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Analysis of chemiluminescence spectra in oxidative degradation of oleic acid
Chemical Physics Letters 565 (2013) 138–142
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Analysis of chemiluminescence spectra in oxidative degradation of oleic acid Tomohiro Sago a, Hiroshi Ishii b, Hideaki Hagihara a,⇑, Noriyuki Takada a, Hiroyuki Suda a a b
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Japan Applied Technology Inc., 1053-1 Yaho, Kunitachi, Tokyo 186-0011, Japan
a r t i c l e
i n f o
Article history: Received 26 December 2012 In final form 16 February 2013 Available online 26 February 2013
a b s t r a c t We studied chemiluminescence spectroscopy to develop an evaluation method for organic material degradation. The thermal oxidation of oleic acid as a monounsaturated fatty acid was investigated using chemiluminescence measurements. The obtained spectra indicated that the luminescence consisted of several peaks, including peaks for singlet oxygen and excited carbonyls. A detailed analysis of the nuclear magnetic resonance and infrared absorption spectra allowed us to determine the probable structures of the oxidative products. The relationship between the spectrum and the analyzed structure implied that the wavelength of the carbonyl group emission peak shifted as a result of the structure. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Chemiluminescence (CL) is known to be a relatively strong emission resulting from chemical reactions such as the luminol reaction and firefly luminescence. Contemporary CL measurement systems are able to detect very weak emissions from various organic reactions like oxidation. Several investigations of polymer degradation using CL have been reported [1–9]. On the other hand, as an evaluation method for material degradation, infrared (IR) spectroscopy has mainly been used. IR spectroscopy can be used to investigate the amount of the carbonyl groups generated by oxidation, i.e., the carbonyl index (CI), which provides a good indication of how far oxidation has proceeded. Osawa et al. reported that CL measurement could be used to observe the emission at the very early stage of oxidation, where IR spectroscopy could not detect any CI changes [6]. Therefore, CL has been used as a highly sensitive method for evaluating degradation [1–9]. However, the conventional CL technique can only be used to observe the emission intensity. CL spectroscopy is expected to elucidate the degradation mechanism at an early stage if an analysis method for the CL spectrum can be established. It was reported that a CL measurement system equipped with band-pass filters could obtain the spectral data of polymer materials, although filter replacement was necessary during the measurement and the measurement required a long acquisition time [10]. Recently, a Fourier-transform (FT) CL spectrometer with a Savart-plate polarization interferometer and CCD area image sensor has been developed. This makes it possible to obtain the CL spectrum quickly [11,12]. The obtained CL spectrum contains information not only about the intensity but also about the
⇑ Corresponding author. E-mail address: [email protected] (H. Hagihara). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.02.035
emission wavelength. Hence, the spectral behavior with a change in the chemical structure and/or degradation conditions has been discussed [12]. However, the relationship between the structure of the emission species and the emission wavelength is still an open problem. To establish CL spectroscopy as a new method for the evaluation of material degradation, we attempted to measure the CL spectra of various materials and systematically investigate the relationships between the CL spectra and the degradation structures determined by other methods. A spectral analysis of the emission species for a simple model compound is expected to be helpful in developing a general analysis method for the degradation of more complicated compounds such as polymers. In this Letter, we investigated the oxidation reaction of an oleic acid, one of the simplest monounsaturated fatty acids, in which the oxidation reaction is assumed to occur in the vicinity of only one doublebond. A detailed analysis of the structures of the degradation products was performed using nuclear magnetic resonance (NMR) and IR spectroscopy. A speculation was given for the oxidative degradation mechanism of oleic acid by considering the structures of the degradation products and the CL spectra.
