Yamane Takeshi, Sugisaki Kenji, Nakagawa Tomoki, Matsuoka Hideto, Nishio Takahisa, Kinjyo Shigemori, Mori Nobuyuki, Yokoyama Satoshi, Kawashima Chika, Yokokura Naoki, Sato Kazunobu, Kanzaki Yuki, Shiomi Daisuke, Toyota Kazuo, Dolphin David H, Lin Wei-Ching, McDowell Charles A, Tadokoro Makoto, Takui Takeji
Department of Chemistry and Molecular Materials Science, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan.
Phys Chem Chem Phys. 2017 Sep 20;19(36):24769-24791. doi: 10.1039/c7cp03850j.
The fictitious spin-1/2 Hamiltonian approach is the putative method to analyze the fine-structure/hyperfine ESR spectra of high spin metallocomplexes having sizable zerofield splitting (ZFS), thus giving salient principal g-values far from around g = 2 without explicitly providing their ZFS parameters in most cases. Indeed, the significant departure of the g-values from g = 2 is indicative of the occurrence of their high spin states, but naturally they never agree with true g-values acquired by quantum chemical calculations such as sophisticated DFT or ab initio MO calculations. In this work, we propose facile approaches to determine the magnetic tensors of high spin metallocomplexes having sizable ZFS, instead of performing advanced high-field/high-frequency ESR spectroscopy. We have revisited analytical expressions for the relationship between effective g-values and true principal g-values for high spins. The useful analytical formulas for the g-g relationships are given for S's up to 7/2. The genuine Zeeman perturbation formalism gives the exact solutions for S = 3/2, and for higher S's it is much more accurate than the pseudo-Zeeman perturbation approach documented so far (A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Metal Ions, 1970; J. R. Pilbrow, J. Magn. Reson., 1978, 31, 479; F. Trandafir et al., Appl. Magn. Reson., 2007, 31, 553; M. Fittipaldi et al., J. Phys. Chem. B, 2008, 112, 3859), in which the E(S - S) term is putatively treated to the second order. To show the usefulness of the present approach, we exploit Fe(Cl)OEP (S = 5/2) (OEP: 2,3,7,8,12,13,17,18-octaethylporphyrin) and CoOEP (S = 3/2) well magnetically diluted in the diamagnetic host crystal lattice of NiOEP. The advantage of single-crystal ESR spectroscopy lies in the fact that the molecular information on the principal axes of the magnetic tensors is crucial in comparing with reliable theoretical results. In high spin states of metallocomplexes with sizable ZFS in pseudo-octahedral symmetry, their fine-structure ESR transitions for the principal z-axis orientation appear in the lower field far from g = 2 at the X-band, disagreeing with the putative intuitive picture obtained using relevant ESR spectroscopy. A Re dinuclear complex in a mixed valence state exemplifies the cases, whose fine-structure/hyperfine ESR spectra of the neat crystals have been analyzed in their principal-axis system. The DFT-based/ab initio MO calculations of the magnetic tensors for all the high spin entities in this work were carried out.
虚构的自旋-1/2哈密顿方法是一种推测性方法，用于分析具有可观零场分裂（ZFS）的高自旋金属配合物的精细结构/超精细电子顺磁共振（ESR）光谱，因此在大多数情况下，无需明确提供其ZFS参数就能给出远离g = 2的显著主g值。实际上，g值与g = 2的显著偏差表明它们处于高自旋态，但自然地，它们与通过复杂的密度泛函理论（DFT）或从头算分子轨道（MO）计算等量子化学计算获得的真实g值并不一致。在这项工作中，我们提出了简便的方法来确定具有可观ZFS的高自旋金属配合物的磁张量，而不是进行先进的高场/高频ESR光谱分析。我们重新审视了高自旋态有效g值与真实主g值之间关系的解析表达式。给出了S高达7/2时g - g关系的有用解析公式。真正的塞曼微扰形式给出了S = 3/2时的精确解，对于更高的S值，它比迄今为止记录的伪塞曼微扰方法（A. 阿布拉加姆和B. 布莱尼，《过渡金属离子的电子顺磁共振》，1970年；J. R. 皮尔布罗，《磁共振杂志》，1978年，31卷，479页；F. 特兰达菲尔等人，《应用磁共振》，2007年，31卷，553页；M. 菲蒂帕尔迪等人，《物理化学杂志B》，2008年，112卷，3859页）更准确，在后者中，E(S - S)项被假定处理到二阶。为了展示本方法的实用性，我们利用在抗磁性主体晶格NiOEP中良好磁稀释的Fe(Cl)OEP（S = 5/2）（OEP：2,3,7,8,12,13,17,18 - 八乙基卟啉）和CoOEP（S = 3/2）。单晶ESR光谱的优势在于，磁张量主轴上的分子信息对于与可靠的理论结果进行比较至关重要。在具有可观ZFS的伪八面体对称的高自旋态金属配合物中，其在X波段对于主z轴取向的精细结构ESR跃迁出现在远离g = 2的低场，这与使用相关ESR光谱获得的推测直观图像不一致。一个处于混合价态的铼双核配合物就是这样的例子，其纯晶体的精细结构/超精细ESR光谱已在其主轴系统中进行了分析。对这项工作中所有高自旋实体的磁张量进行了基于DFT/从头算MO的计算。
