Infrared spectra of carbocations and CH4+ in helium.
Infrared spectra of carbocations and CH4+ in helium.
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氦气中碳正离子和 CH4 的红外光谱。
DOI：
10.1039/d1cp03138d
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发表时间：
2021
期刊：
影响因子：
--
通讯作者：
Davies JA
中科院分区：
文献类型：
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作者：
Davies JA
本文报道了几种烃类阳离子CH3+、CH4+、CH5+、CH5+(CH4)和C2H5+的红外光谱。光谱是由掺杂中性烃前驱体的氦纳米液滴电子电离形成的弱结合氦-阳离子配合物产生的。光谱跃迁是通过光激发配合物加上裸离子的质谱检测来记录的。对于CH3+，我们提供的证据表明，氦键配合物平均含有10-20个氦原子，旋转温度为~ 5 K。我们表明，这种技术非常适合于研究高度对称或流动的离子物种，因为这些固有性质在氦环境中被保存下来。这与使用单个原子或分子的传统标记方法相反，后者可以改变核心离子的点群或刚性，从而改变光谱轮廓。我们在高通量分子离子CH5+中证明了这一点，在目前的研究中，CH5+的光谱与气相离子的光谱相匹配，而当添加甲烷标签时，流动性就会丧失。最后，我们给出了甲烷阳离子CH4+的第一个红外光谱。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拉伸带，这是姜-泰勒扭曲的结果。
Infrared (IR) spectra of several hydrocarbon cations are reported, namely CH3+, CH4+, CH5+, CH5+(CH4) and C2H5+. 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 CH3+, 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 CH5+, 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, CH4+. The spectrum of this fundamental organic ion shows CH stretching bands consistent with a non-tetrahedral structure, a consequence of Jahn–Teller distortion.