¹⁵N and³¹P NMR Insights into Lactam–Lactim Tautomerism Activity Usingcyclo-μ-Imidopolyphosphates

Abstract

The effects of the molecular structure and solution pH on compounds prone to lactam-lactim tautomerism have been evaluated by (15)N NMR spectroscopy. The lactam-lactim tautomerism activities of cP3O6(NH)3(3-) and cP4O8(NH)4(4-) showed a significant pH dependence, with the process being inactivated under alkaline conditions because of the decrease in the number of hydrogen atoms by the deprotonation of the anions. The tautomerism was activated under the acidic conditions by the increase in the number of dissociative hydrogen atoms resulting from the protonation of the anions. cP3O6(NH)3(3-) has much more of a planar molecular structure than cP4O8(NH)4(4-), meaning that the hydrogen atoms in cP3O6(NH)3(3-) would be delocalized over the entire structure to a greater extent than those in cP4O8(NH)4(4-). This difference in the distribution of hydrogen atoms would result in the lactam-lactim tautomerism activity of cP3O6(NH)3(3-) being higher than that of cP4O8(NH)4(4-). The results have shown that the following factors are critical to the achievement of an efficient anhydrous proton conductor: (1) the regular molecular arrangement of highly planar molecules; (2) the existence of a large number of dissociative protons in a molecule; and (3) a molecular structure with a small energy barrier for the structural rearrangement required of the tautomerism process.