Interdisciplinary Center for Scientific Computing, Ruprecht-Karls University, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany.
Phys Chem Chem Phys. 2013 May 14;15(18):6691-8. doi: 10.1039/c3cp44338h. Epub 2013 Mar 13.
ortho-Nitrobenzylacetate (oNBA) is one of the smallest caged-compounds utilizing the famous ortho-nitrobenzyl caging group and thus serves here as a proto-typical example. Crucial for efficient uncaging of, here, acetate from oNBA is the formation of the aci-form of the nitro-group, which can be formed via excited-state intramolecular hydrogen transfer (ESIHT) in the lowest excited singlet S1 or the triplet T1 state. Using state-of-the-art quantum chemical methods, the efficiency of singlet and triplet ESIHT in aci-formation is investigated and the results are discussed in comparison to previous calculations on related caged-compounds as well as with respect to recent experimental findings. It is shown that the majority of the excited molecules undergo singlet ESIHT, which is generally inefficient with respect to aci-formation and subsequent uncaging, while only a minority of excited molecules undergo intersystem crossing and triplet ESIHT, which eventually leads to kinetically stable aci-isomers of oNBA resulting in final uncaging. Our computational results explain all existing experimental findings on ortho-nitrobenzylic caging groups conclusively and general synthetic strategies for their improvement are suggested.
邻硝基苄基乙酸酯（oNBA）是利用著名的邻硝基苄基笼状基团的最小笼状化合物之一，因此可作为典型示例。对于从 oNBA 中有效解笼化的邻硝基苄基乙酸酯，关键是形成硝基的酸式形式，可通过最低激发单线态 S1 或三重态 T1 态中的激发态分子内氢键转移（ESIHT）形成。本文使用最先进的量子化学方法，研究了单重态和三重态 ESIHT 在酸形成中的效率，并将结果与以前对相关笼状化合物的计算结果以及最近的实验结果进行了比较。结果表明，大多数激发分子经历单重态 ESIHT，其对于酸形成和随后的解笼化通常效率较低，而只有少数激发分子经历系间窜越和三重态 ESIHT，最终导致动力学稳定的邻硝基苄基酸异构体形成，从而导致最终的解笼化。我们的计算结果明确解释了所有现有的关于邻硝基苄基笼状基团的实验发现，并提出了改进它们的一般合成策略。
