Infrared Spectroscopy of Fluorenyl Cations at Cryogenic Temperatures

The notion of (anti)aromaticity
is a successful concept in chemistry
to explain the structure and stability of polycyclic hydrocarbons.
Cyclopentadienyl and fluorenyl cations are among the most studied
classical antiaromatic systems. In this work, fluorenyl cations are
investigated by high-resolution gas-phase infrared spectroscopy in
helium droplets. Bare fluorenyl cations are generated in the gas phase
by electrospray ionization. After mass-to-charge selection, ions are
captured in ultracold helium nanodroplets and probed by infrared spectroscopy
using a widely tunable free-electron laser in the 600–1700
cm–1 range. The highly resolved cryogenic infrared
spectra confirm, in combination with DFT computations, that all cations
are present in their singlet states.

（反）芳香性的概念是化学领域一项成功的理论，用以阐释多环烃的结构与稳定性。环戊二烯基和荧光基阳离子是研究最为深入的典型反芳香性系统之一。在本研究中，通过使用高分辨率气相红外光谱技术在氦滴中对荧光基阳离子进行了探究。在气相中，通过电喷雾电离技术生成裸露的荧光基阳离子。在质荷比选择后，离子被捕获在超冷氦纳米滴中，并使用广泛调谐的自由电子激光（波长范围在600–1700 cm–1）进行红外光谱探测。高度解析的低温红外光谱与DFT计算相结合，证实所有阳离子均处于单重态。