Bopp Joseph C, Roscioli Joseph R, Johnson Mark A, Miller Thomas M, Viggiano A A, Villano Stephanie M, Wren Scott W, Lineberger W Carl
Sterling Chemistry Laboratory, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, USA.
J Phys Chem A. 2007 Feb 22;111(7):1214-21. doi: 10.1021/jp0665372. Epub 2007 Feb 1.
Spectroscopic studies of the SF6- and c-C4F8- anions are reported to provide experimental benchmarks for theoretical predictions of their structures and electron binding energies. The photoelectron spectrum of SF6- is dominated by a long progression in the S-F stretching mode, with an envelope consistent with theoretical predictions that the anion preserves the Oh symmetry of the neutral, but has a longer S-F bond length. This main progression occurs with an unexpectedly strong contribution from a second mode, however, whose characteristic energy does not correspond to any of the neutral SF6 fundamental vibrations in its ground electronic state. The resulting doublet pattern is evident when the bare ion is prepared with low internal energy content (i.e., using N2 carrier gas in a free jet or liquid nitrogen-cooling in a flowing afterglow) but is much better resolved in the spectrum of the SF6-.Ar complex. The infrared predissociation spectrum of SF6-.Ar consists of a strong band at 683(5) cm(-1), which we assign to the nu3 (t1u) fundamental, the same mode that yields the strong 948 cm(-1) infrared transition in neutral SF6. One qualitatively interesting aspect of the SF6- behavior is the simple structure of its photoelectron spectrum, which displays uncluttered, harmonic bands in an energy region where the neutral molecule contains about 2 eV of vibrational excitation. We explore this effect further in the c-C4F8- anion, which also presents a system that is calculated to undergo large, symmetrical distortion upon electron attachment to the neutral. The photoelectron spectrum of this species is dominated by a long, single vibrational progression, this time involving the symmetric ring-breathing mode. Like the SF6- case, the c-C4F8- spectrum is remarkably isolated and harmonic in spite of the significant internal excitation of a relatively complex molecular framework. Both these perfluorinated anions thus share the property that the symmetrical deformation of the structural backbone upon photodetachment launches very harmonic motion in photoelectron bands that occur far above their respective adiabatic electron affinities.
据报道，对SF6-和c-C4F8-阴离子的光谱研究为其结构和电子结合能的理论预测提供了实验基准。SF6-的光电子能谱主要由S-F伸缩模式的长序列主导，其包络线与理论预测一致，即该阴离子保留了中性分子的Oh对称性，但S-F键长更长。然而，这一主要序列意外地受到第二种模式的强烈影响，其特征能量与基态电子态下中性SF6的任何基本振动均不对应。当裸离子以低内能制备时（即，在自由射流中使用N2载气或在流动余辉中使用液氮冷却），会出现明显的双峰模式，但在SF6-.Ar络合物的光谱中分辨率更高。SF6-.Ar的红外预解离光谱在683(5) cm(-1)处有一个强带，我们将其归属于ν3 (t1u) 基频，即与中性SF6中产生948 cm(-1) 强红外跃迁的相同模式。SF6-行为的一个定性有趣方面是其光电子能谱的简单结构，在中性分子含有约2 eV振动激发的能量区域显示出清晰、谐波的谱带。我们在c-C4F8-阴离子中进一步探讨了这种效应，该阴离子也是一个经计算在电子附着到中性分子时会发生大的对称畸变的体系。该物种的光电子能谱主要由一个长的单振动序列主导，这次涉及对称环呼吸模式。与SF6-的情况一样，尽管相对复杂的分子框架有显著的内部激发，但c-C4F8-光谱非常孤立且呈谐波状。因此，这两种全氟阴离子都具有这样的特性，即光解离时结构骨架的对称变形在远高于其各自绝热电子亲和能的光电子能谱带中引发非常谐波的运动。
