Polycyclic aromatic hydrocarbons (PAH) are of importance in both terrestrial & extra-terrestrial environments; they are possible candidates for the emitters of the unidentified infrared (UIR) emission bands. Because proton sources (H+ & H3+) are abundant in space and proton affinities of PAH are high, proton transfer reactions to produce protonated PAH are likely to take place in interstellar medium. Protonated PAH are hence also possible candidates for the emitters of the UIR bands. Corannulene (C20H10) is a fragment of C60. It was proposed to be an intermediate in the formation of fullerenes in outer space. Protonated corannulene (H+C20H10) could be of astrophysical significance and hence it is imperative to investigate its infrared features in the laboratory. Our recent technique of using electron bombardment on a p-H2 matrix to produce protonated/hydrogenated species cleanly & efficiently can provide their IR absorption spectra covering a wide spectral range [1, 2]. The experiments were carried out on a gold-plated copper substrate at 3.3 K; the spectra in range 500-4000 cm-1 were recorded with an FTIR. Protonated corannulene (H+C20H10) and their neutral counterparts C20H11 were produced upon electron bombardment of the matrix during deposition of C20H10 mixed in p-H2 over a period of 10 h. The matrix was then maintained in darkness for 30 h followed by secondary photolysis using light at various wavelengths. The H+C20H10 became diminished after maintaining the matrix in darkness, whereas its neutral counterparts were produced. Infrared absorption features, grouped according to the behavior after maintaining the matrix in darkness and upon secondary photolysis, were unambiguously assigned to two protonated species of C20H10 (with proton at the hub and rim sites) and their respective neutral counterparts by comparison with quantum-chemically calculated IR spectra of possible candidates. Similarly, IR spectra of protonated and hydrogenated species of sumanene (C21H12), another fragment of C60, were also investigated with this technique. The implications of the spectra of these molecules on the UIR bands will be discussed.

Infrared spectra of protonated and hydrogenated corannulene (C20H10) and sumanene (C21H12) in solid para-hydrogen

256th National Meeting (2018)

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