Magnetic properties of single crystal alpha
Transkript
Magnetic properties of single crystal alpha
Radiation Physics and Chemistry 81 (2012) 146–151 Contents lists available at SciVerse ScienceDirect Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem Magnetic properties of single crystal alpha-benzoin oxime: An EPR study Ulku Sayin a,n, Ömer Dereli b, Ercan Türkkan b, Ayhan Ozmen a a b Department of Physics, Science Faculty, Selcuk University, Konya, Turkey Department of Physics, Education Faculty, Selcuk University, Konya, Turkey a r t i c l e i n f o a b s t r a c t Article history: Received 10 April 2011 Accepted 15 August 2011 Available online 29 September 2011 The electron paramagnetic resonance (EPR) spectra of gamma irradiated single crystals of alphabenzoinoxime (ABO) have been examined between 120 and 440 K. Considering the dependence on temperature and the orientation of the spectra of single crystals in the magnetic field, we identified two different radicals formed in irradiated ABO single crystals. To theoretically determine the types of radicals, the most stable structure of ABO was obtained by molecular mechanic and B3LYP/6-31G(d,p) calculations. Four possible radicals were modeled and EPR parameters were calculated for the modeled radicals using the B3LYP method and the TZVP basis set. Calculated values of two modeled radicals were in strong agreement with experimental EPR parameters determined from the spectra. Additional simulated spectra of the modeled radicals, where calculated hyperfine coupling constants were used as starting points for simulations, were well matched with experimental spectra. & 2011 Elsevier Ltd. All rights reserved. Keywords: EPR Single crystal Oxime Iminoxy radical Alpha-benzoin oxime Density functional theory (DFT) calculations 1. Introduction Vic-dioximes (R1C(¼NOH)C(¼NOH)R2) and their derivatives have played significant roles as model systems in applied chemistry. Generally, they are used as biological model compounds (i.e., vitamin B12), but they are also used in photography, medicine, agriculture, textiles, technological improvement, dye chemistry, and semi-conductor manufacturing (Schrauzer et al., 1965; Thomas and Underhill, 1972; Underhill et al., 1973; Chakravorty, 1974; Kurita, 1998; Mathur and Narang, 1990; Ravi Kumar, 2000). The synthesis of vic-dioximes and various derivatives has been studied for a long period of time (Schrauzer, 1976; Serin and Bekaroğlu, 1983; Serin et al., 1992; Gök et al., 1993; Dilworth and Parrott, 1998; Wolkert and Hoffman, 1999; Kurtoğlu and Serin, 2002; Wang et al., 2003; Hardy et al., 2004; Macquarrie and Hardy, 2005). In recent years, the discovery of the anti-tumor effects of coordination compounds in cancer research has increased the attention on vic-dioxime complexes. Vic-dioximes have a high tendency to form isomers. (Park et al., 2005; Soga et al., 2001) Electron Paramagnetic Resonance (EPR) spectroscopy is a technique that has been widely used in the identification of irradiation damage centers in substances. The magnetic properties of several vic-dioximes and oximes have also been investigated using the EPR technique (Norman and Gilbert, 1967; Lakkaraju et al., 1994; Jaszewski et al., 2000; Turkkan et al., n Corresponding author. Tel.: þ90 0332 223 1838; fax: þ90 332 241 2499. E-mail address: uakpinar@selcuk.edu.tr (U. Sayin). 0969-806X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2011.08.013 2009; Sayin et al., 2010b; Dereli et al., 2011). Literature surveys indicated that magnetic properties of alpha-benzoinoxime (ABO) have not yet been examined. To explore the properties of vicdioximes in more detail, we have investigated the magnetic properties of the high-energy irradiated ABO by EPR as a continuation of the studies on vic-dioximes. In the present study, the magnetic properties of ABO were investigated with EPR spectroscopy. Density functional theory (DFT) calculations were used to support the interpretation of the experimental results and to assist in the identification of the radical type by comparing the experimental and calculated EPR parameters. 