2. Experimental 2.1. Chemiluminescence spectrum measurement Oleic acid (purity greater than 99.0%, biochemistry grade; Wako Pure Chemical Industries, Osaka, Japan) was used without further purification. The pre-oxidative samples were used for CL spectral measurements. The pre-oxidation of the oleic acid was conducted under air at 150 °C (oil bath) for different periods in a glass apparatus equipped with a magnetic stirrer. For each CL measurement, 300 lL of oleic acid was placed in an aluminum pan. The spectra were
T. Sago et al. / Chemical Physics Letters 565 (2013) 138–142
139
Figure 1. CL spectra of oleic acid degraded by heating at 150 or 180 °C under air: (A) raw spectra with different heating times, (B) their normalized spectra, and (C) peak separation of CL spectrum (sample heated at 150 °C for 780 min) by least-squares fitting using Voigt function.
recorded on a multichannel FT-CL spectrometer (MS-8310, Japan Applied Technology Inc.) under air, with heating at 150 or 180 °C. The interferogram was accumulated for 30 min to obtain a high signal-to-noise ratio, and then converted to the corresponding spectrum by Fourier transformation. After the abscissa of spectrum was converted from wavelength into wavenumber, the peak separation was carried out by the Fityk program [13] using Voigt functions1, and then the abscissa of spectrum was reconverted into wavelength. Unfortunately, automatic separation was difficult due to the broad shape of the spectra; the software divided the spectra into a couple of broad peaks, although the sharp peaks of singlet oxygen (1O2) essentially should be observed. On the other hand, when the spectral pattern was examined carefully, seven shoulders were observed at specific positions in every spectrum with high reproducibility. Peak separation also can be performed by estimating the number of components from the spectral shape [14]. Therefore, the fitting were carried out under conditions that the spectra consist of seven components. Some of the resulting separated peaks were reasonably assigned to the emission of 1O2 as mentioned in Section 3.
using an FT-IR spectrometer (Nicolet 6700, Thermo Fisher Scientific Inc.) equipped with a diamond attenuated total reflectance (ATR) unit. All of the samples were placed directly on the ATR crystal. The 1H NMR and 1H–1H correlation spectroscopy (COSY) spectra were recorded on a Bruker UltraShield 400 Magnet NMR spectrometer. A sample was dissolved in chloroform-d and tetramethylsilane was used as an internal reference.
2.2. IR and NMR measurements The IR and NMR spectra of the sample were measured after preoxidation and CL measurement. The IR spectra were measured 1 The validity of fitting using the Voigt function was verified in the case of the decomposition of hydrogen peroxide (our data). The emissions from singlet oxygen were fitted using Voigt and Gaussian functions. The standard deviation showed that the Voigt function better represented the CL spectra than the Gaussian function did. Therefore, the Voigt function was used for peak separation in this study.
Figure 2. IR spectra of oleic acid degraded by heating at 150 °C under air.
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Figure 3. 1H and 1H–1H COSY NMR spectra of oleic acid degraded by heating at 150 °C under air for 855 min.
3. Results and discussion 3.1. CL spectra of oxidized oleic acid Figure 1 shows the CL spectra of oleic acid obtained while heating under air. A broad emission signal was observed at 400– 850 nm. The intensity increased with increasing heating time (Figure 1A), and the shapes were almost unchanged during heating (Figure 1B). These results showed that the amounts of degradation products that could emit light increased according to the heating time. The spectrum, which obtained by heating for 780 min at 150 °C, fitted using the sum of seven Voigt functions was shown in Figure 1C. The wavelengths of the separated peaks were 450, 490, 550, 590, 640, 690, and 760 nm. Based on Russell’s report, it was expected that some of the peaks in the observed spectrum could be ascribed to 1O2 [15]. He proposed a degradation mechanism for an organic compound by oxidation, in which peroxy radicals are produced by the reaction of carbon radicals with oxygen, after which excited carbonyl and 1O2 are generated by the decomposition of the peroxy radicals. In the case of 1O2, the emissions are observed at several specific wavelengths as a result of the energy levels of the monomol or dimol 1O2. The main emissions of 1O2 have been reported as follows: the monomol emission, i.e.,
Figure 4. Oxidative structures of oleic acid determined by NMR. The numbers on the carbons correspond to the resonances in the NMR spectrum shown in Figure 3.
