High Accuracy ab Initio Calculations on Reactions of OH with 1-Alkenes. The Case of Propene.

The energetics of terminal, central OH-additions as well as allylic H-abstractions by OH in its reaction with propene was studied as proxies for the 1-alkenes + OH reactions using several single and multireference ab initio techniques with basis set extrapolation where possible. Selection of the localized occupied orbitals forming the active space for multireference methods is discussed. Initial geometries of the reactants, prereaction complex (π-complex), and transition states were determined at the [5,5]-CASPT2/cc-pVTZ level of theory. Frequency analysis was also carried out at this level with the introduction of a scale factor. Analyzing the results, it will be concluded that multireference effects are negligible, and from the various single reference models we will opt for UCCSD(T)/cc-pVTZ for final geometry optimizations and vibrational frequency analysis. These results will be compared with those from approximate models yielding information on the reliability of the latter. Triples contributions are found to be very important, except for the π-complex, which has a UCCSD(T)/CBS relative enthalpy of -10.56 kJ/mol compared to infinitely separated propene + OH. The addition transition states are found to have relative enthalpies of -9.93 kJ/mol for the central and -9.84 kJ/mol for the terminal case. Allylic abstraction mechanisms, although lying significantly higher, still have only slightly positive barriers - a value of 3.21 kJ/mol for the direct and 1.67 kJ/mol for the consecutive case. Conventional transition state theory was used as a rough estimation for determining rate constants and turned out to agree well with experimental data.