the lithiation of 196 with BuLi and with LDA . 

As noted above, the hardness or softness of an acidic or basic site is not an inherent property of the particular atom at that site, but can be influenced by the substituent atoms The addition of soft, polarizable substituents can soften an otherwise hard center and the presence of electron-withdrawing substituents can reduce the softness of a site. The acidic boron atom is borderline between hard and soft. Addition of three hard, electronegative fluorine atoms hardens the boron and makes it a hard Lewis acid. Conversely, addition of three soft, electropositive hydrogens54 softens the boron and makes it a soft Lewis acid. Examples of the difference in hardness of these two boron acids are... 

Modulators associated with toxicophore No. 4 cf Table 19.2. Each of the modulators augments or decreases the activity inherent in the toxicophore (i.e., 51.0 units see Figs. 19.1-19.3). (HOMO + LUMO)/2 describesthe electronegativity of the molecule. That value is multiplied by —25.8. Modulator No. 5 and No. 2 are shown embedded in chemicals in Figs. 19.2 and 19.3, respectively. Modulator No. 1 describesa 2D distance descriptor of 6.5 A between two atoms. For interpretation ofthe structures see legend to Table 19.2. 

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