Derivatization Technique To Identify Specifically Carbonyl Groups by

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Derivatisation technique to identify specifically carbonyl groups by infrared spectroscopy: Characterisation of photooxidative ageing products in terpenes and terpeneous resins Stefan Zumbühl, Andreas Brändle, Andreas Hochuli, Nadim C. Scherrer, and Walter Remo Caseri Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04008 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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Analytical Chemistry

Stefan Zumbühl†*, Andreas Brändle‡, Andreas Hochuli†, Nadim C. Scherrer†, Walter Caseri‡

Derivatisation technique to identify specifically carbonyl groups by infrared spectroscopy: Characterisation of photooxidative ageing products in terpenes and terpeneous resins †

Bern University of Applied Sciences BFH/HKB, Art Technological Laboratory, Fellerstrasse 11, 3027 Bern, Switzerland, [email protected]

‡

Swiss Federal Institute of Technology ETH Zurich, Laboratory for Multifunctional Materials, Vladimir-Prelog-Weg 1-5/10, 8093 Zürich, Switzerland

Abstract Analysis of bioorganic materials by infrared spectroscopy (FTIR) 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 oxidised di- and triterpenes, for example, is hindered by the superposition of similar absorption bands of carbonyl functional groups summing up to a broad, non-distinctive signal. This study presents a technique for selective fluorination of various carboxylic acids by exposure to gaseous sulfur tetrafluoride. The derivatisation 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,βunsaturated 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 derivatisation method enables 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 ageing 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. Introduction Natural resins have been widely used as transparent coatings for paintings and as additives in oil based varnishes 1, 2 throughout history . One fundamental problem of these materials is their high sensitivity to oxidation, which is promoted by light, resulting in changes of their chemical, optical and mechanical properties. Despite the known drawbacks and viable modern alternatives 3, 4, natural varnishes remain popular and are still being applied in painting conservation. Their continued use is often justified with the argument, that the light induced ageing is strongly limited in a UVA-free museum light environment. These di- and triterpeneous resins are a complex mixtures of molecules consisting of a planar polycyclic hydrocarbon framework with a wide variety of functional groups 5-7. The high susceptibility to oxidation of these compounds leads to a large variety of components 8, 9, as up 10, 11 . The distinction of the to six oxygen atoms can be built into a triterpene molecule during photooxidative ageing broad range of oxidation products and, as such, detailed monitoring of the oxidative ageing process, is a complex analytical task. While it has been possible to identify some of the oxidation products using gas-chromatography mass spectrometry (GC-MS) 7, it is known that these compounds represent only a marginal fraction of the full range of ageing products. This study is focused on the specific characterisation of functional groups using Fourier transform infrared spectroscopy (FTIR) to gain information on the oxidation pathways. Within this context, the carbonyl groups are the most relevant functional groups in all natural resins. Since these compounds oxidise readily, they may sum up to a stronger and broader absorption in the carbonyl-stretching band region ν(C=O), caused by multiple overlaps from 10-12 . It is thus of considerable analytical interest to isolate the various degradation products formed during ageing these bands in order to characterise the various functional groups. While statistical peak deconvolution of natural resins allows isolation of distinctive carbonyl bands, it does not deliver unconditional characterisation of the 13, 14 functional groups . Therefore, selective fluorination of carbonyl groups offers one possible strategy to separate such overlapping ν(C=O) carbonyl signals chemically. In this context, sulfur tetrafluoride SF4 is of specific interest. It is a corrosive gas at standard temperature and forms hydrogen fluoride HF upon exposure to water in the 15 15-18 material . In this form it is a precursor to many organofluorine compounds . It converts, for example, carboxylic acids R(O)OH into acyl fluorides R(O)F 19, 20, while ketones and ester groups remain intact. This method has successfully been applied to the characterisation of primary carboxylic acids in drying oils using FTIR 21-23. The situation in terpeneous resins, however, is much more complex. The aim of this study is to characterise and localise different carbonyl containing functional groups in terpenes, absorbing in the spectral range between 2000-

