ABSTRACT Rate constants for restricted rotation around the N2-N3 single bond in cis-1,3-diphenyltriazenes (ArN1=N2–N3HAr’), from a set comprising a total of twenty-two meta and para symmetrical and unsymmetrical substituted substrates, are analyzed by means of an extended Hammett equation. A double-variable fitting renders revised Hammett reaction constant values of –1.88 ± 0.08 and 0.70 ± 0.08 for N1 and N3, respectively. Interestingly, the observed rate constants ascribed to restricted rotation in cyano-containing substrates are found to be pH-dependent. The latter is ascribed to a change in the ionization state of the substrate, a conclusion supported by solvent isotope effects. The pH-dependence also reveals the influence of aryl substituents on delocalization of π electrons and, hence, on restricted rotation, as tautomeric cis-1,3-diphenyltriazenes exhibit opposite pH effects. Resulting Hammett reaction constants for restricted rotation in deprotonated (anionic) 4-cyano-containing cis-1,3-diphenyltriazenes are –1.98 ± 0.03 and 0.54 ± 0.07 for N1 and N3, respectively.
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