FMMF method

The Dewar-Golden-Harris approach (DGH), also called FMMF method (i.e. Field, Mesomeric and Mesomeric-Field method) [Dewar et al, 1971a] is a modification of the Dewar-Grisdale approach, where the substituent X is approximated by a finite dipole (represented by two point charges along the /-X bond) and the reaction site at position i as a single point charge. This approach is based on the equation ... 

FMMF method Dewar-Golden-Harris approach -> eiectronic substituent constants... 

All these problems can be avoided in the following general treatment of substituents (FMMF method), based on the PMO approach. In this we take account of all three substituent effects ( ,M,M ) and calculate them explicitly... 

Since the and qij can be calculated for any system, there are just three parameters to be determined for each substituent (F, M, MFg) and two for each reaction center (Pr, Pr ). These are found by fitting suitable experimental data, e.g., dissociation constants of various substituted carboxylic acids. In the original Hammett treatment, there were just two parameters for each substituent (cr, Gp) and one for each reaction center (p) however, this approach was limited to benzene derivatives only. Equation (4.143) should apply to molecules of all kinds, saturated as well as conjugated, and it does indeed seem to do so in quite an effective manner. This approach has been termed the FMMF method. 

Although the FMMF method dramatically reduces the number of substituent constants that one must consider, it suffers from the disadvantage that it is not possible to visually compare two reactions with a three-parameter equation, i.e., one cannot simply plot logs of rates or equilibrium constants against a single parameter as one does in the normal Hammett treatment. It is possible, however, to calculate Hammett substituent constants for any position using the FMMF parameters and the following relationship ... 

FIGURE 4.16. Logarithms of relative rates of solvolysis of ( O) meta- and ( ) para-substituted cumyl chlorides in 90% aqueous acetone at 25°C (relative to the un-substituted compound) plotted against a constants calculated by the FMMF method (rate data from Ref. 7). 

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