Non-covalent interactions in energy storage materials

As part of the BENCh RTG program, our primary interests are in understanding the interactions of energy relevant compounds with possible storage materials. More commonly, these are written about as "hydrogen storage materials" in the media. Our focus is to understand the interactions gases important to clean energy and sustainable industry with potential storage materials. In the example of hydrogen storage, these are found as metal-organic frameworks (MOF) or covalent-organic frameworks (COF). Our projects combine computational methods to predict structures for hydrogen binding on larger molecules, and microwave spectroscopy to determine the accuracy of these structures and to determine secondary properties. 

Dispersion effects on secondary observables in rotational spectroscopy

Microwave / rotational spectroscopy techniques have high-resolution capabilities, allowing our research to look beyond they structure of molecules and complexes. While molecular structure and geometry is the primary use of rotational spectroscopy, but there are other useful components as well. Molecular rotation is a very low energy form of spectroscopy, and we typically conduct our experiments in the microwave region of the spectrum. As such, many small but important nuclear, electronic, and molecular motion effects can split rotational energy levels and be observed in our experiments as fine or hyperfine spectra structures. We often use these as properties as local probes into the subtle chemical to similar molecules, or when different types of van der Waals complexes are formed. The "probes" we can use are:

• Large amplitude motions (internal rotation, ring puckering, proton tunneling)
• Nuclear spin coupling 
• Electron spin coupling

Aspects of these interactions are found in many examples within our group’s research.  

Microwave Spectrometer Experiment

The microwave spectroscopic technique we employ in the laboratory is called pulsed-jet Fabry-Perot cavity Fourier Transform microwave (FTMW) spectroscopy. A pulsed-jet of gas is produced at temperatures of about 1 Kelvin by rapidly expanding high-pressure gases into a high-vacuum chamber. This cold jet expands into a Fabry-Perot microwave cavity created by two large spherical aluminum mirrors whose separation ensures a high Q (about 5000) cavity tunable within the 6 to 18 GHz range. A pulse of microwave radiation timed to coincide with the arrival of the gas pulse is introduced into the cavity. If the molecules in the jet have a spectral transition within the ~ 1 MHz spectral width of the cavity they can absorb the radiation and a macroscopic polarization of the molecules is induced. We then detect the free-induction decay (FID) of the ensemble, similar to NMR techniques. Through averaging, we can obtain signal to noise ratios that allow for the detection of isotopic species in natural abundance. The instrument is controlled by a custom program written in the University of Hannover by Jen-Uwe Grabow.