Abstract Cold ion action spectroscopy is applied to record the gas phase infrared fingerprint spectra of mass-selected C5H5N•+ and C5H5NH+ cations using an infrared free electron laser. The structures of the cations are deduced from the experimental spectra using anharmonic vibrational frequencies from density functional theory calculations. A very good agreement between experimental and theoretical infrared frequencies is observed. The dominant structure of the C5H5N•+ cation is assigned to the aromatic pyridine radical cation form. Additionally, a minor contribution of the lower energetic α-distonic isomer is observed. The C5H5NH+ cation is ascribed to the aromatic pyridinium cation where protonation has taken place on the nitrogen atom. The rare-gas tag used in the action spectroscopic method has a negligible effect on the vibrational frequencies. The observed species, with now accurately determined vibrational frequencies, are good candidates for future rotational spectroscopic studies and infrared observations in astronomical sources such as interstellar clouds or Titan’s atmosphere.

中文翻译：

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冷离子红外光谱阐明吡啶阳离子 (C5H5N•+) 和质子化吡啶 (C5H5NH+) 的稳定异构结构

摘要 应用冷离子作用光谱仪，利用红外自由电子激光记录质量选择的C5H5N•+和C5H5NH+阳离子的气相红外指纹图谱。使用来自密度泛函理论计算的非谐振动频率从实验光谱推导出阳离子的结构。观察到实验和理论红外频率之间的非常好的一致性。C5H5N•+ 阳离子的主要结构属于芳香族吡啶自由基阳离子形式。此外，还观察到了较低能量的 α-不饱和异构体的次要贡献。C5H5NH+ 阳离子归因于芳族吡啶鎓阳离子，其中在氮原子上发生了质子化。作用光谱法中使用的稀有气体标签对振动频率的影响可以忽略不计。观察到的物种，现在准确确定了振动频率，是未来旋转光谱研究和对星际云或泰坦大气等天文来源进行红外观测的良好候选者。