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(CH2CH2)(3)N]-H-[OOC(CH2)(n)COOH] (n = 1-7) have been obtained by mechanochemical reaction of solid dicarboxylic acids of variable chain length HOOC(CH2)(n) COOH (n = 1-7) with solid 1,4-diazabicyclo[2.2.2]octane (dabco). H-1 MAS and N-15 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 center dot center dot center dot O- interactions (with proton transfer) and N center dot center dot center dot H-O interactions (without proton transfer) and between strong and weak hydrogen bonds. Correlations among isotropic H-1, N-15 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 H-1 and N-15 shielding parameters, is in agreement with the experimental values found by solid-state NMR spectroscopy. Hydrogen/deuterium H/D isotope effects on the N-15 NMR chemical shifts have been also investigated.