School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand.
Phys Chem Chem Phys. 2020 Nov 18;22(44):25789-25802. doi: 10.1039/d0cp03718d.
The photochemistry of molecules with peptide bonds is of both theoretical and experimental interest. Among these molecules, formamide (HCONH2) has been frequently selected as a representative in the study of proteins. Because several fundamental questions remained unanswered, particularly the role of thermal energy, the unimolecular photodissociations of HCONH2 were studied using the CASPT2(6,9)/aug-cc-pVDZ method and transition state theory (TST). The theoretical results verify that C-N dissociation is both kinetically and thermodynamically favorable in the S1 state. In addition, the dissociated radical products (HCO and NH2) could be precursors in the other dissociation channels owing to their high reactivities. The NH3-CO and NH3-OC H-bond complexes were reportedly observed to display the highest quantum yield in a UV experiment because the processes for determining the rates of isomerization dissociations of HCO and NH2 (which involve formyl hydrogen transfer) are spontaneous and kinetically favorable. Similarly, the van der Waals complex between HNCO and H2 represents the common end product with HCO and NH2 as the precursors. This is because the S1 and T1 relax-scan potential energy curves for the dehydrogenation straightforwardly intersect the S0 state with approximately similar structures at the intersection points. Therefore, the promotion of the intersystem crossing by a noble-gas cryogenic matrix appears inessential as previously reported. In contrast, we conjectured that the xenon matrix used in the UV experiment helps promote the van der Waals complex by reducing the energy barriers for dehydrogenation rather than by enhancing the rate of intersystem crossing from the singlet to triplet state. It was concluded that HCN, HNC, and H2O (dehydration) were formed via a two-step process, wherein, the cleavage of the C-O bond in formamidic acid first occurs in the S1 state. This is followed by the hydrogen/proton transfer from HCNH to OH on barrierless potentials. Overall, the mechanisms proposed in this work reveal the role of thermal energy and effects of variations in the multiconfigurational characteristics along the potential energy curves in detail. The available information on these aspects is limited, and the present results could provide insights into unimolecular photodissociations; these could be beneficial in future theoretical and experimental studies on this and other similar systems.
具有肽键的分子的光化学既有理论意义又有实验意义。在这些分子中，甲酰胺（HCONH2）经常被选为蛋白质研究的代表。由于仍有几个基本问题没有得到解答，特别是热能的作用，因此使用 CASPT2(6,9)/aug-cc-pVDZ 方法和过渡态理论（TST）研究了 HCONH2 的单分子光解。理论结果证实，在 S1 态下，C-N 键的离解在动力学和热力学上都是有利的。此外，由于其高反应性，分离的自由基产物（HCO 和 NH2）可能是其他离解通道的前体。据报道，由于决定 HCO 和 NH2 异构分解速率的过程（涉及甲酰基氢转移）是自发的且在动力学上有利的，因此在 UV 实验中观察到 NH3-CO 和 NH3-OC H 氢键复合物具有最高的量子产率。同样，HNCO 和 H2 之间的范德华复合物代表了以 HCO 和 NH2 为前体的常见终产物。这是因为脱氢的 S1 和 T1 弛豫扫描势能曲线与 S0 状态直接相交，在交点处具有相似的结构。因此，正如先前报道的那样，通过稀有气体低温基质促进系间窜越似乎并非必不可少。相比之下，我们推测，在 UV 实验中使用的氙气基质通过降低脱氢的能垒而不是通过增强从单重态到三重态的系间窜越速率来帮助促进范德华复合物。得出的结论是，HCN、HNC 和 H2O（脱水）是通过两步过程形成的，其中首先在 S1 态下发生甲酰胺中 C-O 键的断裂。然后，在无势垒的势能上，HCNH 中的氢/质子转移到 OH 上。总的来说，这项工作提出的机制详细揭示了热能的作用以及沿势能曲线的多组态特征变化的影响。关于这些方面的可用信息有限，本研究结果可以为单分子光解提供一些见解，这对今后对该体系和其他类似体系的理论和实验研究可能会有所帮助。
