Estimation of Charge Density on Nitrogen in Amides by Measurement of One-Bond Carbon-Hydrogen Nuclear Coupling Constants in N-CH3 Group

Abstract

Abstract One-bond, 13C-1H coupling constants, J1(C-H), 'in amines, ammonium ions, and carboxylic amides correlate with structure and support the concept that the value of J1(C-H) is related to the charge density on the nitrogen atom; for example, amine oxides have nearly the same charge density at nitrogen as does the tetramethylammonium ion. The J1(C-H) values for methyls bonded to nitrogen in various amides then give an experimental estimate of the charge density at the nitrogen atom that enables an estimate of the bond order in the C-N amide-bond; the data suggest that carboxylic amides have a C-N bond order of about 1.35, that sulfonamides have an S-N bond order of about 1.45, and that phosphinamides, R2 P(O)N(CH3)2 , have a P-N bond order of about 1.3. In contrast, aminephosphines have a P-N single bond. The value for carboxylic amides is in reasonable agreement with bond distances in amides.