Magnetic properties of single crystal alpha

Radiation Physics and Chemistry 81 (2012) 146–151
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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: [email protected] (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.
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