BASICITY OF UREA - NEAR-INFRARED SPECTROSCOPIC AND THEORETICAL-STUDIES ON THE HYDROGEN-BONDING ABILITY OF TMU AND DMDPU

The hydrogen-bonding interactions between thioacetamide (TA) and urea derivatives such as tetramethylurea (TMU) and dimethyldiphenylurea (DM DPU) have been studied using near-infrared absorption spectroscopy. Th ermodynamic parameters for the interactions between TA and urea deriva tives were determined by analyzing the v(N-H)(as)+Amide II combination band of TA at 1970 nm. The Delta H degrees values, indicating the int rinsic strength of hydrogen bonding, are -23.0 kJ/mol and -19.8 kJ/mol for TMU and DMDPU, respectively. This is well explained by the induct ive effects of substituents. Ab initio molecular orbital calculations for the proton affinity of TMU, N,N'-dimethylformamide (DMF), and N,N- dimethylacetamide (DMA) in gas phase have been carried out at HF/3-21G ad HF/6-31G(d) levels. showing that the proton affinity of TMU is lar ger than that of DMA, which agrees with the experimental results.