Planar and nonplanar conformations of (meso-tetraphenylporphinato)nickel(II) in solution as inferred from solution and solid-state Raman spectroscopy
Planar and nonplanar conformations of (meso-tetraphenylporphinato)nickel(II) in solution as inferred from solution and solid-state Raman spectroscopy
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DOI：
10.1021/jp970496f
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发表时间：
1997-08-07
影响因子：
2.9
通讯作者：
Shelnutt, JA
中科院分区：
文献类型：
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作者：
Jentzen, W;Unger, E;Shelnutt, JA
根据溶液和在不同温度（170 - 297 K）和激发波长（413.1和457.9 nm）下获得的单晶共振拉曼光谱，推断（内消旋四苯基卟啉）镍（II）[Ni（TPP）]的至少两种构象异构体在溶液中共存。结构敏感拉曼谱线v（8）和v（2）的形状明显不对称，并随温度变化。这些宽线可以被分解成至少两个子线，低频（LF）和高频（HF）分量。与此相反，晶体中Ni（TPP）的非平面结构的相应单晶拉曼线是窄的和对称的。对于线v（2），宽的LF子线来自非平面构象，窄的HF子线来自更平面的构象。这一分配与观察结果一致，即在将激发波长从413.1 nm改变到457.9 nm时，v（2）的LF子线被更多地增强。选择性共振增强是由紫外-可见吸收带的红移和非平面形式的拉曼激发轮廓引起的。vs的子线的频率分配与v（2）的频率分配相反（即，v（8）的HF亚线来自非平面构象，LF亚线来自更平面的大环）。这种分配是基于使用位于B带红色一侧的激发波长的子线宽度和增强行为。的子线的非平面构象的分配也与单晶的拉曼光谱，其中Ni（TPP）是已知的从X-射线晶体学具有主要是皱褶的非平面构象。具体而言，分配给溶液中的非平面形式的v（8）和v（2）的子线的频率与单晶中Ni（TPP）的频率紧密匹配。我们提出了两个热力学模型的子线的强度比的温度依赖性的解释。两态模型假设溶液中有一个平面构象和一个非平面构象。从范特霍夫曲线的斜率来看，在这个模型中，非平面构象的能量优势约为1.8 kJ mol（-1）。由于分子力学计算预测三种构象的Ni（TPP）在溶液中，我们也考虑了三态模型。这三个结构是两个非平面结构的纯皱褶（ruf）和纯鞍（sad）大环扭曲和平面构象。在计算中，ruf构象是能量最低的结构，平面和sad构象具有几乎相等的较高能量。假设能量之间存在这种关系，但允许实际的能量分离变化，三态分析给出了类似的结果，即ruf构象相对于平面和sad构象稳定约2.3 kJ mol（-1）。
The coexistence in solution of at least two conformers of (meso-tetraphenylporphinato)nickel(II) [Ni(TPP)] is inferred from solution and single-crystal resonance Raman spectra obtained at different temperatures (170 - 297 K) and excitation wavelengths (413.1 and 457.9 nm). The shapes of the structure-sensitive Raman lines v(8) and v(2) are clearly asymmetric and change with temperature. These broad lines can be decomposed into at least two sublines, a low-frequency (LF) and a high-frequency (HF) component. In contrast, the corresponding single-crystal Raman lines of the nonplanar structure of Ni(TPP) in the crystal are narrow and symmetric. For the line v(2), the broad LF subline results from nonplanar conformers and the narrow HF subline arises from a more planar conformer. This assignment is consistent with the observation that the LF subline of v(2) is more enhanced upon changing the excitation wavelength from 413.1 to 457.9 nm. The selective resonance enhancement is caused by the red shifts of the UV-visible absorption bands and Raman excitation profiles of the nonplanar form. The frequency assignment for the sublines of vs is reversed from that of v(2) (i.e., the HF subline of v(8) arises from nonplanar conformers and the LF subline results from a more planar macrocycle). This assignment is based on subline broadness and enhancement behavior using an excitation wavelength located on the red side of the B band. The assignments of the sublines to the nonplanar conformer are also in agreement with the Raman spectra of single crystals in which Ni(TPP) is known from X-ray crystallography to have a predominantly ruffled nonplanar conformation. Specifically, the frequencies of the sublines of v(8) and v(2) that are assigned to the nonplanar form in solution closely match the frequencies of Ni(TPP) in the single crystal. We propose two thermodynamic models for the interpretation of the temperature dependence of the intensity ratios of the sublines. A two-state model assumes one planar and one nonplanar conformer in solution. From the slopes in the van't Hoff plots, the nonplanar conformer is energetically favored by about 1.8 kJ mol(-1) in this model. Since molecular mechanics calculations predict three conformers of Ni(TPP) in solution, we also consider a three-state model. The three structures are two nonplanar structures of purely ruffled (ruf) and purely saddled (sad) macrocyclic distortions and a planar conformer. In the calculations, the ruf conformation is the lowest-energy structure with the planar and sad conformers having almost equal higher energies. Assuming this relationship between the energies, but allowing the actual energy separation to vary, the three-state analysis gives the similar result that the ruf conformation is stabilized by about 2.3 kJ mol(-1) with respect to the planar and sad conformers.