The purpose of this work is to provide spectroscopic evidence of possible chemical pathways that took place in the formation of fossil resins Class I. The natural polymerization process is mimicked in the laboratory using pure communic acids (terpene derivatives) as reactants, which were subjected to controlled temperature treatment. We shall focus here on the trans-, cis-and iso-/ mirceo-communic acid isomers, and the temperature-induced reactivity is discussed on the basis of the different arrangements of the conjugated double-bonds moieties in the monomeric units. Transformations starting at 130, 90 and 127°C, for trans-, cisand iso-communic acids, respectively, led to the formation of intermediate products. The whole process was followed and analyzed by micro-Raman spectroscopy. The absence of several Raman signatures suggests conjugation loss in the side-chain of these intermediate compounds. With increasing temperature, the intermediate products suffer additional maturation reactions to form the final fossil resin analogues object of this work.

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We present Raman spectroscopy experiments in dimethylacetylene (DMA) using a sapphire anvil cell up to 4 GPa at room temperature. DMA presents phase transitions at 0.2 GPa (liquid to phase I) and 0.9 GPa, which have been characterized by changes in the Raman spectrum of the sample. At pressures above 2.6 GPa several bands split into two components, suggesting an additional phase transition. The Raman spectrum of the sample above 2.6 GPa is identical to that found for the monoclinic phase II (C2/m) at low temperatures, except for an additional splitting of the band assigned to the fourfold degenerated asymmetric methyl stretch. The global analysis of the Raman spectra suggests that the observed splitting is due to the loss of degeneracy of the methyl groups of the DMA molecule in phase II. According to the above interpretation, crystal phase II of DMA extends from 0.9 GPa to pressures close to 4 GPa. Between 0.9 and 2.6 GPa, the methyl groups of the DMA molecules rotate almost freely, but the rotation is hindered on further compression.

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