Rearrangements in model peptide-type radicals via intramolecular hydrogen-atom transfer
Rearrangements in model peptide-type radicals via intramolecular hydrogen-atom transfer
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DOI：
10.1002/hlca.200690210
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发表时间：
2006-01-01
影响因子：
1.8
通讯作者：
Radom, Leo
中科院分区：
文献类型：
--
作者：
Moran, Damian;Jacob, Rebecca;Radom, Leo
用高水平量子化学计算方法研究了模型肽自由基中的分子内氢原子转移。用许多理论方法进行1，2-、1，3-、1，5-和1，6 [C N]-H位移、1，4-和1，7 [C C]-H位移以及1，4 [N N]-H位移（方案1）的检查。首先，评估了UB 3-LYP（具有6- 31 G（d）、6- 31 G（2dfP）和6-311+G（d，p）基组）和UMP 2（具有6- 31 G（d）基组）用于确定自由基几何构型的性能。我们发现UB 3-LYP结构只有很小的基组依赖性，并且用UB 3-LYP/6- 31 G（d）优化的几何结构通常足以与高水平复合方法结合使用，以确定改进的H-转移热化学。在这方面评估的方法包括高级复合方法G3（MP2）-RAD、CBS-QB 3和G3//B3-LYP，以及与6-31+G（dp）和6-311++G（3df 3 pd）基组相关的密度泛函方法B3-LYP、MPWB 1 K和BMK。高层次的方法给出的结果是彼此接近，而最近开发的泛函MPWB 1 K和BMK提供了具有成本效益的替代方案。对于所考虑的系统，N-中心自由基到C-中心自由基的转化总是放热的（25 kJ（.）摩尔或更高），并且这可以导致小于60 kJ（.）mol（-1）（特别是1，5 [CN]-H和1，6 [CN]-H位移）。H-迁移势垒似乎随着过渡结构（TS）中的环尺寸增加而降低，其中发现势垒降低，例如当从经由四元环TS进行的重排移动时（例如，1，3 [CN]-H移位，CH 3-C（O）-NH中心点->中心点CH 2-C（O）-NH 2）到经由六元环TS进行的重排（例如，1，5 [CN]-H位移，（NH）-N-中心点-CH 2-C（O）-NH-CH 3 NH 2-CH 2-C（O）-NH-CH 2中心点）。
Intramolecular H-atom transfer in model peptide-type radicals was investigated with high-level quantum-chemistry calculations. Examination of 1,2-, 1,3-, 1,5-, and 1,6[C N]-H shifts, 1,4- and 1,7[C C]-H shifts, and 1,4[N N]-H shifts (Scheme 1), was carried out with a number of theoretical methods. In the first place, the performance of UB3-LYP (with the 6-31G(d), 6-31G(2dfP), and 6-311+G(d,p) basis sets) and UMP2 (with the 6-31G(d) basis set) was assessed for the determination of radical geometries. We found that there is only a small basis-set dependence for the UB3-LYP structures, and geometries optimized with UB3-LYP/6-31G(d) are generally sufficient for use in conjunction with high-level composite methods in the determination of improved H-transfer thermochemistry. Methods assessed in this regard include the high-level composite methods, G3(MP2)-RAD, CBS-QB3, and G3//B3-LYP, as well as the density-functional methods B3-LYP, MPWB1K, and BMK in association with the 6-31+G(dp) and 6-311++G(3df3pd) basis sets. The high-level methods give results that are close to one another, while the recently developed functionals MPWB1K and BMK provide cost-effective alternatives. For the systems considered, the transformation of an N-centered radical to a C-centered radical is always exothermic (by 25 kJ (.) mol' or more), and this can lead to quite modest barrier heights of less than 60 kJ (.) mol(-1) (specifically for 1,5[C N]-H and 1,6[C N]-H shifts). H-Migration barriers appear to decrease as the ring size in the transition structure (TS) increases, with a lowering of the barrier being found, for example when moving from a rearrangement proceeding via a four-membered-ring TS (e.g., the 1,3[C N]-H shift, CH3-C(O)-NH center dot -> center dot CH2-C(O)-NH2) to a rearrangement proceeding via a six-membered-ring TS (e.g., the 1,5[C N]-H shift, (NH)-N-center dot-CH2-C(O)-NH-CH3 NH2-CH2-C(O)-NH-CH2 center dot).