2. Experimental The ABO (C14H13NO2) powder was purchased from Merck. The samples were crystallized in the laboratory by slow evaporation from a concentrated solution in ethanol at room temperature. The grown single crystals were irradiated with a 60Co-g-ray source at 0.91 kGy/h for approximately 110 h. The colorless single crystals turned brown. After irradiation, the EPR spectra of the ABO crystals were recorded at 120 K at 101 intervals in the magnetic field applied along each of the three perpendicular axes (x, y, and z) using a Bruker model EMX 081 X-band EPR spectrometer. Low- and high-temperature measurements were performed using a Bruker variable temperature-control unit, and the Bruker SimFonia software program was used for the spectral simulations. The g factors of the radicals were found by comparison with a DPPH sample (g ¼2.0036). U. Sayin et al. / Radiation Physics and Chemistry 81 (2012) 146–151 3. Computational details To establish the possible stable conformations, the conformational space of ABO was scanned using the MMFF method. The calculation was performed with the Spartan 08 program (Spartan, 2008). DFT proved to be extremely useful in treating the electronic structure of molecules. The 6-31G(d,p) basis set was used as an effective and economical level to study fairly large organic molecules. Geometry optimizations of model radicals were performed with Becke’s three-parameter hybrid-exchange functional combined with the Lee–Yang–Parr correlation functional (Lee et al., 1988; Becke, 1993; Stephens et al., 1994); specifically, the B3LYP method and the standard 6-31G(d,p) basis set were used. The optimizations were performed without any constraints (full optimization). All stationary points were confirmed to be local minima by their harmonic vibration frequencies, and normal-mode calculations were performed at the same level as the geometry optimization. Hfccs and g factors of the modeled radicals were calculated using the B3LYP method and the TZVP basis set combination (Godbout et al., 1992). Conformational analysis was performed by the SPARTAN 08 program package (Spartan, 2008); all other calculations were performed using the GAUSSIAN 03 program (Frisch et al., 2003). 4. Results and discussion Free radicals produced by gamma-irradiation in the single crystal ABO were investigated between 120 K and 440 K with EPR. The spectra were found to be dependent on the temperature and orientation of the magnetic field. The dependence of the spectra Fig. 1. Dependence of spectra to the temperature, between 120 and 450 K, when the magnetic field oriented 01 to the x-axis in yz plane. 147 on the temperatures between 120 and 440 K when the magnetic field was oriented 0o to the x-axis in the yz plane, are shown in Fig. 1. It is understood from Fig. 1 that two different radicals exist and one of the radicals quenched at higher than 340 K, and the spectrum subsequently disappeared. The spectra of the radicals labeled as R1 and R2 overlapped at low temperatures, as seen in Fig. 2(a). Previous EPR studies on similar structures, specifically on other oximes, have provided useful information to resolve the spectra. Several EPR studies on oximes and vic-dioximes have shown that iminoxy radicals, such as R1R2(C¼NO) or R1C(¼NO)C(¼NOH)R2, have been produced after irradiation (Miyagawa and Gordy, 1959; Thomas, 1964; Gilbert and Norman, 1966; Turkkan et al., 2009; Sayin et al., 2010b; Dereli et al., 2011). The investigations indicated that high-energy irradiation removes the hydrogen atom from the oxime branches of oximes and vic-dioximes. It has been reported that the unpaired electron in iminoxy radicals is located in the NO group and is characterized as a s-type radical, and the spin density is shared between the 14N and 16O atoms. The radicals are characterized by a relatively large isotropic 14N hfcc. (aN E30 G) Therefore, it was assumed that the disappeared R1 radical is an iminoxy radical. The spectra of R2 observed at 380 K and higher are very similar to a previously published spectrum of the radical obtained from gamma-irradiated 4-phenylsemicarbazide (Sayin et al., 2010a). In that study, the authors assumed that the unpaired electron was delocalized in the phenyl ring of the radical because of the small para–ortho–meta H-splittings (aH E3–5 G). In addition, H-(aH E 16 G) and N-(aN E6 G) splittings were present, and the assumptions were supported by the ab initio calculations. Fig. 2. EPR spectra of gamma-irradiated ABO single crystal when the magnetic field oriented. (a) 201 to the x-axis in yz plane and (b) 901 to the y-axis in xz plane at room temperature. 148 U. Sayin et al. / Radiation Physics and Chemistry 81 (2012) 146–151 The line intensities in the EPR spectra of ABO were irresolvable due to the large number of overlapping lines. To resolve the number of lines, the magnetic field values of all detectable lines positions were plotted against rotation angles in three perpendicular planes, as shown in Fig. 3. It is clearly observed from symmetries around the center fields of lines in the spectra in Fig. 3 that there are two paramagnetic species. When the behaviors of the line variations in these spectra were considered the most apparent splittings, the g tensors of R1 and R2 radicals could be determined. The first radical, labeled R1, displayed three lines arising from the 14N nuclei. Besides, in some orientations there was seen additional splitting aside from the splitting of 14N nuclei, which can be 1H splitting calculated theoretically as 6.82 G for H3 atom given in Table 3 or because of the superimposition of R1 radicals with different orientation or conformation. Since the splitting cannot be observed explicitly in most orientations, it is difficult to say experimentally what the source of mysterious small splitting is in the radical R1. This situation was observed through the comparison of two experimental spectra given in Fig. 2. Fig. 2a only displays the 14N triplet, and Fig. 2b shows external lines with the splitting of 14N nuclei. In addition to the small ortho–para 1H-splittings, one 14N- and one 1H-splitting are observed for R2. The experimentally determined isotropic and anisotropic components and the directional cosines of the EPR parameters are given in Table 1. A total of 57 spectra were collected at 101 intervals in each of the three axes. The g values were found anisotropic with the average values (giso ¼2.0051 for R1 and giso ¼2.0031 for R2). The angular variations of the hyperfine interactions A(y) and the spectroscopic splitting-factor g(y) are shown in Figs. 4 and 5 for R1 and R2, respectively. To more clearly interpret the EPR spectra and assign appropriate radicals, detailed DFT calculations were performed for several possible model radicals. To make the necessary calculations, models of the possible radicals need to be created. Using the molecular structure as the initial geometry, possible radicals were modeled by theoretical calculations. From the literature survey, crystal data for the title compound were not available. Therefore, a meticulous conformational analysis was carried out for the title compound. Rotating 101 intervals around the free rotation bonds, the conformational space of the title compound was scanned by the molecular mechanic MMFF method, and full geometry optimizations of these structures were performed by the B3LYP/631G(d,p) method. The results of geometry optimizations indicated that the title compound had seven conformers. Ground state energies, relative energies and dipole moments of conformers are presented in Table 2. From the calculated energies of seven conformers, conformer 1 was the most stable. Using the structure of conformer 1 (Fig. 6) as the initial geometry, four radicals (RM-1, RM-2, RM-3 and RM-4) were modeled. Fig. 3. Angular variation of line positions at the low field side of the EPR spectra of gamma irradiated ABO. Table 1 The experimental EPR parameters (hyperfine coupling constant’s and spectroscopic splitting factor) of R1 and R2. R1 radical R2 radical Hyperfine coupling constants (G) Direction cosines A(14N) Axx ¼ 24,97 Ayy ¼37,86 Azz ¼36,96 Aiso ¼ 33,26 0,697 0,268 0,665 0,010 0,924 0,383 0,717 0,274 0,641 gxx ¼ 2,0036 gyy ¼2,0042 gzz ¼ 2,0076 giso ¼2,0051 0,476 0,561 0,678 0,727 0,685 0,056 0,495 0,466 0,733 g values g Hyperfine coupling constants (G) Direction cosines A(1H) Axx ¼19,19 Ayy ¼ 10,76 Azz ¼17,77 Aiso ¼ 15,90 0,895 0,033 0,445 0,316 0,656 0,685 0,314 0,754 0,577 A(14N) Axx ¼8,25 Ayy ¼ 6,37 Azz ¼3,67 Aiso ¼ 6,09 Axx ¼4,2 Ayy ¼ 3,48 Azz ¼2,31 Aiso ¼ 3,33 0,695 0,594 0,406 0,711 0,482 0,512 0,108 0,644 0,757 0,730 0,671 0,133 0,560 0,475 0,678 0,392 0,570 0,722 gxx ¼2,0014 gyy ¼ 2,0033 gzz ¼ 2,0045 giso ¼2,0031 0,780 0,481 0,400 0,226 0,813 0,537 0,584 0,329 0,742 A(1H0 p) g values g U. Sayin et al. / Radiation Physics and Chemistry 81 (2012) 146–151 Fig. 4. Angular variation of (a) hfcc for Fig. 5. Angular variation of (a) hfcc for 1H (b) hfcc for 14 14 N and (b) g values, for radical R1. N (c) hfcc for ortho–para 1H and (d) g values, for radical R2. Table 2 Energetics of the conformers calculated at the B3LYP/6-31G(d,p) level. Conf, E (Hartree) 1 2 3 4 5 6 7 746,4723400 746,4704408 746,4697411 746,4695534 746,4689329 746,4681586 746,4681471 DE (kcal/ mol) E0 (Hartree) 0,000 1,192 1,631 1,749 2,138 2,624 2,631 746,230977 746,229867 746,228851 746,228462 746,228012 746,227469 746,227341 mol) Dip, Mom, (D) 0,0000 0,6965 1,3341 1,5782 1,8606 2,2013 2,2816 2,4166 1,1440 1,8676 1,5649 1,2268 1,5559 1,8676 DE0 (kcal/ E0, Zero point corrected energy RM-1 was an iminoxy radical modeled in a form similar to the suggested radicals in prior EPR studies of oxime derivatives. It was formed by abstraction of the 6H atom from the oxime Fig. 6. The most stable conformer of the ABO. 149 150 U. Sayin et al. / Radiation Physics and Chemistry 81 (2012) 146–151 RM-1 RM-2 RM-3 RM-4 Fig. 7. Optimized geometries model radicals. Table 3 Calculated (B3LYP/TZVP) values of isotropic hfccs (G) and g-factors for model radicals. aiso 3H 4N 6H 8H 26H 27H 28H 29H 30H giso RM-1 RM-2 6,82 31,01 88,33 2,45 1,16 2,0056 2,0168 RM-3 RM-4 13,61 5,96 1,85 6,62 1,07 2,26 2,41 2,21 1,03 1,05 2,41 3,56 3,67 1,49 1,52 4,01 2,0039 2,0035 branch. RM-2 was formed by abstraction of the 8H atom from the hydroxyl group, RM-3 was formed by