Theoretical (DFT) and experimental (Raman and FTIR) spectroscopic study on communic acids, main components of fossil resins Oscar R. Montoro ⁎, José Tortajada, Álvaro Lobato, Valentín G. Baonza, Mercedes Taravillo MALTA-Consolider Team, Departamento de Química-Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Avda, Complutense s/n, 28040, Spain abstract article info Article history: Received 23 May 2019 Received in revised form 8 July 2019 Accepted 17 July 2019 Available online 19 July 2019 The objective of this work is to establish a spectral assignment of several communic acids. The most significant vibrational modes of three stereoisomers of communic acids [trans-, cis-, and iso- (or mirceo-)] are presented. They are showed throughout experimental Raman and IR spectra, and on the basis on calculations with Density Functional Theory (DFT) and the assignment of the spectral bands of different resins found in the literature. These three communic acids studied are the most important isomers present in the scaffold of the fossil resins Class I, as monomers or co-polymerized according to several authors. These kinds of terpenes are used as starting material, for example, for the synthesis of the fungicide and compounds bioactives. In a novel way, it is reported jointly the assignment of the experimental Infrared and Raman modes together with theoretical modes, since normally the authors tend to focus on one or another spectroscopic technique only. These results can be used as a reference for distinguishing amber from less matured resins as copal, determining the local origin of archaeological fossilized resins. Moreover, they will serve as help to differentiate between real and imitation ambers. © 2019 Elsevier B.V. All rights reserved. Keywords: Communic acid Fossil resins IR and Raman spectra Vibrational wavenumbers DFT 1. Introduction Copals and fossil resins, as amber, are gemstones of organic nature which have survived to the present day. Most fossil resins (or resinites, when occurring in coal seams) [1–3], are formed by natural terpene- based compounds mainly by high-molecular weight cross-linked poly- mers, therefore, they have an organic origin. The early stage of the for- mation of fossil resins involves that resins are secreted by higher plants after suffering some damage. The existence of different groups of these organic compounds with specific botanical, chemical, geographical affinities has been treated in the literature [4]. But nowadays, the most extended and accepted clas- sification about fossil resins has been proposed by Anderson et al. [5,6]; this classification is based on the composition and structural char- acteristics of fossil resins. These authors sort fossil resins into five Clas- ses, the most important of which is called Class I, object of this manuscript. Although exact chemical composition of the fossil resins is still unknown, it is accepted that resins based primarily on co- polymers of labdatriene (carboxylic acids and their reduced forms) are the major component of these compounds as such Baltic amber, belong- ing to the Class I [7,8]. According to Anderson's classification, in more detail, fossil resins are classified into five classes: - Class I, derived from resins based on polymers of (primarily) labdatriene carboxylic acids (mainly monomers of 8(17),12 (13) 14-labdatriene-18-oic acid) and theirs reduced forms (see Fig. 1 to view Labdanic skeleton and his nomenclature); then, it will be seen more in depth. The most characteristic example of this kind is Baltic Amber. - Class II, derived from resins based on polymer of bicyclic sesquiterpenoid hydrocarbons, especially cadinene. - Class III, whose basic structural character is polystyrene. - Class IV, character sesquiterpenoid, particularly based on the Cedrane skeleton. - Class V, diterpenoids non-polymeric of carboxylic acid, mainly based on abietanes skeleton. It is important to say that in all Classes, there are other chemical (sec- ondly) constituents present in fossil resins, as mono-, sesqui-, di- and tri-terpenoids and, other low-molecular weight compounds. More specifically, Class I is classified in: - Class Ia (see Fig. 2 to view Labdatriene carboxylic acids, alcohols and hydrocarbons) derived from resins based primarily on polymers of communic acid, partially co-polymerized with communol and biformene, probably as a cross linking agent is characteristic, having Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 224 (2020) 117405 ⁎ Corresponding author at: Departamento de Química-Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Avda, Complutense s/n, 28040, Spain. E-mail address: ormontoro@quim.ucm.es (O.R. Montoro). https://doi.org/10.1016/j.saa.2019.117405 1386-1425/© 2019 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa