Department of Chemistry, University of Southern California , Los Angeles, California 90089-0482, United States.
J Phys Chem A. 2013 Nov 21;117(46):12049-59. doi: 10.1021/jp404552g. Epub 2013 Jul 29.
The photodissociation dynamics of the hydroxymethyl radical (CH2OH, CH2OD, and CD2OD) following excitation to the 3s and 3p(x) Rydberg states is studied using time-sliced velocity map imaging of hydrogen photofragments. Dissociation takes place on the ground potential energy surface reached via conical intersections from the excited states, and formaldehyde and hydrxymethylene are identified as reaction products. The major product, formaldehyde, has a bimodal internal energy distribution. The largest fraction has high kinetic energy (KE), modest rotational excitation, and vibrational excitation mainly in the CO stretch and the CH(D)2 deformations modes (scissors, wag, and rock). The minor fraction has lower KEs and a higher rovibrational excitation that is unresolved. A bimodal internal energy distribution in the formaldehyde fragment has been predicted by Yarkony [J. Chem. Phys. 2005, 122, 084316] for a conical intersection along the O-H bond coordinate. The hydroxymethylene product state distributions depend strongly on the nature of the excited state. In dissociation via the 3s state, the hydroxymethylene products have broad rovibrational state distributions and are produced at low yield. As suggested by Yarkony, they may be produced in the same dissociation events that give rise to low KE formaldehyde. In these events, the bound region of the PES is sampled following the conical intersection along O-H(D). The hydroxymethylene yield is low near its threshold and increases slowly with excitation energy to the 3s state, but its internal energy distribution remains broad and the contributions of the cis- and trans-isomers cannot be resolved. The mechanism changes markedly when exciting to the 3p(x) state. The hydroxymethylene products have less rotational excitation and show separate contributions of cis- and trans-isomers. The trans-isomer is found to be a minor product relative to the higher-energy cis-isomer, as predicted by Yarkony for conical intersections along the C-H coordinate. It appears that the efficiency of dissociation via conical intersections along the O-H and C-H coordinates depends on the initial excited state. While the O-H conical intersection seam (vertical cone) provides an efficient route to the ground state following excitation via the 3s or the 3p(x) Rydberg states, conical intersections along the C-H bond coordinate (tilted cone) are sampled more efficiently via 3p(x) excitation and proceed through different dynamics. The energy separations between formaldehyde and hydroxymethylene and between the cis- and trans-isomers of hydroxymethylene are determined experimentally for all the investigated isotopologs and are in good agreement with theory.
羟基自由基 (CH2OH、CH2OD 和 CD2OD) 在被激发到 3s 和 3p(x) 里德堡态后，通过氢光碎片的时间切片速度映射成像研究其光离解动力学。离解发生在通过从激发态到达的锥形交叉点的地面势能面上，并且甲醛和羟甲基被确定为反应产物。主要产物甲醛具有双峰内部能量分布。最大部分具有高动能 (KE)、适度的旋转激发以及主要在 CO 拉伸和 CH(D)2 变形模式（剪刀、摇摆和摇滚）中的振动激发。较小的部分具有较低的 KE 和更高的无法分辨的 rovibrational 激发。对于沿着 O-H 键坐标的锥形交叉点，Yarkony [J. Chem. Phys. 2005, 122, 084316] 预测了甲醛碎片中的双峰内部能量分布。羟甲基产物状态分布强烈依赖于激发态的性质。通过 3s 态离解时，羟甲基产物具有广泛的 rovibrational 状态分布并且产率低。正如 Yarkony 所建议的那样，它们可能是在导致低 KE 甲醛的相同离解事件中产生的。在这些事件中，沿着 O-H(D) 沿着锥形交叉点在 PES 的束缚区域中被采样。在接近其阈值时，羟甲基的产率低，并且随着 3s 态的激发能缓慢增加，但它的内部能量分布仍然很宽，并且无法分辨顺式和反式异构体的贡献。当激发到 3p(x) 态时，机制明显改变。羟甲基产物的旋转激发较少，并显示顺式和反式异构体的单独贡献。发现反式异构体相对于高能顺式异构体是次要产物，这正如 Yarkony 所预测的那样，沿着 C-H 坐标的锥形交叉点。似乎沿着 O-H 和 C-H 坐标的锥形交叉点的离解效率取决于初始激发态。虽然通过 3s 或 3p(x) 里德堡态激发的 O-H 锥形交叉缝（垂直锥）为后续激发到基态提供了有效途径，但通过 3p(x) 激发更有效地采样 C-H 键坐标（倾斜锥）的锥形交叉点，并通过不同的动力学进行。对于所有研究的同位素，实验确定了甲醛和羟甲基之间以及羟甲基的顺式和反式异构体之间的能量分离，并且与理论很好地一致。
