Search for Hydrogenated C60 (Fulleranes) in Circumstellar Envelopes

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, C60Hm, m = 1-60) may be abundant in space. In this paper, we present theoretical studies of the vibrational modes of isomers of C60Hm. 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 mu m 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.