School of Chemistry, University Park, University of Nottingham, NG7 2RD Nottingham, United Kingdom.
J Chem Phys. 2011 Feb 14;134(6):064306. doi: 10.1063/1.3536500.
The photoionization of enantiomerically pure epichlorohydrin (C(3)H(5)OCl) has been studied using linearly and circularly polarized vacuum ultraviolet synchrotron radiation. The threshold photoelectron spectrum was recorded and the first three bands assigned using molecular orbital calculations for the expected conformers, although uncertain experimental conformer populations and an anticipated breakdown in Koopmans' theorem leave some ambiguity. Measurements of the photoelectron circular dichroism (PECD) were obtained across a range of photon energies for each of these bands, using electron velocity map imaging to record the angular distributions, during which a record PECD chiral asymmetry factor of 32% was observed. A comparison with calculated PECD curves clarifies the assignment achieved using ionization energies alone and further suggests a likely relative population of the conformers. Threshold photoelectron-photoion coincidence methods were used to study the ionic fragmentation of epichlorohydrin. Fragment ion appearance energies show nonstatistical behavior with clear indications that the cationic epoxide ring is unstable and lower energy decay channels proceeding via ring breaking are generally open. Extensive neutral homochiral clusters of epichlorohydrin may be formed in supersonic molecular beam expansions seeded in Ar. Electron angular distribution measurements made in coincidence with dimer and trimer ions are used to effect an examination of the PECD associated with ionization of size-selected neutral cluster species, and these results differ clearly from PECD of the neutral monomer. The shifted ionization thresholds of the n-mers (n = 2, ..., 7) are shown to follow a simple linear relationship, but under intense beam expansion conditions the monomer deviates from this relationship, and the monomer electron spectra tail to below the expected monomer adiabatic ionization potential (IP). PECD measurements made in coincidence with monomer ions obtained under different beam expansion conditions were used to identify unambiguously a contribution from dissociative photoionization of larger clusters to the monomer parent mass ion yield above and below its adiabatic IP.
用线偏振和圆偏振真空紫外同步辐射研究了对映纯表氯醇（C（3）H（5）OCl）的光致电离。记录了阈光电电子能谱，并使用预期构象的分子轨道计算对前三个带进行了分配，尽管实验构象群体不确定和预期的 Koopmans 定理失效留下了一些模糊性。在这些带的每个带的一系列光子能量下，使用电子速度图成像测量了光电圆二色性（PECD），以记录角分布，在此期间观察到了 32%的记录 PECD 手性不对称因子。与单独使用电离能获得的计算 PECD 曲线的比较进一步澄清了分配，并进一步表明构象的可能相对群体。使用阈光电电子-光致离子符合方法研究了表氯醇的离子碎裂。碎片离子出现能表现出非统计行为，清楚地表明阳离子环氧化物不稳定，并且通常通过开环断裂的较低能量衰减通道是开放的。在 Ar 中种入超音速分子束膨胀中，可能形成大量的表氯醇同手性中性团簇。在与二聚体和三聚体离子符合的电子角分布测量中，用于检查与尺寸选择的中性团簇物种电离相关的 PECD，这些结果与中性单体的 PECD 明显不同。n-mer（n = 2，...，7）的偏移电离阈值被证明遵循简单的线性关系，但在强烈的光束扩展条件下，单体偏离了这种关系，并且单体电子光谱向下延伸到低于预期的单体绝热电离势（IP）。在不同的光束扩展条件下获得的单体离子符合的 PECD 测量用于明确识别较大团簇的离解光致电离对单体母体质量离子产额的贡献，该贡献高于和低于其绝热 IP。
