Phung Quan Manh, Nam Ho Ngoc, Ghosh Abhik
Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan.
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan.
Inorg Chem. 2023 Dec 11;62(49):20496-20505. doi: 10.1021/acs.inorgchem.3c03689. Epub 2023 Nov 27.
A first DMRG/CASSCF-CASPT2 study of a series of paradigmatic {FeNO}, {FeNO}, and {FeNO} heme-nitrosyl complexes has led to substantial new insight as well as uncovered key shortcomings of the DFT approach. By virtue of its balanced treatment of static and dynamic correlation, the calculations have provided some of the most authoritative information available to date on the energetics of low- versus high-spin states of different classes of heme-nitrosyl complexes. Thus, the calculations indicate low doublet-quartet gaps of 1-4 kcal/mol for {FeNO} complexes and high singlet-triplet gaps of ≳20 kcal/mol for both {FeNO} and {FeNO} complexes. In contrast, DFT calculations yield widely divergent spin state gaps as a function of the exchange-correlation functional. DMRG-CASSCF calculations also help calibrate DFT spin densities for {FeNO} complexes, pointing to those obtained from classic pure functionals as the most accurate. The general picture appears to be that nearly all the spin density of FeP is localized on the Fe, while the axial ligand imidazole (ImH) in FeP(ImH) pushes a part of the spin density onto the NO moiety. An analysis of the DMRG-CASSCF wave function in terms of localized orbitals and of the resulting configuration state functions in terms of resonance forms with varying NO(π*) occupancies has allowed us to address the longstanding question of local oxidation states in heme-nitrosyl complexes. The analysis indicates NO(neutral) resonance forms [i.e., Fe(II)-NO and Fe(III)-NO] as the major contributors to both {FeNO} and {FeNO} complexes. This finding is at variance with the common formulation of {FeNO} hemes as Fe(II)-NO species but is consonant with an Fe L-edge XAS analysis by Solomon and co-workers. For the {FeNO} complex {FeP}, our analysis suggests a resonance hybrid description: Fe(I)-NO ↔ Fe(II)-NO, in agreement with earlier DFT studies. Vibrational analyses of the compounds studied indicate an imperfect but fair correlation between the NO stretching frequency and NO(π*) occupancy, highlighting the usefulness of vibrational data as a preliminary indicator of the NO oxidation state.
对一系列典型的{FeNO}、{FeNO}和{FeNO}血红素亚硝酰配合物进行的首次DMRG/CASSCF - CASPT2研究带来了重要的新见解，同时也揭示了DFT方法的关键缺陷。凭借其对静态和动态相关性的平衡处理，这些计算提供了迄今为止关于不同类血红素亚硝酰配合物低自旋态与高自旋态能量学方面一些最具权威性的信息。因此，计算结果表明{FeNO}配合物的 doublet - quartet 能隙较低，为1 - 4千卡/摩尔，而{FeNO}和{FeNO}配合物的 singlet - triplet 能隙较高，大于等于20千卡/摩尔。相比之下，DFT计算得出的自旋态能隙随交换相关泛函而有很大差异。DMRG - CASSCF计算也有助于校准{FeNO}配合物的DFT自旋密度，表明从经典纯泛函获得的自旋密度最为准确。总体情况似乎是，FeP的几乎所有自旋密度都局域在Fe上，而FeP(ImH)中的轴向配体咪唑(ImH)将一部分自旋密度推到了NO部分上。通过对基于定域轨道的DMRG - CASSCF波函数以及基于具有不同NO(π*)占据情况的共振形式的所得组态态函数进行分析，我们得以解决血红素亚硝酰配合物中局部氧化态这一长期存在的问题。分析表明NO(中性)共振形式[即Fe(II) - NO和Fe(III) - NO]是{FeNO}和{FeNO}配合物的主要贡献者。这一发现与将{FeNO}血红素普遍表述为Fe(II) - NO物种的观点不同，但与Solomon及其同事进行的Fe L边XAS分析一致。对于{FeNO}配合物{FeP}，我们的分析表明是一种共振杂化描述：Fe(I) - NO ↔ Fe(II) - NO，这与早期的DFT研究一致。对所研究化合物的振动分析表明，NO伸缩频率与NO(π*)占据情况之间存在不完美但合理的相关性，突出了振动数据作为NO氧化态初步指标的有用性。
