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(³P) 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.

系统间杂交在光化学中起重要作用。当由于强自旋-轨道耦合而存在重原子时，或者当形成牢固结合的长寿命络合物（增加了找到单线态-三重态相交接缝的机会）时，这被认为是有效的。在这里，我们提供了O（^3）双分子反应中不同的系统间交叉机制的证据。P）与烷基胺。在横梁实验中，测量了从甲基中提取H所产生的氨基烷基自由基的产物速度-通量图，这也将OH自由基作为共碎片。产物的低平移能量释放和各向同性角分布表明，这种反应在生成OH和氨烷基之前经历了络合物的形成。但是，三重态势能表面上没有井可以支撑这种复合物。相反，多参考从头计算表明，由于新生自由基产物对之间的长距离偶极-偶极相互作用以及长距离的单线态-三重态分裂消失，系统间交叉发生在出口通道区域。