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) 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). 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.

Hydrogen Bonding and Dynamic Behaviour in Crystals and Polymorphs of Dicarboxylic–Diamine Adducts: A Comparison between NMR Parameters and X‐ray Diffraction Studies

“…Frey and Sternberg [49,50] have observed direct relationships between d In a previous paper we have investigated the proton transfer reaction along a hydrogen bond between aliphatic dicarboxylic acids and the diamine base 1,4-diazabicyclo-[2.2.2]octane (DABCO), by means of 1 H, 13 C, and 15 N solidstate NMR spectroscopy. [51] We founded that the isotropic 13 C chemical shift moved to higher frequencies as it passes from the carboxylic acid to the carboxylate form. The 13 C chemical shift tensor analysis of the carboxylic group showed that d 22 is the most sensitive parameter, changing from 192 AE 6 for the deprotonated form to 162 AE 5 ppm for the protonated form.…”

Solid‐State NMR Studies on Supramolecular Chemistry

“…Direct relationships between δ 1 H and HB strength and between δ 1 H and X-H distance for different classes of hydrogenbonded compounds have been reported. 37 Thus, NMR shift data can be used to detect and to estimate the presence and the strength of HBs (Scheme 1).…”

“…The 1 H spectrum of the adduct between malonic acid and DABCO, reported in Figure 4(a), clearly shows in the high frequency region two well defined HB signals at 17.6 and 12.4 ppm that can be assigned to the O-H· · ·O and the N + -H· · ·O − protons, respectively (Figure 4b). 37 Owing to the intrinsic difficulty of X-ray techniques in detecting the hydrogen atom position, an effective method that combines 1 H fast MAS NMR and density functional theory (DFT) calculation for the determination of the O-H distance has been proposed. 38 It consists in refining the hydrogen atom position within the HB, in which the computed 1 H δ are reported as a function of the O-H distance while all other atoms are retained at their positions.…”

“…Using the experimental δ value of 16.5 ppm (Figure 5a), the plot for the succinic acid-DABCO adduct (Figure 5b) gives an O-H distance of 1.043Å, which is in good agreement with the value obtained by geometry optimization (1.039Å). 37 The 1 H chemical shift is not the only NMR parameter that can provide insights in HB interactions but also the 13 and δ 22 change their values in opposite direction. δ 22 , the chemical shift tensor that lies perpendicular to the plane of symmetry of the C=O group, is the most diagnostic parameter that reflects the HB strength ( Figure 6).…”

Coupling Solid-State NMR with GIPAW ab Initio Calculations in Metal Hydrides and Borohydrides

“…Refs. [14][15][16][17][18] ), ab initio calculations of NMR properties in the solid state are less developed. A first attempt to compute the magnetic shielding in molecules was performed by using localized atom-centred basis sets, based on the Molecular Orbital Theory.…”