| Abstract: | Formation and thermodynamic characteristics of C-H ... O hydrogen bonding of methylated uracils and caffeine have been studied by nmr along two lines. 1. The concentration and temperature dependencies of the PMR spectra of 1,3-dimethyluracil (m2 1,3Ura), 1,3-dimethylthymine (m2 1,3Thy), and 1,3,6-trimethyluracil (m3 1,3,6Ura) in chloroform at high concentrations of base analogs indicated the self-association of m2 1,3Ura and m2 1,3Thy via C(6)H ... O hydrogen bonding and the competitive formation of C-H ... O bonds between carbonyl oxygens and chloroform. The intermolecular interaction energy and the arrangement of molecules in the local minima of various m2 1,3Ura dimers were calculated by the method of atom-atom potentials. The deepest minimum for the m2 1,3Ura coplanar dimer corresponds to a C(6)-H ... O hydrogen-bond formation. 2. At low concentration of m2 1,3Ura and caffeine in CCl4, C(6)-H ... O bonding for m2 1,3Ura and C(8)-H ... O bonding for caffeine with oxygens of dimethyl sulfoxide (DMSO) and acetone were observed. The association constants of these complexes were obtained at different temperatures. The enthalpies delta H, of the m2 1,3Ura-DMSO, m2 1,3Ura-accetone, caffeine-DMSO, and caffeine-acetone complexes were -2 +/- 0.1 kcal/mol. The calculations showed that the deepest minimum of the caffeine-acetone coplanar complex corresponds to C(8)-H ... O bonding with energy of -3.5 kcal/mol and that of the m2 1,3Ura-acetone complexes corresponds to C(6)-H ... O bonding with energy of -3.4 kcal/mol. The approximate correction for the solvent effect provides good agreement of the experimental data with the calculations. |
