NPE Effects in Bimolecular Nucleophilic Substitution

By use of the force fields established in the two preceding papers, relative NPE effects are calculated for bromide ion exchanges (the Finkelstein reactions) for a series of simple alkyl bromides. The effects on rate constants cover a range of about 30:1, neglecting contributions (if any) from variation in rotational symmetry numbers. These calculated results are analyzed in detail and compared with related literature results, both experimental and calculated. Extension of the force fields permits computation of relative NPE effects for reaction of methyl bromide with hydroxide ion and with a series of alkoxides. Effects for alkoxides differ only slightly within the group, but the NPE contribution to the rate constant for reaction with hydroxide is nearly twice that for reaction with methoxide. Interpretations for these findings are suggested.