T. Sago et al. / Chemical Physics Letters 565 (2013) 138–142
(1Dg) ? (3Rg ), is at 1270 nm, and the dimol emissions, i.e., (1Dg)(1Dg) ? (3Rg )(3Rg ) are at 578, 633, and 703 nm from different vibration-band transitions [16,17]. Thus, in the obtained CL spectra, the emission components observed at 590, 640, and 690 nm might have been emitted from the dimol of 1O2. However, it is inadequate to represent the spectra using only the sum of the oxygen emissions. It was speculated that the short wavelength components were emissions from excited carbonyl compounds. Therefore, the degradation structure was analyzed using IR and NMR to determine the relationship between the emission wavelength and the structure of the emission species. 3.2. Structure of oxidized oleic acid determined by IR and NMR spectroscopies The IR spectra of oleic acid after heat treatment, shown in Figure 2, indicated the formation of functional groups as a result of oxidation. The new peaks observed at 1169 and 1735 cm 1 could be assigned to the C–O (stretching) and C@O (stretching), respectively, of the carbonyl groups newly generated by oxidation. The peaks observed at 690 and 966 cm 1 could also be assigned to the @C–H (out of plane bending) of cis and trans isomers, respectively [18,19]. An attempt was then made to clarify the chemical structure of the oxidative products in detail, using NMR spectroscopy. The 1H NMR spectrum of degraded oleic acid (heated at 150 °C for 855 min) shown in Figure 3 shows that the new resonances appeared at the C@C double-bond region (5–7 ppm) and oxygenbinding region (around 4 ppm) after heating. The 1H–1H COSY spectrum of the oleic acid revealed a resonance at 6.84 ppm (C@C double-bond region) coupled with those at 6.11 ppm (double-bond next to a carbonyl group) and 2.20 ppm (methylene). Thus, an a,b-unsaturated carbonyl structure (Figure 4a) was suggested. In addition, the resonance at around 5.5 ppm (C@C double-bond), coupled with those at 3.08 ppm (CH2 next to a carbonyl group) and around 2.0 ppm (methylene), can be assigned to a b,c-unsaturated carbonyl compound (Figure 4b). The resonance at 4.06 ppm coupled with those at 5.64 ppm (C@C doublebond) and 1.50 ppm (methylene) can be assigned to an alcohol or an ester (Figure 4c). The oxidative products of oleic acid that are shown are not inconsistent with the IR results. These degradation structures can be explained in terms of the Russell mechanism, which suggests that the carbonyl group, alcohol, and oxygen could be generated by oxidation [15]. In the Russell mechanism, it is also known that the excited carbonyl and the 1O2 are emission species [12]. 3.3. Assignment of CL spectra It has previously been reported by Wenli et al. [20] that excited carbonyl groups emit light at around 400–500 nm. In the present CL spectra, two peaks appear at around 450 and 490 nm. The absorption and emission peaks of unsaturated carbonyl compounds differ from those of saturated carbonyl compounds in photoluminescent studies [21–23], and it was reported that oxidized polybutadiene exhibited two photoluminescence peaks, at 475 and 520 nm, for saturated aldehyde and the a,b-unsaturated carbonyl compounds, respectively [21]. A similar peak shift caused by the structure of the emission species is also expected to occur in CL measurements. In the present spectra, it might be possible to assign the small signals at around 450 and 490 nm to b,c- and a,b-unsaturated carbonyl compounds, respectively. These emissions assignable to carbonyl compounds are relatively weak compared to the total emission amounts. This weakness could be attributed in part to the yield of the excited carbonyl that could emit light. Peroxy compounds decompose into
