rH NMR Spectroscopy of the Hydrogen-Bonded Imide Groups of Hub(M)a:3CA Provides a Useful Method for the Characterization of These Aggregates

Variable-temperature lH NMR spectroscopy of the region between 13 and 16 ppm can be used in the conformational analysis of hydrogen-bonded aggregates comprising 1 equiv-of a trismelamine (hub(M)s) and 3 equiv of an isocyanuric acid (CA). T*o typer of isomersare obsenred: a symmetric C3 isomer (identified by two lines in this spectral region at low temperature) and an asymmetric C1 isomer (identifred by six lines at low temperature), each existi.rg ir a pair of enantiomers. The isomers have similar structures: the N-H connectivity established by nuclear Overhauser effects are consistent with a rosette motif. The relative concentrations of isomers depend slightly on solvent (CDzCIz, CDCls, CzDzCI+) and on the structure of the CA (barbital, dibromobarbituric acid, triphenylpropylisocyanuric acid, and neoherylisocyanuric acid); the Ca isomer is favored over the C1-isomer by ca. 0-7 kcaVmol in all cases. This N-// region of the NMR spectrum also carries information about the dynamic behavior of these "gg""g"ter. The exchange of hydrogen-bonded imide protons between different environments leadslo tti" coalescence of pairs of inride lines. This exchange-mediated coalescence occurs at different temperatures and allows (with additional NOE data) individual CA groups to be identifred. The activation energy for the exchange process or p_rocesses leading to coalescence of pairs of imide lines is -14 kcaVmol. This proc".r i, faster than the interconversion between Cs and C1 isomers.