A comparative study on the hydrogen bonding ability of amide and thioamide using near IR spectroscopy

The hydrogen bonding abilities of amide and thioamide as a proton acceptor have been compared experimentally and theoretically. The near IR absorption spectroscopic technique has been employed to measure the thermodynamic data for the hydrogen-bonding interactions of N,N- dimethylthioformamide (DMTF) with proton donors such as thioacetamide (TA) and thiopropionamide (TPA). The v(N-H)/(as) + Amide II combination band of TA and TPA at ~ 1970 nm has been chosen. The enthalpy changes for the formation of DMTF:TA (1:1) and DMTF:TPA (1:1) hydrogen-bonded complexes are measured to be - 5.8 ± 0.2 and - 5.0 ± 0.2 kcal/mol, respectively, which are almost 2 times larger than those of N,N-dimethylformamide (DMF):TA and DMF:TPA complexes. However, the equilibrium constant for the hydrogen bonding formation of DMTF with TA (TPA) is ~ 6 times smaller than that of DMF. The proton affinity (PA) of O-site on DMF and that of S-site on DMTF were studied by ab initio molecular orbital calculations using the 6-311G** basis set at the B3LYP level, showing that the PA value of DMTF is larger than that of DMF. For the first time, we present the experimental and theoretical comparison on the ability of hydrogen bonded complex of thioamide and amide.