Department of Chemistry and §Department of Physics, University of California, Berkeley , California 94720, United States.
J Am Chem Soc. 2013 Sep 25;135(38):14229-39. doi: 10.1021/ja405511v. Epub 2013 Sep 11.
The photoionization and dissociative photoionization of glycerol are studied experimentally and theoretically. Time-of-flight mass spectrometry combined with vacuum ultraviolet synchrotron radiation ranging from 8 to 15 eV is used to investigate the nature of the major fragments and their corresponding appearance energies. Deuterium (1,1,2,3,3-D5) and (13)C (2-(13)C) labeling is employed to narrow down the possible dissociation mechanisms leading to the major fragment ions (C3H(x)O2(+), C2H(x)O2(+), C2H(x)O(+), CH(x)O(+)). We find that the primary fragmentation of the glycerol radical cation (m/z 92) occurs only via two routes. The first channel proceeds via a six-membered hydrogen-transfer transition state, leading to a common stable ternary intermediate, comprised of neutral water, neutral formaldehyde, and a vinyl alcohol radical cation, which exhibits a binding energy of ≈42 kcal/mol and a very short (1.4 Å) hydrogen bond. Fragmentation of this intermediate gives rise to experimentally observed m/z 74, 62, 44, and 45. Fragments m/z 74 and 62 both consist of hydrogen-bridged ion-molecule complexes with binding energy >25 kcal/mol, whereas the m/z 44 species lacks such stabilization. This explains why water- or formaldehyde-loss products are observed first. The second primary fragmentation route arises from cleaving the elongated C-C bond. Also for this channel, intermediates comprised of hydrogen-bridged ion-molecule complexes exhibiting binding energies >24 kcal/mol are observed. Energy decomposition analysis reveals that electrostatic and charge-transfer interactions are equally important in hydrogen-bridged ion-molecule complexes. Furthermore, the dissociative photoionization of the glycerol dimer is investigated and compared to the main pathways for the monomeric species. To a first approximation, the glycerol dimer radical cation can be described as a monomeric glycerol radical cation in the presence of a spectator glycerol, thus giving rise to a dissociation pattern similar to that of the monomer.
甘油的光电离和光解光电离通过实验和理论进行了研究。飞行时间质谱结合从 8 到 15 eV 的真空紫外同步辐射用于研究主要碎片的性质及其相应的出现能。氘（1,1,2,3,3-D5）和（13）C（2-（13）C）标记用于缩小导致主要碎片离子（C3H（x）O2（+），C2H（x）O2（+），C2H（x）O（+），CH（x）O（+））的可能的解离机制。我们发现，甘油自由基阳离子（m/z 92）的主要断裂仅通过两条途径发生。第一条途径通过六元氢转移过渡态进行，导致中性水、中性甲醛和乙烯醇自由基阳离子的常见稳定三元中间体，其结合能约为 42 kcal/mol，氢键非常短（1.4 Å）。该中间体的断裂导致实验观察到的 m/z 74、62、44 和 45。碎片 m/z 74 和 62 都由具有大于 25 kcal/mol 的结合能的氢键离子-分子复合物组成，而 m/z 44 物种则缺乏这种稳定性。这解释了为什么首先观察到水或甲醛损失产物。第二条主要断裂途径来自于切断伸长的 C-C 键。对于这个通道，也观察到具有大于 24 kcal/mol 的结合能的氢键离子-分子复合物的中间体。能量分解分析表明，静电和电荷转移相互作用在氢键离子-分子复合物中同样重要。此外，研究了甘油二聚体的光解光电离，并与单体的主要途径进行了比较。在第一近似中，甘油二聚体自由基阳离子可以被描述为在存在旁观者甘油的情况下的单体甘油自由基阳离子，从而导致与单体相似的解离模式。
