School of Chemistry, University of Bristol, Bristol, United Kingdom.
J Chem Phys. 2013 Apr 28;138(16):164318. doi: 10.1063/1.4802058.
The wavelength dependences of C-Y and O-H bond fission following ultraviolet photoexcitation of 4-halophenols (4-YPhOH) have been investigated using a combination of velocity map imaging, H Rydberg atom photofragment translational spectroscopy, and high level spin-orbit resolved electronic structure calculations, revealing a systematic evolution in fragmentation behaviour across the series Y = I, Br, Cl (and F). All undergo O-H bond fission following excitation at wavelengths λ ≲ 240 nm, on repulsive ((n∕π)σ∗) potential energy surfaces (PESs), yielding fast H atoms with mean kinetic energies ∼11,000 cm(-1). For Y = I and Br, this process occurs in competition with prompt C-I and C-Br bond cleavage on another (n∕π)σ∗ PES, but no Cl∕Cl∗ products unambiguously attributable to one photon induced C-Cl bond fission are observed from 4-ClPhOH. Differences in fragmentation behaviour at longer excitation wavelengths are more marked. Prompt C-I bond fission is observed following excitation of 4-IPhOH at all λ ≤ 330 nm; the wavelength dependent trends in I∕I∗ product branching ratio, kinetic energy release, and recoil anisotropy suggest that (with regard to C-I bond fission) 4-IPhOH behaves like a mildly perturbed iodobenzene. Br atoms are observed when exciting 4-BrPhOH at long wavelengths also, but their velocity distributions suggest that dissociation occurs after internal conversion to the ground state. O-H bond fission, by tunnelling (as in phenol), is observed only in the cases of 4-FPhOH and, more weakly, 4-ClPhOH. These observed differences in behaviour can be understood given due recognition of (i) the differences in the vertical excitation energies of the C-Y centred (n∕π)σ∗ potentials across the series Y = I < Br < Cl and the concomitant reduction in C-Y bond strength, cf. that of the rival O-H bond, and (ii) the much increased spin-orbit coupling in, particularly, 4-IPhOH. The present results provide (another) reminder of the risks inherent in extrapolating photochemical behaviour measured for one molecule at one wavelength to other (related) molecules and to other excitation energies.
在紫外光激发 4-卤代苯酚（4-YPhOH）后，通过速度映射成像、H 里德堡原子光碎片平移光谱和高级自旋轨道分辨电子结构计算的组合，研究了 C-Y 和 O-H 键断裂的波长依赖性，揭示了 Y = I、Br、Cl（和 F）系列中碎片行为的系统演化。所有化合物在激发波长 λ ≲ 240nm 时都会发生 O-H 键断裂，这发生在排斥的 ((n∕π)σ∗) 势能面（PES）上，产生平均动能约为 11,000cm(-1)的快速 H 原子。对于 Y = I 和 Br，这个过程与另一个 (n∕π)σ∗PES 上的快速 C-I 和 C-Br 键断裂竞争，但从 4-ClPhOH 中没有观察到可明确归因于单光子诱导的 C-Cl 键断裂的 Cl∕Cl∗产物。在较长激发波长下，碎片行为的差异更为显著。在所有 λ ≤ 330nm 时，4-IPhOH 的激发都观察到了快速的 C-I 键断裂；I∕I∗产物分支比、动能释放和反冲各向异性的波长依赖性趋势表明（就 C-I 键断裂而言），4-IPhOH 的行为类似于轻度扰动的碘苯。在长波长激发 4-BrPhOH 时也观察到 Br 原子，但它们的速度分布表明解离发生在内部转换到基态之后。仅在 4-FPhOH 和较弱的 4-ClPhOH 情况下，才通过隧道（如苯酚）观察到 O-H 键断裂。考虑到（i）沿系列 Y = I < Br < Cl 的 C-Y 中心（n∕π）σ∗势的垂直激发能的差异以及相应的 C-Y 键强度的降低，与竞争的 O-H 键相比，以及（ii）特别是在 4-IPhOH 中自旋轨道耦合的大大增加，这些观察到的行为差异是可以理解的。本研究结果再次提醒人们，将在一个波长下测量的一个分子的光化学反应行为外推到其他（相关）分子和其他激发能是存在风险的。
