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 acce ptor have been compared experimentally and theoretically. The near IR absorption spectroscopic technique has been employed to measure the th ermodynamic data for the hydrogen-bonding interactions of N,N-dimethyl thioformamide (DMTF) with proton donors such as thioacetamide (TA) and thiopropionamide (TPA), The nu(N-H)(as) + Amide II combination band o f TA and TPA at similar to 1970 nm has been chosen. The enthalpy chang es for the formation of DMTF:TA (1:1) and DMTF:TPA (1:1) hydrogen-bond ed complexes are measured to be - 5.8 +/- 0.2 and - 5.0 +/- 0.2 kcal/m ol, respectively, which are almost 2 times larger than those of N,N-di methylformamide (DMF):TA and DMF:TPA complexes, However, the equilibri um constant for the hydrogen bonding formation of DMTF with TA (TPA) i s similar to 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 initi o molecular orbital calculations using the 6-311G* basis set at the B 3LYP level, showing that the PA value of DMTF is larger than that of D MF. For the first time, we present the experimental and theoretical co mparison on the ability of hydrogen bonded complex of thioamide and am ide. (C) 1998 Elsevier Science B.V, All rights reserved.