Laser spectroscopic study on model negative ion systems of biological electron-transfer reaction intermediates

用光脱附谱研究了带负电荷的甲酰胺团簇(FA_n^-，n=5-21)中电子局域化的尺寸特征。在所研究的所有尺寸的光电子能谱中，发现了两种类型的带，它们是由不同的阴离子异构体引起的。通过在近红外区的相对光剥离截面测量，将峰值在1 eV附近的低结合能带指定为溶剂化电子态。结果表明，新生电子捕获主要由溶剂化电子的形成所控制。较高的能带来源于显著弛豫过程后产生的共价阴离子态，它表现出电子结合能随团簇大小的快速增加。此外，还测量了微溶剂化的苯腈阴离子(C_6H_5CN-S；S=H_2O和CH_3OH)在3μm区的电子剥离效率，代表了溶剂分子通过OH伸缩振动自剥离的共振带。与相应的中性团簇相比，两个团簇阴离子的氢键OH键向低能侧移动了约300 cm^~(-1)~(-1)。在用密度泛函理论优化的团簇结构中，溶剂分子共线键合在苯腈阴离子的CN基团的边缘，模拟的振动能与观察到的能带位置很好地吻合。用自然布居理论分析了电子吸附时溶剂取向变化的定性含义，并结合密度泛函计算，从理论上讨论了不对称的振动模对自脱离过程动力学的影响。

Size-dependent features of the electron localization in negatively charged formamide clusters (FA_n^-, n=5-21) have been studied by photodetachment spectroscopy. In the photoelectron spectra for all the sizes studied, two types of bands due to different isomers of anions were found. The low binding energy band peaking around 1 eV is assigned to the solvated electron state by relative photodetachment cross-section measurements in the near-infrared region. It is suggested that nascent electron trapping is dominated by formation of the solvated electron. The higher energy band originates from the covalent anion state generated after a significant relaxation process, which exhibits a rapid increase of electron binding energy as a function of the cluster size. A unique behavior showing a remarkable band intensity of the higher energy band was found only for n=9. In addition, vibrational spectra of microsolvated benzonitrile radical anions (C_6H_5CN-S; S=H_2O and CH_3OH) were measured by probing the electron detachment efficiency in the 3μm region, representing resonance bands of autodetachment via OH stretching vibrations of the solvent molecules. The hydrogen-bonded OH band for both the cluster anions exhibited a large shift to the lower energy side with approximately 300 cm^<-1> comparing to those for the corresponding neutral clusters. The solvent molecules are bound collinearly to the edge of the CN group of the benzonitrile anion in the cluster structures optimized with the density functional theory, in which the simulated vibrational energies are in good agreement with the observed band positions. Natural population analyses were performed for a qualitative implication in changes of solvent orientation upon electron attachment.Asymmetric band shapes depending on the vibrational modes are discussed with respect to dynamics of the autodetachment process from a theoretical aspect incorporated with density functional calculations.