¹⁵N and ³¹P NMR Insights into Lactam–Lactim Tautomerism Activity Using cyclo-μ-Imidopolyphosphates

The effects of the molecular structure and solution pH on compounds prone to lactam–lactim tautomerism have been evaluated by ¹⁵N 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.