H-1, C-13, and N-15 Solid-State NMR Studies of Imidazole- and Morpholine-Based Model Compounds Possessing Halogen and Hydrogen Bonding Capabilities

The halogen and hydrogen bonding interactions present in solid 1-(2,3,3-triiodoallyl)imidazole (1), morpholinium iodide (2), the 1:1 cocrystal 1-(2,3,3-triiodoallyl)imidazole-morpholinium iodide (3), morpholine (4), imidazole (5), and 1-(3-iodopropargyl)imidazole (6) have been investigated by solid-state H-1, C-13, and N-15 NMR spectroscopies. Comparison of the N-15 CP MAS NMR spectrum of 3 with that of 2 indicates that protonated morpholine is present in solid 3, but this conclusion must be taken with caution as GIPAW calculations predict a N-15 chemical shift for morpholine similar to that of the morpholinium cation. Conclusive evidence for the presence of a morpholinium cation in crystalline 3 was obtained by recording the static N-15 NMR spectrum of this host-guest complex and comparing the morpholiniun/morpholine part of the spectrum with the static spectra of 3 and 4 as obtained from ab initio calculations of NMR parameters based on the X-ray structures of these compounds. Concerning the imidazolyl group, N-15 NMR spectroscopy has proven quite valuable to identify changes in the bonding situation of the C-N = C nitrogen on passing from 1 to 3. In addition, slight differences are observed between the N-15 chemical shifts of 1 and 6 that are ascribed to differences in halogen bond strengths between the two compounds. Attempts have also been made to study halogen bonding by C-13 NMR spectroscopy, but this method did not provide exploitable results as signals corresponding to the sp and sp(2) carbon atoms bonded to iodine could not be observed experimentally. H-1 NMR spectroscopy is a powerful tool to study hydrogen bonding interactions of moderate energies such as +NH2 center dot center dot center dot X (X = N, O, I). Indeed, we have found that the chemical shifts of the NH hydrogens were quite sensitive to the nature of X and to the N-H center dot center dot center dot X distance. This is demonstrated by the fact that the chemical shifts of the +NH2 protons of the morpholinium cation in 2 and 3 are noticeably different.