Theoretical and experimental approaches to evaluate the intermolecular hydrogen-bonding ability of tertiary amides

The intermolecular hydrogen-bonding abilities of five tertiary amides, N,N-dimethylacetamide (DMA), N,N-dimethylpropionamide (DMP), N,N-dimethylisobutylamide (DMIB), N,N-diethylacetamide (DEA) and N,N-diethylpropionamide (DEP), have been investigated experimentally and theoretically. The strength of the hydrogen-bonding interaction with thioacetamide (TA) as proton donor in CCl4 has been measured using near-infrared and infrared absorption spectroscopy and 1H NMR spectroscopy. All the experimental results reveal clearly a decrease in the hydrogen-bonding ability in the order DMA>DMIB>DMP and DEA>DEP. The near-infrared spectrum of TA provides the standard enthalpy change for 1:1 hydrogen-bonded complex formation as − 18.9, − 17.3, − 18.5, − 19.3 and − 18.3 kJ mol−1 for DMA, DMP, DMIB, DEA and DEP, respectively. The ab initio proton affinity of tertiary amides calculated at the DFT/B3LYP/6-31G** level follows the same sequence as that of the experimental results. To confirm these notable results, the association energies for DMA, DMP and DMIB complexes with TA were computed at the DFT/B3LYP/6-31G** and 6-311++G** levels, showing consistently the order DMA>DMIB>DMP. We suggest that the repulsion between alkyl substituents at the carbonyl carbon-site and the nitrogen-site could influence the hydrogen-bonding ability of tertiary amides.