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products with various energy levels. Mendenhall et al. [24,25] suggested that the yield of the excited carbonyl (triplet) is much lower than that of 1O2 in the reaction of an alkyl hydroperoxide. Another reason might be related to the difference between the quantum yields of the emissions of the carbonyl group and 1O2. On the other hand, a couple of non-assignable emission peaks overlap with the emission peaks of 1O2 and carbonyl compounds. Although the details are not clear at present, this extra emission might be assigned to a complex formed between an excited carbonyl group and a ground state carbonyl group (similar to excimer). Because of the stabilization, emission of the complex is expected to shift to longer wavelength than that of monomeric species. For a more detailed discussion, we will conduct further investigations, including theoretical calculations of the peak shift caused by the structure of the emission species and the development of a higher-resolution spectrometer. We believe that the development of a CL analysis method will assist in the evaluation of organic material degradation by providing information about how oxidation occurs, along with an understanding of the precise structure of the excited intermediate product. 4. Conclusions To elucidate the emission species in organic material oxidation, CL spectroscopy was used to investigate oleic acid. A broad CL spectrum with several shoulders was observed and separated into seven components. A detailed analysis of the CL spectra and IR and NMR measurements suggested the structures of the oxidative products, as well as the plausible emission species, i.e., 1O2 and two excited carbonyl compounds. The separated peaks of the CL spectra at 590, 640 and 690 nm were assigned to dimol 1O2 with different vibration band transitions. It was also suggested that the separated peaks at 450 and 490 nm were emissions from different excited carbonyls, namely the b,c- and a,b-unsaturated carbonyl compounds. The results implied that the structure of an excited compound affects the emission wavelength. It was also found that the spectra contained signals from other emission species, which might be attributed to formation of a complex formed between an excited carbonyl group and a ground state carbonyl group. A better understanding of CL spectroscopy must be obtained to assist the further development of a material degradation analysis technique. References [1] K. Jacobson, Polym. Degrad. Stab. 91 (2006) 2126. [2] B.G.S. Goss, I. Blakey, M.D. Barry, G.A. George, Polym. Degrad. Stab. 74 (2001) 523. [3] X. Zhong, F. Yoshii, T. Sasaki, T. Yagi, K. Makuuchi, Polym. Degrad. Stab. 51 (1996) 159. [4] S. Jipa, Z. Osawa, H. Otsuki, M. Nishimoto, Polym. Degrad. Stab. 56 (1997) 45. [5] A. Kron, T. Reitberger, B. Stenberg, Polym. Int. 42 (1997) 131. [6] Z. Osawa, M. Someya, Y.S. Fu, F. Konoma, Polym. Degrad. Stab. 43 (1994) 461. [7] S. Hosoda, Y. Seki, H. Kihara, Polymer 34 (1993) 4602. [8] P. Cerruti, C. Carfagna, J. Rychly, L. Matisova-Rychla, Polym. Degrad. Stab. 82 (2003) 477. [9] B. Schartel, M. Hennecke, Polym. Degrad. Stab. 67 (2000) 249. [10] G.S. Timmins, R.E. dos Santos, A.C. Whitwood, L.H. Catalani, P.D. Mascio, B.C. Gilbert, E.J.H. Bechara, Chem. Res. Toxicol. 10 (1997) 1090. [11] T. Karakisawa, T. Yamada, M. Sekine, H. Ishii, C. Satoh, K.R. Millington, M. Nakata, Luminescence 27 (2012) 362. [12] T. Hironiwa, T. Yamada, H. Ishii, C. Satoh, K.R. Millington, M. Nakata, Polym. J. 44 (2012) 1015. [13] M. Wojdyr, J. Appl. Crystallogr. 43 (2010) 1126. [14] Y. Rodriguez-Lazcano, V. Correcher, J. Garcia-Guinea, Radiat. Phys. Chem. 81 (2012) 126. [15] G.A. Russell, J. Am. Chem. Soc. 79 (1957) 3871. [16] A.U. Khan, M. Kasha, J. Chem. Phys. 39 (1963) 2105. [17] A.U. Khan, M. Kasha, J. Am. Chem. Soc. 92 (1970) 3293. [18] A. Rohman, Y.B. Che Man, Vib. Spectrosc. 55 (2011) 141.
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[19] N.E. Maurer, B. Hatta-Sakoda, G. Pascual-Chagman, L.E. Rodriguez-Saona, Food Chem. 134 (2012) 1173. [20] Y. Wenli, Z. Yaping, Biochem. Biophys. Acta 1725 (2005) 30. [21] N.S. Allen, J. Homer, J.F. Mckellar, J. Appl. Polym. Sci. 21 (1977) 2261. [22] S.W. Beavan, P.A. Hackett, D. Phillips, Polym. J. 10 (1974) 925.
