Intermolecular 2hJNN coupling in multiply hydrogen-bonded ureidopyrimidinone dimers in solution

15N-Labeled ureido-4[1H]-pyrimidinones 4a and 5a were synthesized in order to investigate hydrogen bonding in the strongly hydrogen-bonded dimers in solution with intermolecular 2hJNN coupling. Both direct-detection 15N NMR and one-dimensional 15N INADEQUATE (for smaller scalar coupling constants) were employed to determine the coupling constants. For dimers of 4 in CDCl3, a temperature-dependent 2hJNN was observed ranging from 2 Hz at +10 C to 5.1 Hz at -20 C. In dimers of more slowly exchanging bifunctional derivative 5, the coupling constants could be determined at room temperature from an inverse-gated 1H-decoupled 15N NMR experiment. Coupling constants in different isomers of the dimer of 5a (4.96, 5.13, 4.37, and 5.27 Hz) were used to test the relationship between 2hJNN values and N-N distances as proposed by Del Bene et al. The N-N distances calculated with the aid of this relationship show excellent agreement with the distances observed in the X-ray crystal structures of 5b, particularly when the nonlinearity of the hydrogen bonds is taken into account.