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1500 cm-1. This region is of particular interest to follow the degradation reactions with FTIR spectroscopy, allowing verification of the individual reaction pathways for each material. Experimental Materials: To characterise the reaction selectivity, different terpenes containing characteristic functional groups were investigated: deoxycholic acid 98% (Sigma/Aldrich D2510), valerenic acid (PhytoLab 89288), abietic acid 85% (Acros Organics, 173130250), abietic ester 99% (methyl abietate: Aldrich 03840) and hydroxydamarenone 98% (“Dipterocarpol”: Aldrich 365653). Peracidic oleanone was produced from oleanolic acid 98% (Aldrich 05504) by treatment with hydrogen peroxide for 24h 24. Further experiments were performed with dammar and colophony resins. These materials were collected from the historic material collection at the Bern University of Applied Sciences BFH/HKB. All materials and reference samples were dissolved in Shellsol A and applied to a film thickness of 100 µm with a Film Applicator Model 360 (Erichsen GmbH & Co. KG) on glass slides. The samples ® ® were aged under light conditions generated by light fluorescence tubes True Lite 5500K and Philips UV-20W/08 2 -1 2 F20 T12 BLB of ≈5800 lm/m und 557mW lm / ≈3200 mW/m at ≈40°C, and atmospheric oxygen content. The light was filtered through window glass used as a long pass filter at 330nm (LP 330) to simulate standard indoor light conditions, or a bloomed Mirogard Protect ® laminated glass with a long pass filter at 390 nm (LP 390) to simulate UVA-free (99%) museum light conditions. Sample preparation: Derivatisation: Selective fluorination of terpenes and natural di- and triterpene resins with ® gaseous SF4 (ABCR Dr. Braunagel GmbH & Co. KG, Art No. AB10417) was performed as reported previously for other materials. The micro-samples were rolled on to a CVD diamond window prior to placement into a gas reaction chamber. The derivatisation treatment was performed in an all-teflon® chamber (volume 10cm3) 23. Prior to SF4 exposure, the chamber was purged with gaseous nitrogen N2 for 10 Min. Then the water free chamber was flushed with gaseous sulfur tetrafluoride SF4. The samples were exposed to the reactive gas for 12h. After treatment the chamber was purged with N2 again and the toxic gas was adsorbed to active charcoal. The FTIR measurements were performed immediately after derivatisation. Instrumental: FTIR measurements were performed on a Bruker® Hyperion 3000/Tensor 27 infrared spectrometer and the micro-samples were analysed in transmission mode at 4 -1 cm spectral resolution and 32 scans. The detection area of a measurement was 100 µm in diameter (spot size). 13 C NMR measurements were performed on a Bruker AV300 MHz spectrometer. About 80 mg of material were dissolved in 0.7 mL solvent. The fresh material and the reference compounds were dissolved in deuterated chloroform, aged samples in perdeuterated acetone. Results and Discussion Derivatisation of individual terpenes Distinction of functional groups is a key to the successful characterisation of specific oxidation products of terpenes and resins. Therefore, in a first step, commercial terpenes (Fig 1) were used to investigate the selective reactivity of different functional units with carboxylic groups towards SF4. Signal shifting upon derivatisation of primary carboxylic acids was studied using deoxycholic acid. SF4 treatment caused a signal shift of the correlating acyl fluoride R(C=O)F from ≈1705 cm-1 to ≈1840 cm-1, in line with the literature for polymers 19, 25. The reactivity of α,βunsaturated, primary carboxyl groups was studied on valeric acid, which exhibits a ν(C=O) absorption at 1681 cm-1. Upon transformation by SF4, ν(C=O) absorption of the unsaturated acyl fluorides appeared at 1796 cm-1. Fluorination of saturated, as well as of α,β-unsaturated carboxylic acid groups led to a positive shift of -1 approximately 130 cm . Based on the polycyclic ring structures of terpenes, carboxylic acids may also be located in a tertiary position, which is true for many diterpenes. These compounds generally show ν(C=O)-absorptions -1 6, 12 around 1700-1690 cm . Fluorination of oleanolic acid documents a comparable signal shift of the tertiary acyl fluoride R3(C=O)F to ≈1826 cm-1. In contrast to the carboxylic acid groups mentioned above, the weakly acidic peroxy acids do not form fluorinated counterparts and show an unchanged ν(C=O) absorption band at ≈1765-1775 cm-1. Similarly, the ester group of abietic ester and the ketone group of hydroxydammarenone remained intact upon exposure to SF4 and did not show any signal shifts. The selectivity of the fluorination reaction was tested for abietic acid and hydroxydammarenone used for ageing experiments. The FTIR data as well as the 1H NMR and 13C NMR data show, that the carbonyl group of the hydroxydammarenone is inert. The tertiary carboxylic acid group of the abietic acid, on the contrary, was fluorinated to the corresponding acyl fluoride. Upon completion of the reaction with SF4, the νO-H and νC-O bands in the FTIR spectrum are fully eliminated. In addition to the formation of acyl fluoride, the fluorination process leads to a strong -1 shift of the ν(C=O) absorption band in the range of ≈ +130cm , as well as a new signal caused by the νC-F group. 13 The C NMR measurements confirm the formation of acyl fluoride. Prior to derivatisation, the tertiary carboxylic acid shows a signal at 185.3 ppm, while the formation of acyl fluoride leads to a characteristic doublet signal of the


