Delocalization interactions anomeric effect

Hence, the anomeric effect [positive value of AE(AE3)] can appear at a suitable combination of the V and V coefficients. Because the V, term is associated with the dipole-dipole interactions of polar groups, and the V2 term with delocalization interactions of lone-pair orbitals on heteroatoms, it may be concluded that the latter equation also effectively describes a balance... 

The balance of electrostatic and delocalization interactions in an isolated molecule may be perturbed by the influence of the solvent. In calculations based on Eq. 7, the analysis of solvation-energy terms suggested that the electrostatic contribution stabilizing the ap orientation of the acetal s ment is the conformationally dominant term. For example, in 2-methoxyoxane, the difference in energy of the (ap, ap) and (ap, sc) conformers in water, compared to that in the isolated molecule, caused by solute-solvent electrostatic interactions alone, amounts to 4 kJ.mor. Accordingly, the inter-and intra-molecular, electrostatic interactions operate in reverse directions in acetals. Whereas the intramolecular, electrostatic interactions are responsible, together with delocalization interactions, for the aiq)earance of the anomeric, reverse anomeric, and exo-anomeric effects, the solute-solvent electrostatic interactions lessen their im nitude, and may even cancel them. Of course, the solvent may also influence the electron distribution and energy of MO s in a molecule. In this way, the orbital interactions of lone-pairs and delocalization contributions to the anomeric effect may be scaled by the solvent, but this mechanism of the environmental effect is, in most cases, of only minor importance. 

The theoretical basis of the anomeric effect is a matter of controversy. Electrostatic interactions, electron delocalization and negative hyperconjugation have all been postulated as the dominant forces involved (Deslong-champs, 1983 Kirby, 1983 Schleyer and Kos, 1983 Sinnott, 1988). The view that the anomeric effect may be based on a combination of contributions from electrostatic interactions and electron delocalization (Romers et al., 1969), does not seem to have gained much currency. 

The failure of n->o interactions and the electron-delocalization model to rationalize the reverse anomeric effect is argued by Sinnott (1988) to be the major flaw in this model. 

The axial conformation in MM4 is stabilized by the anomeric effect, which is represented largely by substantial (-1.85) and V2 (-1.30) torsion potentials (Fig. 7.1). The bond dipoles in the equatorial conformation add up to give a greater dipole moment, and a higher dipole-dipole repulsion energy than in the axial conformation. Thus, increasing the dielectric constant tends to stabilize the equatorial conformation more than it stabilizes the axial, as shown in Table 7.5. The anomeric effect in MM4 is the summation of the electronic delocalization effect included in the torsion potential, plus contributions from torsion-stretch and torsion-bend interactions, plus the electrostatic effect from the interaction of the bond dipoles. 

Product or reactant stabilizing factors that have been studied thus far include resonance/charge delocalization, solvation, hyperconjugation, intramolecular hydrogen bonding, aromaticity, inductive, jr-donor, polarizability, steric, anomeric, and electrostatic effects, as well as ring strain and soft-soft interactions. Product or reactant destabilization factors are mainly represented by anti-aromaticity, steric effects in some types of reactions, and, occasionally, electrostatic effects. What makes the PNS particularly useful is that it is completely general, mathematically provable,4 and knows no exception. 

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