Relationships Involving Olefinic Carbons

Selected results are presented in Table 6.3 for typical olefinic carbon atoms. The Mulliken net charges were obtained [40] from full (geometry and exponents) optimizations in the STO-3G basis. 

TABLE 6.3. Atomic Charges (me) and NMR Shifts of Olefinic Carbon Atoms 

Here again, a and tt populations vary in opposite directions. Using now Eqs. (6.3) and (6.5), that is, the slope 

This rough estimate is to be taken cum grano salts (with a grain of salt)— it follows from energy calculations and bmte-force fits with experimental energy data [44]. It is probably not precise, because the contributions of the ip -carbon charge variations are rather small and likely to be blurred by uncertainties of the experimental energy data. 

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Nucleophilic attack by this species at the a-carbon atom will also be governed by the same steric considerations as in thermal decomposition and hence the inverse relationship of thermal stability and resistance to nucleophilic attack, and anti-wear activity and ease of nucleophilic attack. Further thermal processes involve olefin elimination from alkyl groups and lead to the formation of phosphorus acids. Nucleophilic substitutions of one phosphorus species by another leads to P-O-P structures and zinc mercaptide Zn(SR)2 as a reaction intermediate. Reaction of this mercaptide with dithiophosphate leads to trithiophosphates and eventually tetrathiophosphates. Finally, an oil-insoluble deposit is formed of a mixture of zinc thiophosphate and zinc pyro- and polypyrothiophosphates. 

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