Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom.
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom.
J Chem Phys. 2019 Nov 21;151(19):194307. doi: 10.1063/1.5124137.
Messenger spectroscopy is a well-established method for recording infrared (IR) spectra of molecular ions. It relies upon the tagging of weakly bound atoms or molecules, known as the "messenger," to the ion of interest. The ideal tag species is helium since it has the weakest possible interaction with any molecular ion and is consequently the least likely to alter the structure and function. However, the attachment of a helium tag is challenging because of the exceptionally cold conditions that are inherently required. In this work, electron ionization of doped liquid helium nanodroplets has been used to create cations tagged with a variable number (N) of helium atoms. Mass-selective ion detection has made it possible to record IR spectra as a function of N, thus revealing the effect on the structure and charge distribution within the ionic core as solvation becomes more extensive. We illustrate this capability for protonated carbon dioxide tagged with up to 14 helium atoms, He-HOCO. The first atom preferentially binds to the proton and results in a substantial redshift of 44 cm for the OH stretching vibration, while the stepwise attachment of additional atoms up to N = 7 causes small and progressive blueshifts, which are attributed to the gradual formation of a ring of helium around the carbon atom. The methodology described herein offers a new route to obtain IR spectra of He-tagged ions and provides an insight into ion-solvent interactions at the molecular level.
信使光谱学是一种成熟的方法，用于记录分子离子的红外（IR）光谱。它依赖于将所谓的“信使”标记到感兴趣的离子上，这些标记物与离子的结合较弱。理想的标记物是氦气，因为它与任何分子离子的相互作用都尽可能弱，因此最不可能改变结构和功能。然而，由于需要非常低的温度条件，氦气标签的附着具有挑战性。在这项工作中，使用掺杂的液氦纳米液滴的电子电离来创建标记有可变数量（N）氦原子的阳离子。质量选择性离子检测使得能够记录随 N 变化的 IR 光谱，从而揭示了在溶剂化变得更加广泛时对离子核内结构和电荷分布的影响。我们展示了这种能力，即最多可标记 14 个氦原子的质子化二氧化碳，He-HOCO。第一个原子优先与质子结合，导致 OH 伸缩振动的红移 44cm，而逐步附加额外的原子直到 N=7 导致小的和渐进的蓝移，这归因于氦原子周围逐渐形成氦环。本文描述的方法为获得 He 标记离子的 IR 光谱提供了新途径，并提供了对分子水平上离子-溶剂相互作用的深入了解。
