Methylhydroxycarbene: Tunneling Control of a Chemical Reaction
Methylhydroxycarbene: Tunneling Control of a Chemical Reaction
复制标题
DOI：
10.1126/science.1203761
复制
发表时间：
2011-06-10
期刊：
影响因子：
56.9
通讯作者：
Allen, Wesley D.
中科院分区：
文献类型：
--
作者：
Schreiner, Peter R.;Reisenauer, Hans Peter;Allen, Wesley D.
化学反应性通常从动力学控制与热力学控制的广义角度来理解，其中决定性因素分别是各种反应路径中的最低活化势垒或最终产物的最低自由能。我们证明，量子力学隧道可以取代传统的动力学控制和直接反应专门的产品，其反应路径具有较高的势垒。具体地说，我们通过在约1200开尔文（K）下真空热解棕榈酸，然后在11 K下进行氩气基质捕获来制备甲基羟基卡宾（H（3）C-C-OH）。以前难以捉摸的卡宾，其特征在于紫外和红外光谱以及严格的量子力学计算，在28.0千卡/摩尔（kcal mol（-1））的势垒下，通过隧穿进行[1，2]氢转移到乙醛，半衰期约为1小时。与乙烯醇类似的异构化具有22.6 kcal mol（-1）的低得多的势垒，但在低温下被隧穿势能分布的较大宽度所排除。
Chemical reactivity is conventionally understood in broad terms of kinetic versus thermodynamic control, wherein the decisive factor is the lowest activation barrier among the various reaction paths or the lowest free energy of the final products, respectively. We demonstrate that quantum-mechanical tunneling can supersede traditional kinetic control and direct a reaction exclusively to a product whose reaction path has a higher barrier. Specifically, we prepared methylhydroxycarbene (H(3)C-C-OH) via vacuum pyrolysis of pyruvic acid at about 1200 kelvin (K), followed by argon matrix trapping at 11 K. The previously elusive carbene, characterized by ultraviolet and infrared spectroscopy as well as exacting quantum-mechanical computations, undergoes a facile [1,2] hydrogen shift to acetaldehyde via tunneling under a barrier of 28.0 kilocalories per mole (kcal mol(-1)), with a half-life of around 1 hour. The analogous isomerization to vinyl alcohol has a substantially lower barrier of 22.6 kcal mol(-1) but is precluded at low temperature by the greater width of the potential energy profile for tunneling.