School of Chemistry, University of Bristol, Bristol, UK BS8 1TS.
Phys Chem Chem Phys. 2011 Aug 28;13(32):14646-62. doi: 10.1039/c1cp21260e. Epub 2011 Jul 13.
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.
使用 H（Rydberg）原子光碎片平动能谱（HRA-PTS）和完全活性空间二阶微扰理论（CASPT2）方法研究了 4-和 5-羟基吲哚（HI）和甲氧基吲哚（MI）的竞争 N-H 和 O-H 键断裂途径。当 4-HI 被激发到束缚（1）L（b）能级时，通过在所得总动能释放（TKER）谱内分配结构，证明了 O-H 键的断裂。与苯酚类似，推断解离是通过与较低的（1）L（b）/（1）πσ*（（OH））锥形交叉（CI）下的势垒相关的 H 原子隧穿发生的。在这些或更短的激发波长（284.893≥λ（phot）≥193.3nm）下，没有发现明显的 N-H 键解离产率的证据。对 4-MI 的伴随研究揭示了不同的反应动力学。在这种情况下，通过直接激发到（1）πσ*（（NH））态，推断 N-H 键的断裂发生在 λ（phot）≤271.104nm。对测量的 TKE 谱的分析意味着，与吡咯类似，（1）πσ*（（NH））态通过从具有较高振动态强度的附近束缚态强度借用获得振动态强度。相比之下，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。本 CASPT2 计算有助于解释这一观察结果：在 5-HI 中，（1）L（b）/（1）πσ* CI diabats 下方的面积比 4-HI 中的面积大约 60%，因此隧穿的 O-H 键解离的可能性小得多。只有将波长降至≤255nm，才能发现 N-H 和/或 O-H 键解离的迹象。与伴随的 5-MI 研究相比，我们推断在 λ（phot）>235nm 时，5-HI 中 O-H 键的断裂很少，并且 N-H 键的断裂是 255>λ（phot）>235nm 波长区域中 H 原子的主要来源。4-和 5-HI 的非常不同的解离动力学可追溯到-OH 取代基的位置及其对整体电子结构的影响。
