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-dimethylisobut ylamide (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 i n CCl4 has been measured using near-infrared and infrared absorption spectr oscopy and H-1 NMR spectroscopy. All the experimental results reveal clearl y 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 entha lpy 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/B3L YP/6-31G** level follows the same sequence as that of the experimental resu lts. To confirm these notable results, the association energies for DMA, DM P and DMIB complexes with TA were computed at the DFT/B3LYP/6-31G** and 6-3 11++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 terti ary amides.