High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of 2,5-dimethylpyrrole.
High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of 2,5-dimethylpyrrole.
复制标题
2,5-二甲基吡咯近紫外光解的高分辨率照片碎片平移光谱研究。
DOI：
--
复制
发表时间：
2006
期刊：
影响因子：
--
通讯作者：
M. Ashfold
中科院分区：
文献类型：
--
作者：
B. Cronin;M. Nix;A. Devine;R. Dixon;M. Ashfold
用H Rydberg原子光碎片平移谱（PTS）研究了2，5-二甲基吡咯（2，5-dichloropyrrole，2，5-dichloropyrrole，2，5-dichloropyrrole）在193.3nm和244 <λ（phot）< 282 nm范围内的近紫外（near ultraviolet，UV）激发下的光解动力学。互补的紫外吸收，并在最长的激发波长，一个光子共振多光子电离光谱的2，5-dichloro也报告;后者的分析突出了甲基扭转运动的作用，促进母吸收。推导出的碎裂动力学与最近报道的（B.克罗宁，M. G. D.尼克斯河H. Qadiri和M. N. R. Ashfold，物理化学物理，2004，6，5031）。在较长波长处的激发导致2，5-π的1（1）A（2）（pisigma*）激发态的（振动诱导的）布居，但是一旦λ（phot）降低到约250 nm更强，偶极允许的跃迁开始在母体吸收中变得明显。H +2，5-二甲基吡咯基的全动能释放（TKER）谱在λ（phot）>或=244 nm处测量的（2，5-DMPyl）片段显示出结构化的快组分，其中许多由TKER约为5100 cm（-1）的峰主导;对该结构的分析揭示了2，5-DMP 1的选定振动能级的λ（phot）依赖性布居，并且能够测定2，5-四氟乙烷中的N-H键强度：D（0）= 30 530 +/- 100 cm（-1）。提出了两类行为来解释所观察到的能量划分的细节。两者都假设N-H键裂变涉及通过（或隧穿）1（1）A（2）势能面上的小出口通道势垒，但根据光制备分子的振动能含量而不同。促进1（1）A（2）-X（1）A（1）吸收的特定母体面外骨架模式似乎以螺旋方式演变成2，5-DMPyl产物的相应振动。甲基扭转也可以促进2，5-dichloro中的1（1）A（2）<- X（1）A（1）吸收，并提供一种比吡咯中高得多的激发振动能级密度的手段。这样的激发能级被推断为通过重新分配克服N-H解离坐标中的出口通道势垒所需的最小量的内部能量来解离。通过扩展的N-H键长处的锥形交叉与基态表面耦合被提出作为分离产物内的适度平移->振动能量转移的进一步机制。母吸收截面在波长约250 nm处显著增加，并且在λ（phot）≤ 254 nm处记录的PTS光谱在较低TKER处显示第二个非结构化峰。如在吡咯中，该较慢的组分归因于来自高度振动激发的基态分子的单分子衰变的H原子，所述基态分子通过来自位于1（1）A（2）状态之上的光激发态的无辐射衰变形成。
The photodissociation dynamics of 2,5-dimethylpyrrole (2,5-DMP) has been investigated following excitation at 193.3 nm and at many near ultraviolet (UV) wavelengths in the range 244 < lambda(phot) < 282 nm using H Rydberg atom photofragment translational spectroscopy (PTS). Complementary UV absorption and, at the longest excitation wavelengths, one photon resonant multiphoton ionisation spectra of 2,5-DMP are reported also; analysis of the latter highlights the role of methyl torsional motions in promoting the parent absorption. The deduced fragmentation dynamics show parallels with that reported recently (B. Cronin, M. G. D. Nix, R. H. Qadiri and M. N. R. Ashfold, Phys. Chem. Chem. Phys., 2004, 6, 5031) for the bare pyrrole molecule. Excitation at the longer wavelengths leads to (vibronically induced) population of the 1(1)A(2)(pisigma*) excited state of 2,5-DMP, but once lambda(phot) decreases to approximately 250 nm stronger, dipole allowed transitions start to become apparent in the parent absorption. All total kinetic energy release (TKER) spectra of the H + 2,5-dimethylpyrrolyl (2,5-DMPyl) fragments measured at lambda(phot)> or=244 nm show a structured fast component, many of which are dominated by a peak with TKER approximately 5100 cm(-1); analysis of this structure reveals lambda(phot) dependent population of selected vibrational levels of 2,5-DMPyl, and enables determination of the N-H bond strength in 2,5-DMP: D(0) = 30 530 +/- 100 cm(-1). Two classes of behaviour are proposed to account for details of the observed energy partitioning. Both assume that N-H bond fission involves passage over (or tunnelling through) a small exit channel barrier on the 1(1)A(2) potential energy surface, but differ according to the vibrational energy content of the photo-prepared molecules. Specific parent out-of-plane skeletal modes that promote the 1(1)A(2)-X(1)A(1) absorption appear to evolve adiabatically into the corresponding vibrations of the 2,5-DMPyl products. Methyl torsions can also promote the 1(1)A(2)<-- X(1)A(1) absorption in 2,5-DMP, and provide a means of populating a much higher density of excited vibrational levels than in pyrrole. Such excited levels are deduced to dissociate by redistributing the minimum amount of internal energy necessary to overcome the exit channel barrier in the N-H dissociation coordinate. Coupling with the ground state surface via a conical intersection at extended N-H bond lengths is proposed as a further mechanism for modest translational --> vibrational energy transfer within the separating products. The parent absorption cross-section increases considerably at wavelengths approximately 250 nm, and PTS spectra recorded at lambda(phot)< or = 254 nm display a second, unstructured, peak at lower TKER. As in pyrrole, this slower component is attributed to H atoms from the unimolecular decay of highly vibrationally excited ground state molecules formed via radiationless decay from photo-excited states lying above the 1(1)A(2) state.
影响因子：
3.8
作者：
Cohen, SD;Pumford, NR;Hinson, JA
通讯作者：
Hinson, JA