Infrared Spectroscopy of Aromatic Molecular Ions Cooled at 0.37 KIguchi, A.,1,2* Singh, A.,3 Kuma, S., 2 Tanuma, H.4, Azuma, T.2,4*presenter
1aiguchi@lowtem.hokudai.ac.jp, Institute of Low Temperature Science, Hokkaido University,
Japan
2RIKEN, Japan
3
University of Southern California, United States
4Tokyo Metropolitan University, JapanOne of the high-sensitivity methods for cold molecular ion spectroscopy is with superfluid
helium nanodroplets. Due to the superfluidity at a low temperature of 0.37 K and the small
number of atoms, helium droplets are an ideal matrix to explore unstable molecular species
such as radicals and cations. In these regards, the rotational and vibrational spectra of molecular
cations embedded in helium droplets are perturbed to a very minimal extent. However, whilerotational spectra of neutral molecules have been observed widely, the observations of ionicspecies at 0.37 K have yet to progress.
An infrared spectroscopic method for cold molecular ions in helium droplets has recently been
established. 1 The helium droplets doped with neutral precursors are ionized by an electron
impact ionizer. Since the ionization potential of 4He is very high, the dopants are ionized via
charge transfer and cooled to 0.37 K. Due to the rapid cooling of cations by the surrounding
low-temperature helium droplets, doped molecular ions can remain in their metastable states.2
We performed the infrared spectroscopy of one of the aromatic molecular ions, aniline cation(c-C6H5–NH2+), and the H-loss cation in helium droplets in the N–H stretching region. In thegas phase, some H-loss cations of the 5-membered ring were reported by infrared spectra at thefingerprint region.3 Some neutral 5-membered rings with a –CNH substitute were observed in
the interstellar space. On the other hand, ourexperimental results of the infrared spectra inhelium droplets indicated the N–H vibrationalband at 3406 cm–1, which disagreed with the 5membered ring structures. Theoretical calculationsuggests several candidate species, including a 7membered ring structure. These results imply thatthe excess energy opened a new reaction pathway
to this cation and competed with the pathway to
the 5-membered rings.4
Figure 1. Some of the reaction pathways fromaniline cations.
Erukala, S.; Feinberg, A.; Singh, A.; Vilesov, A. F. Infrared Spectroscopy of Carbocations upon Electron
Ionization of Ethylene in Helium Nanodroplets. J. Chem. Phys. 2021, 155 (8), 084306.
2Iguchi, A.; Singh, A.; Bergmeister, S.; Azhagesan, A. A.; Mizuse, K.; Fujii,A.; Tanuma, H.; Azuma, T.;
Scheier, P.; Kuma, S.; Vilesov, A. F. Isolation and Infrared Spectroscopic Characterization of Hemibonded
Water Dimer Cation in Superfluid Helium Nanodroplets. J. Phys. Chem. Lett. 2023, 14, 8199−8204.
3Zeh, D.; Bast, M.; Rap, D.B.; Schmid, P.C.; Thorwirth, S.; Brünken, S.; Schlemmer, S.; Schäfer, M. Cryogenic
messenger-IR ion spectroscopy study of phenol & aniline molecular ions and of the common fragment ion[C5H6]⋅+ formed by EI-MS. J. Mol. Spectrosc. 2021, 378, 111453.
4
Iguchi, A; Singh, A.; Kuma, S.; Tanuma, H.; Azuma, T. Vibrational spectroscopy of aniline cations and their
H-loss cations in helium droplets. J. Mol. Spectrosc. 2024, 401, 111903.
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