Proton magnetic resonance studies on the self-association and hydrogen bonding of imidazole in chloroform solutions

Self-association of imidazole and its hydrogen bonding to N, N-dimethylacetamide (DMA), tri-n-butyl phosphate (TBP) and tri-n-octyl phosphine oxide (TOPO)in chloroform have been investigated by PMR method. It was found that imidazole exists predominantly as trimers in chloroform solution. The trimerization constants were found to be 54.0, 32.9, 27.1 and 19.9 at −13°, 13°, 27°, and 43°C, respectively. The value of ΔH for trimerization was determined to be −3.1 kcal/mol; ΔG° and ΔS at 27°C were −2.0 kcal/mol and −3.6 e.u., respectively. The phenomenon of up-field shifts of the NH proton frequency in imidazole with increasing amounts of donor molecule could be interpreted as the result of competition between imidazole hydrogen bonding to the donor and its self-association. The association constants for imidazole hydrogen bonding to DMA, TBP and TOPO were found to be 7.1, 8.6 × 10, and 2.3 × 102 1./mol, respectively, at 25°C, and the values of ΔH were found to be −4.2, −6.5, and −5.3 kcal/mol, respectively.