Table 12.5. Substituent Effects on Stability of AilyUc and Benzyiic Radicals from 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. [Pg.150]

Very recently there has been an experimental and theoretical study of electronic substituent effects in 4-aminoaryl (4-substituted aryl) sulfones PMR, 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. [Pg.519]

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. [Pg.2]

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. [Pg.179]

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. [Pg.243]

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