Proton and carbon‐13 nuclear magnetic resonance studies of substituted pyrimidines. V-13C, 1H coupling contsants for some methyl‐ and aminopyrimidines

Abstract
J(13C1H) coupling constants for some methyl‐ and aminopyrimidines have been determined by 13C NMR. Both the one‐bond and long‐bond and long‐range coupling constants follow general trends which can be summarized in a few simple rules. In particular, the 3J(C‐i,H) coupling constants between a ring carbon C‐i and the ring protons are larger than the 2J(C‐i,H) coupling constants. The opposite is observed for the couplings between the ring carbons and the methyl protons: 3J(C,Me). These general rules are very useful for the assignment of resonances in complex 13C spectra of pyrimidines and seem to be valid for other 6‐membered aromatic nitrogen heterocycles. Furthermore, the additivity of substituent effects on 1J (CH) for monosubstituted pyrimidines allows the estimation of 1J (CH) for polysubstituted pyrimidines with a very good accuracy.