Davies Julia A, Yang Shengfu, Ellis Andrew M
Department of Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, UK.
Phys Chem Chem Phys. 2021 Dec 15;23(48):27449-27459. doi: 10.1039/d1cp03138d.
Infrared (IR) spectra of several hydrocarbon cations are reported, namely CH, CH, CH, CH(CH) and CH. The spectra were generated from weakly-bound helium-cation complexes formed by electron ionization of helium nanodroplets doped with a neutral hydrocarbon precursor. Spectroscopic transitions were registered by photoexcitation of the complexes coupled with mass spectrometric detection of the bare ions. For CH, we provide evidence showing that the helium-bound complexes contain 10-20 helium atoms (on average) and have a rotational temperature of ∼5 K. We show that this technique is well-suited to the study of highly symmetric or fluxional ionic species, as these intrinsic properties are preserved in the helium environment. This is in contrast to conventional tagging methods that use a single atom or molecule, which can change the point group or rigidity of the core ion and therefore the spectral profile. We demonstrate this for the highly fluxional molecular ion CH, whose spectrum in the current study matches that of the gas phase ion, whereas the fluxionality is lost when a methane tag is added. Finally, we present the first IR spectrum of methane cation, CH. The spectrum of this fundamental organic ion shows CH stretching bands consistent with a non-tetrahedral structure, a consequence of Jahn-Teller distortion.
报道了几种烃阳离子的红外（IR）光谱，即CH、CH、CH、CH(CH)和CH。这些光谱是由掺杂有中性烃前体的氦纳米液滴经电子电离形成的弱束缚氦阳离子络合物产生的。通过对络合物进行光激发并结合对裸离子的质谱检测来记录光谱跃迁。对于CH，我们提供的证据表明，与氦结合的络合物平均含有10 - 20个氦原子，旋转温度约为5K。我们表明，该技术非常适合研究高度对称或易变的离子物种，因为这些固有特性在氦环境中得以保留。这与使用单个原子或分子的传统标记方法形成对比，传统方法可能会改变核心离子的点群或刚性，从而改变光谱轮廓。我们以高度易变的分子离子CH为例进行了说明，在当前研究中其光谱与气相离子的光谱匹配，而添加甲烷标记时易变性消失。最后，我们展示了甲烷阳离子CH的首个红外光谱。这种基本有机离子的光谱显示出与非四面体结构一致的CH伸缩带，这是 Jahn - Teller 畸变的结果。
