Fullerene-indene adducts (ICMA & ICBA) in an astrochemical perspective part 1: chemical thermodynamics, stability and electronic absorption spectroscopy

Abstract

Both C60 fullerene and indene were detected in certain astrophysical objects. It is then possible that these two molecules may react together in space forming adducts like indene-C60 mono-adduct (ICMA) as well as the indene-C60 bis-adducts (ICBA). In view of the potential detection of such adducts in space, the chemical thermodynamics of the reaction between C60 and indene was analyzed using the thermochemical group increment approach. ICMA and ICBA were synthesized and their composition and stability was studied with thermogravimetric analysis and derivative thermogravimetry (TGA-DTG). ICBA and moreover ICMA resulted stable and strong adducts. In particular, the latter is stable up to 350 °C and this fact increases the chances to find it in space. Furthermore, the retro Diels-Alder reaction was accurately studied on ICMA with differential scanning calorimetry (DSC) and an unusually high activation energy for the decomposition was determined and explained with the chemical thermodynamic data. The electronic absorption spectra of ICMA and ICBA were studied in n-hexane and the molar extinction coefficients of the main absorption bands have been determined. A possible way to detect such adducts is through electronic absorption spectroscopy, while the infrared spectroscopy of ICMA and ICBA is analyzed and discussed in the following second part of this work.