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fluorinated carbon at 170.7 ppm and 165.7 ppm (Fig.2). These results show that exposure to SF4 is a very powerful tool in separating ketones and carboxylic acids, generally yielding IR-absorption bands in a narrow spectral range. Upon the splitting by the fluorination process, the bands of the three types of carboxylic acids are well isolated from each other, as well as from the inert saturated and unsaturated ketones (Fig. 1). This result indicates that, in principle, no less than 7 relevant and otherwise interfering carbonyl bands can be separated spectrally by exposure to SF4, which is remarkably powerful in the context of these complex materials. Ageing of terpenes promoted by light Terpenes generally exhibit a complex ageing behaviour and oxidation reactions follow the principle of radical chain 9, 26, 27 reactions . These materials are readily oxidised as they may contain photosensitive groups. Their reactivity is additionally fostered by the characteristic structure of the hydrocarbon backbone promoting propagative chain reactions. Earlier research has unveiled that the components within natural resins vary in their ease to oxidise readily 10. Based on this observation, ageing of two pure terpenes with characteristic individual structural properties 13 was followed with FTIR and C NMR spectroscopy. Accelerated light ageing was performed at indoor conditions behind a window glass, and at museum conditions behind a UV-protective glass, accumulating 6 weeks of exposure. Photooxidation of hydroxydammarenone The triterpene hydroxydammarenone is the main component of dammar resin and possesses a photochemically -1 reactive ketone group (Fig. 3A). Accordingly, this material exhibits a narrow ν(C=O) absorption band at ≈1705 cm of the ketone group, which is inert to derivatisation with SF4 (Fig. 3B,C). After 6 weeks of ageing, the carbonyl band became much broader with a shoulder at ≈1760 cm-1. After treatment of sample material with gaseous SF4, the carbonyl band split into four distinct ν(C=O) signals (Fig. 3B,C). The two new main bands were attributed to peroxy -1 -1 acids (≈1765 cm ) and carboxylic acids (≈1840 cm ), respectively. Notably, ageing in UVA-free environment resulted in identical reaction products at the same ratio, albeit in much lower quantities. These reaction products demonstrate the relevance of the Norrish I reaction during the ageing process, leading to the formation of dammarenolic acid (reaction I) (Fig. 5) 7. The presence of these reaction products implies that the oxidation of saturated ketones cannot be prevented by UV-protected conditions. Interestingly, however, a pronounced ν(C=O) band at ≈1765 cm-1 suggests that the peroxy acids (oxidative intermediates), are quite stable and decay relatively slowly to primary carboxylic acids with a ν(C=O) band at ≈1838 cm-1. Furthermore, a ν(C=O) band at ≈1738 cm-1 unveils a minor amount of ester groups. This observation was confirmed by 13C-NMR spectroscopy (Fig. 3D,E). Fresh hydroxydammarenone shows signals such as that of the C3 ketone group (218.4 ppm), the C20 hydroxyl group (75.1 ppm) and the C24/C25 double bond (124.7 ppm and 131.3 ppm). After an accumulated 6 weeks of ageing, new (weak) signals between 220-170 ppm hint at the formation of ketones, carboxylic acids and peroxy acids, confirming the results derived from FTIR spectroscopy. However, the C3 ketone group is still clearly present, which implies that oxidation at this position proceeds at a low rate. Some new and weak bands between 14 ppm and 58 ppm point at reactions progressing at the hydrocarbon backbone. Furthermore, the presence of signals at ≈90 ppm, attributed to tertiary hydrogen peroxides, confirm that propagation reactions are very limited. The slightly reduced relative signal intensities of the tertiary C-H group C13 (42.3 ppm) and C17 (49.7 ppm), as well as of the CH2 group C23 adjacent to the double bond (22.5 ppm) suggest that these represent potentially reactive groups. Other hydrogen atoms are largely inert. Yet, formation of small amounts of new double bonds (≈145-120 ppm) was observed and is related to a shoulder -1 around ≈1650-1550 cm in the FTIR spectrum. Overall, the readiness of hydroxydammarenone to oxidise can be judged as very low, despite the photochemical sensitivity of the saturated ketones in either light condition. Photooxidation of abietic acid Abietic acid (Fig. 4A) is the main component of colophony, a highly oxidisable natural resin. It possesses a -1 carboxylic acid at a tertiary carbon atom, which responds with a characteristic ν(C=O) absorption at ≈1825 cm upon derivatisation with SF4. Besides this signal, several other ν(C=O) absorption bands are present after derivatisation, relating to distinct functional groups appearing in variable proportions, depending on the ageing conditions (Fig. 4B,C). The band at ≈1705 cm-1 drastically diminished after irradiation, although exposure to light was reduced to 3 weeks compared to 6 weeks with hydroxydammarenone. In a UVA-free environment, the ν(C=O)signal of abietic acid at ≈1685 cm-1 is the dominant band. This band is assigned to unsaturated ketones, which form 28 as primary oxidation products by the decay of hydrogen peroxides . While it was not possible to detect hydrogen peroxides in these experiments, the fast formation of ketones hints at a fast homolytic decay of these unstable intermediates (step 2 of reaction II in Fig 5). It was observed that hydrogen atoms of C-H groups next to double bonds (leading to α,β-unsaturated ketones) are being abstracted faster than hydrogen atoms of tertiary carbon



