Conceptual Method Development
Understanding the connection between electronic structure and material properties is a principal challenge in chemistry and material science. Useful insights into chemical bonding allow for “chemical intuition”, which guides and dramatically accelerates experimental efforts.
We are particularly interested in the development of universally applicable chemical descriptors, and methods capable of analyzing chemical bonding in molecules, in extended forms of matter (polymers and crystals), and in matter under high compression. To that end, we are developing ”Experimental Quantum Chemistry”, a unique framework through which the energy of any transformation (such as a bond formation) is described as a sum of terms, all of which can all be interchangeably obtained by experimental or computational means. Other interests include quantification of electron localization, the refinement and furthering of atomic and group electronegativity, and estimates of atomic and effective group radii.
Aside from facilitating chemical rationales across different fields of disciplines and thermodynamic conditions, our research is generating promising input data for future machine learning approaches aimed at high throughput material discovery.
- M. Rahm, T. Zeng, R. Hoffmann, Electronegativity Seen as the Ground State Average Valence Electron Binding Energy, J. Am. Chem. Soc, 141,342-351, 2019 [A new scale of electronegativity]
- M. Rahm, R. Hoffmann, Distinguishing Bonds, J. Am. Chem. Soc., 138, 3731-3744, 2016 [Introduction to the Q-descriptor, shown in figure]
- M. Rahm, R. Hoffmann, N.W. Ashcroft, Atomic and Ionic Radii of Elements 1-96, Chem. Eur. J. 22, 14625-14632, 2016
- M. Rahm, R. Hoffmann, Toward an Experimental Quantum Chemistry: Exploring a New Energy Partitioning, J. Am. Chem. Soc., 137, 10282–10291, 2015
What is the molecular basis of life, and how did life originate? These are fundamental questions that ultimately will involve systems of extreme complexity. To help evaluate which initial chemistry might have given rise to such complexity, we are studying one of the Universe’s simplest and most ubiquitous molecules.
Hydrogen cyanide, HCN, is believed to be a key ingredient for prebiotic chemistry, and a versatile building block for the construction of biomolecules. However, the chemical structure of HCN-based polymers remains an open question. There are significant challenges associated with the characterization of polymeric HCN, and highly complex mixtures of soluble and insoluble materials are typically generated in laboratories.
To tackle this challenge, we are using a combination quantum chemistry and theoretical condensed matter physics methodology to explore well-defined HCN-based materials and polymerization mechanisms. It is possible that such structures might form under kinetic controlled conditions (low temperature) and over long time-scales. To evaluate how HCN chemistry plays into observations we are collaborating with planetary scientists, whose particular focus is Saturn’s moon Titan. Titan can, in some respects, be considered a frozen over version of the early Earth, and the former provides a natural laboratory for studying prebiotic chemistry occurring under cryogenic conditions. One of the most abundant products of Titan’s atmospheric chemistry is HCN, where it contributes to the formation of not yet well-understood photochemical hazes.
- M. Rahm, J. I. Lunine, D. A. Usher, D. Shalloway, Polymorphism and Electronic Structure of Polyimine and its Potential Significance for Prebiotic Chemistry on Titan, PNAS, 113, 8121-8126, 2016
- H. E. Maynard-Casely, R. Hodyss, M. L. Cable, T. H. Vub, M. Rahm, A co-crystal between benzene and ethane, an potential evaporite material for Saturn’s moon Titan, IUCrJ, 3, doi:10.1107/S2052252516002815, 2016
Green Energetic Materials
High-energy-density materials (HEDMs) are necessary in a range of applications, for
example, for heavy orbital launchers, satellite navigation, airbags, high explosives, rocketry and in pyrotechnics. We have a long-standing interest in the development of better performing, safer, and more environmentally friendly alternatives.
Today’s state of the art HEDMs approach a fundamental limit of science – the maximal energy density that can be stored by chemical means. Identifying realistic targets nearer to this limit is a monumental challenge, and one that can help identify new principles for chemical design. By exploring this limit, we are developing insight and methodology necessary to impact several technologically driven fields centered around metastable materials. The ability to computationally predict structures, properties, detection characteristics and routes to the synthesis of materials far from thermodynamic equilibrium, is, for example, essential for identifying environmentally sustainable energy storage solutions.
- M. Rahm, G. Bélanger-Chabot, R. Haiges, K. O. Christe, Nitryl Cyanide, NCNO2, Angew. Chem. Int. Ed. 53, 6893-6897, 2014
[Prediction, synthesis and characterization of the world’s most energetic molecule (kcal/g), kinetically stable under ambient conditions]
- M. Rahm, S. V. Dvinskikh, I. Furó, T. Brinck, Experimental Detection of Trinitramide, N(NO2)3 Angew. Chem. Int. Ed. 50, 1145, 2011
[Prediction and detection of the world’s largest nitrogen oxide]