Edgewood Chemical Biological Center, RDCB-DRI-I, 5183 Blackhawk Road, Aberdeen Proving Ground , Maryland 21010, United States.
J Phys Chem A. 2013 Sep 19;117(37):9039-46. doi: 10.1021/jp403778a. Epub 2013 Sep 6.
An understanding of how molecular environment and structure are reflected in optical absorption spectra offers a number of advantages, such as improved detection of materials or providing an easy means of distinguishing crystal polymorphs of the same molecular solid. This study advances this understanding by comparing near IR laser photoacoustic absorption measurements of the first C-H stretch overtones around 5975 cm(-1) of β-2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL20) to simulated spectra using density functional calculations and the local mode model of C-H stretches. The calculations reveal that accounting for movement of charge throughout the model crystal unit cell with a pure quantum mechanical method in the calculation of the transition dipole moment is critical to matching the experimental data. Vibrational modes in a given molecule induce movement of charge in neighboring molecules, such that calculation of the transition dipole moment had to include the entire crystal unit cell. Movement of charge across the periodic boundary conditions (PBC) of the model needs to be accounted for to calculate a spectrum validated by the experimental measurement. The Hirshfeld population analysis minimizes discontinuities for movement of charge across the PBC.
理解分子环境和结构如何反映在光吸收光谱中具有许多优势，例如提高对材料的检测能力，或者提供一种简便的方法来区分同一种分子固体的晶体多形体。本研究通过比较β-2,4,6,8,10,12-六硝基-2,4,6,8,10,12-六氮杂异伍兹烷（CL20）在 5975cm(-1)左右的第一个 C-H 伸缩倍频的近红外激光光声吸收测量值与使用密度泛函计算和 C-H 伸缩局部模式模型的模拟光谱，推进了这一理解。计算结果表明，在计算跃迁偶极矩时，通过纯量子力学方法考虑整个模型晶体单元中的电荷移动对于匹配实验数据至关重要。给定分子中的振动模式会引起相邻分子中的电荷移动，因此必须包括整个晶体单元来计算跃迁偶极矩。需要考虑穿过模型的周期性边界条件 (PBC) 的电荷移动，以计算出与实验测量相符的光谱。Hirshfeld 布居分析最大限度地减少了电荷穿过 PBC 时的不连续性。