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atoms (leading to saturated ketones). The different types of ketones have a decisive influence on the photosensitivity of the material. In an UVA-free environment, the unsaturated ketones were barely excited and proved to be stable, unlike the saturated ones. Thus, secondary oxidation reactions are limited. Such a reduced light sensitivity of unsaturated ketones was also observed from GALDI-MS (Graphite-assisted laser desorption/ionization mass spectrometry) measurements on other pure terpenes 29. Exposure to UVA containing light, however, leads to different ageing pathways. The subsequent oxidation was expressed by a strong increase in intensity of the ν(C=O) band of the peroxy acids at ≈1775 cm-1 (Fig. 4B,C), in analogy to reaction I (Fig. 5). The decay of these peroxy acids proceeded slowly to primary carboxylic acids, which remained undetected with FTIR spectroscopy. This particular case unveils a small limitation of the derivatisation technique, as low contents of -1 primary acids (≈1840 cm ) remain difficult to detect due to interference with carboxylic acids at tertiary carbon atoms (≈1825 cm-1) of the diterpene molecule. 13

The reactions were, nevertheless, confirmed by C NMR spectroscopy (Fig. 4D,E). The fast propagation reaction considerably alters the terpenes, even in a UVA-free environment. Those processes dramatically increase upon exposure to UVA environments. This is illustrated by the strong reduction of the signal response at C9 (51.0 ppm) and C15 (34.9 ppm, 34.5 ppm) of the α,β-unsaturated CH2 groups, reporting the oxidation of these C-H groups in next to a double bond. Yet, also tertiary C5 (44.9 ppm) atoms and α,β-unsaturated C-H groups at C9 (51.0 ppm) and C15 (34.9 ppm, 34.5 ppm) exhibit a considerable reactivity. The data suggests that conjugated double bonds greatly influence the rate of oxidation. Accordingly, based on the fast increase in the carbonyl absorption measured by FTIR spectroscopy, oxidative propagation reactions proceed rapidly in abietic acid. The fast rate of this reaction can be explained by the high accessibility of the reactive positions in abietic acid. The formation of α,β-unsaturated ketones in the context of a general increase of double bonds (170-120 ppm) (Fig. 4D) is of paramount importance, as these components are responsible for the strong yellowing in chromophoric systems. In the case of abietic acid, FTIR and 13C-NMR analyses demonstrate clearly that primary and secondary oxidation reactions run sequentially, rather than simultaneously. Depending on the light filtering system applied, reactions proceed at variable ratios, increasingly comprising all structural elements of the molecule at higher levels of UVA irradiation. Photooxidation of natural resins As the derivatisation method was successfully tested on pure terpenes, natural resins exhibiting a higher degree of complexity were subsequently employed. Two natural resins (colophony and dammar) were investigated. Dammar contains photochemically excitable ketones at ≈1710 cm-1 (Fig. 6A), which are present