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.