Ultraviolet Photodissociation Spectroscopy of the Cold K+路Calix[4]arene Complex in the Gas Phase

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• 作者：Yoshiya Inokuchi ; Kazuki Soga ; Kenta Hirai ; Motoki Kida ; Fumiya Morishima ; Takayuki Ebata
• 刊名：Journal of Physical Chemistry A
• 出版年：2015
• 出版时间：August 6, 2015
• 年：2015
• 卷：119
• 期：31
• 页码：8512-8518
• 全文大小：398K
• ISSN：1520-5215

文摘

The cooling of ionic species in the gas phase greatly simplifies the UV spectrum, which is of special importance when studying the electronic and geometric structures of large systems, such as biorelated molecules and host鈥揼uest complexes. Many efforts have been devoted to achieving ion cooling with a cold, quadrupole Paul ion trap (QIT), but one problem was the insufficient cooling of ions (up to 鈭?0 K) in the QIT. In this study, we construct a mass spectrometer for the ultraviolet photodissociation (UVPD) spectroscopy of gas-phase cold ions. The instrument consists of an electrospray ion source, a QIT cooled with a He cryostat, and a time-of-flight mass spectrometer. With great care given to the cooling condition, we can achieve 鈭?0 K for the vibrational temperature of ions in the QIT, which is estimated from UVPD spectra of the benzo-18-crown-6 (B18C6) complex with a potassium ion, K⁺路B18C6. Using this setup, we measure a UVPD spectrum of cold calix[4]arene (C4A) complex with potassium ion, K⁺路C4A. The spectrum shows a very weak band and a strong one at 36018 and 36156 cm^鈥?, respectively, accompanied by many sharp vibronic bands in the 36000鈥?6600 cm^鈥? region. In the geometry optimization of the K⁺路C4A complex, we obtain three stable isomers: one endo and two exo forms. On the basis of the total energy and UV spectral patterns predicted by density functional theory calculations, we attribute the structure of the K⁺路C4A complex to the endo isomer (C₂ symmetry), in which the K⁺ ion is located inside the cup of C4A. The vibronic bands of K⁺路C4A at 36鈥?18 and 36鈥?56 cm^鈥? are assigned to the S₁(A)鈥揝₀(A) and S₂(B)鈥揝₀(A) transitions of the endo isomer, respectively.