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

Abstract

Supramolecular adducts of formula [N(CH2CH2)3N]−H−[OOC(CH2)nCOOH] (n = 1−7) have been obtained by mechanochemical reaction of solid dicarboxylic acids of variable chain length HOOC(CH2)nCOOH (n = 1−7) with solid 1,4-diazabicyclo[2.2.2]octane (dabco). 1H MAS and 15N 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 1H, 15N 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 1H and 15N shielding parameters, is in agreement with the experimental values found by solid-state NMR spectroscopy. Hydrogen/deuterium H/D isotope effects on the 15N NMR chemical shifts have been also investigated.