Department of Chemistry, University of Missouri, Columbia, MO, USA.
Department of Chemistry, Temple University, Philadelphia, PA, USA.
Nat Chem. 2019 Feb;11(2):123-128. doi: 10.1038/s41557-018-0186-5. Epub 2018 Dec 10.
Intersystem crossing plays an important role in photochemistry. It is understood to be efficient when heavy atoms are present due to strong spin-orbit coupling, or when strongly bound long-lived complexes are formed that increase the chance of finding the singlet-triplet intersection seam. Here we present evidence for a different intersystem crossing mechanism in the bimolecular reaction of O(P) with alkylamines. In crossed-beam experiments, product velocity-flux maps are measured for aminoalkyl radicals produced from H abstraction from the methyl group, which also gives OH radicals as co-fragments. The low translational-energy release and isotropic angular distributions of the products indicate that such reactions undergo the formation of a complex before OH and aminoalkyl are produced. However, there is no well on the triplet potential energy surface that could support such a complex. Multi-reference ab initio calculations suggest, instead, that intersystem crossing occurs in the exit-channel region due to the long-range dipole-dipole interaction between the nascent radical product pair coupled with the vanishing singlet-triplet splitting at long range. Intersystem crossing then leads to a deep hydroxylamine well before OH elimination.
体系间窜越在光化学中起着重要作用。由于强自旋轨道耦合的存在，或者当形成强束缚的长寿命复合物时，体系间窜越被认为是有效的，这增加了找到单重态-三重态交叉缝的机会。在这里，我们提出了在 O(P)与烷基胺的双分子反应中存在不同的体系间窜越机制的证据。在交叉束实验中，测量了从甲基氢提取产生的氨基烷基自由基的产物速度通量图，该反应也产生 OH 自由基作为共碎片。产物的低平移能释放和各向同性角分布表明，这种反应在产生 OH 和氨基烷基之前经历了复合物的形成。然而，三重势能表面上没有可以支持这样的复合物的势阱。多参考从头算计算表明，相反，由于新生自由基产物对之间的远程偶极-偶极相互作用以及长程 singlet-triplet 分裂的消失，体系间窜越发生在出口通道区域。然后，体系间窜越导致在 OH 消除之前进入深羟胺阱。
