JILA, National Institute of Standards and Technology and Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309, United States.
J Phys Chem A. 2013 Oct 3;117(39):10047-57. doi: 10.1021/jp400702p. Epub 2013 Apr 17.
Phenyl radical has been studied via sub-Doppler infrared spectroscopy in a slit supersonic discharge expansion source, with assignments for the highest frequency b2 out-of-phase C-H symmetric stretch vibration (ν19) unambiguously confirmed by ≤6 MHz (0.0002 cm(-1)) agreement with microwave ground state combination differences of McMahon et al. [Astrophys. J. 2003, 590, L61-64]. Least squares analysis of over 100 resolved rovibrational peaks in the sub-Doppler spectrum to a Watson Hamiltonian yields precision excited-state rotational constants and a vibrational band origin (ν0 = 3071.8915(4) cm(-1)) consistent with a surprisingly small red-shift (0.9 cm(-1)) with respect to Ar matrix isolation studies of Ellison and co-workers [J. Am. Chem. Soc. 2001, 123, 1977]. Nuclear spin weights and inertial defects confirm the vibrationally averaged planarity and (2)A1 rovibronic symmetry of phenyl radical, with analysis of the rotational constants consistent with a modest C2v distortion of the carbon backbone frame due to partial sp rehybridization of the σ C radical-center. Most importantly, despite the number of atoms (N = 11) and vibrational modes (3N - 6 = 27), phenyl radical exhibits a remarkably clean jet cooled high-resolution IR spectrum that shows no evidence of intramolecular vibrational relaxation (IVR) phenomena such as local or nonlocal perturbations due to strongly coupled nearby dark states. This provides strong support for the feasibility of high-resolution infrared spectroscopy in other aromatic hydrocarbon radical systems.
苯自由基已通过亚多普勒红外光谱学在狭缝超声速放电扩展源中进行研究，通过与 McMahon 等人的微波基态组合差的≤6 MHz（0.0002 cm^-1）的吻合，明确地确认了最高频率 b2 反相 C-H 对称伸缩振动（ν19）的归属[天体物理学报。2003, 590, L61-64]。通过对亚多普勒光谱中 100 多个分辨出的转动振动峰进行最小二乘分析，得到了高精度的激发态转动常数和振动带的起源（ν0 = 3071.8915（4）cm^-1），与 Ellison 及其同事在 Ar 基质隔离研究中令人惊讶的小红移（0.9 cm^-1）一致[J. Am. Chem. Soc. 2001, 123, 1977]。核自旋权重和惯性缺陷证实了苯自由基振动平均的平面性和（2）A1 转动对称性，对转动常数的分析与由于 σ C 自由基中心的部分 sp 重杂化导致的碳骨架框架的适度 C2v 变形一致。最重要的是，尽管有 11 个原子（N = 11）和 27 个振动模式（3N - 6 = 27），但苯自由基表现出非常干净的喷射冷却高分辨率红外光谱，没有显示出分子内振动弛豫（IVR）现象的证据，例如由于强耦合的附近暗态而导致的局部或非局部扰动。这为在其他芳香烃自由基体系中进行高分辨率红外光谱学提供了强有力的支持。
