Resonance effect parameters interpretations

Although cTi estimates by different methods or from different data sets may disagree, it is generally held that the inductive effect of a substituent is essentially independent of the nature of the reaction. It is otherwise with the resonance effect, and Ehrenson et al. have defined four different ctr values for a substituent, depending upon the electronic nature of the reaction site. An alternative approach is to add a third term, sometimes interpreted as a polarizability factor, and to estimate the inductive and resonance contribution statistically with the added parameter the resonance effect appears to be substantially independent of reaction site. " " ... 

The error in Hiickel s treatment lies not in the quantum mechanical calculations themselves, which are correct as far as they go, but in the oversimplification of the problem and in the incorrect interpretation of the results. Consequently it has seemed desirable to us to make the necessary extensions and corrections in order to see if the theory can lead to a consistent picture. In the following discussion we have found it necessary to consider all of the different factors mentioned heretofore the resonance effect, the inductive effect, and the effect of polarization by the attacking group. The inclusion of these several effects in the theory has led to the introduction of a number of more or less arbitrary parameters, and has thus tended to remove significance from the agreement with experiment which is achieved. We feel, however, that the effects included are all justified empirically and must be considered in any satisfactory theory, and that the values used for the arbitrary parameters are reasonable. The results communicated in this paper show that the quantum mechanical theory of the structure of aromatic molecules can account for the phenomenon of directed substitution in a reasonable way. 

In contrast to other oxidative processes run under silent conditions, no PTC is necessary. In a two-phase system consisting of aqueous KMn04 and indane in benzene, an 80% yield can be obtained, provided the pressure in the reaction vessel is reduced to ca. 450 torr. This effect is interpreted by a resonance between the ultrasonic frequency and the vibration frequency of the bubbles, the radii of which is a function of the pressure (Ch. 2, p. 54). Optimal energy transfer is ensured under these conditions, and the importance of the pressure parameter, not frequently evidenced in sonochemistry, is illustrated. 

Taft and Topsom s article57 and also Topsom s66 should be consulted for details of the setting up of the scales of substituent parameters. The equation has been applied to a wide range of gas-phase reactivities. (In the multiple regressions an intercept term is often permitted, but usually this turns out to be indistinguishable from zero, as it should be if equation 16 is valid.) For aliphatic and alicyclic saturated systems the resonance term is duly negligible. The roles of field, resonance and polarizability effects are discussed and the interpretation of the various p values is attempted. 

C chemical shifts in aromatic compounds are dependent on the polarity of the substituent. Appendix 3, Table A3.14 shows the substituent effects for a range of substituted benzenes. The 13C spectra of substituted benzenes can often be interpreted on the basis of these substituent parameters in association with data from off-resonance decoupled spectra. 

The simplest, most direct, and most precise determination of bond distances and bond angles from rotational constants is from equilibrium values of these constants. Equilibrium parameters have a well-defined interpretation and are virtually invariant to isotopic substitution. Unfortunately, the required spectra in the first excited vibrational states are nearly always very difficult to obtain. In addition, the rotational constants must be free of the effects of perturbations and resonances. As a result, equilibrium structures have been obtained only for diatomic molecules and a few small polyatomic molecules. An example is the structure of S02 obtained by Morino et al.19 (Table 1). Also shown in the table is the approximate re structure called the rm structure by Watson.17... 

The calculated energy differences give a good correlation with Hammett ct+ values. The p parameter (p = -17) is considerably larger than that observed experimentally for proton exchange (p -8). A physical interpretation of this difference is that the computational results pertain to the gas phase, where substituents are at a maximum because of the absence of any leveling effect owing to solvation. Note that the numerical results parallel the conclusions from qualitative application of resonance... 

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