Letter - spectral assignment Received: 11 November 2013
Revised: 5 March 2014
Accepted: 11 March 2014
Published online in Wiley Online Library: 7 April 2014
(wileyonlinelibrary.com) DOI 10.1002/mrc.4070
EPR spectrum of a radical from a nontypical antioxidant Carolina Aliagaa,b* and Marcos C. Rezendea Introduction Isobenzofuranone HP-136 (5,7-ditert-butyl-3-(3,4-dimethylphenyl) benzofuran-2(3H)-one) is a commercially available product, which has found applications as a good antioxidant in high-temperature polymerization processes, also showing less solvent dependence than common phenolic antioxidants.[1] H-abstraction from this compound by radicals leads to the formation of the carboncentered radical 1 (Scheme 1), which shows a surprisingly low reactivity toward oxygen.[2] This unexpected low reactivity has been explained in terms of the low electron spin density on its benzyl carbon atom. Molecules that exhibit such a low reactivity have been shown to develop a maximum of 50% of the total molecular electron spin density on the carbon centered radical.[3] Radical 1 meets a few criteria, which are believed to be responsible for its unusually low reactivity towards oxygen, such as benzylic resonance stabilization and extensive spin delocalization by heteroatoms and aromatic rings. In addition, the rate of dimerization of 1 was found to be higher than of its reaction with oxygen, its dimer being also an excellent antioxidant at relatively high temperatures (40–50 C).[4] Some controversy is found in the literature regarding the actual species that gives origin to radical 1 and whether hydrogen abstraction occurs from a keto – or from an enolic tautomer of HP-136.[5,6] Whichever is its precursor, in both cases, radical 1 is postulated as the final product from the hydrogen-abstraction process. Evidence for its existence has been derived from the UV-vis spectrum of this transient species, with a band at 530 nm, characteristic of a benzyl radical, produced when a solution of this isobenzofuranone is irradiated in the presence of di-tert-butyl peroxide.[2] In spite of the applications of HP-136 and of the interest generated by the radical formed from it,[5–8] it is a bit surprising that direct and unequivocal evidence of radical 1, provided by its electron paramagnetic resonance (EPR) spectrum, is not yet available. In this work, we therefore describe for the first time the EPR spectrum of radical 1, analyzing its electron spin density distribution, in relation to its characteristically low reactivity.
photolysed at room temperature for 15 min in a 3-mm-diameter quartz tube in the cavity of the spectrometer, employing a mercury lamp with a cutoff filter of 550 nm. At the end of the reaction time, the EPR spectrum of the persistent radical product 1 was recorded. The experimental EPR parameters were frequency, 9.466297 GHz; microwave power, 1 mW; modulation amplitude, 0.1 G; time constant, 5.12 ms; and conversion time, 20.97 ms. The theoretical calculations (geometrical optimization and electron spin densities) were performed with the B3LYP/6-31G (d) method available in the Gaussian 09 package.[9]
Results and Discussion Radical 1 was obtained by the photosensitized oxidation of a basic ethanolic solution of HP-136 by eosin.[10,11] Total consumption of the substrate was evident from the UV-vis spectra of the mixture, recorded in the course of the reaction (Fig. 1). Because of its low concentration, formation of radical 1 could not be detected in the UV-vis spectra of the mixture but was clearly evident from the clean EPR spectrum recorded at the end of the reaction, which is shown in Fig. 2(a). No radical species was detected when the EPR spectrum of a blank sample of irradiated eosin in basic ethanol was recorded. This was a final indication that the radical species detected at the end of the photosensitized reaction was 1. Figure 2(b) reproduces the simulated EPR spectrum of radical 1, employing parameters listed in Table 1. Optimization of the structure of radical 1 by the B3LYP/6-31G (d) method yielded electron spin density values that allowed a good assignment of the hyperfine coupling constants. The rather low electron spin density on the benzyl carbon atom of 1 (0.49) was in agreement with the poor reactivity of this radical with O2. As can be gathered from the calculated electron spin densities, the unpaired electron is very delocalized in the molecule, being shared by the benzylic carbon atom with the neighboring carbonyl group and with all aromatic carbon atoms ortho and para to it (Fig. 3).
Experimental
Magn. Reson. Chem. 2014, 52, 409–411
* Correspondence to: Carolina Aliaga, Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile. E-mail: carolina. [email protected] a Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile b Centro para el Desarrollo de la Nanociencia y la Nanotecnología (CEDENNA), Universidad de Santiago de Chile, Santiago, Chile
Copyright © 2014 John Wiley & Sons, Ltd.
