Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA.
Department of Chemistry, University of Louisiana at Lafayette, Lafayette, LA 70504, USA.
Phys Chem Chem Phys. 2023 Mar 8;25(10):7453-7465. doi: 10.1039/d3cp00207a.
The photodissociation dynamics of the dimethyl-substituted acetone oxide Criegee intermediate [(CH)COO] is characterized following electronic excitation to the bright ππ* state, which leads to O (D) + acetone [(CH)CO, S] products. The UV action spectrum of (CH)COO recorded with O (D) detection under jet-cooled conditions is broad, unstructured, and essentially unchanged from the corresponding electronic absorption spectrum obtained using a UV-induced depletion method. This indicates that UV excitation of (CH)COO leads predominantly to the O (D) product channel. A higher energy O (P) + (CH)CO (T) product channel is not observed, although it is energetically accessible. In addition, complementary MS-CASPT2 trajectory surface-hopping (TSH) simulations indicate minimal population leading to the O (P) channel and non-unity overall probability for dissociation (within 100 fs). Velocity map imaging of the O (D) products is utilized to reveal the total kinetic energy release (TKER) distribution upon photodissociation of (CH)COO at various UV excitation energies. Simulation of the TKER distributions is performed using a hybrid model that combines an impulsive model with a statistical component, the latter reflecting the longer-lived (>100 fs) trajectories identified in the TSH calculations. The impulsive model accounts for vibrational activation of (CH)CO arising from geometrical changes between the Criegee intermediate and the carbonyl product, indicating the importance of CO stretch, CCO bend, and CC stretch along with activation of hindered rotation and rock of the methyl groups in the (CH)CO product. Detailed comparison is also made with the TKER distribution arising from photodissociation dynamics of CHOO upon UV excitation.
二甲基取代的丙酮氧化物 Criegee 中间体 [(CH3)2COO] 的光解动力学特性，是通过将其电子激发到明亮的 ππ* 态来进行研究的，这导致 O(D) + 丙酮 [(CH3)CO, S] 产物的生成。在喷射冷却条件下，用 O(D) 检测记录的 (CH3)2COO 的 UV 作用光谱是宽的、无结构的，并且基本上与使用 UV 诱导耗散方法获得的相应电子吸收光谱没有变化。这表明，(CH3)2COO 的 UV 激发主要导致 O(D) 产物通道。虽然能量上是可及的，但没有观察到更高能量的 O(P) + (CH3)CO (T) 产物通道。此外，互补的 MS-CASPT2 轨道表面跳跃 (TSH) 模拟表明，导致 O(P) 通道的种群最小，并且离解的总概率不为 1（在 100 fs 内）。利用 O(D) 产物的速度图成像，揭示了在不同的 UV 激发能量下，(CH3)2COO 光解时总动能释放 (TKER) 分布。使用一种混合模型来模拟 TKER 分布，该模型将冲动模型与统计部分相结合，后者反映了在 TSH 计算中确定的寿命较长（>100 fs）的轨迹。冲动模型解释了 Criegee 中间体和羰基产物之间的几何变化引起的 (CH3)2CO 的振动活化，表明 CO 拉伸、CCO 弯曲和 CC 拉伸以及甲基的受阻旋转和晃动的活化在 (CH3)2CO 产物中非常重要。还与 UV 激发时 CHOO 光解动力学产生的 TKER 分布进行了详细比较。
