-
氰基是典型的吸电子基团, 吸引了相关研究人员的兴趣. 许多论文报道了邻羟基苯腈及其二聚物、与小分子的复合物的散射荧光谱, 其主要目的是研究分子间的氢键作用和电子基态的振动特性. 也有采用荧光激发谱研究激发态振动的报道, 但缺少系统分析激发态光谱振动模式的相关工作. 相比于荧光谱, 共振多光子电离(Resonance Enhanced Multiphoton Ionization, REMPI)光谱探测离子获得激发态能级信息, 具有质量分辨能力, 排除了不同荷质比杂质的干扰. 氰基的强的吸电子能力导致包含氰基的分子具有较高的电离能, 许多苯腈衍生物的REMPI实验需要双色激光才能进行. 本文在自制的直线式飞行时间质谱仪上进行了双色共振增强双光子电离实验, 首次获得了振动分辨的邻羟基苯腈的REMPI光谱, 结合高精度密度泛函理论计算和Franck-Condon光谱模拟, 详细分析了光谱特征, 发现了大量基频、泛频和组合振动, 并进行了尽可能准确的光谱归属. 大部分苯环的基频振动归属为环在平面内的畸变或平面内的摇摆, 这与分子激发过程中苯环的扩张有关. 理论和实验结果都表明, REMPI光谱的低频段信号强, 背景低, 谱带少, 分辨率好. 随着振动频率的增加, 信号向相反的方向变化. 这是由于低频段光谱主要来自于低频的基频振动、少量泛频的贡献, 随着振动频率增加, 泛频和各种模的组合振动逐渐增多, 导致了高频区谱带稠密, 分辨率变差. 高阶振动和多模的组合振动通常有较低的Franck-Condon因子, 因此信号随频率增加逐渐变弱, 信噪比变差.The cyano group is a typical electron withdrawing group, which has attracted the interest of related researchers. Many papers reported the dispersed fluorescence spectra of o-hydroxybenzonitrile, its dimers, and complexes with small molecules, the main purpose of which is to study the intermolecule hydrogen bond and the vibration features of the electronic ground state. There are also reports using fluorescence excitation spectra to study excited state vibrations, but it is a lack of related work to systematically analyze the vibration features of excited state spectra. Compared with fluorescence spectroscopy, resonance enhanced multiphoton ionization (REMPI) spectroscopy detects ions to obtain excited state energy level information, which has mass resolution capability, and eliminates the interference of impurities with different charge-to-mass ratios. The strong electron-withdrawing ability of cyano group results in higher ionization energy for molecules containing cyano groups. Many REMPI experiments of benzonitrile derivatives require two-color lasers. In this paper, two-color resonance enhanced two-photon ionization experiment was performed on a home-made linear time-of-flight mass spectrometer, and the vibration-resolved REMPI spectrum of o-hydroxybenzonitrile was obtained for the first time. Combined with high-precision density functional theory calculations and Franck-Condon spectral simulations, the spectral characteristics were analyzed in detail, and a large number of fundamental, overtone and combined vibrations were found. The spectral assignment was carried out as accurately as possible. Most of the fundamental vibrations located at ring are assigned to the in-plane distortion or swing of the ring, which is related to the expansion of the ring during the molecular excitation. Theoretical and experimental results show that the low-frequency signal of REMPI spectrum is strong, the background is low, the band is less, and the resolution is good. As the vibration frequency increases, the signal changes in the worse direction. This is because the low-frequency spectrum mainly comes from the low-frequency fundamental vibrations and the contribution of a small amount of overtones. As the vibration frequency increases, the quantity of overtone and combined vibrations gradually increases, resulting in dense bands and low resolution. Theoretical calculations show that the high-order vibration and combination of multi-mode vibration usually have a lower Franck-Condon factor, so the signal gradually becomes weak as the frequency increases, and the signal-to-noise ratio becomes worse.
计量
- 文章访问数: 2470
- PDF下载量: 13
- 被引次数: 0