Electronegativity perturbation

Earlier in this chapter we considered the effect of orbital interactions on a previously noninteracting system. But suppose now we take as starting point two interacting orbitals 4>A and B of equal energy and we introduce a change in electronegativity at centers A and B. The qualitative results of such a perturbation are again well known from elementary quantum chemistry ... 

When an electronegativity perturbation occurs, two molecular orbitals originally containing equal components of two atomic orbitals localize so that the lower one is primarily on the more electronegative atom, the upper one on the less electronegative atom. 

Correlation diagrams include the product orbitals while perturbation approaches require knowledge of the empty orbitals of the reactant. However, the occupied molecular orbitals of diazirine (II), compared with those of cyclopropene (I), do seem to give some indication of a preferred thermal decomposition of (II) compared with the rearrangement of (I). Moreover these molecular orbitals are a typical illustration of the localization obtained in the presence of an electronegativity perturbation. 

The E-state indices are atomic descriptors composed of an intrinsic state value I and a perturbation AI that measures the interactions with all other atoms in a molecule. The Kier-Hall electronegativity is the starting point in the definition of the intrinsic state of an atom, which encodes its potential for electronic interactions and its connectivity with adjacent atoms. The intrinsic state of an atom i is [19, 21] ... 

Another consequence of this surface sol-gel process on SBA-15 is the reduction of the surface silanol groups induced by the condensation reaction with titaniumbutoxide. The reaction, after hydrolysis, converts surface silanol functionality, (SiO)3SiOH, into Ti-OH functionality. Given the similar electronegativity of silicon (1.8) and titanium (1.7), the expected consequence of this modification in the solid state Si NMR is a decrease in the band and a corresponding increase in the Q band that is, the OTi(OH)3 and OSi(OH)3 ligands will cause equivalent NMR shift perturbations on the substituted silicon site. 

The calculations of Feibelman and Hamann have expressly addressed the surface electronic perturbation by sulfur as well as by Cl and The sulfur-induced total charge density vanishes beyond the immediately adjacent substrate atom site. However, the Fermi-level density of states, which is not screened, and which governs the ability of the surface to respond to the presence of other species, is substantially reduced by the sulfur even at nonadjacent sites. Finally, the results for several impurities indicate a correlation between the electronegativity of the impurity and its relative perturbation of the Fermi-level density of states, a result which could be very relevant to the poisoning of H2 and CO chemisorption by S,C1, and as discussed above. 

These qualitative conclusions coincide well with the observed geometries of E(NSN)2E molecules. In where the electronegative perturbation is greatest,... 

Fig. 12 a—c. A schematic representation of the frontier % MO s of an S3N3X molecule, as derived from those of SsNf (a). The diagram illustrates the effects of (b) electronegative perturbation at the tricoordinate sulfur atom and (c) conjugation with the n symmetry orbitals of the substituent... 

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