Perturbation method qualitative applications

The previous section was designed merely to provide some indication of the accuracy and limitations of perturbation methods. However, there is little point in doing approximate calculations by hand when the cheapest computers can give the correct results in seconds. The real utility of perturbation theory lies in qualitative applications. We will begin to explore its potential by asking, for example (1) will an end be more reactive exercise a given electrophile than ethylene and (2) will an electrophile attack the end at C, or at Os ... 

The study of photoinduced electron transfer between molecular donors and acceptors provides a means to assess the electronic coupling provided by the DNA helix. Early applications of this method to DNA-mediated reactions utilized reactants noncovalently bound to DNA [28-30] These studies provided qualitative information concerning the efficiency and distance dependence of electron transfer, but the ambiguity associated with random distributions of reactants along the DNA helix precluded a quantitative analysis. Once chemical methods were developed for the covalent attachment of donors and acceptors to DNA [31], the distance dependence of these reactions, as well as the effects of perturbations within the intervening medium, could be systematically studied. The identification of unnatural DNA bases with appropriate photophysical and redox properties has also helped to define the extent of electronic coupling provided by the base stack [16]. 

The application of Mossbauer spectroscopy in diverse fields of qualitative and quantitative analysis is based on the ease with which hyperfine interactions can be observed. The information obtained from Mossbauer spectroscopy may be correlated with other methods by which HI can be examined such as NMR, EPR, ENDOR, PAC (perturbed angular correlations), nuclear orientation and neutron scattering. However, Mossbauer spectroscopy often proves to be experimentally simpler, more illustrative and an efficient method for studying applied problems. Mossbauer nuclei are ideal spies supplying information on both the microscopic and macroscopic properties of solids. 

MNDOC is a correlated version of MNDO. Unlike all previously discussed methods, MNDOC includes electron correlation explicitly and thus differs from MNDO at the level of the underlying quantum chemical approach (a) while being completely analogous to MNDO in all other aspects (b)-(d) except for the actual values of the parameters. In MNDOC electron correlation is treated conceptually by full configuration interaction, and practically by second-order perturbation theory in simple cases (e.g., closed-shell ground states) and by a variation-perturbation treatment in more complicated cases (e.g., electronically excited states).The MNDOC parameters have been determined at the correlated level and should thus be appropriate in all MNDO-type applications which require an explicit correlation treatment for a qualitatively suitable zero-order description. In closed-shell ground states... 

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