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

Joint Experimental and Computational Investigation of the Flexibility of a Diacetylene-Based Mixed-Linker MOF: Revealing the Existence of Two Low-Temperature Phase Transitions and the Presence of Colossal Positive and Giant Negative Thermal Expansions

International audienceSolvothermal reaction in N,N-dimethylformamide(DMF) between 1,6-bis(1-imidazolyl)-2,4-hexadiyne monohydrate(L1·H2O), isophthalic acid (H2L2), and Zn(NO3)2·6H2Ogives the diacetylene-based mixed-ligand coordination polymer{[Zn(L1)(L2)](DMF)2}n (UMON-44) in 38% yield. Combinationof DSC with variable-temperature single-crystal X-raydiffraction revealed the occurrence of two phase transitionsspanning the ranges 129–144 K and 158–188 K. Furthermore,the three structurally similar phases of UMON-44 show giantnegative and/or colossal positive thermal expansions. Theseunusual phenomena exist without any change in the contentsof the unit cell. DFT calculations using the PBE+D3dispersion scheme were able to distinguish between thesepolymorphs by accurately reproducing their salient structuralfeatures, although corrections in the size of the unit cellturned out to be necessary for the high-temperature phaseto account for its large thermal expansion. In addition, theinfrared spectra (vibration frequencies and peak intensities)of these theoretical models were calculated, allowing forunivocal identification of the corresponding polymorphs.Last, the limits of our computational method were tested bycalculating the phase transition temperatures and their associatedenthalpies, and the derived figures compare favorablywith the values determined experimentally