School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, Scotland, United Kingdom.
ISM-CNR, Istituto di Struttura della Materia, LD2 Unit, 34149 Trieste, Italy.
J Chem Phys. 2018 Jul 21;149(3):034305. doi: 10.1063/1.5041249.
New synchrotron based studies of the photoelectron ionization spectra (PES) for the isomeric 1- and 2-methyltetrazoles (1- and 2-MeTet) show markedly higher resolution than previous reports. The unusual spectral profiles suggest that a considerable overlay of the ionic states occurs for both molecules. Under these circumstances of near degeneracy of two or more ionic states, mutual annihilation of vibrational fine structure occurs for all except the strongest vibrational states; the PES just reflects the resultants rather than full spectra. Theoretical determination of the adiabatic ionization energies (AIEs) proved a challenge; the most successful method was second order Møller-Plesset perturbation theory (MP2). These calculations suggest that the lowest PES bands for both isomers contain ionization both from lone pair σ-orbitals (A') on the N-atoms (LP) and π-orbitals (A″). The lowest experimental AIEs are as follows: 1-MeTet is 10.315 eV assigned to 1A', while 2-MeTet is 10.543 eV assigned to 1A″. Franck-Condon analysis shows that the lowest ionization energy regions of both spectra are dominated by IE from the LPA' manifold, even though the A″ states have a higher absolute intensity. In this example, we have utilized a VUV Rydberg state to assist simplification of the PES; more frequently, the PES assignment is simpler and assists the location of Rydberg states in the VUV. The very slow spectral onset for 2-MeTet demonstrates the importance of vertical ionization energy calculations since maxima are more readily measured than slow onsets. These were performed at the equilibrium structure of the XA' state, using both multi-reference multi-root configuration interaction and the ionization potential variant of the equations-of-motion coupled cluster method, with single and double excitations (EOMIP-CCSD). This enabled the principal ionization bands to be identified over a wider range of energy. Attempts to study the higher ionic states by EOMIP-CCSD showed that several states of each symmetry are close to degenerate for 1-MeTet, in particular. A multi-configuration self-consistent field study confirmed the small separation of ionic states, but state switching during the optimization process largely disabled this method.
基于同步辐射的新型研究表明，1-和 2-甲基四唑（1-和 2-MeTet）的光电子电离光谱（PES）具有比以往报道更高的分辨率。不寻常的光谱特征表明，这两种分子的离子态有相当大的重叠。在这种两个或多个离子态接近简并的情况下，除了最强的振动态之外，所有振动精细结构都会相互湮灭；PES 仅反映结果而不是完整的光谱。绝热电离能（AIE）的理论确定是一个挑战；最成功的方法是二阶 Møller-Plesset 微扰理论（MP2）。这些计算表明，两种异构体的最低 PES 带都包含来自 N 原子上的孤对σ轨道（A'）（LP）和π轨道（A″）的电离。最低的实验 AIE 如下：1-MeTet 为 10.315 eV，分配给 1A'，而 2-MeTet 为 10.543 eV，分配给 1A″。Franck-Condon 分析表明，两个光谱的最低电离能区域都由来自 LPA'能级的 IE 主导，尽管 A″态具有更高的绝对强度。在这个例子中，我们利用 VUV 里德堡态来帮助简化 PES；更常见的情况是，PES 分配更简单，并有助于在 VUV 中定位里德堡态。2-MeTet 的光谱起始非常缓慢，这表明垂直电离能计算非常重要，因为最大值比缓慢起始更容易测量。这些是在 XA'态的平衡结构下进行的，使用多参考多根组态相互作用和运动方程耦合簇方法的电离势变体，包括单和双激发（EOMIP-CCSD）。这使得主要的电离带能够在更宽的能量范围内被识别。尝试使用 EOMIP-CCSD 研究更高的离子态表明，对于 1-MeTet，每个对称的几个态非常接近简并。多组态自洽场研究证实了离子态的小分离，但在优化过程中状态切换很大程度上禁用了这种方法。
