Spectroscopic Signature of Noncovalent Bonds

在这项由化学系化学结构、动力学和机制A(CSDM-A)计划资助的项目中，犹他州州立大学的Steve Scheiner教授正在使用现代量子化学计算来预测当两个分子以弱(非共价)方式相互作用时，分子光谱受到影响的方式。化学家和其他科学家使用不同类型的光谱学来确定原子和分子的结构和行为。红外光谱(IR)探测分子的振动，其频率特定于分子的几何形状和所包含的原子类型。核磁共振波谱(核磁共振)测量了某些原子核在外部磁场中翻转方向的频率。在大多数化学样品中，感兴趣的分子不是孤立的，而是在附近的其他分子存在的情况下移动(特别是在液体和固体样品的情况下)。Scheiner教授的研究试图揭示分子与其邻居的相互作用如何改变其振动和磁性，以及这些变化将如何在红外和核磁共振光谱测量中显示出来。这些相互作用被称为非共价键，因为它们不涉及相邻分子之间的实际成键(换句话说，共享电子)，因此比实际的共价键弱。然而，非共价键对许多重要体系所采用的结构非常重要，包括蛋白质、材料和晶体，其中一些在现代技术中非常有用。量子计算是一种模拟非共价相互作用，然后用引起光谱变化的基本力来解释它们的方法。这个项目涉及两名研究生和一名本科生。参与这一研究项目的学生在尖端量子化学及其在现实系统中的应用方面获得了宝贵的经验。该项目专注于非共价键的一个特定子集，所有这些键都与普遍存在且研究得很好的氢键有关，这些氢键已经成为化学和生物结构和反应活性的支柱。用一大群电负性元素中的任何一种取代氢键的桥质子，会产生所谓的卤键、硫族键、镍键和四氢键，这取决于这个桥原子来自哪一族的元素周期表。这些键的新颖性导致了最初的努力，即将它们的性质和强度与光谱特征联系起来，主要是红外和核磁共振。人们正在设计各种类型的络合物，将这些类型的键结合到各种各样的系统中，跨越各种强度和原子类型。计算光谱并将其与成键的其他特征进行比较，从而制定出一套规则和基本原则，将所有这些属性彼此联系起来。这个项目的目标是开发概念，帮助实验者检查他们特定的系统，以了解存在哪些相互作用，以及哪些可能影响结构。这项工作的更广泛影响包括增加对控制生物和化学结构的力的理解的潜在社会益处，以及培训学生应用一系列量子化学方法了解真实系统的机会。除了为学生提供正式的研究培训外，Scheiner教授还为研究生和高级本科生开发了一门课程，旨在向计算和实验学生介绍现代理论化学方法。参与CSDM-A项目的研究生也参与了这门课程的指导。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 Steve Scheiner of Utah State University is using modern quantum chemical calculations to predict the way in which the molecular spectra of molecules are affected when two molecules interact with one another in a weak (noncovalent) manner. Chemists and other scientists use different types of spectroscopy to determine the structure and behaviors of atoms and molecules. Infrared spectroscopy (IR) probes the vibrations of molecules, the frequencies of which are specific to a molecule’s geometry and the types of atoms it contains. Nuclear magnetic resonance spectroscopy (NMR) measures the frequencies at which certain atomic nuclei flip direction when exposed to an external magnetic field. In most chemical samples, the molecules of interest are not isolated, but rather moving in the presence of other nearby molecules (especially in the case of liquid and solid samples). Professor Scheiner’s research seeks to uncover how molecules’ interaction with their neighbors changes their vibrational and magnetic properties, and how these changes would show up in IR and NMR spectroscopic measurements. These interactions are called “non-covalent” because they do not involve actual bonding (in other words, sharing of electrons) between neighboring molecules, and are therefore weaker than actual covalent bonds. Yet non-covalent bonds are very important to the structure adopted by many important systems, including proteins, materials, and crystals, some of which are very useful in modern technologies. Quantum calculations are a means of simulating non-covalent interactions and then explaining them in terms of fundamental forces that cause spectroscopic changes. This project involves two graduate students and one undergraduate researcher. The students engaged in this research project are gaining valuable experience in both cutting-edge quantum chemistry and their application to real-world systems. The project focuses on a particular subset of noncovalent bonds, all of them related to the ubiquitous and well-studied H-bonds that have become a mainstay of chemical and biological structure and reactivity. Replacement of the bridging proton of a H-bond by any of a large group of electronegative elements, lead to what have been called halogen, chalcogen, pnicogen, and tetrel bonds, depending upon what family of the periodic table this bridging atom is drawn from. The novelty of these bonds has led to only initial efforts to link their properties and strength to the spectroscopic features, primarily IR and NMR. Complexes of various sorts are being designed that incorporate these types of bonds in a wide variety of systems, spanning a wide range of strength and atom type. Spectra are calculated and compared to other features of the bonding, so as to draw up a set of rules and underlying principles that relate all of these properties to one another. The goal of this project is to develop concepts that will aid experimentalists as they examine their particular systems to understand what interactions are present and which might be influencing the structures. The broader impacts of this work include potential societal benefits from an increased understanding of the forces that control biological and chemical structures, as well as opportunities for the training of students in the application of a range of quantum chemical methods to understand real systems. In addition to the formal research training for students, Prof. Scheiner has developed a course for graduate students and advanced undergraduate designed to introduce both computational and experimentalist students to modern methods of theoretical chemistry. The graduate students working on this CSDM-A project also take part in the instruction ofthis course.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.