[23] P. Tiemblo, J.M. Gomez-Elvira, G. Teyssedre, F. Massines, C. Laurent, Polym. Degrad. Stab. 65 (1999) 113. [24] Q. Niu, G.D. Mendenhall, J. Am. Chem. Soc. 112 (1990) 1656. [25] G.D. Mendenhall, X.C. Sheng, J. Am. Chem. Soc. 113 (1991) 8976.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Analysis of chemiluminescence spectra in oxidative degradation of oleic acid Tomohiro Sago a, Hiroshi Ishii b, Hideaki Hagihara a,⇑, Noriyuki Takada a, Hiroyuki Suda a a b
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Japan Applied Technology Inc., 1053-1 Yaho, Kunitachi, Tokyo 186-0011, Japan
a r t i c l e
i n f o
Article history: Received 26 December 2012 In final form 16 February 2013 Available online 26 February 2013
a b s t r a c t We studied chemiluminescence spectroscopy to develop an evaluation method for organic material degradation. The thermal oxidation of oleic acid as a monounsaturated fatty acid was investigated using chemiluminescence measurements. The obtained spectra indicated that the luminescence consisted of several peaks, including peaks for singlet oxygen and excited carbonyls. A detailed analysis of the nuclear magnetic resonance and infrared absorption spectra allowed us to determine the probable structures of the oxidative products. The relationship between the spectrum and the analyzed structure implied that the wavelength of the carbonyl group emission peak shifted as a result of the structure. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Chemiluminescence (CL) is known to be a relatively strong emission resulting from chemical reactions such as the luminol reaction and firefly luminescence. Contemporary CL measurement systems are able to detect very weak emissions from various organic reactions like oxidation. Several investigations of polymer degradation using CL have been reported [1–9]. On the other hand, as an evaluation method for material degradation, infrared (IR) spectroscopy has mainly been used. IR spectroscopy can be used to investigate the amount of the carbonyl groups generated by oxidation, i.e., the carbonyl index (CI), which provides a good indication of how far oxidation has proceeded. Osawa et al. reported that CL measurement could be used to observe the emission at the very early stage of oxidation, where IR spectroscopy could not detect any CI changes [6]. Therefore, CL has been used as a highly sensitive method for evaluating degradation [1–9]. However, the conventional CL technique can only be used to observe the emission intensity. CL spectroscopy is expected to elucidate the degradation mechanism at an early stage if an analysis method for the CL spectrum can be established. It was reported that a CL measurement system equipped with band-pass filters could obtain the spectral data of polymer materials, although filter replacement was necessary during the measurement and the measurement required a long acquisition time [10]. Recently, a Fourier-transform (FT) CL spectrometer with a Savart-plate polarization interferometer and CCD area image sensor has been developed. This makes it possible to obtain the CL spectrum quickly [11,12]. The obtained CL spectrum contains information not only about the intensity but also about the
⇑ Corresponding author. E-mail address: [email protected] (H. Hagihara). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.02.035
emission wavelength. Hence, the spectral behavior with a change in the chemical structure and/or degradation conditions has been discussed [12]. However, the relationship between the structure of the emission species and the emission wavelength is still an open problem. To establish CL spectroscopy as a new method for the evaluation of material degradation, we attempted to measure the CL spectra of various materials and systematically investigate the relationships between the CL spectra and the degradation structures determined by other methods. A spectral analysis of the emission species for a simple model compound is expected to be helpful in developing a general analysis method for the degradation of more complicated compounds such as polymers. In this Letter, we investigated the oxidation reaction of an oleic acid, one of the simplest monounsaturated fatty acids, in which the oxidation reaction is assumed to occur in the vicinity of only one doublebond. A detailed analysis of the structures of the degradation products was performed using nuclear magnetic resonance (NMR) and IR spectroscopy. A speculation was given for the oxidative degradation mechanism of oleic acid by considering the structures of the degradation products and the CL spectra.
2. Experimental 2.1. Chemiluminescence spectrum measurement Oleic acid (purity greater than 99.0%, biochemistry grade; Wako Pure Chemical Industries, Osaka, Japan) was used without further purification. The pre-oxidative samples were used for CL spectral measurements. The pre-oxidation of the oleic acid was conducted under air at 150 °C (oil bath) for different periods in a glass apparatus equipped with a magnetic stirrer. For each CL measurement, 300 lL of oleic acid was placed in an aluminum pan. The spectra were
T. Sago et al. / Chemical Physics Letters 565 (2013) 138–142
139
Figure 1. CL spectra of oleic acid degraded by heating at 150 or 180 °C under air: (A) raw spectra with different heating times, (B) their normalized spectra, and (C) peak separation of CL spectrum (sample heated at 150 °C for 780 min) by least-squares fitting using Voigt function.
