Group of Theoretical Chemistry and Molecular Modelling, Department SBG, Hasselt University, Agoralaan, Gebouw D, B-3590 Diepenbeek, Belgium.
J Phys Chem A. 2010 Apr 1;114(12):4400-17. doi: 10.1021/jp9116358.
The main purpose of the present work is to simulate from many-body quantum mechanical calculations the results of experimental studies of the valence electronic structure of n-hexane employing photoelectron spectroscopy (PES) and electron momentum spectroscopy (EMS). This study is based on calculations of the valence ionization spectra and spherically averaged (e, 2e) electron momentum distributions for each known conformer by means of one-particle Green's function [1p-GF] theory along with the third-order algebraic diagrammatic construction [ADC(3)] scheme and using Kohn-Sham orbitals derived from DFT calculations employing the Becke 3-parameters Lee-Yang-Parr (B3LYP) functional as approximations to Dyson orbitals. A first thermostatistical analysis of these spectra and momentum distributions employs recent estimations at the W1h level of conformational energy differences, by Gruzman et al. [J. Phys. Chem. A 2009, 113, 11974], and of correspondingly obtained conformer weights using MP2 geometrical, vibrational, and rotational data in thermostatistical calculations of partition functions beyond the level of the rigid rotor-harmonic oscillator approximation. Comparison is made with the results of a focal point analysis of these energy differences using this time B3LYP geometries and the corresponding vibrational and rotational partition functions in the thermostatistical analysis. Large differences are observed between these two thermochemical models, especially because of strong variations in the contributions of hindered rotations to relative entropies. In contrast, the individual ionization spectra or momentum profiles are almost insensitive to the employed geometry. This study confirms the great sensitivity of valence ionization bands and (e, 2e) momentum distributions on the molecular conformation and sheds further light on spectral fingerprints of through-space methylenic hyperconjugation, in both PES and EMS experiments.
本工作的主要目的是从多体量子力学计算模拟实验研究的结果，采用光电子能谱（PES）和电子动量谱（EMS）研究正己烷的价电子结构。这项研究是基于对每个已知构象的价层电离谱和球平均（e,2e）电子动量分布的计算，采用单粒子格林函数[1p-GF]理论与三阶代数图论构造[ADC(3)]方案，并使用 Kohn-Sham 轨道，这些轨道是通过密度泛函理论（DFT）计算，采用 Becke 3-参数 Lee-Yang-Parr（B3LYP）泛函作为 Dyson 轨道的近似值来推导的。这些谱和动量分布的首次统计分析采用了 Gruzman 等人在 W1h 水平上对构象能差的最新估计[J. Phys. Chem. A 2009, 113, 11974]，以及在超出刚性转子-谐振子近似水平的配分函数的统计计算中，使用 MP2 几何、振动和旋转数据相应获得的构象权重。比较了使用 B3LYP 几何和相应的振动和旋转配分函数的焦点分析结果与这些能量差异的结果。这两种热化学模型之间存在很大差异，尤其是由于受阻旋转对相对熵的贡献存在很大差异。相比之下，单独的电离谱或动量分布几乎不受所采用的几何形状的影响。这项研究证实了价层电离带和（e,2e）动量分布对分子构象的高度敏感性，并进一步阐明了 PES 和 EMS 实验中通过空间亚甲基超共轭的光谱特征。
