Harvey Jeremy N
Centre for Computational Chemistry, School of Chemistry, University of Bristol, Cantock's Close, Bristol, UKBS8 1TS.
Phys Chem Chem Phys. 2007 Jan 21;9(3):331-43. doi: 10.1039/b614390c. Epub 2006 Nov 20.
Many chemical reactions involve a change in spin-state and are formally forbidden. This article summarises a number of previously published applications showing that a form of Transition State Theory (TST) can account for the kinetics of these reactions. New calculations for the emblematic spin-forbidden reaction HC + N(2) are also reported. The observed reactivity is determined by two factors. The first is the critical energy required for reaction to occur, which in spin-forbidden reactions is often defined by the relative energy of the Minimum Energy Crossing Point (MECP) between potential energy surfaces corresponding to the different spin states. The second factor is the probability of hopping from one surface to the other in the vicinity of the crossing region, which is largely defined by the spin-orbit coupling matrix element between the two electronic wavefunctions. The spin-forbidden transition state theory takes both factors into account and gives good results. The shortcomings of the theory, which are largely analogous to those of standard TST, are discussed. Finally, it is shown that in cases where the surface-hopping probability is low, the kinetics of spin-forbidden reactions will be characterised by unusually unfavourable entropies of activation. As a consequence, reactions involving a spin-state change can be expected to compete poorly with spin-allowed reactions at high temperatures (or energies).
许多化学反应涉及自旋态的变化，从形式上来说是禁阻的。本文总结了一些此前发表的应用，表明一种过渡态理论（TST）的形式可以解释这些反应的动力学。还报道了对标志性的自旋禁阻反应HC + N(2)的新计算。观察到的反应活性由两个因素决定。第一个是反应发生所需的临界能量，在自旋禁阻反应中，这通常由对应于不同自旋态的势能面之间的最小能量交叉点（MECP）的相对能量定义。第二个因素是在交叉区域附近从一个表面跃迁到另一个表面的概率，这在很大程度上由两个电子波函数之间的自旋 - 轨道耦合矩阵元定义。自旋禁阻过渡态理论考虑了这两个因素并给出了良好的结果。讨论了该理论的缺点，这些缺点在很大程度上与标准TST的缺点类似。最后，结果表明在表面跃迁概率较低的情况下，自旋禁阻反应的动力学将表现出异常不利的活化熵。因此，预计涉及自旋态变化的反应在高温（或高能）下与自旋允许反应的竞争能力较差。