only in the main components of triterpenes. The main components of the diterpenes in colophony (pine resin), on the contrary, -1 comprise mainly tertiary carboxylic acids, with an intense ν(C=O)-absorption band at ≈1825 cm (Fig. 6B). Despite the structural differences, all resins are rapidly oxidised, as disclosed by an enormous increase in relative intensity and broadening of the carbonyl band in IR spectra (Fig. 6). The concentration of excitable ketones was shown not to be a crucial factor with respect to the rate of reaction, as the concentration of radicals may be substantial in the 9 raw material already . Furthermore, it was observed in pure terpenes that propagation is the key factor in the kinetics of the oxidation process. The development of individual functional groups was documented with FTIR spectroscopy at regular intervals during the 6 weeks of ageing. Within the first phase of oxidation, mainly ketones were formed, which are generated by the decay of intermediate peroxides 28. Since these primary formed ketones exhibit a different light sensitivity, as described above, the subsequent progression of oxidation is dominated by the prevalent light conditions. UV-filtering (LP 390 nm) strongly reduces, albeit not inhibits, excitation of saturated ketones and thereby decelerates secondary reactions in dammar resin. According to the observation made on abietic acid, the excitation of α,β-unsaturated ketones in colophony is limited and secondary reactions are strongly reduced. When exposed to UVA-containing light conditions, excitation of all ketones via the Norrish I reaction leads -1 to rapid generation of peroxy acids (≈1770 cm ) as the main reaction products in all resins (steps 3 and 4 in Fig. 5). The bond dissociation energy of the O-O bond is significantly higher for peroxy acid than for other peroxides 30, 31 . Thus, aliphatic peroxy acids have no characteristic UV absorption >300nm 32 and can therefore be regarded as much more stable than other peroxides under the light conditions tested. These, nevertheless, do decay slowly to -1 primary carboxylic acids (IR band at ≈1840 cm ), which then represent the stable end products. Conclusion The results show that the derivatisation technique with gaseous SF4 enables to detect and identify various carbonyl compounds using FTIR spectroscopy, even in complex mixtures of terpenes. It allows the spectral separation of overlapping carbonyl bands from primary acids, tertiary acids, α,β-unsaturated acids, peroxy acids, esters, ketones, and α,β-unsaturated ketones into spectrally distinct absorption bands. The method has proven to be a powerful tool to investigate the light-induced oxidative ageing of natural resins that are commonly used as varnish materials. The possibility to characterise the various reaction products during their process of ageing with this technique may shed new light on respective reaction pathways. Although no kinetic data was generated, the change in relative signal intensities by FTIR clearly indicate, that different terpenes oxidise at individual rates. Some compounds are readily