recorded on a multichannel FT-CL spectrometer (MS-8310, Japan Applied Technology Inc.) under air, with heating at 150 or 180 °C. The interferogram was accumulated for 30 min to obtain a high signal-to-noise ratio, and then converted to the corresponding spectrum by Fourier transformation. After the abscissa of spectrum was converted from wavelength into wavenumber, the peak separation was carried out by the Fityk program [13] using Voigt functions1, and then the abscissa of spectrum was reconverted into wavelength. Unfortunately, automatic separation was difficult due to the broad shape of the spectra; the software divided the spectra into a couple of broad peaks, although the sharp peaks of singlet oxygen (1O2) essentially should be observed. On the other hand, when the spectral pattern was examined carefully, seven shoulders were observed at specific positions in every spectrum with high reproducibility. Peak separation also can be performed by estimating the number of components from the spectral shape [14]. Therefore, the fitting were carried out under conditions that the spectra consist of seven components. Some of the resulting separated peaks were reasonably assigned to the emission of 1O2 as mentioned in Section 3.
using an FT-IR spectrometer (Nicolet 6700, Thermo Fisher Scientific Inc.) equipped with a diamond attenuated total reflectance (ATR) unit. All of the samples were placed directly on the ATR crystal. The 1H NMR and 1H–1H correlation spectroscopy (COSY) spectra were recorded on a Bruker UltraShield 400 Magnet NMR spectrometer. A sample was dissolved in chloroform-d and tetramethylsilane was used as an internal reference.
2.2. IR and NMR measurements The IR and NMR spectra of the sample were measured after preoxidation and CL measurement. The IR spectra were measured 1 The validity of fitting using the Voigt function was verified in the case of the decomposition of hydrogen peroxide (our data). The emissions from singlet oxygen were fitted using Voigt and Gaussian functions. The standard deviation showed that the Voigt function better represented the CL spectra than the Gaussian function did. Therefore, the Voigt function was used for peak separation in this study.
Figure 2. IR spectra of oleic acid degraded by heating at 150 °C under air.
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T. Sago et al. / Chemical Physics Letters 565 (2013) 138–142
Figure 3. 1H and 1H–1H COSY NMR spectra of oleic acid degraded by heating at 150 °C under air for 855 min.
3. Results and discussion 3.1. CL spectra of oxidized oleic acid Figure 1 shows the CL spectra of oleic acid obtained while heating under air. A broad emission signal was observed at 400– 850 nm. The intensity increased with increasing heating time (Figure 1A), and the shapes were almost unchanged during heating (Figure 1B). These results showed that the amounts of degradation products that could emit light increased according to the heating time. The spectrum, which obtained by heating for 780 min at 150 °C, fitted using the sum of seven Voigt functions was shown in Figure 1C. The wavelengths of the separated peaks were 450, 490, 550, 590, 640, 690, and 760 nm. Based on Russell’s report, it was expected that some of the peaks in the observed spectrum could be ascribed to 1O2 [15]. He proposed a degradation mechanism for an organic compound by oxidation, in which peroxy radicals are produced by the reaction of carbon radicals with oxygen, after which excited carbonyl and 1O2 are generated by the decomposition of the peroxy radicals. In the case of 1O2, the emissions are observed at several specific wavelengths as a result of the energy levels of the monomol or dimol 1O2. The main emissions of 1O2 have been reported as follows: the monomol emission, i.e.,
Figure 4. Oxidative structures of oleic acid determined by NMR. The numbers on the carbons correspond to the resonances in the NMR spectrum shown in Figure 3.