409
EPR spectra were obtained with a Bruker EMX-1572 spectrometer, operating at the X band. The sodium salt of eosin B (Aldrich) was used without any further purification. Irganox® HP-136 was supplied by Ciba Specialty Chemicals Inc. HP-136 (0.5 mM) and eosin B (27 μM) were dissolved in 1 ml of a 0.1-M NaOH ethanolic solution. The resulting solution was
C. Aliaga and M. C. Rezende Table 1. Hyperfine coupling constants aH used for the simulation of the EPR spectrum of 1, with the corresponding calculated electron spin densities on the neighboring carbon atoms Hydrogen atoma H-1 {1} H-2 {1} H-3 {3} H-4 {1} H-5 {1}
aH, gauss
Calculated electron spin densityb
2.6 0.35 0.32 0.27 0.23
0.20 0.17 0.14 0.13 0.12
Scheme 1. Formation of radical 1 from antioxidant HP-136. a
As numbered in Fig. 3 (right), with the number of hydrogen atoms in brackets. b Calculated densities on neighboring carbon atoms, as depicted in Fig. 3 (left).
Figure 1. Variation of the UV-vis spectra of a mixture of HP-136 (0.5 mM) and eosin (27 μM) in a 0.1-M NaOH ethanolic solution irradiated for 15 min with an Hg lamp provided with a 550-nm cutoff filter. The first intermediate curve was recorded 3 min after the beginning of the reaction and the following curves at 1-min intervals.
Figure 3. Graphical representation of the calculated electron spin densities on radical 1 (left), with an indication of the hydrogen atoms (right) associated with the hyperfine coupling constants used to simulate its EPR spectrum (Table 1). Relatively small carbon electron spin densities (0.12–0.2) are depicted as yellowish green, a high electron spin density (0.49) as blue. C–H bonds involved in the hyperfine coupling with aromatic carbon atoms have been explicitly drawn in the chemical structure.
As can be seen in the table, there is only a qualitative correlation between calculated electron spin densities (ρ) and aH values. The linear correlation between these parameters, anticipated by McConnell equation, is only crudely verified in our case. This equation works at its best with fully planar polycyclic aromatic systems such as naphthalene, anthracene, fluorenide, and carbazolide radicals.[12,13] This is not exactly what happens to radical 1, leading to the expectation that a correlation between aH and ρ should be only qualitative. In conclusion, in the present communication, we recorded for the first time the EPR spectrum of the benzylic radical 1, generated from the commercially available isobenzofuranone HP-136, a valuable antioxidant in high-temperature polymerization processes. Its existence had been postulated by different groups before and indirectly detected by its transient UV-vis spectrum.[2] The rationalization of its low reactivity toward O2, in terms of its highly delocalized electron spin density, finds now support in the analysis of its EPR spectrum and on theoretical calculations that were in agreement with the hyperfine coupling constants employed for the simulated spectrum. Acknowledgement This work was financed by FONDECYT project 1110736.
References
410
Figure 2. Experimental (a) and simulated (b) EPR spectra of radical 1, assuming the hyperfine coupling constants and multiplicities listed in Table 1.
wileyonlinelibrary.com/journal/mrc
[1] V. Filippenko, M. Frenette, J. C. Sacaiano. Org. Lett. 2009, 16, 3634–3637. [2] J. C. Scaiano, A. Martin, G. P. A. Yap, J. U. Ingold. Org. Lett. 2000, 2, 899–901.
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2014, 52, 409–411
EPR spectrum of a radical from a nontypical antioxidant [3] E. V. Bejan, E. Font-Sanchis, J. C. Scaiano. Org. Lett. 2001, 3, 4059–4062. [4] M. Frenette, P. D. MacLean, L. R. C. Barclay, J. C. Scaiano. J. Am. Chem. Soc. 2006, 128, 16432–16433. [5] C. Aliaga, D. R. Stuart, A. Aspée, J. C. Scaiano. Org. Lett. 2005, 7, 3665–3668. [6] X.-Q. Zhu, J. Zhou, C.-H. Wang, X.-T. Li, S. Jing. J. Phys. Chem. B 2011, 115, 3588–3603. [7] C. Aliaga, A. Aspée, J. C. Scaiano. Org. Lett. 2003, 5, 4145–4148. [8] C. Aliaga, J. M. Juarez-Ruiz, J. C. Scaiano, A. Aspée. Org. Lett. 2008, 10, 2147–2150. [9] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
[10] [11] [12] [13]
Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, Revision D.01. Gaussian, Inc., Wallingford CT, 2009. I. H. Leaver. Aust. J. Chem. 1971, 24, 891–894. L. I. Gross Weiner, E. F. Zwicker. J. Chem. Phys. 1961, 34, 1411–1417. P. W. Atkins, Physical Chemistry, 3rd edn, Oxford University Press, Oxford, 1990, pp. 562. N. L. Bauld, J. H. Zoeller. Tetrahedron Lett. 1967, 8, 885–889.