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excited photochemically and thus become efficient sources of radicals, while others oxidise easily during oxidative propagation due to their abstractability of weakly bound hydrogen atoms. Natural resins are always a combination of multiple compounds and, as a consequence, are prone to oxidise much faster than most of the pure terpenes. Accelerated light ageing under indoor and museum conditions unveiled that different functional groups are formed at variable proportions during individual stages of the aging process.



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Fig 1 Various commercially available terpenes (top) used to investigate the selective reactivity of different functional groups (highlighted in grey): 1) deoxycholic acid, 2) oleanolic acid, 3) valeric acid, 4) oleanolic peroxy acid, 5) abietic ester, 6) hydroxydammarenone. The measured peak position before (regular) and after SF4 treatment (bold) are marked. Characteristic IR-absorption bands before (centre) and after exposure to SF4 (bottom). Functional groups with overlapping bands (highlighted in grey) are separated from each other upon fluorination with SF4.


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Fig 2 Derivatisation of abietic acid with SF4. FTIR-Spectrum before (A) and after sample pre-treatment using 13 sulfur tetrafluoride (SF4) (B). C NMR data (C) before (left) and after fluorination (SF4) of the tertiary acid group (right).



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Fig.3 Chemical structure of hydroxydammarenone acid (A). FTIR spectrum of hydroxydammarenone acid upon irradiation for 6 weeks in daylight which was filtered through a window glass used as a long pass filter at 330 nm (LP 330) or a museum glass with a long pass filter at 390 nm (LP 390) before (B) and after derivatisation with SF4 13 (C). C NMR spectra of pristine and aged hydroxydammarenone acid (D) and (E), the main changes in the hydrocarbon backbone upon ageing are labelled. The eliminated chloroform signal at 77.2 ppm are denoted by an asterisk (*).


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Fig. 4 Chemical structure of abietic acid (A). FTIR spectra of abietic acid upon exposure to daylight filtered through a window glass used as a long pass filter at 330 nm (LP 330) or a museum glass with a long pass filter at 390 nm (LP 390) before (B) and after derivatisation with SF4 (C). 13C NMR spectra of pristine and aged abietic acid (D) and (E). The eliminated chloroform signal at 77.2 ppm is denoted by an asterisk (*).



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Fig. 5 Schematic illustration of oxidation reaction paths of terpenes. A) Reaction I: Light-induced radical initiation by the Norrish I reaction. B) Reaction II: Oxidative propagation reaction by abstraction of tertiary bonded hydrogen atoms, forming different carbonyl-containing functional groups with characteristic absorption bands in FTIR spectra (highlighted in grey).


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Fig 6 Sequential FTIR spectra of triterpeneous resins over a time period of 6 weeks of exposure to light. Dammar resin aged in UVA-containing light (LP 330) (A) and UV-free light (LP 390) (B), and colophony resin aged in UVAcontaining light (C) and UVA-free light (D). These plots show FTIR spectra before (left) and after derivatisation with SF4 (right).



