Search for Hydrogenated C₆₀ (Fulleranes) in Circumstellar Envelopes

Abstract

The recent detection of fullerene (C 60 ) in space and the positive assignment of five diffuse interstellar bands to C + 60 reinforce the notion that fullerene-related compounds can be efficiently formed in circumstellar envelopes and be present in significant quantities in the interstellar medium. Experimental studies have shown that C 60 can be readily hydrogenated, raising the possibility that hydrogenated fullerenes (or fulleranes, C 60 H m , m = 1 − 60) may be abundant in space. In this paper, we present theoretical studies of the vibrational modes of isomers of C 60 H m . Our results show that the four mid-infrared bands from the C 60 skeletal vibrations remain prominent in slightly hydrogenated C 60 , but their strengths diminish in different degrees with increasing hydrogenation. It is therefore possible that the observed infrared bands assigned to C 60 could be due to a mixture of fullerenes and fulleranes. This provides a potential explanation for the observed scatter of the C 60 band ratios. Our calculations suggest that a feature around 15 µm due to the breathing mode of heavily hydrogenated C 60 may be detectable astronomically. A preliminary search for this feature in 35 C 60 sources is reported.

The Infrared Spectrum of Protonated C70

“…Fullerenes can form derivatives, such as partially hydrogenated fullerenes, which have also been suggested to be present in the interstellar matter (ISM; Stoldt et al 2001;Zhang et al 2017Zhang et al , 2020Díaz-Luis et al 2018). However, the additional hydrogens in such species destabilize the carbon cage due to the change in orbital hybridization from sp 2 to sp 3 (Van Lier et al 2002).…”

Hydrogenated fullerenes (fulleranes) in space

“…Intriguingly, a strong feature peaking at 21 µm was recently revealed in two supernova remnants although its observational properties differ from those of the 21 µm feature in proto-PNe (Rho et al 2018). It is noticeable that the experimental and computational spectra of fulleranes exhibit prominent features in the 18-22 µm spectral range (Iglesias-Groth et al 2012;Zhang et al 2017). Theoretical calculations show that with increasing hydrogen coverage, the central wavelength monotonically shifts from 18 µm to 22 µm (Webster 1995;Zhang 2020).…”

“…Theoretical computations show that the C-H stretching mode is greatly suppressed in cations or slightly hydrogenated fullerenes (Pauzat et al 1995;Zhang et al 2017). In such cases, the feature around 3.4 µm is not an ideal proxy to search for fulleranes.…”

Methodological Investigation for Hydrogen Addition to Small Cage Carbon Fullerenes

“…This suggests that the relative stability of smaller fullerenes may be modified in the presence of hydrogen. While there has been extensive experimental [13,[18][19][20][21] and theoretical [13,18,22] studies of hydrogenated C 60 , less attention has been given to smaller hydrogenated fullerenes. It was theoretically proposed that T d -C 28 H 4 , a tetrahedral fullerene with triply fused pentagons on each corner of the tetrahedron, might be stable [15] and indeed has since been experimentally identified [23].…”