Dynamic H NMR investigation along with a theoretical study around the C-C and C = C bonds in a particular phosphorus ylide

摘要

In the present work, the dynamic ~1H NMR effects were investigated at variable temperatures within a particular phosphorus ylide involving a 2-benzoxazolinone around the carbon-carbon single bond and also partial carbon-carbon double bond in two Z- and E-rotational isomers. Activation and kinetic parameters including ?H~≠, ?G~≠, ?S~≠ and E_a were determined in accord with the dynamic H NMR data for three rotational processes. In addition, theoretical studies based upon rotation around the same bonds were investigated using ab initio and DFT methods at the HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory. Theoretical activation and kinetic parameters including ?H~≠,?G~≠, ?S~≠ and E_a were calculated at 298 K and experimental temperatures for five rotational processes. These results (experimental and theoretical), taken together, indicate that the rotational energy barrier around the C = C double bond was considerably high and the observation of the two rotational isomers was impossible (seen as a single isomer) at the high temperatures, in this case rotation around the C = C bond was too fast on the NMR time scale. When the temperature was relatively low, the rate of rotation was sufficiently slow; therefore, observation of the two Z- and f-isomers was possible. In addition, calculations at the HF/6-31G(d,p) level of theory showed very favorable results in agreement with the experimental data on rotation around the C = C bond. While, B3LYP level using the 6-31G(d,p) basis set was provided the reasonable data for the restricted rotations around the C-C and C-N single bond.