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Mills, J. S.; White, R. Studies in Conservation 1977, 22, 12-31. De la Rie, R. E. Analytical Chemistry 1989, 61, 1228A-1240A. Samet, W. Varnishes and Surface Coatings; American Institute for Conservation AIC: Washington D.C., 1998. De la Rie, R. E.; Quillen Lomax, S.; Palmer, M., Rio de Janeiro 2002; James & James; 881-887. Dev, S.; Gupta, A. S.; Patwardhan, S. A. Handbook of Terpenoids - Triterpenoids; CRC Press Inc.: Boca Raton, 1986. Dev, S.; Misra, R. Handbook of Terpenoids - Diterpenoids; CRC Press Inc.: Boca Raton, 1986. van der Doelen, G. A., University of Amsterdam, Amsterdam, 1999. Dietemann, P.; Higgitt, C.; Kälin, M.; Edelmann, M. J.; Knochenmuss, R.; Zenobi, R. Journal of Cultural Heritage 2009, 10, 30-40. Dietemann, P.; Kälin, M.; Zumbühl, S.; Knochenmuss, R.; Wülfert, S.; Zenobi, R. Analytical Chemistry 2001, 73, 20872096. Zumbühl, S.; Knochenmuss, R.; Wülfert, S.; Dubois, F.; Dale, M. J.; Zenobi, R. Analytical Chemistry 1998, 70, 707-715. Dietemann, P.; Edelmann, M. J.; Meisterhans, C.; Pfeiffer, C.; Zumbühl, S.; Knochenmuss, R.; Zenobi, R. Helvetica Chimica Acta 2000, 83, 1766-1777. Beltran, V.; Salvado, N.; Buti, S.; Pradell, T. Anaytical and Bioanalytical Chemistry 2016, 4073-4083. Daher, C.; Bellot-Gurlet, L.; Le Ho, A. S.; Paris, C.; Regert, M. TALANTA: The International Journal of Pure and Applied Analytical Chemistry 2013, 115, 540-547. Azémard, C.; Vieillescazes, C.; Ménager, M. Microchemical Journal 2014, 112, 137-149. Wang, C.-L. J. Fluorination by Sulfur Tetrafluoride; John Wiley & Sons Inc., 2004. Baasner, B.; Hagemann, H.; Tatlow, J. C. Organo-Fluorine Compounds; Thieme Verlag: Stuttgart, 1999. Dmowski, W. In Organo-Fluorine Compounds - Houben-Weyl: Methods of Organic Chemistry; Baasner, B., Hagemann, H., Tatlow, J. C., Eds.; Thieme Verlag: Stuttgart, 1999; Vol. Volume E 10 a, pp 321-431. Lagow, R. J. In Organo-Fluorine Compounds - Houben-Weyl: Methods of Organic Chemistry; Baasner, B., Hagemann, H., Tatlow, J. C., Eds.; Thieme Verlag: Stuttgart, 1999; Vol. Volume E 10 a, pp 188-208. Wilhelm, C.; Gardette, J.-L. Journal of Applied Polymer Science 1994, 51, 1411-1420. Siesler, H. W.; Holland-Moritz, K. Infrared and Raman Spectroscopy of Polymers Dekker New York, Basel, 1980. Mallégol, J.; Gardette, J.-L.; Lemaire, J. Journal of the American Oil Chemist’s Society 1999, 76, 967-976. Zumbühl, S.; Scherrer, N.; Ferreira, E.; Hons, S.; Müller, M.; Kühnen, R.; Navi, P. Zeitschrift für Kunsttechnologie und Konservierung 2011, 25, 139-151. Zumbühl, S.; Scherrer, N. C.; Eggenberger, U. Applied Spectroscopy 2014, 68. Klenk, H.; Götz, P. H.; Siegmeier, R.; Mayr, W. In Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, 2005, pp doi 10.1002/14356007.a14356019_14356199. Mailhot, B.; J.-L., G. Macromolecules 1992, 25, 4119-4126. Rabek, J. F. Polymer Degradation - Mechanisms and Experimental Methods; Chapman & Hall: London, 1995. Rangby, B.; Rabek, J. F. In Photodegradation, Photo-oxidation and Photostabilization of Polymers; Rangby, B., Rabek, J. F., Eds.; John Wiley & Sons: London, 1973, pp 97-119. Richardson, W. H.; Yelvington, M. B.; Andrist, A. H.; Ertley, E. W.; Smith, R. S.; Johnson, T. D. The Journal of Organic Chemistry 1973, 38, 4219-4225. Dietemann, P.; Herm, C. In Organic Mass Spectrometry in Art and Archeology; Colombini, M. P., Modugno, F., Eds.; Wiley & Sons Ltd: Chichester, 2009. Lefort, D.; Fossey, J.; Gruselle, M.; Nedelec, J.-Y. Tetrahedron 1985, 41, 4237-4252. Bach, R. D. In The Chemistry of Peroxides; Rappoport, Z., Ed.; John Wiley & Sons Ltd: Chichester, 2006; Vol. Volume 2, Part 1, pp 1-91. Ogata, Y.; Tomizawa, K.; Furuta, K. In The Chemistry of Peroxides; Patai, S., Ed.; John Wiley & Sons Ltd.: Chichester, 1983, pp 711-775.


