Substituent effects theoretical calculations

Recently, with the advent of efficient programs for carrying out ab initio molecular orbital calculations (5), it has become possible to examine substituent effects theoretically. In one such study, Hehre et al. (6) have conducted STO-3G calculations on a large range of monosubstituted benzenes. In an extension of that work, we present here an account of similar calculations on both disubsti-tuted and polysubstituted benzenes. 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

Table 12.4. Substituent Effects on Radical Stability from Measurements of Bond Dissociation Energies and Theoretical Calculations of Radical Stabilization Energies...

Substituent effects as evaluated on the basis of the Hammett equation and its extended forms, are - this has to be emphasized again — empirical results. Nevertheless, it is very soothing to know that theoretical approaches, i. e., calculations of substituent effects using ab initio molecular orbital theory (Topsom, 1976, 1981, 1983 Taft and Topsom, 1987, STO-3G and 4-31G level), give results that are consistent with the experimental data. However, it is not recommended to use only theoretically calculated substituent constants and values for F, R, and other parameters for the interpretation of experimental data. 

Very recently there has been an experimental and theoretical study of electronic substituent effects in 4-aminoaryl (4-substituted aryl) sulfones146. PMR, 13C NMR and infrared measurements were involved and semi-empirical all-valence CNDO/2 calculations, with and without sulfur d orbitals, were carried out. Various correlations between spectral results and substituent constants are presented. There is good agreement between experimental and theoretical data, which does not depend on the inclusion or exclusion of the sulfur d orbitals from the calculations. 

A theoretical value for the magnitude of dJ/dEz was obtained using the delocalized molecular orbital approach of Gil and Teixeira-Dias 17> who calculated substituent effects on. The Pople expression for the contact contribution to the coupling constant 1 c-h1 °f a methyl group can be written... 

There is considerable need for exploration of interaction effects If SCSs are to be used for signal assignments or structure determinations, it is essential to know about alterations of SCSs by interactions with other substituent(s) to avoid misinterpretations. Additionally, interaction effects provide valuable information about the o-electron distribution and its dependence on structure, since it is well known that 13C chemical shifts are highly sensitive to changes in the geometry and/or electronic state of the molecule. This research area is not easily accessible experimentally by other spectroscopic methods, at least for larger molecules, which are also beyond the reach of most theoretical calculations. 

In addition to these probes, theoretical calculations are being made in efforts to decide upon the inherently most stable stmcture of the ion. These will be discussed along with the conclusions which have been derived from the study of isotope effects in solvolysis and substituent effects in stabilizing media. 

The nature and position of substitution has a profound influence upon the oxepin-arene oxide equilibrium position. The effect of substituents on the relative energies of each valence tautomer has been calculated (80JA1255) and these theoretical results are in accord with the limited experimental data which are available. In general terms, oxepins substituted at the 3-position are less favored than the corresponding arene oxides, while the reverse obtained for 2- and 4-substituted oxepins. This substituent effect has been rationalized in terms of a preference for the maximum number of alternative resonance contributors. The influence of both 7r donating and v withdrawing substituents oh the oxepin contribution is summarized in Scheme 2. This latter effect may be considered as an electronic substituent effect. 

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