School of Chemistry, University of Bristol, Cantock's Close, Bristol, UK BS8 1TS.
Phys Chem Chem Phys. 2014 Jan 14;16(2):588-98. doi: 10.1039/c3cp53450b.
We report a combined experimental (H (Rydberg) atom photofragment translational spectroscopy) and theoretical (ab initio electronic structure and vibronic coupling calculations) study of the effects of symmetry on the photodissociation dynamics of phenols. Ultraviolet photoexcitation to the bound S1((1)ππ*) state of many phenols leads to some O-H bond fission by tunneling through the barrier under the conical intersection (CI) between the S1 and dissociative S2((1)πσ*) potential energy surfaces in the R(O-H) stretch coordinate. Careful analysis of the total kinetic energy release spectra of the resulting products shows that the radicals formed following S1 ← S0 excitation of phenol and symmetrically substituted phenols like 4-fluorophenol all carry an odd number of quanta in vibrational mode ν(16a), whereas those deriving from asymmetrically substituted systems like 3-fluorophenol or 4-methoxyphenol do not. This contrasting behavior can be traced back to symmetry. Symmetrically substituted phenols exist in two equivalent rotamers, which interconvert by tunneling through the barrier to OH torsional motion. Their states are thus best considered in the non-rigid G4 molecular symmetry group, wherein radiationless transfer from the S1 to S2 state requires a coupling mode of a2 symmetry. Of the three a2 symmetry parent modes, the out-of-plane ring puckering mode ν(16a) shows much the largest interstate coupling constant in the vicinity of the S1/S2 CI. The nuclear motions associated with ν(16a) are orthogonal to the dissociation coordinate, and are thus retained in the radical products. Introducing asymmetry (even a non-linear substituent in the 4-position) lifts the degeneracy of the rotamers, and lowers the molecular symmetry to Cs. Many more parent motions satisfy the reduced (a'') symmetry requirement to enable S1/S2 coupling, the most effective of which is OH torsion. This motion 'disappears' on O-H bond fission; symmetry thus imposes no restriction to forming radical products with vibrational quantum number v = 0. The present work yields values for the O-H bond strengths in 3-FPhOH and 4-MeOPhOH, and recommends modest revisions to the previously reported O-H bond strengths in other asymmetrically substituted phenols like 3- and 2-methylphenol and 4-hydroxyindole.
我们报告了一项组合实验（H（里德堡）原子光碎片平移光谱学）和理论（从头算电子结构和振动态耦合计算）研究，研究了对称性对酚类光解动力学的影响。许多酚类物质的紫外光激发到束缚 S1((1)ππ*)态，通过在 S1 和解离 S2((1)πσ*)势能表面之间的锥形交叉（CI）下的势垒隧道，导致一些 O-H 键断裂在 R（OH）伸缩坐标中。对所得产物的总动能释放光谱进行仔细分析表明，苯酚和对称取代的酚（如 4-氟苯酚）的 S1 ← S0 激发后形成的自由基，其振动模式 ν（16a）均携带奇数个量子，而来自不对称取代体系的自由基（如 3-氟苯酚或 4-甲氧基苯酚）则没有。这种对比的行为可以追溯到对称性。对称取代的酚类物质存在于两个等效的构象异构体中，它们通过穿过 OH 扭转运动的势垒隧道相互转化。因此，它们的状态最好在非刚性 G4 分子对称群中考虑，其中从 S1 到 S2 态的无辐射转移需要 a2 对称的耦合模式。在三个 a2 对称的母体模式中，面外环弯曲模式 ν（16a）在 S1/S2 CI 附近显示出最大的体系间耦合常数。与 ν（16a）相关的核运动与离解坐标正交，因此保留在自由基产物中。引入不对称性（甚至在 4-位引入非线性取代基）会使构象异构体的简并度降低，并将分子对称性降低到 Cs。更多的母体运动满足降低（a''）对称性要求，以实现 S1/S2 耦合，其中最有效的是 OH 扭转。这个运动在 O-H 键断裂时“消失”；因此，对称性对形成振动量子数 v = 0 的自由基产物没有限制。本工作给出了 3-FPhOH 和 4-MeOPhOH 中 O-H 键强度的值，并建议对其他不对称取代的酚（如 3-和 2-甲基苯酚和 4-羟基吲哚）中先前报道的 O-H 键强度进行适度修订。
