Lai Wenzhen, Chen Hui, Shaik Sason
Institute of Chemistry and The Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of Jerusalem, Givat Ram Campus, 91904 Jerusalem, Israel.
J Phys Chem B. 2009 Jun 4;113(22):7912-7. doi: 10.1021/jp902288q.
QM/MM calculations of Mossbauer parameters lead to assignments of the three isomeric species of CPO-II, "major", "minor", and unassigned "6% species", which were recently observed in experiment and posed two puzzles (Stone, K. L.; Hoffart, L. M.; Behan, R. K.; Krebs, C.; Green, M. T. J. Am. Chem. Soc. 2006, 128, 6147). Both the minor and major species were found to be iron(IV)-hydroxides, thus accounting for the observed ratio of their relative yield that is pH-independent. The difference between the minor and major species is a single water molecule that acts as a H-bond acceptor from the ferryl in the minor species (2b) and it is essential to get a good match of the calculated Mossbauer parameters to the experimentally observed ones for the minor species. The major species (2c-2e, 2e-NW) may or may not have a water molecule. The calculations reveal also two candidates for the unassigned 6% species, which are a Por+*FeIIIOH species 2e-Fe(III), without or with a water molecule, or the corresponding aqua complex Por+*FeIIIOH2 3c formed by adding an additional proton to the system. These species have DeltaEQ parameters of the same magnitude but with opposite signs: negative (-2.30 mm/s) for the two 2e-Fe(III) species and positive (2.39 mm/s) for 3c. The above assignments were further consolidated by an extended correlation (Figure 2) between the iron spin density and the DeltaEQ parameters of the species calculated in the present study and by relating DeltaEQ to the d-electronic configuration on iron. A bonding model of the FeO(H) moiety (Figure 3) was used to account for the variation of the spin density and provided further support for the correlation in Figure 2 and the assignment. Experimental determination of the sign of the quadruple parameter will finally confirm the identity of this species. In addition, since 3c possesses an additional proton, its identity can be revealed by pH-dependent yield. All in all, the present paper shows that QM/MM calculations can conduct a useful dialogue with experiment in this complex field.
对穆斯堡尔参数进行量子力学/分子力学（QM/MM）计算，得出了CPO-II的三种异构体的归属，即“主要”物种、“次要”物种和未归属的“6%物种”，这些异构体最近在实验中被观测到，并带来了两个谜题（斯通，K.L.；霍法特，L.M.；贝汉，R.K.；克雷布斯，C.；格林，M.T.《美国化学会志》2006年，128卷，6147页）。结果发现，次要物种和主要物种均为铁（IV）-氢氧化物，因此解释了所观测到的它们相对产率的比例与pH无关的现象。次要物种和主要物种之间的差异在于一个水分子，该水分子在次要物种（2b）中作为来自铁酰基的氢键受体，对于使计算得到的穆斯堡尔参数与次要物种的实验观测值良好匹配而言至关重要。主要物种（2c - 2e，2e - NW）可能有水分子，也可能没有。计算还揭示了未归属的6%物种的两个候选物，一个是没有或带有一个水分子的Por + *FeIIIOH物种2e - Fe(III)，或者是通过向体系中额外添加一个质子形成的相应水合配合物Por + *FeIIIOH2 3c。这些物种具有相同大小但符号相反的四极分裂参数（ΔEQ）：两种2e - Fe(III)物种为负（-2.30 mm/s），而3c为正（2.39 mm/s）。通过本研究中计算的物种的铁自旋密度与ΔEQ参数之间的扩展相关性（图2），以及将ΔEQ与铁上的d电子构型相关联，上述归属得到了进一步巩固。使用FeO(H)部分的键合模型（图3）来解释自旋密度的变化，并为图2中的相关性和归属提供了进一步支持。四重参数符号的实验测定最终将确认该物种的身份。此外，由于3c含有一个额外的质子，其身份可以通过pH依赖的产率来揭示。总而言之，本文表明QM/MM计算能够在这个复杂的领域与实验进行有益的对话。
