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首页> 外文期刊>Physical chemistry chemical physics: PCCP >The electronic spectroscopy of transition metal di-hydrides H2M(CO)(4) (M = Fe,Os): a theoretical study based on CASSCF/MS-CASPT2 and TD-DFT
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The electronic spectroscopy of transition metal di-hydrides H2M(CO)(4) (M = Fe,Os): a theoretical study based on CASSCF/MS-CASPT2 and TD-DFT

机译：过渡金属二氢化物H2M（CO）（4）（M = Fe，Os）的电子光谱：基于CASSCF / MS-CASPT2和TD-DFT的理论研究

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Transition energies to the low-lying singlet electronic excited states of H2M(CO)(4) (M = Fe,Os) are calculated at the CASSCF/MS-CASPT2 level of theory using relativistic effective core potentials in the ab initio model potential (AIMP) approach. The main features of the absorption spectra of both molecules differ significantly. The spectrum of H2Fe(CO)(4) is dominated by low-lying excited states corresponding mainly to 3d(Fe) --> sigma(u)* and 3d(Fe) --> sigma(g)* excitations calculated between 39 310 cm(-1) (4.9 eV) and 43 210 cm(-1) (5.4 eV). These allowed transitions contribute to the shoulder observed in the experimental spectrum around 270 nm (37 040 cm(-1)) and are responsible for the photoreactivity of H2Fe(CO)(4) at 254 nm (39 370 cm(-1)). In contrast the lowest part of the absorption spectrum of the osmium analog H2Os(CO)(4) is characterized by a high density of metal-ligand-charge-transfer (MLCT) states between 47 220 cm(-1) (5.9 eV) and 52 430 cm(-1) (6.55 eV) and corresponding to 5d(Os) --> pi(CO)* excitations. The transitions corresponding to 5d(Os) --> sigma(u)* and 5d(Os) --> sigma(g)* excitations are displaced to the upper part of the absorption spectrum of H2Os(CO)(4) (beyond 60 000 cm(-1) or 166 nm) and cannot induce H-2 elimination under irradiation at 254 nm like in the iron complex. A series of sigma-bond-ligand-charge-transfer (SBLCT) states corresponding to sigma(g) --> pi(CO)* and sigma(u) --> pi(CO)* excitations not present in H2Fe(CO)(4) is found exclusively in the spectrum of H2Os(CO)(4) beyond 55 000 cm(-1). The upper part of the spectrum of H2Fe(CO)(4) (below 220 nm) is assigned to intense MLCT transitions corresponding to 3d(Fe) --> pi(CO)* excitations. The MS-CASPT2 and TD-DFT methods agree qualitatively as far as the assignment and transition energies of the low-lying states are concerned in both molecules whereas a comparison of the calculated oscillator strengths is more problematic. The transitions are generally underestimated at the TD-DFT level, an effect that is even more pronounced for the charge transfer excitations to the carbonyl ligands. The high density of singlet electronic states in these simple transition metal hydrides carbonyls illustrates the complexity of the electronic spectroscopy in this class of molecules. This explains the poor resolution of these overcrowded spectra and the difficulty in understanding the photoreactivity of these molecules. [References: 60]
机译：在理论上的CASSCF / MS-CASPT2水平上，使用从头算模型电势中的相对论有效核心电势，计算出H2M（CO）（4）（M = Fe，Os）的低单重态电子激发态的跃迁能AIMP）方法。两种分子的吸收光谱的主要特征明显不同。 H2Fe（CO）（4）的光谱以低激发态为主，主要对应于39d之间计算的3d（Fe）-> sigma（u）*和3d（Fe）-> sigma（g）*激发310 cm（-1）（4.9 eV）和43210 cm（-1）（5.4 eV）。这些允许的跃迁有助于在270 nm（37040 cm（-1））的实验光谱中观察到的肩部，并负责H2Fe（CO）（4）在254 nm（39 370 cm（-1））的光反应性。相反，analog类似物H2Os（CO）（4）的吸收光谱的最低部分的特征是在47 220 cm（-1）（5.9 eV）之间的高密度金属-配体电荷转移（MLCT）状态和52430 cm（-1）（6.55 eV）并对应于5d（Os）-> pi（CO）*激发。与5d（Os）-> sigma（u）*和5d（Os）-> sigma（g）*激发相对应的跃迁被位移到H2Os（CO）（4）的吸收光谱的上部60 000 cm（-1）或166 nm），并且不能像铁络合物一样在254 nm的辐射下诱导H-2消除。对应于H2Fe（CO）中不存在的sigma（g）-> pi（CO）*和sigma（u）-> pi（CO）*激发的一系列sigma-bond-ligand-charge-transfer（SBLCT）状态）（4）仅在H2Os（CO）（4）的光谱中超过55000 cm（-1）。 H2Fe（CO）（4）的光谱的上限（低于220 nm）分配给与3d（Fe）-> pi（CO）*激发相对应的强烈MLCT跃迁。 MS-CASPT2和TD-DFT方法在两个分子中都涉及低位态的赋能和跃迁能时，在质量上是一致的，而比较计算出的振荡器强度则存在更多问题。通常在TD-DFT水平上低估了这些跃迁，这种影响对于电荷转移激发到羰基配体上更为明显。这些简单的过渡金属氢化物羰基中的单重态电子态的高密度说明了这类分子中电子光谱的复杂性。这解释了这些过度拥挤的光谱的较差的分辨率以及难以理解这些分子的光反应性。 [参考：60]

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