Dewar’s method

Goldstein and Hoffmann have extended their arguments to a number of possible geometries of interaction in addition to the simple single ring.15 Dewar has also presented a perturbation approach to aromaticity that is historically antecedent to that of Goldstein and Hoffmann. We shall return to the topic of aromaticity, and describe Dewar s method, when we discuss pericyclic reactions in Chapter 11. 

Dewar s method has received considerable criticism, and has not been used in all-valence electrons calculations. 

Trinajstic et examined some type A systems containing sulfur, including various thienopyrroles 460 (R = H), 389 (R H), 89 (R = H), and 461 (R — H). Dewar s method was used for ground-state calculations and the method of Pariser and Parr for the excited state, though sulfur c/ orbital contributions were ignored. Calculated electronic transitions corresponded to those found from UV spectra (Section V,E) for 460 (R = 389 (R = CH Ph), and 89 (R = Et), " and a... 

In the Dewar s method, the mixture of noble gases is introduced from a gasholder into a bulb filled with coconut charcoal. This bulb is placed in a cold bath (-100°C). The mixture of gases is allowed to remain there for about half an hour during which time Ar, Kr and Xe get adsorbed. He and Ne, which remain unadsorbed, are pumped out of the bulb and kept in contact with another lot of charcoal maintained at -180°C whereby Ne is adsorbed, leaving He in the free state and the latter is pumped out. Ne is recovered by heating the charcoal. 

Dewar s method of working forward from reactant and backward from product,... 

Use Dewar s method in combination with other more qualitative reasoning to estimate the relative ease of displacement of chlorine by ethoxide ion for the mono-chloroquinolines and the monochloroisoquinolines. 

The most convenient way of defining MINDO/3 is in relation to the earlier methods. The basic form of the equations was similar to those in INDO, only in MINDO/3 the origin of the parameters was different. For example, rather than use atomic spearal data to define 17, the one-electron integral was made an adjustable parameter. MINDO/3 had several of these parameters, which were adjusted to give the best fit to experimental data for molecules. Here we see a basic difference between Dewar s methods and those that had gone before. Slater s rules were abandoned instead the orbital exponent was made a parameter, as were the multipliers in the resonance integral, By adjusting the parameters to fit molecular data rather than theory, as with Slater s rules, or spectra, as with the values, a vastly improved result was obtained. 

It remains to consider the performance of Dewar s method (p. 45) for evaluating localization energies. The essential equation, already discussed there, is... 

Dewar s method builds on Hiickel s earlier work regarding tt bonding in hydrocarbons. 

One example showing a serious discrepancy of the frontier electron method was reported by Dewar H8,ii9). This is 10,9-borazaphenanthrene, and the value of / -B) was reported to have been calculated by the Pople method, but the parameters usyd were not indicated. Fujimoto s calculation by the Pariser-Parr-Pople method 120>, in perfect disagreement with Dewar s, gives the most reactive position as 8, which parallels experiment. The ambiguity involved in the integral values adopted seems to be serious, so that the establishment of parametrization for boron heterocycles is desirable. 

Dewar s perturbation molecular orbital (PMO) method analyzes the interactions that take place on assembling p orbitals in various ways into chains and rings.44 It is similar to the methods we have used in Section 10.4 in considering aromaticity, but lends itself better to a semiquantitative treatment. We shall nevertheless be concerned here only with the qualitative aspects of the theory as it applies to pericyclic transition states. 

Algebraic expressions for terms M and C were derived using Dewar s PMO method (for C in a version similar to the co-technique [57] in order to calculate carbocation stabilization energies). The size factor S is simply a cubic function of the number of carbon atoms [97], The three independent variables of the model were assumed to be linearly related to the experimental Iball indices (vide supra). By multilinear regression analysis (sample size = 26) an equation was derived for calculating Iball indices from the three theoretical parameters. The correlation coefficient for the linear relation between calculated and experimental Iball indices is r = 0.961. 

Semiempirical molecular orbital methods23-25 incorporate parameters derived from experimental data into molecular orbital theory to reduce the time-consuming calculation of two-electron integrals and correlation effects. Examples of semiempirical molecular orbital methods include Dewar s AMI, MNDO, and MINDO/3. Of the three quantum chemical types, the semiempirical molecular orbital methods are the least sophisticated and thus require the least amount of computational resources. However, these methods can be reasonably accurate for molecules with standard bond types. 

After participating in the development of the MINDO method in Michael Dewar s group, Colin Baird initiated a research program at Western Ontario based on ab initio calculations on the ground and excited states of molecules.220 221 Over the years his interests shifted more to chemical education, with a special interest in developing concepts and materials for environmental chemistry. 

To expand a bit on Dewar s cautious endorsement of the SCF procedure [20] ( SCF calculations are by no means foolproof . ..Usually one finds a reasonably rapid convergence to the required solution ) occasionally a wavefunction is obtained that is not the best one available from the chosen basis set. This phenomenon is called wavefunction instability. To see how this could happen note that the SCF method is an optimization procedure somewhat analogous to geometry optimization (Section 2.4). In geometry optimization we seek a relative minimum or a transition state on a hypersurface in a mathematical energy versus nuclear coordinates space defined by E =/(nuclear coordinates) in wavefunction... 

The first (1967) of the Dewar-type methods was PNDDO [35], partial NDDO), but because further development of the NDDO approach turned out to be unexpectedly formidable [33], Dewar s group temporarily turned to INDO, creating MINDO/1 [36] (modified INDO, version 1). The third version of this method, MINDO/3, was said [33] [to have] so far survived every test without serious failure , and it became the first widely-used Dewar-type method. Keeping their promise to return to NDDO the group soon came up with MNDO (modified NDDO). MINDO/3 was made essentially obsolete by MNDO, except perhaps for the study of carbocations (Clark has summarized the strengths and weaknesses of MINDO/3, and the early work on MNDO [37]). MNDO (and MNDOC and MNDO/d) and its descendants, the very popular AMI and PM3, are discussed below. Briefly mentioned are a modification of AMI called SAMI and an... 

SAMI (semi ab initio method number 1) was the last semiempirical method to be reported (1993, [76]) by Dewar s group. SAMI is essentially a modification of AMI in which the two-electron integrals are calculated ab initio using contracted... 

Cyclooctatetraene itself does not undergo 4 + 2 cycloadditions it is its valence isomer bicyclo[4.2.0]octatriene which reacts with dienophiles.36 Use Dewar s PMO method to explain why the direct reaction with cyclooctatetraene is disfavored. 

A modification of Dewar and Harget s method has been employed to predict the electronic spectra of a range of heterocyclic compounds, including pyrido[2,3- ]quinoline and all known phenanthrolines, and give satisfactory agreement with experimental values <82IJC(B)334>. 

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