Spectroscopic Signature of Noncovalent Bonds

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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.