Substituent Effect of N,N-Dialkylamides on the Intermolecular Hydrogen Bonding with Thioacetamide

The alkyl substituent effect on the intermolecular hydrogen-bonding properties of tertiary amides has been investigated experimentally and theoretically. N,N-Dimethylformamide (DMF), N,N-diethylformamide (DEF), N,N-diisopropylformamide (DIF), N,N-dimethylacetamide (DMA), N,N-diethylacetamide (DEA), and N,N-diisopropyl acetamide (DIA) were chosen as proton acceptors for thioacetamide (TA) in CCl4 solution. Thermodynamic parameters for the interaction of tertiary amides with TA have been measured by near-infrared (NIR) absorption spectroscopy. For DMF, DEF, and DIF, the standard enthalpy change of 1:1 hydrogen-bonded complex formation is about −19.5 kJ/mol and the equilibrium constant at 298 K is about 60 M-1. For DMA, DEA, and DIA, the standard enthalpy change increases from −18.9 to −19.8 kJ/mol and the equilibrium constant at 298 K decreases from 65 to 50 M-1, as the alkyl group becomes bulky. The IR data of N−H stretching vibrations of TA are very consistent qualitatively with the NIR thermodynamic data. The temperature dependence of proton NMR chemical shift has been examined for N−Hs of TA to understand the characteristics of hydrogen bonding. We suggest that more hydrogen-bonding interaction via the hydrogen syn to the sulfur atom of TA could take place in the formamide series, compared to the acetamide series. The proton affinity of these tertiary amides is calculated by using HF/6-31+G*, HF/6-31G**, and B3LYP/ 6-31G** levels of ab initio molecular orbital theory.