Infrared spectroscopic study of the formation of fossil resin analogs with temperature using trans-communic acid as precursor

For million years resin exudates have undergone chemical alterations by heat, pressure, radiation, water, oxygen, microorganisms, and have suffered processes of sedimentation and diagenesis. These agents have affected the organic functional groups present in the terpenes of the resins, giving rise to what we nowadays know as fossil resins. In this work, we try to get further insight in the chemical formation of fossil resins. As the simulation of the natural process is quite complex, we have focused on the temperature induced reactivity of the trans-communic acid, the main component of the Class I resins. Using this terpene derivate as a very basic model of a resin exudate, we have monitored their thermal changes by infrared spectroscopy, Differential Scanning Calorimetry and Thermogravimetric Analysis within the range of 25 to 340 °C. The temperature-induced transformation, both in presence and absence of inert atmosphere, is discussed on the basis of the reactivity of the conjugated double-bond, the exocyclic bond and the carboxylic acid group present in the trans-communic acid. The results obtained in these series of experiments agree with the maturation scheme accepted in the literature for natural resins, i.e. an initial cross-linked polymerization and a subsequent maturation reaction. From combined DSC/TGA and infrared spectroscopy results, we conclude that chemical changes produced in the trans-communic acid in the range 130-175 °C may mimic the initial polymerization-like process in the natural resins, whereas those produced between 180 and 340 °C seem to correspond to the maturation pathways described in the literature for fossil resins Class Ib. Spectral assignment of the most relevant infrared-active modes of the trans-communic acid at 25 °C is also provided with the aid of Density Functional Theory calculations.