Friday, October 03, 2008

ASAP Cryst. Growth Des., ASAP Article, 10.

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

Karim Bouchmella,† Sylvain G. Dutremez,*† Bruno Alonso,‡ Francesco Mauri,§ and Christel Gervais*#

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 1H, 13C, and 15N NMR spectroscopies. Comparison of the 15N 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 15N 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 15N NMR spectrum of this host−guest complex and comparing the morpholinium/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, 15N NMR spectroscopy has proven quite valuable to identify changes in the bonding situation of the C−NC nitrogen on passing from 1 to 3. In addition, slight differences are observed between the 15N 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 13C NMR spectroscopy, but this method did not provide exploitable results as signals corresponding to the sp and sp2 carbon atoms bonded to iodine could not be observed experimentally. 1H NMR spectroscopy is a powerful tool to study hydrogen bonding interactions of moderate energies such as +NH2···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···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.

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