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Fig 1 Various commercially available terpenes (top) used to investigate the selective reactivity of different functional groups (highlighted in grey): 1) deoxycholic acid, 2) oleanolic acid, 3) valeric acid, 4) oleanolic peroxy acid, 5) abietic ester, 6) hydroxydammarenone. The measured peak position before (regular) and after SF4 treatment (bold) are marked. Characteristic IR-absorption bands before (centre) and after exposure to SF4 (bottom). Functional groups with overlapping bands (highlighted in grey) are separated from each other upon fluorination with SF4. 124x183mm (600 x 600 DPI)



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Fig 2 Derivatisation of abietic acid with SF4. FTIR-Spectrum before (A) and after sample pre-treatment using sulfur tetrafluoride (SF4) (B). 13C NMR data (C) before (left) and after fluorination (SF4) of the tertiary acid group (right). 86x87mm (600 x 600 DPI)


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Fig.3 Chemical structure of hydroxydammarenone acid (A). FTIR spectrum of hydroxydammarenone acid upon irradiation for 6 weeks in daylight which was filtered through a window glass used as a long pass filter at 330 nm (LP 330) or a museum glass with a long pass filter at 390 nm (LP 390) before (B) and after derivatisation with SF4 (C). 13C NMR spectra of pristine and aged hydroxydammarenone acid (D) and (E), the main changes in the hydrocarbon backbone upon ageing are labelled. The eliminated chloroform signal at 77.2 ppm are denoted by an asterisk (*). 151x127mm (600 x 600 DPI)



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Fig. 4 Chemical structure of abietic acid (A). FTIR spectra of abietic acid upon exposure to daylight filtered through a window glass used as a long pass filter at 330 nm (LP 330) or a museum glass with a long pass filter at 390 nm (LP 390) before (B) and after derivatisation with SF4 (C). 13C NMR spectra of pristine and aged abietic acid (D) and (E). The eliminated chloroform signal at 77.2 ppm is denoted by an asterisk (*). 150x126mm (600 x 600 DPI)


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Fig. 5 Schematic illustration of oxidation reaction paths of terpenes. A) Reaction I: Light-induced radical initiation by the Norrish I reaction. B) Reaction II: Oxidative propagation reaction by abstraction of tertiary bonded hydrogen atoms, forming different carbonyl-containing functional groups with characteristic absorption bands in FTIR spectra (highlighted in grey). 81x36mm (600 x 600 DPI)



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Fig 6 Sequential FTIR spectra of triterpeneous resins over a time period of 6 weeks of exposure to light. Dammar resin aged in UVA-containing light (LP 330) (A) and UV-free light (LP 390) (B), and colophony resin aged in UVA-containing light (C) and UVA-free light (D). These plots show FTIR spectra before (left) and after derivatisation with SF4 (right). 111x69mm (600 x 600 DPI)


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Derivatization Technique To Identify Specifically Carbonyl Groups by

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