Theoretical Investigation Of The Reactions Of First-Row Transition Metal Ions With Acetaldehyde And Acetone

Because the reaction of transition metal ions with carbonyl containing compounds is of fundamental importance in catalysis, oxidation, corrosion, and biological processes, the reactivity of ketones and aldehydes with various transition metal ions has been systematically studied by a number of experimental techniques. The experimental results have provided a wealth of accurate thermochemical data and proposed some reaction mechanisms for this type of reactions. In spite of this impressive progress, a complete characterization of the actual reaction mechanisms and the information of different intermediates involved need for a close collaboration between experiment and theory. In this thesis, acetone and acetaldehyde are chosen as models for carbonyl compounds, and the reaction mediated by Sc+, Fe+, and Ni+ is theoretically investigated by density functional theory (DFT). All the stationary points on the potential energy surfaces (PESs) are completely characterized. As a representation of earlier first-row transition metal ions (Sc+, Ti+, and V+), the theoretical investigation of reaction Sc+(3D and 1D) + acetone indicates that oxidation of Sc+ by (CH3)2CO can take place on both triplet and singlet PESs, starting with the formation of encounter complex followed by three possible production pathways: (i) metal-mediated H migration, (ii) direct methyl H shift, and/or (iii) C-O insertion. The singlet pathways are always energetically preferable with respect to the corresponding triplet routes. Along the singlet paths, intersystem crossings take place for the triplet ground-state reactants crossing to the surface. The most favorable pathway passes through the metal-mediated H migration on the singlet surface and the exclusive oxidation of the Sc+/(CH3)2CO system is the result of the extremely high oxygen affinity of Sc+. As the representations of late first-row transition metal ions (Fe+, Co+, and Ni+), the reaction of acetone with Fe+ and Ni+ are investigated to gain new insight into the reaction of Ni+ and Fe+ with acetone. The C2H6 and CO loss of (CH3)2CO by M+ proceeds through five elementary steps, i.e., encounter complexation, C-C activation, methyl migration, C-C coupling, and nonreactive dissociation, giving strongly exothermic products M+(CO) + C2H6. For the Fe+/acetone system, the reaction can take place on both quartet and sextet PESs. The sextet reaction coordinate constitutes a higher-energy required pathway, whereas the quartet pathway through intersystem crossing from the sextet ground-state reactants is always preferable. The alternative concerted C-C coupling mechanism proposed early for a C-C insertion minimum to rearrange directly to an exit-channel precursor cannot take place. Theory strongly suggests that the rate-determining transition state in the M+/acetone reaction is methyl migration rather than the earlier-invoked initial C-C activation. The theoretical work on the Ni+ + CH3CHO reaction provides further insight into the mechanistic details of decarbonylation of acetaldehyde with late first-row transition metal ions. Combining with our previous studies concerning the reaction of the series of Cr+-Fe+ with acetaldehyde, it is found that for the Cr+, Co+, and 4Fe+ mediated systems decarbonylation of CH3CHO only take place via C-C activation, and aldehyde C-H activation is unlikely to be important. On the other hand, both C-C and aldehyde C-H activations could result in the decarbonylation of CH3CHO with Ni+ and 6Fe+, where potential minimum M+(H)(CO)(CH3) is located to be a common species on the decarbonylation pathways.