Ultrafast decarboxylation of organic peroxides in solution: interplay of different spectroscopic techniques, quantum chemistry, and theoretical modeling.
Ultrafast decarboxylation of organic peroxides in solution: interplay of different spectroscopic techniques, quantum chemistry, and theoretical modeling.
复制标题
DOI：
10.1002/anie.200390100
复制
发表时间：
2003-01-20
期刊：
影响因子：
--
通讯作者：
Schroeder, Jorg
中科院分区：
文献类型：
--
作者：
Abel, Bernd;Assmann, Jens;Schroeder, Jorg
有机过氧化物热分解的研究对面向应用的聚合物化学具有重要意义。自由基中间体的形成和后续过程的时间尺度，包括中间体的单分子分解，最终决定了自由基聚合的引发剂效率。[1ą4]大量关于热过氧化氢分解的文献已经积累起来。[5，6]光化学分解几乎只在纳秒到微秒的时间域内通过可见吸收和电子顺磁共振光谱进行研究。[7ą10]由于时间分辨率较差，不可能完全从机理上了解过氧化氢分解的基本反应。即使是随后用皮秒(Ps)时间分辨率进行的实验也没有提供详细而明确的机制洞察，[1ą3，11，12]，主要是因为时间分辨率不足和瞬时吸收带的光谱重叠。最近，通过瞬时红外光谱可以以大约2ą5-ps的时间分辨率监测过氧化氢分解后二氧化碳的形成及其随后的振动冷却。[1ą3]这些研究清楚地表明，需要飞秒(Fs)的时间分辨率才能详细地了解其机理。[1ą3]虽然最近的一篇论文集中在飞秒(Fs)时间尺度上的过氧化氢分解的实验研究，[13]我们在这里描述了利用超快FS光谱结合高水平量子化学和理论模型对过氧化氢脱羧基的瞬时中间体和产物的时间分辨监测。虽然我们已经研究了相当数量的不同的过氧化氢，但我们只讨论二-1-萘酰基过氧化氢(DNPO)作为我们的策略的一个例子(图1)。DNPO光解生成1-萘氧基自由基中间体，作为产物，生成萘基和二氧化碳。如图1所示，对于DNPO的光解，可以在不同的光谱区域直接用超快瞬时吸收光谱来探测瞬时1-萘氧基的衰减和产物的形成：在可见光区可以方便地检测到1-萘氧基，[14]而CO2是通过n3基团的红外吸收检测到的。[1]竞争过程，如热中间体的振动冷却[15ą18]，也可能产生与溶液中光诱导的过氧化氢脱羧基动力学类似的时间尺度的重叠光谱吸收。因此，有必要在较宽的波长范围(UV到IR)上扫描探测器波长，以确定合适的光谱观察窗口。图1的左侧显示了DNPO在碳酸亚丙酯(PC)溶液中激发后在266 nm处的瞬时吸收曲线，并探测了随后在300ą800 nm光谱范围内的瞬时吸收。这些痕迹的一个特征是在1ps内吸光度迅速上升，这归因于™热∫1-萘甲氧基自由基的极快形成。而热自由基反过来在PS时间尺度上解离(如快速吸收衰减所示)，而松弛自由基在更长的时间尺度上分解，这体现在衰变轨迹的™Offset∫中。在长延迟时间，可以识别出1-萘氧基的两个宽吸收带，这与Tateno等人从几个吸收信号分析中报告的瞬时光谱一致。
Studies of the decomposition of organic peroxides are of fundamental interest and importance for application-oriented polymer chemistry. The time scales of the formation of freeradical intermediates and of subsequent processes, including unimolecular decomposition of intermediates, ultimately determine the initiator efficiency in radical polymerizations.[1ą4] A large body of literature on thermal peroxide decomposition has accumulated.[5, 6] Photochemical decomposition has almost exclusively been investigated in the nanosecond to microsecond time domain by means of visible absorption and EPR spectroscopy.[7ą10] As a result of the poor time resolution, a complete mechanistic understanding of the elementary reactions of peroxide decomposition has not been possible. Even subsequent experiments with picosecond (ps) time resolution did not provide detailed unambiguous mechanistic insight,[1ą3, 11, 12] mostly because of insufficient time resolution and spectral overlap of transient absorption bands. More recently, the formation of CO2 and its subsequent vibrational cooling after peroxide decomposition could be monitored with about 2ą5-ps time resolution by transient IR spectroscopy.[1ą3] These studies clearly indicate that femtosecond (fs) time resolution is required for a detailed understanding of the mechanisms.[1ą3] While a recent paper focuses on experimental studies of peroxide decomposition on the femtosecond (fs) timescale,[13] we describe herein time-resolved monitoring of transient intermediates and products in peroxide decarboxylation with ultrafast fs spectroscopy in combination with high-level quantum chemistry and theoretical modeling. Although we have investigated a considerable number of different peroxides, we only discuss di-1-naphthoyl peroxide (DNPO) as an example of our strategy (Figure 1). DNPO photodissociation yields 1-naphthoyloxy freeradical intermediates and, as products, naphthyl radicals and CO2. As shown for the photodecomposition of DNPO in Figure 1, the decay of transient 1-naphthoyloxy radicals and the formation of products can be probed directly by ultrafast transient absorption spectroscopy in different spectral regions: 1-naphthoyloxy radicals can conveniently be detected in the visible region,[14] whereas CO2 is detected through the IR absorption of n3 fundamental.[1] Competing processes such as vibrational cooling of hot intermediates [15ą18] may also produce overlapping spectral absorptions on a similar time scale to that of photoinduced peroxide decarboxylation kinetics in solution. It was thus necessary to scan the probe wavelength over broad wavelength ranges (UV to IR) to identify suitable spectral observation windows. Transient absorption profiles after excitation of DNPO in a solution of propylene carbonate (PC) at 266 nm and probing the subsequent transient absorption in the 300ą800-nm spectral range are shown on the left-hand side of Figure 1. A characteristic feature of these traces is the rapid rise in absorbance within 1 ps, which is attributed to the extremely fast formation of™ hot∫ 1-naphthoyloxy radicals. Whereas the hot radicals in turn dissociate on a ps time scale (as is indicated by the fast absorbance decay), the relaxed radicals decompose on a much longer timescale, which manifests itself in the™ offset∫ of the decay traces. At long delay times, two broad absorption bands of 1-naphthoyloxy radicals can be identified, which agree with the transient spectra reported by Tateno et al.[14] From an analysis of several absorption signals,