Structure and Spectra of Hydrogenated Amorphous Carbon: A DFT comparison between proposed models.

Abstract

Carbon containing grains are widespread in the ISM and in Carbon-rich stars. Observational data are taken mainly in the IR region both in emission and in absorption. Characteristic signature of these species can be found in the 3.4 m absorption band. This band is associated with carbon hydrogen stretchings of both –CH3 and –CH2 subunits. CH aromatic stretching are also found in the near 3.3 m band. Along with that, bending vibrations of the aforementioned subunits are found in the region from 8 to 6 m, also in the region where we could find vibrations from other functional groups, like olefinic ones. More complex torsions and other vibrations can be found below the 10 m region. The present work is devoted to the study of the structure of two of the main HAC frameworks : one formed by small aromatic rings connected by aliphatic chains (RC=rings and chains) proposed by Dartois and coworkers [1] and the other with a graphitic like backbone with aliphatic substituents in the edges of the structure, as can be seen in the work by Steglich and coworkers [2] (SG = substituted graphite). Both proposals can be found in Figure 1. Along with the study of the structure, the study of the infrared spectra can help to elucidate the nature of the ISM observations.