Chang-Ju Yoon

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.

The energetically favorable cis peptide bond for the azaglycine-containing peptide: For-AzGly-NH2 model

The conformational preferences of the azaglycine-containing peptide model, For-AzGly-NH2 (1), were investigated with ab initio and DFT methods for the cases when the formyl group has either a trans (1a) or cis (1b) peptide bond. Based on the HF/6-31G* potential energy surfaces, the minimum energy conformations for 1 were characterized. The structures and energetic relations between the resulting minima for 1 were systematically examined using various basis sets (6-31G*, 6-31G**, 6-311G** and 6-31 + G**). An important result, that the global minimum of 1b (ω0≈0°) is more stable than that of 1a (ω0≈180°), was found. This is comparable to the glycine peptide model, For-Gly-NH2 (2), whose global minimum energy conformation prefers the trans peptide bond to the cis peptide bond as already known. The resulting minima for 1 demonstrate its utility as a valuable biological probe.

NEAR-INFRARED SPECTROSCOPIC STUDIES OF THE HYDROGEN BONDING BETWEEN THIOACETAMIDE AND N,N-DISUBSTITUTED BENZAMIDE DERIVATIVES IN CCL4

Abstract The hydrogen bonding interactions between thioacetamide (TA) and several N , N -disubstituted benzamides ( N , N -dimethylbenzamide (DMBA), N -methoxy- N -methylbenzamide (MMBA), N , N -diethyl- m -toluamide (DEMT), and N , N -diethyl-2,5-difluorobenzamide (DEDF)) have been studied using near-infrared absorption spectroscopy. Thermodynamic parameters for the interactions between TA and benzamides were determined by analyzing the ν N-H as + amide II combination band of TA. The − ΔH 0 values, indicating the intrinsic strength of hydrogen bonding, are −17.4, −21.6, −21.9 and −20.8 kJ mol −1 for DMBA, MMBA, DEMT and DEDF, respectively. The results show that the inductive and resonance effects of substituents appear to influence the formation of hydrogen bonds.