Derivatization Technique To Identify Specifically Carbonyl Groups by Infrared Spectroscopy: Characterization of Photooxidative Aging Products in Terpenes and Terpeneous Resins

Abstract

Analysis of bioorganic materials by infrared spectroscopy (FT-IR) is frequently limited due to overlapping of diagnostic bands from the various components, which poses a fundamental problem to this analytical technique. The distinction of oxidized di- and triterpenes, for example, is hindered by the superposition of similar absorption bands of carbonyl functional groups summing up to a broad, nondistinctive signal. This study presents a technique for selective fluorination of various carboxylic acids by exposure to gaseous sulfur tetrafluoride. The derivatization treatment leads to characteristic band shifts, allowing the separation of otherwise overlapping bands. Accordingly, the IR bands of primary acids, α,β-unsaturated acids, tertiary acids, peroxy acids, esters, ketones, and α,β-unsaturated ketones are split into distinct absorption bands. The capability of this method is demonstrated on the example of natural resins and their ingredients, which are commonly known to be susceptible to oxidation at ambient conditions. The derivatization method enables one to identify various carbonyl containing functional groups by infrared spectroscopy, even in complex mixtures of terpenes. It unveils previously hidden degradation reactions running in terpenes and natural resins exposed to artificial aging by irradiation with light. New insight is presented on the individual reaction pathways of the terpenes hydroxydammarenone and abietic acid as well as of natural resin varnishes made from dammar and colophony.

Amber imitation? Two unusual cases of Pinus resin-coated beads in Iberian Late Prehistory (3rd and 2nd millennia BC)

“…Terpenous resins are a complex mixture of molecules that changes considerably with time due to their high susceptibility to oxidation or polymerization. Their identification has the reputation of being a complex analytical task due to the broad range of oxidation products resulting from the oxidative ageing process observed [29–33]. In contrast, the first molecular changes to natural resins can be observed a few weeks after collection [32].…”

Photoreactivity of Oil and Alcoholic Diterpenic Varnishes: Key Role of the Polymerization Process

“…Such result is in good accordance with other studies that prove the importance of the formation of peresters in the photochemical process. [45,[57][58][59] Moreover, the tertiary CH (2890 and 1047 cm À 1 ) exhibit an important decrease as they are the most susceptible to form peroxide because of their lower energy of bond dissociation in regard of all other types of CÀ H bond, [57,58] as proposed on Scheme 2. It is also noticeable that in position C9, the energy of CH bond is lowered by the presence of an unsaturation on the beta carbon C8.…”

“…In alcoholic varnishes, the photoreactivities of the carboxylic groups of abietane and pimarane compounds were crucial in their degradation mechanism and led to the formation of dimers and polymers. Indeed, we could follow the infrared evolution of most characteristic bands of -COOH during the irradiation (cf Figure 3), with a special attention at the wavenumbers: (i) 2647 and 2535 cm À 1 for their OÀ H stretching, overtone and combination bands, [28,47] (ii) 1695 cm À 1 for their ketonic group, [45] (iii) 1276 cm À 1 for the stretching of the OH group. [45,48] Such effect can be correlated to the huge loss of all free abietianes (72 % for palustric and abietic acid, 53 % for DHA) and pimaranes (52 % for pimaric acid, 64 % and 69 % for sandaracopimaric and isopimaric acids) detected by GC-MS during the phase 1 of the irradiation (cf Figure 4) with no major photoproducts detected.…”

“…The high reactivity of all tertiary CH was detected by NMR during the irradiation of pure abietic acid in other publication. [45] Besides, IR characteristic doublets of quaternary CÀ C ( Figure 6), around 1397 and 1373 cm À 1 , [53] increased all over irradiation, most probably because of the reactivity of terpenes tertiary carbons C5, C9 and C15. Such peroxides easily induce the homolytic breakdown of the OÀ O bonds in radicals.…”

“…[21,40] The low molecular weight fraction is generally composed of sandaracopimaric, hydroxy-sandaracopimaric, isopimaric, (iso)agathic, dihydroagathic, dihydroagathalic, pinifolic, and cis-and transcommunic acids, and phenols (manool, totarol, sugiol and ferruginol). [21,30,39,41,42] The photodegradation of alcoholic varnishes of Pinus resin was studied upon different irradiation devices (Suntest, [38,43] with λ exc > 295 nm Solar Box, [44] with λ exc > 290 nm, fluorescence tubes, [45] with λ exc > 330 nm and λ exc > 390 nm, UV light, [46] at λ exc = 365 nm). Pure Pinus resins were generally reported as undergoing photochemical polymerization, oxidation and dehydrogenation which undergo the formation of aromatic, hydroxy-and oxo-compounds.…”