¹H MAS, ¹⁵N CPMAS, and DFT Investigation of Hydrogen-Bonded Supramolecular Adducts between the Diamine 1,4-Diazabicyclo-[2.2.2]octane and Dicarboxylic Acids of Variable Chain Length

Supramolecular adducts of formula [N(CH 2 CH 2 ) 3 N]−H−[OOC(CH 2 ) n COOH] ( n = 1−7) have been obtained by mechanochemical reaction of solid dicarboxylic acids of variable chain length HOOC(CH 2 ) n COOH ( n = 1−7) with solid 1,4-diazabicyclo[2.2.2]octane (dabco). 1 H MAS and 15 N CPMAS spectra have been measured to investigate the presence of intermolecular hydrogen bonds between acid and base. Proton and nitrogen chemical shifts allow a distinction to be made between N + −H···O - interactions (with proton transfer) and N···H−O interactions (without proton transfer) and between strong and weak hydrogen bonds. Correlations among isotropic 1 H, 15 N chemical shift data and the N−O distances of the atoms involved in the hydrogen bond interaction are found. Density functional theory, applied to explore changes upon hydrogen bonding in the 1 H and 15 N shielding parameters, is in agreement with the experimental values found by solid-state NMR spectroscopy. Hydrogen/deuterium H/D isotope effects on the 15 N NMR chemical shifts have been also investigated.