abstraction of the 3H atom, and RM-4 was formed by abstraction of the hydroxyl group (7O8H) from conformer 1. To provide an accurate calculation for hfccs and g-factors, accurate descriptions of the geometric structures of these radicals were necessary. The B3LYP/6-31G(d,p)-level geometry optimizations were performed for the modeled radicals. The optimized radical geometries are shown schematically in Fig. 7. The optimized radical geometries were used as initial points in the calculations for the hfccs and g factors. The parameters of the model radicals were calculated using the B3LYP/TZVP level of density functional theory. The theoretically calculated isotropic values of the EPR parameters of relevant radicals are given in Table 3 in accordance with the atom numbering scheme shown in Fig. 6. For most interpretation and assignment purposes, isotropic hfcc calculations on isolated molecules deviating approximately 20% from the experimental values would be acceptable (Chipman, 1995). Additionally, it is difficult to measure giso values more accurately than by 10 3. Thus, a deviation of 500 ppm between theory and experimental values usually falls within the error limits; an agreement with the theory within 1000 ppm (1 ppt) is considered satisfactory (Neese, 2001). By comparing the calculated and experimentally observed values (Tables 3 and 1), it is apparent that the calculated values of RM-1 and RM-4 are in excellent agreement with the experimentally observed values of R1 and R2, respectively. The calculated parameters of RM-1 in Table 2 are closer than any of the other model radicals to the experimental parameters of R1. The calculated isotropic hfcc of the N4 atom of RM-1 (30.01 G) is in excellent agreement with the experimentally observed value of the 14N-splitting of R1 (33.26 G). Deviation from the experimental value was less than 20%. In addition to the splitting, RM-1 also had H3-(6.82 G), H8-( 1.16 G) and H15-(1.11 G) splittings. Because the calculated values of the H8- and H15-splittings are small for the RM-1-type model radical, the experimental measurements were difficult to perform. As mentioned in the experimental discussion above, and as seen from comparisons of the spectra in Fig. 2(a) and (b), R1 had a small splitting in addition to the 14 N-splitting. The external lines, seen in some orientations, can come from the hyperfine splitting of H3 atom, which was determined theoretically (6.82 G) or occur because of the superimposition of R1 radicals with different orientation or conformation. Table 1 indicates that the calculated giso value of the RM-1 is closer to the experimental giso value of R1 than that of other model radicals. The deviation of the calculated giso from the experimental giso value of R4 was 500 ppm. For all other modeled radicals, the deviation was larger than 1000 ppm. As observed in Table 2, the calculated parameters of RM-4 are closer than any of the other model radicals to the experimental parameters of R2. The calculated isotropic hfccs of the H3 (13.61 G), N4(5.96), 26H ( 3.56 G), 27H ( 3.67 G) and 30H (4.01 G) atoms of RM-4 are in excellent agreement with the experimentally observed values of the 1H (15.90 G), 14N (6.09 G) and ortho–para 1 H-(3.33 G) splittings of R2. Deviations from the experimental value were less than 20%. Because the calculated isotropic hfcc of meta hydrogen splittings are small (approximately 1.5 G), their values cannot be determined experimentally. Additionally, the calculated giso value of the RM-4 was closer to the experimental giso value of R2. The deviation of the calculated giso from the experimental value of R4 was 400 ppm. RM-1 and RM-4 were identified as radicals R1 and R2, respectively, and were produced in the gamma-irradiated ABO molecule. 5. Conclusions In the presented study, two radicals (R1 and R2), which were formed by abstraction of an H atom from an oxime branch and by abstraction of an OH group from ABO, respectively, were identified in the gamma-irradiated ABO single crystal. The hypothesis regarding the radical identity was strongly supported by the DFT calculations. The theoretical and experimental parameters of R1 were in strong agreement with the values found in the literature for iminoxy radicals (Norman and Gilbert, 1967; Lakkaraju et al., 1994; Jaszewski et al., 2000; Miyagawa and Gordy, 1959; Thomas, 1964; Gilbert and Norman, 1966; Turkkan et al., 2009; Sayin et al., 2010b; Dereli et al., 2011), and the experimental parameters of R2 were in strong agreement with the values found in the literature (Sayin et al., 2010a). Acknowledgment This work was financially supported by the BAP, Selcuk University in Turkey. References Becke, A.D., 1993. Density-functional thermochemistry. III. The role of exact exchange. 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