Theoretical modelling of hydrogen transfer reactions

• Main Author: Izsak, Robert
• Published: Cardiff University 2010
• Subjects: 541.2; QD Chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.584770

The first part of this thesis deals with some general aspects of hydrogen transfer reactions.  Based on the idea of similarity between localized orbitals of functional groups in different molecules, an attempt is made to reflect this transferability in segments of the correlation energy belonging to the set of orbitals of a certain functional group.  Various possibilities are examined for such partitioning.  It turns out that localized orbitals are the best choice for this purpose since other transformations delocalize orbitals, and transferability is lost. In the second half, the energetics of terminal and central OH-additions as well as allylic H-abstractions by OH in its reaction with  propene was studied using several single and multireference ab initio techniques. Selection of the localized occupied orbitals forming the active space for multireference methods is discussed.  Initial geometries optimizations and vibrational frequency analysis were carried out at the [5,5]-CASPT2/cc-pVTZ level of theory.  Multireference effects turned out to be negligible and the UCCSD(T)/cc-pVTZ model was chosen for final geometry optimizations and vibrational frequency analysis.  Triples contributions are found to be very important, except for the pi-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 experiment.