T. Sago et al. / Chemical Physics Letters 565 (2013) 138–142
(1Dg) ? (3Rg ), is at 1270 nm, and the dimol emissions, i.e., (1Dg)(1Dg) ? (3Rg )(3Rg ) are at 578, 633, and 703 nm from different vibration-band transitions [16,17]. Thus, in the obtained CL spectra, the emission components observed at 590, 640, and 690 nm might have been emitted from the dimol of 1O2. However, it is inadequate to represent the spectra using only the sum of the oxygen emissions. It was speculated that the short wavelength components were emissions from excited carbonyl compounds. Therefore, the degradation structure was analyzed using IR and NMR to determine the relationship between the emission wavelength and the structure of the emission species. 3.2. Structure of oxidized oleic acid determined by IR and NMR spectroscopies The IR spectra of oleic acid after heat treatment, shown in Figure 2, indicated the formation of functional groups as a result of oxidation. The new peaks observed at 1169 and 1735 cm 1 could be assigned to the C–O (stretching) and C@O (stretching), respectively, of the carbonyl groups newly generated by oxidation. The peaks observed at 690 and 966 cm 1 could also be assigned to the @C–H (out of plane bending) of cis and trans isomers, respectively [18,19]. An attempt was then made to clarify the chemical structure of the oxidative products in detail, using NMR spectroscopy. The 1H NMR spectrum of degraded oleic acid (heated at 150 °C for 855 min) shown in Figure 3 shows that the new resonances appeared at the C@C double-bond region (5–7 ppm) and oxygenbinding region (around 4 ppm) after heating. The 1H–1H COSY spectrum of the oleic acid revealed a resonance at 6.84 ppm (C@C double-bond region) coupled with those at 6.11 ppm (double-bond next to a carbonyl group) and 2.20 ppm (methylene). Thus, an a,b-unsaturated carbonyl structure (Figure 4a) was suggested. In addition, the resonance at around 5.5 ppm (C@C double-bond), coupled with those at 3.08 ppm (CH2 next to a carbonyl group) and around 2.0 ppm (methylene), can be assigned to a b,c-unsaturated carbonyl compound (Figure 4b). The resonance at 4.06 ppm coupled with those at 5.64 ppm (C@C doublebond) and 1.50 ppm (methylene) can be assigned to an alcohol or an ester (Figure 4c). The oxidative products of oleic acid that are shown are not inconsistent with the IR results. These degradation structures can be explained in terms of the Russell mechanism, which suggests that the carbonyl group, alcohol, and oxygen could be generated by oxidation [15]. In the Russell mechanism, it is also known that the excited carbonyl and the 1O2 are emission species [12]. 3.3. Assignment of CL spectra It has previously been reported by Wenli et al. [20] that excited carbonyl groups emit light at around 400–500 nm. In the present CL spectra, two peaks appear at around 450 and 490 nm. The absorption and emission peaks of unsaturated carbonyl compounds differ from those of saturated carbonyl compounds in photoluminescent studies [21–23], and it was reported that oxidized polybutadiene exhibited two photoluminescence peaks, at 475 and 520 nm, for saturated aldehyde and the a,b-unsaturated carbonyl compounds, respectively [21]. A similar peak shift caused by the structure of the emission species is also expected to occur in CL measurements. In the present spectra, it might be possible to assign the small signals at around 450 and 490 nm to b,c- and a,b-unsaturated carbonyl compounds, respectively. These emissions assignable to carbonyl compounds are relatively weak compared to the total emission amounts. This weakness could be attributed in part to the yield of the excited carbonyl that could emit light. Peroxy compounds decompose into
