The Local Mode Analysis - Decoding Chemical Information From Vibrational Spectroscopy Data

In this project funded by the Chemical Structure Dynamics and Mechanisms-A (CSDM-A) Program of the Chemistry Division, Professor Elfi Kraka and her Computational and Theoretical Chemistry Group (CATCO) at Southern Methodist University (SMU) will use vibrational spectroscopy data to gain new insights on molecules. Over the past decades, vibrational spectroscopy has developed into an important analytical tool with ample applications in chemical synthesis, biological assays, materials quality control, forensic science, or the health sector.

Infrared (IR) light has the proper energy to excite vibrations in a molecule. Depending on the nature of the molecule, different absorption patterns allow molecules to be identified, but vibrational spectroscopy has much more to offer. The vibrational motions of the atoms a molecule induced by the light source conceal information on molecular structure and bonding.

Professor Kraka and her team are developing and applying special software to untangle these complex motions utilizing SMU’s high-performance computer to uncover how the atoms in a molecule are connected and interact. Detailed knowledge about these interactions is expected to lead to progress in the field, ranging from fine-tuning and designing of new materials to understanding and modifying the biological activity in enzymes.

The students engaged in this research project are gaining valuable experience in both cutting-edge modern computational spectroscopy and the use of a supercomputer as a powerful resource to help solving pending chemical problems, preparing them for a future career in a science, technology, engineering, and mathematics (STEM) field.

Information on the electronic structure of a molecule, the strength of its bonds, its geometry, and its conformational flexibility is encoded in the vibrational motions of a molecule induced by IR light. However, these so-called normal vibrational modes are generally delocalized over the molecule caused by a coupling of the atomic movements during the vibration, which hinders the direct access to this valuable information.

Local Vibrational Mode Analysis (LMA) being conducted by Professor Kraka and her team provides a unique solution to this problem by extracting local vibrational modes and related local properties, e.g., local mode force constants related to the intrinsic bond strengths for single molecules in the gas phase as well as for periodic systems. Local and normal vibrational modes are uniquely connected--the physical foundation of LMA--allowing for a unique decomposition of normal modes into local mode contributions.

Specific applications of this approach include the exploration of metal-ligand bonding in a range of molecular settings from heavy-metal materials such as uranium compounds, to heme proteins of importance in biology. Topics in periodic systems to be studied include (i) the investigation of the complex bonding network in ice structures, (ii) a systematic study of bonding in ionic crystals and (iii) the development of a protocol for identifying salts and co-crystals.

Broader impacts of this project include the release of a general-purpose open source LMA software package via the Github development platform to be shared with the spectroscopic community, the promotion of fruitful interactions between experimental and computational chemists working in the field of spectroscopy, and outreach to students from underserved populations.

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.