Investigation Of The Gas-Phase Oxygen-Atom Transport Catalyzed By Transition Metal Ions

The oxygen-atom transport catalyzed by transition metal ions is of paramount importance in chemical engineering, biology, and environmental process. Now, it is more and more popular in designing and improving catalysts at a molecular level, which is based on the extensively understanding of the catalytic reaction mechanisms. In"real-life situation", however, the solvents and other disturbing factors obscure the intrinsic features of a reaction center or the reactive intermediates. Gas-phase experiments combining with theoretical calculations are particularly well-suited for the elucidation of basic properties of isolated molecules and probing the elementary reactions because they are not hampered by various disturbing factors. In this thesis, the reaction of N₂O with CO, alkane (C₂H₆), alkine (C₂H2), and arene (C6H₆) catalyzed by Mn+, Fe⁺, Co⁺, and Ni+ is chosen as a model for studying the catalysis of oxygen-atom transport by quantum chemistry calculations. The intrinsic mechanisms of these reactions are systematically investigated and the rate-determining steps are elucidated. The calculated results are as follows.In the gas-phase Co⁺ and Ni+-mediated N₂O reduction by CO, firstly, metal ions reaction with N₂O forms N₂ and MO+ via the N-O insertion mechanism. Then, the nascent oxide reduction by CO could account for the M+-regenerated product, CO₂, through the C-O coupling mechanism. The NiO+ formation is inhibited by both the high energy barrier and the spin inversion. On the other hand, spin inversion also slows down the reaction rate in CoO+ formation process. Thus, both metal ions are unable to work as a catalyst in N₂O reduction by CO. Coordination of second N₂O by Co⁺ has a positive effect, leading the catalytic reaction could proceed along a thermodynamically and kinetically highly favorable path.The Fe⁺-catalyzed oxidation of acetylene by N₂O starts with Fe⁺(6D and 4F) reaction with N₂O yielding FeO+ through direct O-abstraction and N-O insertion mechanisms, respectively. For the second leg of catalytic cycles, after the coordination of acetylene by nascent FeO+, the reaction could yield ethynol via direct H abstraction mechanism. Alternatively, the system also converts into a"metallaoxacyclobutene"structure, followed by four possible pathways, i.e., direct dissociation (for producing formylcarbene), C-C insertion (for products ketene), C-to-O hydrogen shift (for ethynol), and/or C-to-C hydrogen shift (for ketene and CO). The most favorable channel is oxidation to ketene and carbon monoxide along the cyclization - C-to-C hydrogen shift pathway. Reduction of the CO loss partner FeCH2+ by another N₂O molecule constitutes the third step of the catalytic cycle, which involves a direct abstraction of O-atom from N₂O giving OFeCH2+ and then an intramolecular rearrangement to form formaldehyde adduct Fe⁺-OCH2. Considering the energy acquired from the initial reactants, this reaction is also energetically favored.In the gas-phase Fe⁺ and Co⁺-mediated oxidation of ethane by N₂O, Fe⁺ and Co⁺ reduction N₂O gives rise to metal oxide ion firstly. For second step of the catalytic cycle, after the coordination of ethane by nascent oxide, the direct and/or stepwise (metal ion mediated) H shift could carry the system into hydroxyl intermediate (HO)M+(CH2CH3), which could account for H2O and C₂H4 loss products viaβ-H shift channel and ethanol elimination product through C-O coupling pathway. The H2O loss partner MC₂H4+ could react with another N₂O molecule. N₂O coordinates to MC₂H4+ and gets activated by metal ion to yield (C₂H4)M+O(N₂) through a N-O insertion mechanism. The thermal (C₂H4)M+O(N₂) would release a nitrogen molecule and then is further oxidized through two different mechanisms, i.e., direct H abstraction and/or cyclization. The former mechanism accounts for the ethenol formation. Additionally, for Fe⁺, the reaction also could form the byproducts of FeC₂H2+ and FeOH+. The other mechanism involves a c-CH2CH2OM+ structure, followed by four possible pathways, i.e., C-to-O hydrogen shift (for producing ethenol), C-O coupling (for products oxirane), C-to-C hydrogen shift (for acetaldehyde), and/orα-H abstraction (for acetaldehyde). Additionally, for Fe⁺, the reaction also could form formaldehyde via C-C insertion pathway and methane throughα-H abstraction channel. The C₂H4 loss partner MOH2+ also could react with N₂O generating active MO+ after stepwise loss of N₂ and H2O. Fe⁺ could catalyze the reaction of ethane with N₂O, but its selectivity is very poor because yielding a lot of byproducts. For Co⁺, the mainly products are acetaldehyde and ethanol, indicating a good selectivity. However, Co⁺ is unable to work as a catalyst due to the poor efficiency of the CoO+ formation at room temperature.The Mn+, Co⁺, and Ni+-catalyzed oxidation of benzene by N₂O may involve two different catalytic cycles, i.e., mediated by M+(benzene) and MO+, respectively. In the M+(benzene)-mediated catalytic cycle, for both Mn+ and Co⁺, after the initial formation of M+(benzene), the oxidant N₂O coordinates to the nascent complex and gets activated by metal ion to yield (C6H₆)M+O(N₂) through a N-O insertion mechanism. The thermal (C6H₆)M+O(N₂) would release a nitrogen molecule and then is further oxidized to phenol regenerating the active catalyst M+ through two different mechanisms, i.e., nonradical and/or O-insertion. The large energy acquired from the benzene association not only provide the driving force for the N-O activation as well as the whole oxidation reaction but also make it possible for the benzene oxidation to proceed on a same potential energy surface (PES, septet for Mn+ and triplet for Co⁺) under the single-state reactivity (SSR) paradigm. For the alternative MO+-mediated oxidation mechanism, spin inversion as well as high energy barrier in the course of the N-O activation imply that both Mn+, Co⁺, and Ni+ are unable to work as a catalyst.