Experimental and Computational Maps of Electron Density Distributions in Molecules: New Benchmarks for Quantum Chemistry

在这个由化学系化学结构、动力学和机制A(CSDM-A)计划资助的项目中，布朗大学的彼得·M·韦伯教授和他的学生正在使用X射线自由电子激光来探索电子在分子中是如何排列的。分子可以粗略地描述为带正电的原子核，周围环绕着一团电子。当正原子核之间的负电荷电子密度很高时，相邻原子之间就会发生成键。韦伯研究小组试图生成分子内电子密度的地图。这些电子密度分布决定了所有分子的性质，也就决定了所有材料的性质。此外，电子密度分布决定了化学反应如何进行，以及产生的反应产物的数量和类型。因此，测量分子中反应状态的电子密度分布对我们理解基础化学很重要，并在工业化学中具有广泛的意义。韦伯教授研究的一个独特方面是，不仅测量了处于最低松弛能态的分子的电子密度分布，还测量了处于激发态的分子的电子密度，这些激发态的电子密度已经被激光脉冲改变。通过直接将实验确定的密度图与计算模型生成的密度图进行比较，可以在计算技术方面取得进步。该项目为学生提供高级实验方法方面的培训，为他们提供在国家设施(SLAC)的经验，并为他们提供复杂的计算机模拟经验。该项目首先关注一系列模型系统中的基态密度分布。它们包括六氟化硫、苯酚和1，3-环己二烯等芳香族和共轭体系，以及吡嗪等杂环分子。绝对散射强度是通过用Ar和Ne等原子系统校准得到的。在该项目的第二阶段，获得了激发态电子密度分布，以激发到价态或里德堡态。该部分的模型体系包括1，3-环己二烯和双胺，如二氮杂双环[2，2，2]辛烷。散射实验的技术创新发展了对散射强度的绝对校准，测量了将分子带入电子激发状态的激光脉冲的穿透脉冲能量，以及投射散射图案的x射线脉冲，并提高了时间分辨率。后者是通过用非常短的谐波产生晶体实现一组特殊的上转换光学元件来实现的。最后，数据分析方面的创新集中在实验数据和计算模型之间的迭代，目的是确定三维电子密度分布。基于高能级从头算波函数，开发了计算激发态散射方向图的数据代码。这项研究的更广泛的影响包括改进在化学和相关学科中广泛使用的计算代码。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

In this project funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Chemistry Division, Professor Peter M. Weber and his students at Brown University are using an X-Ray Free Electron Laser to explore how electrons are arranged within molecules. Molecules can be roughly described as positively charged atomic nuclei surrounded by a cloud of electrons. Bonding between adjacent atoms occurs when the density of negatively charged electrons between the positive atomic nuclei is high. The Weber research group seeks to generate maps of electron density within molecules. These electron density distributions determine all molecular properties and, by implication, the properties of all materials. In addition, electron density distributions determine how chemical reactions proceed, and the quantities and types of reaction products that are generated. Measuring electron density distributions in molecules for reacting states is therefore important to our understanding of fundamental chemistry and has myriad implications in industrial chemistry. A unique aspect of Prof. Weber’s research is that electron density distributions are being measured not only for molecules in their lowest “relaxed” energy states, but also in their excited states, where the electron densities have been altered by laser pulses. By directly comparing the experimentally determined density maps to those generated by computational models, advances in computational techniques can be made. The project is providing training for students in advanced experimental methodologies, offers them experience at a National facility (SLAC), and also gives them experience in complex computer simulations.The project focuses first on the ground state density distributions in a range of model systems. They include sulfur hexafluoride, aromatic and conjugated systems such as phenol and 1,3 cyclohexadiene, and heterocyclic molecules such as pyrazine. Absolute scattering intensities are obtained by calibrating with atomic systems such as argon and neon. In a second phase of the project, excited state electron density distributions are obtained for excitation to valence states or Rydberg states. Model systems for this part include 1,3 cyclohexadiene as well as di-amines, such as diazabicyclo[2,2,2]octane. Technical innovations of the scattering experiment develop an absolute calibration of the scattering intensity, measurement of the shot-by shot pulse energies of the laser pulse that brings the molecules to the electronically excited state and the x-ray pulse that casts the scattering pattern, and an improvement in the time resolution. The latter is achieved by implementing a special set of up-conversion optics with very short harmonic generation crystals. Finally, innovations in the analysis of the data focus on the iteration between experimental data and computational models with the aim to determine the three-dimensional electron density distributions. Data codes are developed to calculate excited-state scattering patterns based on high-level ab-initio wave functions. The broader impact of the research includes the improvement of computational codes that are widely used in chemistry and related disciplines.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.