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products with various energy levels. Mendenhall et al. [24,25] suggested that the yield of the excited carbonyl (triplet) is much lower than that of 1O2 in the reaction of an alkyl hydroperoxide. Another reason might be related to the difference between the quantum yields of the emissions of the carbonyl group and 1O2. On the other hand, a couple of non-assignable emission peaks overlap with the emission peaks of 1O2 and carbonyl compounds. Although the details are not clear at present, this extra emission might be assigned to a complex formed between an excited carbonyl group and a ground state carbonyl group (similar to excimer). Because of the stabilization, emission of the complex is expected to shift to longer wavelength than that of monomeric species. For a more detailed discussion, we will conduct further investigations, including theoretical calculations of the peak shift caused by the structure of the emission species and the development of a higher-resolution spectrometer. We believe that the development of a CL analysis method will assist in the evaluation of organic material degradation by providing information about how oxidation occurs, along with an understanding of the precise structure of the excited intermediate product. 4. Conclusions To elucidate the emission species in organic material oxidation, CL spectroscopy was used to investigate oleic acid. A broad CL spectrum with several shoulders was observed and separated into seven components. A detailed analysis of the CL spectra and IR and NMR measurements suggested the structures of the oxidative products, as well as the plausible emission species, i.e., 1O2 and two excited carbonyl compounds. The separated peaks of the CL spectra at 590, 640 and 690 nm were assigned to dimol 1O2 with different vibration band transitions. It was also suggested that the separated peaks at 450 and 490 nm were emissions from different excited carbonyls, namely the b,c- and a,b-unsaturated carbonyl compounds. The results implied that the structure of an excited compound affects the emission wavelength. It was also found that the spectra contained signals from other emission species, which might be attributed to formation of a complex formed between an excited carbonyl group and a ground state carbonyl group. A better understanding of CL spectroscopy must be obtained to assist the further development of a material degradation analysis technique. References [1] K. Jacobson, Polym. Degrad. Stab. 91 (2006) 2126. [2] B.G.S. Goss, I. Blakey, M.D. Barry, G.A. George, Polym. Degrad. Stab. 74 (2001) 523. [3] X. Zhong, F. Yoshii, T. Sasaki, T. Yagi, K. Makuuchi, Polym. Degrad. Stab. 51 (1996) 159. [4] S. Jipa, Z. Osawa, H. Otsuki, M. Nishimoto, Polym. Degrad. Stab. 56 (1997) 45. [5] A. Kron, T. Reitberger, B. Stenberg, Polym. Int. 42 (1997) 131. [6] Z. Osawa, M. Someya, Y.S. Fu, F. Konoma, Polym. Degrad. Stab. 43 (1994) 461. [7] S. Hosoda, Y. Seki, H. Kihara, Polymer 34 (1993) 4602. [8] P. Cerruti, C. Carfagna, J. Rychly, L. Matisova-Rychla, Polym. Degrad. Stab. 82 (2003) 477. [9] B. Schartel, M. Hennecke, Polym. Degrad. Stab. 67 (2000) 249. [10] G.S. Timmins, R.E. dos Santos, A.C. Whitwood, L.H. Catalani, P.D. Mascio, B.C. Gilbert, E.J.H. Bechara, Chem. Res. Toxicol. 10 (1997) 1090. [11] T. Karakisawa, T. Yamada, M. Sekine, H. Ishii, C. Satoh, K.R. Millington, M. Nakata, Luminescence 27 (2012) 362. [12] T. Hironiwa, T. Yamada, H. Ishii, C. Satoh, K.R. Millington, M. Nakata, Polym. J. 44 (2012) 1015. [13] M. Wojdyr, J. Appl. Crystallogr. 43 (2010) 1126. [14] Y. Rodriguez-Lazcano, V. Correcher, J. Garcia-Guinea, Radiat. Phys. Chem. 81 (2012) 126. [15] G.A. Russell, J. Am. Chem. Soc. 79 (1957) 3871. [16] A.U. Khan, M. Kasha, J. Chem. Phys. 39 (1963) 2105. [17] A.U. Khan, M. Kasha, J. Am. Chem. Soc. 92 (1970) 3293. [18] A. Rohman, Y.B. Che Man, Vib. Spectrosc. 55 (2011) 141.
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[19] N.E. Maurer, B. Hatta-Sakoda, G. Pascual-Chagman, L.E. Rodriguez-Saona, Food Chem. 134 (2012) 1173. [20] Y. Wenli, Z. Yaping, Biochem. Biophys. Acta 1725 (2005) 30. [21] N.S. Allen, J. Homer, J.F. Mckellar, J. Appl. Polym. Sci. 21 (1977) 2261. [22] S.W. Beavan, P.A. Hackett, D. Phillips, Polym. J. 10 (1974) 925.
[23] P. Tiemblo, J.M. Gomez-Elvira, G. Teyssedre, F. Massines, C. Laurent, Polym. Degrad. Stab. 65 (1999) 113. [24] Q. Niu, G.D. Mendenhall, J. Am. Chem. Soc. 112 (1990) 1656. [25] G.D. Mendenhall, X.C. Sheng, J. Am. Chem. Soc. 113 (1991) 8976.