411
Magn. Reson. Chem. 2014, 52, 409–411
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc
Revised: 5 March 2014
Accepted: 11 March 2014
Published online in Wiley Online Library: 7 April 2014
(wileyonlinelibrary.com) DOI 10.1002/mrc.4070
EPR spectrum of a radical from a nontypical antioxidant Carolina Aliagaa,b* and Marcos C. Rezendea Introduction Isobenzofuranone HP-136 (5,7-ditert-butyl-3-(3,4-dimethylphenyl) benzofuran-2(3H)-one) is a commercially available product, which has found applications as a good antioxidant in high-temperature polymerization processes, also showing less solvent dependence than common phenolic antioxidants.[1] H-abstraction from this compound by radicals leads to the formation of the carboncentered radical 1 (Scheme 1), which shows a surprisingly low reactivity toward oxygen.[2] This unexpected low reactivity has been explained in terms of the low electron spin density on its benzyl carbon atom. Molecules that exhibit such a low reactivity have been shown to develop a maximum of 50% of the total molecular electron spin density on the carbon centered radical.[3] Radical 1 meets a few criteria, which are believed to be responsible for its unusually low reactivity towards oxygen, such as benzylic resonance stabilization and extensive spin delocalization by heteroatoms and aromatic rings. In addition, the rate of dimerization of 1 was found to be higher than of its reaction with oxygen, its dimer being also an excellent antioxidant at relatively high temperatures (40–50 C).[4] Some controversy is found in the literature regarding the actual species that gives origin to radical 1 and whether hydrogen abstraction occurs from a keto – or from an enolic tautomer of HP-136.[5,6] Whichever is its precursor, in both cases, radical 1 is postulated as the final product from the hydrogen-abstraction process. Evidence for its existence has been derived from the UV-vis spectrum of this transient species, with a band at 530 nm, characteristic of a benzyl radical, produced when a solution of this isobenzofuranone is irradiated in the presence of di-tert-butyl peroxide.[2] In spite of the applications of HP-136 and of the interest generated by the radical formed from it,[5–8] it is a bit surprising that direct and unequivocal evidence of radical 1, provided by its electron paramagnetic resonance (EPR) spectrum, is not yet available. In this work, we therefore describe for the first time the EPR spectrum of radical 1, analyzing its electron spin density distribution, in relation to its characteristically low reactivity.
photolysed at room temperature for 15 min in a 3-mm-diameter quartz tube in the cavity of the spectrometer, employing a mercury lamp with a cutoff filter of 550 nm. At the end of the reaction time, the EPR spectrum of the persistent radical product 1 was recorded. The experimental EPR parameters were frequency, 9.466297 GHz; microwave power, 1 mW; modulation amplitude, 0.1 G; time constant, 5.12 ms; and conversion time, 20.97 ms. The theoretical calculations (geometrical optimization and electron spin densities) were performed with the B3LYP/6-31G (d) method available in the Gaussian 09 package.[9]
Results and Discussion Radical 1 was obtained by the photosensitized oxidation of a basic ethanolic solution of HP-136 by eosin.[10,11] Total consumption of the substrate was evident from the UV-vis spectra of the mixture, recorded in the course of the reaction (Fig. 1). Because of its low concentration, formation of radical 1 could not be detected in the UV-vis spectra of the mixture but was clearly evident from the clean EPR spectrum recorded at the end of the reaction, which is shown in Fig. 2(a). No radical species was detected when the EPR spectrum of a blank sample of irradiated eosin in basic ethanol was recorded. This was a final indication that the radical species detected at the end of the photosensitized reaction was 1. Figure 2(b) reproduces the simulated EPR spectrum of radical 1, employing parameters listed in Table 1. Optimization of the structure of radical 1 by the B3LYP/6-31G (d) method yielded electron spin density values that allowed a good assignment of the hyperfine coupling constants. The rather low electron spin density on the benzyl carbon atom of 1 (0.49) was in agreement with the poor reactivity of this radical with O2. As can be gathered from the calculated electron spin densities, the unpaired electron is very delocalized in the molecule, being shared by the benzylic carbon atom with the neighboring carbonyl group and with all aromatic carbon atoms ortho and para to it (Fig. 3).
Experimental
Magn. Reson. Chem. 2014, 52, 409–411
* Correspondence to: Carolina Aliaga, Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile. E-mail: carolina. [email protected] a Facultad de Química y Biología, Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile b Centro para el Desarrollo de la Nanociencia y la Nanotecnología (CEDENNA), Universidad de Santiago de Chile, Santiago, Chile
Copyright © 2014 John Wiley & Sons, Ltd.
