Detection of Secondary Amides in HCN Polymers by Dipolar Rotational Spin-Echo¹⁵N NMR

The solids formed by base-catalyzed polymerization of HCN or by electrical discharge through an atmosphere of methane and ammonia have been examined by cross-polarization magic-angle spinning dipolar rotational spin-echo¹⁵N NMR. This experiment measures nitrogen-proton dipolar coupling and so can be used to count the average number of protons attached to a nitrogen whose NMR resonance is resolved. On the basis of such measurements, we conclude that the majority of the amidine or amide nitrogens in the base-catalyzed HCN polymers examined are secondary and hence peptide-like. This is not proof for the direct formation of heteropolypeptides from HCN, however, since none of the amidine or amide nitrogens observed are bonded to aliphatic carbons, as established by double cross-polarization¹³C and¹⁵N NMR. The anhydrous solids formed by the electrical discharge have aliphatic carbons but little or no amidine functionality, and so they too are unlikely precursors for the direct formation of heteropolypeptides.