Chang Chih-Hsuan, Nesbitt David J
JILA, National Institute of Standards and Technology, University of Colorado, Boulder, Colorado 80309, USA and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA.
J Chem Phys. 2016 Jul 28;145(4):044304. doi: 10.1063/1.4955295.
A series of CH stretch modes in phenyl radical (C6H5) has been investigated via high resolution infrared spectroscopy at sub-Doppler resolution (∼60 MHz) in a supersonic discharge slit jet expansion. Two fundamental vibrations of a1 symmetry, ν1 and ν2, are observed and rotationally analyzed for the first time, corresponding to in-phase and out-of-phase symmetric CH stretch excitation at the ortho/meta/para and ortho/para C atoms with respect to the radical center. The ν1 and ν2 band origins are determined to be 3073.968 50(8) cm(-1) and 3062.264 80(7) cm(-1), respectively, which both agree within 5 cm(-1) with theoretical anharmonic scaling predictions based on density functional B3LYP/6-311g++(3df,3dp) calculations. Integrated band strengths for each of the CH stretch bands are analyzed, with the relative intensities agreeing remarkably well with theoretical predictions. Frequency comparison with previous low resolution Ar-matrix spectroscopy [A. V. Friderichsen et al., J. Am. Chem. Soc. 123, 1977 (2001)] reveals a nearly uniform Δν ≈ + 10-12 cm(-1) blue shift between gas phase and Ar matrix values for ν1 and ν2. This differs substantially from the much smaller red shift (Δν ≈ - 1 cm(-1)) reported for the ν19 mode, and suggests a simple physical model in terms of vibrational mode symmetry and crowding due to the matrix environment. Finally, the infrared phenyl spectra are well described by a simple asymmetric rigid rotor Hamiltonian and show no evidence for spectral congestion due to intramolecular vibrational coupling, which bodes well for high resolution studies of other ring radicals and polycyclic aromatic hydrocarbons. In summary, the combination of slit jet discharge methods with high resolution infrared lasers enables spectroscopic investigation of even highly reactive combustion and interstellar radical intermediates under gas phase, jet-cooled (Trot ≈ 11 K) conditions.
通过在超音速放电狭缝射流膨胀中以亚多普勒分辨率（约60兆赫兹）进行的高分辨率红外光谱，研究了苯基自由基（C6H5）中的一系列C-H伸缩振动模式。首次观测并对两种a1对称的基本振动ν1和ν2进行了转动分析，它们分别对应于相对于自由基中心的邻位/间位/对位和邻位/对位C原子处同相和异相对称C-H伸缩激发。ν1和ν2带的起源分别确定为3073.968 50(8)厘米-1和3062.264 80(7)厘米-1，这两个值与基于密度泛函B3LYP/6-311g++(3df,3dp)计算的理论非谐标度预测在5厘米-1范围内相符。分析了每个C-H伸缩带的积分带强度，其相对强度与理论预测非常吻合。与先前的低分辨率氩基质光谱[A. V. Friderichsen等人，《美国化学会志》123，1977（2001）]进行频率比较，结果显示ν1和ν2在气相和氩基质值之间存在近均匀的Δν≈+10 - 12厘米-1蓝移。这与报道的ν19模式的小得多的红移（Δν≈ - 1厘米-1）有很大不同，并暗示了一个基于振动模式对称性和基质环境引起的拥挤的简单物理模型。最后，红外苯基光谱可以用一个简单的不对称刚性转子哈密顿量很好地描述，并且没有显示出由于分子内振动耦合导致的光谱拥挤的证据，这对于其他环状自由基和多环芳烃的高分辨率研究是个好兆头。总之，狭缝射流放电方法与高分辨率红外激光的结合使得在气相、射流冷却（Trot≈11 K）条件下对甚至高度反应性的燃烧和星际自由基中间体进行光谱研究成为可能。
