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\title{EXPLORING THE DIFFERENCES IN \textit{ORTHO}- AND \textit{PARA}-H$_2$-FLUOROBENZALDEHYDE
COMPLEXES USING FTMW METHODS}

\author{\underline{\FirstName B. \LastName KEMPKEN},
  \FirstName R. M.  \LastName DOHMEN, \FirstName D. A. \LastName OBENCHAIN}
\address{Institute of Physical Chemistry, Tammannstrasse 6, Georg-August-Universit\"at G\"ottingen, 37077 G\"ottingen}

\author{\FirstName P. \LastName PINACHO, \FirstName M. \LastName SCHNELL}
\address{Deutsches Elektronen-Synchrotron Notkestrasse 85, 22607 Hamburg}

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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France,
  Aug. 28 - Sept. 1, 2023}}}
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\maketitle 

\index{FTMW}

Understanding the intermolecular binding of molecular hydrogen to other compounds
is a basic requirement for the design of possible H$_2$ storage materials.
Nevertheless, studies on H\textsubscript{2} van der Waals complexes with e.g.
H$_2$O,\footnote{\href{https://doi.org/10.1063/1.3533230}{doi:10.1063/S0009-2614(03)00476-7},
X.-G. Wang, T. Carrington, Theoretical study of the rovibrational spectrum of H$_2$O-H$_2$, J. Chem. Phys. 134, 044313 (2011).}
OCS\footnote{\href{https://doi.org/10.1063/1.2756534}{doi:10.1080/00268970210138904},
Z. Yu, K. J. Higgins, W. Klemperer, M. C. McCarthy, P. Thaddeus, K. Liao, W. J\"ager, Rotational spectra of the van der Waals complexes of molecular hydrogen and OCS, J. Chem. Phys. 127, 054305 (2007).},
HCN\footnote{\href{https://doi.org/10.1063/1.1377032}{doi:10.1063/1.462117}, M. Ishiguro,
T. Tanaka, K. Harada, C. J. Whitham, K. Tanaka, Rotational spectra of the
H\textsubscript{2}-HCN cluster observed by millimeter-wave spectroscopy combined with a pulsed supersonic jet technique, J. Chem. Phys. 115, 5155 (2001).},
and AgCl\footnote{\href{https://doi.org/10.1063/1.4895904}{doi:10.1364/AO.36.008533},
G. S. Grubbs II, D. A. Obenchain, H. M. Pickett, S. E. Novick, H\textsubscript{2}-AgCl:
A spectroscopic study of a dihydrogen complex, J. Chem. Phys. 141, 114306 (2014).}
show that to date, there are still gaps in the comprehension especially concerning differences in the binding of \textit{ortho}- and \textit{para}-H$_2$ to a specific host molecule.
Our group investigates the structure of bimolecular \textit{ortho}- and \textit{para}-H$_2$ complexes with 2-, 3-, and 4-fluorobenzaldehyde, applying chirped pulse and high-resolution cavity Fourier-transform microwave (FTMW) spectroscopy.
Rotational and centrifugal distortion constants were determined, supported by DFT-based
predictions.  
Furthermore, the cavity FTMW measurements, taken with a Fabry-P\'erot  spectrometer,
enabled us to resolve the nuclear spin-spin interaction for complexed \textit{ortho}-H$_2$.
The measured rotational constants were used to fit the intermolecular distance.
We found that the H$_2$'s center of mass distance to the aromatic plane is 0.36 -- 0.37 nm for \textit{ortho}-H$_2$ and 0.34 -- 0.35 nm for \textit{para}-H$_2$ complexes.
This result is surprising since the mentioned studies encourage the conclusion
that \textit{ortho}-H$_2$ generally binds more strongly to a host molecule than
\textit{para}-H$_2$.
A possible explanation could be a coupling distortion of the \textit{ortho}-H$_2$'s
rotational energy levels due to internal rotation of the H$_2$ group, which we
investigate here.
DFT-predicted rotational barriers are between 16 and 32 cm$^{-1}$.

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