409
EPR spectra were obtained with a Bruker EMX-1572 spectrometer, operating at the X band. The sodium salt of eosin B (Aldrich) was used without any further purification. Irganox® HP-136 was supplied by Ciba Specialty Chemicals Inc. HP-136 (0.5 mM) and eosin B (27 μM) were dissolved in 1 ml of a 0.1-M NaOH ethanolic solution. The resulting solution was
C. Aliaga and M. C. Rezende Table 1. Hyperfine coupling constants aH used for the simulation of the EPR spectrum of 1, with the corresponding calculated electron spin densities on the neighboring carbon atoms Hydrogen atoma H-1 {1} H-2 {1} H-3 {3} H-4 {1} H-5 {1}
aH, gauss
Calculated electron spin densityb
2.6 0.35 0.32 0.27 0.23
0.20 0.17 0.14 0.13 0.12
Scheme 1. Formation of radical 1 from antioxidant HP-136. a
As numbered in Fig. 3 (right), with the number of hydrogen atoms in brackets. b Calculated densities on neighboring carbon atoms, as depicted in Fig. 3 (left).
Figure 1. Variation of the UV-vis spectra of a mixture of HP-136 (0.5 mM) and eosin (27 μM) in a 0.1-M NaOH ethanolic solution irradiated for 15 min with an Hg lamp provided with a 550-nm cutoff filter. The first intermediate curve was recorded 3 min after the beginning of the reaction and the following curves at 1-min intervals.
Figure 3. Graphical representation of the calculated electron spin densities on radical 1 (left), with an indication of the hydrogen atoms (right) associated with the hyperfine coupling constants used to simulate its EPR spectrum (Table 1). Relatively small carbon electron spin densities (0.12–0.2) are depicted as yellowish green, a high electron spin density (0.49) as blue. C–H bonds involved in the hyperfine coupling with aromatic carbon atoms have been explicitly drawn in the chemical structure.
As can be seen in the table, there is only a qualitative correlation between calculated electron spin densities (ρ) and aH values. The linear correlation between these parameters, anticipated by McConnell equation, is only crudely verified in our case. This equation works at its best with fully planar polycyclic aromatic systems such as naphthalene, anthracene, fluorenide, and carbazolide radicals.[12,13] This is not exactly what happens to radical 1, leading to the expectation that a correlation between aH and ρ should be only qualitative. In conclusion, in the present communication, we recorded for the first time the EPR spectrum of the benzylic radical 1, generated from the commercially available isobenzofuranone HP-136, a valuable antioxidant in high-temperature polymerization processes. Its existence had been postulated by different groups before and indirectly detected by its transient UV-vis spectrum.[2] The rationalization of its low reactivity toward O2, in terms of its highly delocalized electron spin density, finds now support in the analysis of its EPR spectrum and on theoretical calculations that were in agreement with the hyperfine coupling constants employed for the simulated spectrum. Acknowledgement This work was financed by FONDECYT project 1110736.
References
410
Figure 2. Experimental (a) and simulated (b) EPR spectra of radical 1, assuming the hyperfine coupling constants and multiplicities listed in Table 1.
wileyonlinelibrary.com/journal/mrc
[1] V. Filippenko, M. Frenette, J. C. Sacaiano. Org. Lett. 2009, 16, 3634–3637. [2] J. C. Scaiano, A. Martin, G. P. A. Yap, J. U. Ingold. Org. Lett. 2000, 2, 899–901.
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2014, 52, 409–411
EPR spectrum of a radical from a nontypical antioxidant [3] E. V. Bejan, E. Font-Sanchis, J. C. Scaiano. Org. Lett. 2001, 3, 4059–4062. [4] M. Frenette, P. D. MacLean, L. R. C. Barclay, J. C. Scaiano. J. Am. Chem. Soc. 2006, 128, 16432–16433. [5] C. Aliaga, D. R. Stuart, A. Aspée, J. C. Scaiano. Org. Lett. 2005, 7, 3665–3668. [6] X.-Q. Zhu, J. Zhou, C.-H. Wang, X.-T. Li, S. Jing. J. Phys. Chem. B 2011, 115, 3588–3603. [7] C. Aliaga, A. Aspée, J. C. Scaiano. Org. Lett. 2003, 5, 4145–4148. [8] C. Aliaga, J. M. Juarez-Ruiz, J. C. Scaiano, A. Aspée. Org. Lett. 2008, 10, 2147–2150. [9] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.
[10] [11] [12] [13]
Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09, Revision D.01. Gaussian, Inc., Wallingford CT, 2009. I. H. Leaver. Aust. J. Chem. 1971, 24, 891–894. L. I. Gross Weiner, E. F. Zwicker. J. Chem. Phys. 1961, 34, 1411–1417. P. W. Atkins, Physical Chemistry, 3rd edn, Oxford University Press, Oxford, 1990, pp. 562. N. L. Bauld, J. H. Zoeller. Tetrahedron Lett. 1967, 8, 885–889.
411
Magn. Reson. Chem. 2014, 52, 409–411
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc
