Position matters: competing O-H and N-H photodissociation pathways in hydroxy- and methoxy-substituted indoles.
Position matters: competing O-H and N-H photodissociation pathways in hydroxy- and methoxy-substituted indoles.
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位置很重要：羟基和甲氧基取代的吲哚中竞争的 O-H 和 N-H 光解离途径。
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发表时间：
2011
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影响因子：
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通讯作者：
M. Ashfold
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作者：
T. Oliver;G. King;M. Ashfold
采用氢原子光碎片平移谱（HRA-PTS）和完全活性空间二级微扰理论（CASPT 2）方法研究了4-和5-羟基吲哚（HI）和甲氧基吲哚（MI）的N-H和O-H键竞争解离途径.当4-HI被激发到（1）L（B）能级时，（λ（phot）≤ 284.893 nm）O-H键发生了断裂。通过与苯酚的类比，推断解离是通过H原子在与（1）L（B）/（1）πσ*（（OH））圆锥相交（CI）的较低的diabat相关的势垒下隧穿而发生的。在这些或更短的激发波长（284.893 ≥ λ（phot）≥ 193.3 nm）下，未发现显著的N-H键解离产率。4-MI的伴随研究揭示了不同的反应动力学。在这种情况下，N-H键裂变发生在λ（phot）≤ 271.104 nm处，直接激发到（1）πσ*（（NH））态。测量的TKER光谱的分析暗示了一种机制，其中，如在吡咯中，（1）πσ*（（NH））状态通过从具有较高振子强度的附近束缚态借用强度来获得振子强度。相反，5-HI的HRA-PTS研究表明，当在（1）L（B）能级上激发时，没有O-H键解离的证据。目前的CASPT 2计算有助于使这一观察合理化：5-HI中（1）L（B）/（1）πσ* CI diabats下方的面积比4-HI中的相应面积大约60%，因此通过隧道效应解离O-H键的可能性要小得多。只有将波长降低至≤ 255 nm，才能鉴别出N-H和/或O-H键解离的迹象。通过与5-MI研究结果的比较，我们推断在λ（phot）> 235 nm处，5-HI中O-H键几乎不发生裂变，而在255 > λ（phot）> 235 nm处，N-H键裂变是H原子的主要来源。4-和5-HI的非常不同的解离动力学被追踪到-OH取代基的位置，以及其对整体电子结构的影响。
H (Rydberg) atom photofragment translational spectroscopy (HRA-PTS) and complete active space with second order perturbation theory (CASPT2) methods have been used to explore the competing N-H and O-H bond dissociation pathways of 4- and 5-hydroxyindoles (HI) and methoxyindoles (MI). When 4-HI was excited to bound (1)L(b) levels, (λ(phot) ≤ 284.893 nm) O-H bond fission was demonstrated by assignment of the structure within the resulting total kinetic energy release (TKER) spectra. By analogy with phenol, dissociation was deduced to occur by H atom tunnelling under the barrier associated with the lower diabats of the (1)L(b)/(1)πσ*((OH)) conical intersection (CI). No evidence was found for a significant N-H bond dissociation yield at these or shorter excitation wavelengths (284.893 ≥ λ(phot) ≥ 193.3 nm). Companion studies of 4-MI revealed different reaction dynamics. In this case, N-H bond fission is deduced to occur at λ(phot) ≤ 271.104 nm, by direct excitation to the (1)πσ*((NH)) state. Analysis of the measured TKER spectra implies a mechanism wherein, as in pyrrole, the (1)πσ*((NH)) state gains oscillator strength by intensity borrowing from nearby bound states with higher oscillator strengths. HRA-PTS studies of 5-HI, in contrast, showed no evidence for O-H bond dissociation when excited on (1)L(b) levels. The present CASPT2 calculations assist in rationalizing this observation: the area underneath the (1)L(b)/(1)πσ* CI diabats in 5-HI is ~60% greater than the corresponding area in 4-HI and O-H bond dissociation by tunnelling is thus much less probable. Only by reducing the wavelength to ≤ 255 nm were signs of N-H and/or O-H bond dissociation identified. By comparison with companion 5-MI studies, we deduce little O-H bond fission in 5-HI at λ(phot) > 235 nm and that N-H bond fission is the dominant source of H atoms in the wavelength region 255 > λ(phot) > 235 nm. The very different dissociation dynamics of 4- and 5-HI are traced to the position of the -OH substituent, and its effect on the overall electronic structure.