Mayer James M, Hrovat David A, Thomas Jennie L, Borden Weston Thatcher
Contribution from the Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA.
J Am Chem Soc. 2002 Sep 18;124(37):11142-7. doi: 10.1021/ja012732c.
Degenerate hydrogen atom exchange reactions have been studied using calculations, based on density functional theory (DFT), for (i) benzyl radical plus toluene, (ii) phenoxyl radical plus phenol, and (iii) methoxyl radical plus methanol. The first and third reactions occur via hydrogen atom transfer (HAT) mechanisms. The transition structure (TS) for benzyl/toluene hydrogen exchange has C(2)(h)() symmetry and corresponds to the approach of the 2p-pi orbital on the benzylic carbon of the radical to a benzylic hydrogen of toluene. In this TS, and in the similar C(2) TS for methoxyl/methanol hydrogen exchange, the SOMO has significant density in atomic orbitals that lie along the C-H vectors in the former reaction and nearly along the O-H vectors in the latter. In contrast, the SOMO at the phenoxyl/phenol TS is a pi symmetry orbital within each of the C(6)H(5)O units, involving 2p atomic orbitals on the oxygen atoms that are essentially orthogonal to the O.H.O vector. The transferring hydrogen in this reaction is a proton that is part of a typical hydrogen bond, involving a sigma lone pair on the oxygen of the phenoxyl radical and the O-H bond of phenol. Because the proton is transferred between oxygen sigma orbitals, and the electron is transferred between oxygen pi orbitals, this reaction should be described as a proton-coupled electron transfer (PCET). The PCET mechanism requires the formation of a hydrogen bond, and so is not available for benzyl/toluene exchange. The preference for phenoxyl/phenol to occur by PCET while methoxyl/methanol exchange occurs by HAT is traced to the greater pi donating ability of phenyl over methyl. This results in greater electron density on the oxygens in the PCET transition structure for phenoxyl/phenol, as compared to the PCET hilltop for methoxyl/methanol, and the greater electron density on the oxygens selectively stabilizes the phenoxyl/phenol TS by providing a larger binding energy of the transferring proton.
基于密度泛函理论（DFT），通过计算研究了简并氢原子交换反应，涉及以下反应：（i）苄基自由基与甲苯，（ii）苯氧基自由基与苯酚，以及（iii）甲氧基自由基与甲醇。第一个和第三个反应通过氢原子转移（HAT）机制发生。苄基/甲苯氢交换的过渡结构（TS）具有C(2)(h)()对称性，对应于自由基苄基碳上的2p-π轨道接近甲苯的苄基氢。在这个TS中，以及在甲氧基/甲醇氢交换的类似C(2) TS中，单占据分子轨道（SOMO）在原子轨道中具有显著密度，在前一个反应中沿着C-H向量，在后一个反应中几乎沿着O-H向量。相比之下，苯氧基/苯酚TS处的SOMO是每个C(6)H(5)O单元内的π对称轨道，涉及氧原子上的2p原子轨道，这些轨道基本上与O.H.O向量正交。该反应中转移的氢是典型氢键的一部分质子，涉及苯氧基自由基氧上的σ孤对和苯酚的O-H键。由于质子在氧σ轨道之间转移，电子在氧π轨道之间转移，该反应应描述为质子耦合电子转移（PCET）。PCET机制需要形成氢键，因此不适用于苄基/甲苯交换。苯氧基/苯酚通过PCET发生反应而甲氧基/甲醇交换通过HAT发生的偏好归因于苯基比甲基具有更大的π供电子能力。这导致与甲氧基/甲醇的PCET山顶相比，苯氧基/苯酚的PCET过渡结构中氧上的电子密度更大，并且氧上更大的电子密度通过提供转移质子的更大结合能选择性地稳定了苯氧基/苯酚TS。
