Distortion-interaction model

A review of pericyclic reactions has included discussion of the distortion-interaction model of reactivity, the importance of CH- r interactions in controlling selectivity, and the role of transition metal and organocatalysts. The presence of water in pericyclic reactions has also been reviewed. A density response function has been reported as a test for an allowed pericyclic reaction in conjunction with the chemical hardness at the onset of the reaction. ... 

We have followed a phenomenological approach and used the cluster model [18]. In this model the eg-type distortion interacts more strongly with the electronic state of an octahedral coordinated Cr3+ ion than the distortions of t2g symmetry. According to Ham [19], we assume that the continuum of vibrational modes with eg character can be approximated by a single mode with an effective frequency o>, mass /r and coupling constant V. The collective coordinates of the eg mode are conventionally known as Qe x2 — y2) and Qs ( 3z2 — r2). The linear Jahn-Teller Hamiltonian in equation (1) for the X state is [18] ... 

The second aspect of this paper is one of presenting a structural picture of the polymer-water interface, based on the potential-distortion adsorption model. Adjacent polymer chains Interact by non-bonded, that is, van der Waals forces, so that, locally, the surface should show resemblances to that of a molecular solid. A type B situation Is postulated, with local restructuring to from pockets into which water molecules have Inserted themselves, as depicted in Figure 12. As the water activity (vapor pressure, if vapor phase Is present) Increases, a layer several molecules thick builds up. The layer Is sufflclent-... 

Although sophisticated electronic structure methods may be able to accurately predict a molecular structure or the outcome of a chemical reaction, the results are often hard to rationalize. Generalizing the results to other similar systems therefore becomes difficult. Qualitative theories, on the other hand, are unable to provide accurate results but they may be useful for gaining insight, for example why a certain reaction is favoured over another. They also provide a link to many concepts used by experimentalists. Frontier molecular orbital theory considers the interaction of the orbitals of the reactants and attempts to predict relative reactivities by second-order perturbation theory. It may also be considered as a simplified version of the Fukui function, which considered how easily the total electron density can be distorted. The Woodward-Hoffmann rules allow a rationalization of the stereochemistry of certain types of reactions, while the more general qualitative orbital interaction model can often rationalize the preference for certain molecular structures over other possible arrangements. 

Ess DH, Houk KN (2008) Theory of 1,3-dipolar cycloadditions distortion/interaction and frontier molecular orbital models. J Am Chem Soc 130(31) 10187-10198. doi 10.1021/ja800009z... 

It is not a trivial task to make a good calibration set. The set should be designed so that each constituent spans the anticipated concentrations. As the number of constituents increases, this becomes progressively more difficult. The temptation is to omit constituents or use pure constituents, so that one mixture is 100% of a single constituent. This should be avoided unless it is possible that the constituent could indeed be pure in an unknown. It is more likely that a constituent could be missing from an unknown, in which case a mixture with a missing constiment is acceptable. The reason that pure samples should not be used is that none of the constituent interactions will be present in the spectra. Therefore, more than one parameter of the mixmre will be missing, which can distort the model considerably. 

Equation (23) predicts a dependence of xR on M2. Experimentally, it was found that the relaxation time for flexible polymer chains in dilute solutions obeys a different scaling law, i.e. t M3/2. The Rouse model does not consider excluded volume effects or polymer-solvent interactions, it assumes a Gaussian behavior for the chain conformation even when distorted by the flow. Its domain of validity is therefore limited to modest deformations under 0-conditions. The weakest point, however, was neglecting hydrodynamic interaction which will now be discussed. 

Since the hydrodynamic interaction decreases as the inverse distance between the beads (Eq. 27), it is expected that it should vary with the degree of polymer chain distortion. This is not considered in the Zimm model which assumes a constant hydrodynamic interaction given by the equilibrium averaging of the Oseen tensor (Eq. 34). 

If the central atom has different groups or atoms around it, or if one or more of the vertices of the polyhedron is occupied by a lone pair, then variations in bond angles will occur such that distorted polyhedral arrangements are obtained. In its quantitative forms, the VSEPR model parameterizes each individual interaction and makes very accurate predictions of the distortions which are to be expected. 

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