Zhao Lianming, Wang Yong, Guo Wenyue, Shan Honghong, Lu Xiaoqing, Yang Tianfang
College of Physics Science and Technology, China University of Petroleum, Dongying, Shandong 257061, People's Republic of China.
J Phys Chem A. 2008 Jun 26;112(25):5676-83. doi: 10.1021/jp800511h. Epub 2008 May 31.
The gas-phase Fe(+)-mediated oxidation of acetylene by N2O on both sextet and quartet potential energy surfaces (PESs) is theoretically investigated using density functional theory. Geometries and energies of all the stationary points involved in the catalytic reaction are located. For the catalytic cycles, the crucial step is the initial N2O reduction by Fe(+) to form FeO(+), in which a direct O-abstraction mechanism is located on the sextet PES, whereas the quartet pathway favors a N-O insertion mechanism. Spin inversion moves the energy barrier for this process downward to a position below the ground-state entrance channel. The second step of the catalytic cycles involves two mechanisms corresponding to direct hydrogen abstraction and cyclization. The former mechanism accounts for the ethynol formation with the upmost activation barrier below the entrance channel by about 5 kcal/mol. The other mechanism involves a "metallaoxacyclobutene" structure, followed by four possible pathways, i.e., direct dissociation, C-C insertion, C-to-O hydrogen shift, and/or C-to-C hydrogen shift. Among these pathways, strong exothermicities as well as energetically low location of the intermediates suggest oxidation to ketene and carbon monoxide along the C-to-C hydrogen shift pathway is the most favorable. Reduction of the CO loss partner FeCH2(+) by another N2O molecule constitutes the third step of the catalytic cycles, which contains direct abstraction of O from N2O giving OFeCH2(+), intramolecular rearrangement to form Fe(+)-OCH2, and nonreactive dissociation. This reaction is also energetically favored considering the energy acquired from the initial reactants.
采用密度泛函理论对六重态和四重态势能面（PES）上气相Fe(+)介导的N₂O氧化乙炔反应进行了理论研究。确定了催化反应中所有驻点的几何结构和能量。对于催化循环，关键步骤是Fe(+)将N₂O还原生成FeO(+)，其中六重态PES上存在直接氧提取机制，而四重态途径则倾向于N - O插入机制。自旋反转将该过程的能垒向下移动到基态入口通道以下的位置。催化循环的第二步涉及对应于直接氢提取和环化的两种机制。前一种机制导致乙炔醇的形成，其最高活化能垒比入口通道低约5千卡/摩尔。另一种机制涉及“金属氧杂环丁烯”结构，随后有四种可能的途径，即直接解离、C - C插入、C到O的氢转移和/或C到C的氢转移。在这些途径中，强烈的放热以及中间体在能量上的低位表明，沿着C到C的氢转移途径氧化为乙烯酮和一氧化碳是最有利的。另一个N₂O分子还原CO损失伙伴FeCH₂(+)构成催化循环的第三步，该步骤包括从N₂O直接提取O生成OFeCH₂(+)、分子内重排形成Fe(+) - OCH₂以及非反应性解离。考虑到从初始反应物获得的能量，该反应在能量上也是有利的。
