Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany.
Laboratory for Femtochemistry and Synchrotron Radiation, Paul Scherrer Institut (PSI), CH-5232 Villigen, Switzerland.
J Chem Phys. 2017 Aug 28;147(8):084303. doi: 10.1063/1.4999906.
The photodissociation dynamics of the CH isomers ortho- and para-xylyl are investigated in a free jet. The xylyl radicals are generated by flash pyrolysis from 2-(2-methylphenyl)- and 2-(4-methylphenyl) ethyl nitrite and are excited into the D state. REMPI- spectra show vibronic structure and the origin of the transition is identified at 32 291 cm for the para- and at 32 132 cm for the ortho-isomer. Photofragment H-atom action spectra show bands at the same energy and thus confirm H-atom loss from xylyl radicals. To gain further insight into the photodissociation dynamics, velocity map images of the hydrogen atom photofragments are recorded. Their angular distribution is isotropic and the translational energy release is in agreement with a dissociation to products in their electronic ground state. Photodissociation of para-xylyl leads to the formation of para-xylylene (CH), while the data for ortho-xylyl agree much better with the isomer benzocyclobutene as the dominant molecular fragment rather than ortho-xylylene. In computations we identified a new pathway for the reaction ortho-xylyl → benzocyclobutene + H with a barrier of 3.39 eV (27 340 cm), which becomes accessible at the employed excitation energy. It proceeds via a combination of scissoring and rotational motion of the -CH and -CH groups. However, the observed rate constants measured by delaying the excitation and ionization laser with respect to each other are significantly faster than computed ones, indicating intrinsic non-RRKM behaviour. A comparably high value of around 30% of the excess energy is released as translation of the H-atom photofragment.
邻位和对位二甲苯异构体的光解动力学在自由喷射中进行了研究。二甲苯自由基通过从 2-(2-甲基苯基)-和 2-(4-甲基苯基)乙基亚硝酸盐的闪蒸热解产生，并被激发到 D 态。REMPI-光谱显示出振子结构，跃迁的起源在对位异构体为 32291cm，在邻位异构体为 32132cm 处被识别。光碎片 H 原子作用光谱在相同的能量处显示出谱带，因此证实了从二甲苯自由基中失去 H 原子。为了更深入地了解光解动力学，记录了氢原子光碎片的速度图图像。它们的角分布是各向同性的，并且翻译能量释放与产物处于其电子基态的解离一致。对位二甲苯的光解导致对位二甲苯（CH）的形成，而对于邻位二甲苯，数据更符合作为主要分子碎片的苯并环丁烯，而不是邻位二甲苯。在计算中，我们确定了反应邻位二甲苯→苯并环丁烯+H 的新途径，其势垒为 3.39eV（27340cm），在所用的激发能量下可以达到。它通过-CH 和-CH 基团的剪切和旋转运动的组合进行。然而，通过彼此延迟激发和电离激光来测量的观察到的速率常数明显快于计算值，表明存在内在的非 RRKM 行为。H 原子光碎片的翻译释放了大约 30%左右的过剩能量。
