Samanta B R, Fernando R, Rösch D, Reisler H, Osborn D L
Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, USA.
Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551-0969, USA.
J Chem Phys. 2020 Aug 21;153(7):074307. doi: 10.1063/5.0018582.
Photodissociation of pyruvic acid (PA) was studied in the gas-phase at 193 nm using two complementary techniques. The time-sliced velocity map imaging arrangement was used to determine kinetic energy release distributions of fragments and estimate dissociation timescales. The multiplexed photoionization mass spectrometer setup was used to identify and quantify photoproducts, including isomers and free radicals, by their mass-to-charge ratios, photoionization spectra, and kinetic time profiles. Using these two techniques, it is possible to observe the major dissociation products of PA photodissociation: CO, CO, H, OH, HCO, CHCO, CHCO, and CH. Acetaldehyde and vinyl alcohol are minor primary photoproducts at 193 nm, but products that are known to arise from their unimolecular dissociation, such as HCO, HCO, and CH, are identified and quantified. A multivariate analysis that takes into account the yields of the observed products and assumes a set of feasible primary dissociation reactions provides a reasonable description of the photoinitiated chemistry of PA despite the necessary simplifications caused by the complexity of the dissociation. These experiments offer the first comprehensive description of the dissociation pathways of PA initiated on the S excited state. Most of the observed products and yields are rationalized on the basis of three reaction mechanisms: (i) decarboxylation terminating in CO + other primary products (∼50%); (ii) Norrish type I dissociation typical of carbonyls (∼30%); and (iii) O-H and C-H bond fission reactions generating the H atom (∼10%). The analysis shows that most of the dissociation reactions create more than two products. This observation is not surprising considering the high excitation energy (∼51 800 cm) and fairly low energy required for dissociation of PA. We find that two-body fragmentation processes yielding CO are minor, and the expected, unstable primary co-fragment, methylhydroxycarbene, is not observed because it probably undergoes fast secondary dissociation and/or isomerization. Norrish type I dissociation pathways generate OH and only small yields of CHCO and HOCO, which have low dissociation energies and further decompose via three-body fragmentation processes. Experiments with d-PA (CHCOCOOD) support the interpretations. The dissociation on S is fast, as indicated by the products' recoil angular anisotropy, but the roles of internal conversion and intersystem crossing to lower states are yet to be determined.
利用两种互补技术，在气相中于193nm波长下研究了丙酮酸（PA）的光解离。采用时间切片速度映射成像装置来确定碎片的动能释放分布，并估算解离时间尺度。复用光电离质谱仪装置则用于通过质荷比、光电离光谱和动力学时间分布来识别和量化光产物，包括异构体和自由基。运用这两种技术，可以观察到PA光解离的主要产物：CO、CO、H、OH、HCO、CHCO、CHCO和CH。乙醛和乙烯醇是193nm波长下的次要初级光产物，但对已知由它们单分子解离产生的产物，如HCO、HCO和CH，进行了识别和量化。尽管由于解离的复杂性导致必要的简化，但考虑到观察到的产物产率并假设一组可行的初级解离反应进行的多变量分析，对PA的光引发化学提供了合理描述。这些实验首次全面描述了PA在S激发态引发的解离途径。基于三种反应机制，对观察到的大多数产物和产率进行了合理化解释：（i）脱羧反应生成CO和其他初级产物（约50%）；（ii）羰基典型的Norrish I型解离（约30%）；（iii）产生H原子的O-H和C-H键裂变反应（约10%）。分析表明，大多数解离反应产生的产物不止两种。考虑到PA的高激发能（约51800cm）和解离所需的相当低的能量，这一观察结果并不令人惊讶。我们发现产生CO的双体碎片化过程较少，且未观察到预期的不稳定初级共碎片甲基羟基卡宾，因为它可能经历快速的二次解离和/或异构化。Norrish I型解离途径产生OH，且仅产生少量具有低解离能并通过三体碎片化过程进一步分解的CHCO和HOCO。用d-PA（CHCOCOOD）进行的实验支持了这些解释。如产物的反冲角各向异性所示，在S态上的解离很快，但内转换和系间窜越到较低态的作用尚待确定。
