School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia.
J Phys Chem A. 2013 Feb 14;117(6):1124-35. doi: 10.1021/jp3046142. Epub 2012 Aug 13.
The gas-phase reactivity of the vanadium hydroxides VO(2)(OH)(2) and V(2)O(5)(OH) toward methanol was examined using a combination of ion-molecule reactions (IMRs) and collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer. Isotope-labeling experiments with CD(3)OH, (13)CH(3)OH, and CH(3)(18)OH were used to confirm the stoichiometry of ions and the observed sequence of reactions. The experimental data were interpreted with the aid of density functional theory calculations, carried out at the B3LYP/SDD6-311++G** level of theory. While VO(2)(OH)(2) is unreactive, V(2)O(5)(OH) undergoes a metathesis reaction to yield V(2)O(5)(OCH(3)). The DFT calculations reveal that the metathesis reaction of methanol with VO(2)(OH)(2) suffers from a barrier of +0.52 eV (relative to separated reactants) but that the reaction of V(2)O(5)(OH) with methanol readily proceeds via addition/elimination reactions with both transition states being below the energy of the separated reactants. CID of V(2)O(5)(OCH(3)) (m/z 213) yields three ions arising from activation of the methoxo ligand: V(2), O(6), C, H (m/z 211); V(2), O(5), H (m/z 183); and V(2), O(4), H (m/z 167). Additional experiments and DFT calculations suggest that these ions arise from losses of H(2), formaldehyde and the sequential losses of H(2) and CO(2), respectively. The use of an (18)O-labeled methoxo ligand in V(2)O(5)((18)OCH(3)) (m/z 215) showed the competing losses of H(2)C(16)O and H(2)C(18)O and [H(2) and C(16)O(18)O] and [H(2) and C(16)O(2)], highlighting that (16)O/(18)O exchange between the methoxo ligand and the vanadium oxide occurs prior to the subsequent fragmentation of the ligand. DFT calculations reveal that a key step involves hydrogen atom transfer from the methoxo ligand to the oxo ligand of the same vanadium center, producing the intermediate V(2)O(4)(OH)(OCH(2)) containing a ketyl radical ligand and a hydroxo ligand. This intermediate can either undergo CH(2)O loss, or the ketyl radical can couple with an oxo ligand of the adjacent vanadium center, producing V(2)O(3)(μ(2)-O(2)CH(2)), which is a key intermediate in the (16)O/(18)O scrambling and in the H(2) loss channel.
采用离子-分子反应 (IMR) 和在四极离子阱质谱仪中进行的碰撞诱导解离 (CID) 的组合，研究了钒氢氧化物 VO(2)(OH)(2) 和 V(2)O(5)(OH) 与甲醇的气相反应性。使用 CD(3)OH、(13)CH(3)OH 和 CH(3)(18)OH 的同位素标记实验证实了离子的化学计量和观察到的反应序列。实验数据通过在 B3LYP/SDD6-311++G**理论水平上进行的密度泛函理论计算进行了解释。虽然 VO(2)(OH)(2) 没有反应性，但 V(2)O(5)(OH) 会经历复分解反应，生成 V(2)O(5)(OCH(3))。DFT 计算表明，甲醇与 VO(2)(OH)(2) 的复分解反应的势垒为+0.52 eV（相对于分离的反应物），但 V(2)O(5)(OH) 与甲醇的反应很容易通过加成/消除反应进行，两个过渡态都低于分离反应物的能量。V(2)O(5)(OCH(3)) (m/z 213) 的 CID 产生三个来自甲氧基配体活化的离子：V(2), O(6), C, H (m/z 211)；V(2), O(5), H (m/z 183)；和 V(2), O(4), H (m/z 167)。进一步的实验和 DFT 计算表明，这些离子分别来自 H(2)、甲醛的损失和 H(2)和 CO(2)的顺序损失。在 V(2)O(5)((18)OCH(3)) (m/z 215) 中使用 (18)O 标记的甲氧基配体表明，H(2)C(16)O 和 H(2)C(18)O 和 [H(2)和 C(16)O(18)O] 和 [H(2)和 C(16)O(2)] 的竞争损失，突出表明甲氧基配体和钒氧化物之间的 (16)O/(18)O 交换发生在随后的配体断裂之前。DFT 计算表明，一个关键步骤涉及氢原子从甲氧基配体转移到同一钒中心的氧配体，生成含有酮基自由基配体和羟配体的中间产物 V(2)O(4)(OH)(OCH(2))。该中间体可以失去 CH(2)O，或者酮基自由基可以与相邻钒中心的氧配体结合，生成 V(2)O(3)(μ(2)-O(2)CH(2))，这是 (16)O/(18)O 混合和 H(2)损失通道中的关键中间体。
