Perturbational molecular orbital approach

Biemann (1962a) noted that his type-D reaction was the mass-spectrometric equivalent of the reverse Diels-Alder reaction. Several examples of this type of fragmentation are now known (Budzikiewicz et al., 1965 Maccoll, 1968a). Dougherty (1968b), assumed that both the thermal and electron impact-induced processes were electrocyclic in his explanation of the apparent similarity between the type D process and the reverse Diels-Alder reaction in terms of a simple perturbation molecular orbital approach (Section VIIID). 

The SPARC (Sparc Performs Automated Reasoning in Chemistry) approach was introduced in the 1990s by Karickhoff, Carreira, Hilal and their colleagues [16-18]. This method uses LSER [19] to estimate perturbed molecular orbitals [20] to describe quantum effects such as charge distribuhon and delocalizahon, and polarizability of molecules followed by quanhtative structure-activity relationship (QSAR) studies to correlate structure with molecular properties. SPARC describes Gibbs energy of a given process (e.g. solvation in water) as a sum of ... 

One of the most used approaches for predicting homoaromaticity has been the perturbational molecular orbital (PMO) theory of Dewar (1969) as developed by Haddon (1975). This method is based on perturbations in the Hiickel MO theory based on reducing the resonance integral (/3) of one bond. This bond represents the homoaromatic linkage. The main advantage of this method is its simplicity. PMO theory predicted many potential homoaromatic species and gave rise to several experimental investigations. 

Antiaromaticity [1] is the phenomenon of destabilization of certain molecules by interelectronic interactions, that is, it is the opposite of aromaticity [2], The SHM indicates that when the n-system of butadiene is closed the energy rises, i.e. that cyclobutadiene is antiaromatic with reference to butadiene. In a related approach, the perturbation molecular orbital (PMO) method of Dewar predicts that union of a C3 and a Ci unit to form cyclobutadiene is less favorable than union to form butadiene [3]. 

It is clear that there are quite a few possible theoretical approaches to the formulation of a comprehensive model for the adsorption processes discussed above. The most fundamental one would be based on the perturbation molecular orbital (PMO) theory of chemical reactivity [730,731] in which the wave functions of the products are approximated using the wave functions of the reactants. A key issue in the use of Klopman s PMO theory is the relative importance of the two terms in the expression for the total energy change of the system, Afpen. which is taken to be a good index of reactivity, [732,733] ... 

Up to a few years ago chemical reactivity was discussed in term of reactivity indexes. These approaches, although valuable, will not be discussed here, since they have been frequently reviewed in the past40-44). Nor will we discuss the perturbation molecular orbital theory for reactants, which has been the subject of extensive reviews 45—47) Extensions of this method can be found in papers by Klopman 48 5°) and Dougherty 51). I shall now mention some methods which have not yet found wide popularity but seem very promising. I mean the criterion of maxi-... 

The group transfer reactions can be easily explained by perturbation molecular orbital (PMO) approach. If both the R groups migrate without inversion, in a... 

In this chapter the molecular orbital theory covered in Chapter 1 is applied to the problem of aromaticity. The HMO approach is shown to be somewhat limited in its usefulness. However the PMO (perturbational molecular orbital) method, which uses Hiickel orbitals, does lead to satisfactory criteria of... 

In a recent book Dewar (1969) has given a general account of the perturbation molecular orbital (PMO) approach and its relevance in discusrions on physical-organic chemistry. Of particular interest is one application which provides a general theory of pericyclic reactions (Dewar, 1969, 1971), The PMO treatment in this context has but one central concept, namely the potential aromaticity of pericyclic transition states (see p. 61), and may be generalized into a single definitive rule ... 

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. 

However, due to the availability of numerous techniques, it is important to point out here the differences and equivalence between schemes. To summarize, two EDA families can be applied to force field parametrization. The first EDA type of approach is labelled SAPT (Symmetry Adapted Perturbation Theory). It uses non orthogonal orbitals and recomputes the total interaction upon perturbation theory. As computations can be performed up to the Coupled-Cluster Singles Doubles (CCSD) level, SAPT can be seen as a reference method. However, due to the cost of the use of non-orthogonal molecular orbitals, pure SAPT approaches remain limited... 

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