Abstract
Abstract With the detection of C60, C70, and in the interstellar medium, fullerenes are currently the largest molecules identified in space. The relatively high proton affinities of C60 and C70 support the hypothesis that protonated fullerenes may also be abundant in the interstellar matter. Here, we present the first experimental vibrational spectrum of C70H+, recorded in the gas phase. The attachment of a proton to C70 causes a drastic symmetry lowering, which results in a rich vibrational spectrum. As compared to C60, where all C-atoms are equivalent due to the icosahedral symmetry, C70 belongs to the D5h point group and has five nonequivalent C-atoms, which are available as protonation sites. Combined analysis of the experimental spectrum and spectra computed at the density functional theory level enables us to evaluate the protonation isomers being formed. We compare the IR spectra of C60H+ and C70H+ to IR emission spectra from planetary nebulae, which suggests that a mixture of these fullerene analogs could contribute to their IR emission.
Highlights
Like its carbonaceous sibling C60, the C70 molecule has received much attention as the most abundant member of the fullerene family
Protonated C70 was produced in an atmospheric pressure chemical ionization (APCI) source, which is efficient for apolar molecules such as polycyclic aromatic hydrocarbons (Marvin et al 1999; Grosse & Letzel 2007) and fullerenes; it was used to produce protonated C60 in our previous study (Palotás et al 2020)
As compared to C70, symmetry breaking upon addition of a proton leads to the appearance of a wealth of new peaks
Summary
Like its carbonaceous sibling C60, the C70 molecule has received much attention as the most abundant member of the fullerene family. Similar to C60, it follows the isolated pentagon rule that makes this allotrope of carbon stable (Kroto 1987; Fowler 1996). Upon their discovery in the 1980s, both fullerenes attracted the attention of the astrophysical community and initiated a search for their presence in interstellar environments (Ehrenfreund & Foing 1997). Based on accurate gas-phase laboratory spectra in the near-IR, C+60 was more recently identified (Campbell et al 2015), representing a rare case in which a few of the enigmatic diffuse interstellar bands (DIBs) were convincingly assigned. Apart from the identification of these fullerenes themselves, their interstellar abundance (Campbell et al 2016; Berné et al 2017) provides further support to the PAH hypothesis (Berné & Tielens 2012), as facile interconversion between fullerenes and PAHs has been suggested upon hydrogenation of fullerenes or dehydrogenation of PAHs (Álvaro Galué 2014; Zhen et al 2014)
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