γ-irradiated [(CH3)4N]2SiF6 single crystal investigation by electron paramagnetic resonance technique

Article abstract of DOI:10.1016/j.molstruc.2006.09.034

In this study, γ-irradiation damage centers in [(CH₃)₄N]₂SiF₆ single crystal has been investigated by electron paramagnetic resonance spectroscopy (EPR) at room temperature. It has been found that γ-irradiation produces the (CH₃)₃ over(N, ?)⁺ radicals in this compound. The g factor and the hyperfine constant for H atoms, measured as 28.7 G, were found to be isotropic. The hyperfine coupling constant of the N nucleus, observed (CH₃)₃ over(N, ?)⁺ hole radical in [(CH₃)₄N]₂SiF₆ was found to be anisotropic with A_// = 15 and A_⊥ = 23 G, respectively. This means that each methyl groups rotate around the C_{3 ν}-axis and they also rotate around the C_{3 ν}-axis of (CH₃)₃ over(N, ?)⁺ radical. These results were compared with the earlier studies in (CH₃)₃ over(N, ?)⁺ radical and discussed.

Full text of DOI:10.1016/j.molstruc.2006.09.034

Journal of Molecular Structure 834–836 (2007) 308–310
www.elsevier.com/locate/molstruc
c-irradiated [(CH3)4N]2SiF6 single crystal investigation
by electron paramagnetic resonance technique
_
*
E. Bozkurt , I. Kartal, B. Karabulut
Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayis University, Kurupelit, 55139 Samsun, Turkey
Received 12 September 2006; received in revised form 27 September 2006; accepted 28 September 2006
Available online 13 November 2006
Abstract
In this study, c-irradiation damage centers in [(CH3)4N]2SiF6 single crystal has been investigated ꢀby electron paramagnetic resonance
spectroscopy (EPR) at room temperature. It has been found that c-irradiation produces the ðCH3Þ3Nþ radicals in this compound. The g
factor and the hyperfine constant for H atoms, measured as 28.7 G, were found to be isotropic. The hyperfine coupling constant of the N
ꢀ
nucleus, observed ðCH3Þ3Nþ hole radical in [(CH3)4N]2SiF6 was found to be anisotropic with A// = 15 and A^ = 23 G, respectively. This
ꢀ
means that each methyl groups rotate around the ꢀC3m-axis and they also rotate around the C3m-axis of ðCH3Þ3Nþ radical. These results
were compared with the earlier studies in ðCH3Þ3Nþ radical and discussed.
ꢀ 2006 Elsevier B.V. All rights reserved.
ꢀ
Keywords: EPR; c-irradiation; Single crystal; ðCH3Þ3Nþ radical
1. Introduction
in the latter compounds the rotation or jumping
ꢀ
reorientation of the C3m-axis of the ðCH3Þ3Nþ radical
has been observed [4]. This radical species could be pro-
duced by c-irradiation, through the breakage of the
N–H bond of the [(CH3)4N]+ ions or the breakage of an
N–C bond of the [(CH3)4N]+ ions [3]. As a continuation
of this kind of studies, we report in this paper the EPR
spectra of c-irradiation products in [(CH3)4N]2SiF6 and
to our knowledge this compound was not previously
It is well known that free radicals can be produced in
crystals via c-, x-, or uv-irradiation and these free radicals
can be studied by the electron paramagnetic resonance
(EPR) technique. The electron paramagnetic resonance
(EPR) investigations provide the most complete informa-
tion about the structure and the motional behaviour of
radicals. In literature, several authors have studied the
ꢀ
ðCH3Þ3Nþ radical produced in various substances [1–4].
investigated in single crystal form at room temperature.
ꢀ
As a result of these studies, it can be concluded that the
In this study, the motional behaviour of ðCH3Þ3Nþ radi-
cal was investigated in single crystal form at room
temperature.
crystal field effects are not much effective on the structure
ꢀ
of the ðCH3Þ3Nþ, but the motional behaviour of this rad-
ical was found to be different in (CH3)4NCl, (CH3)4NBF4,
and (CH3)4NPF6 from those of which are trapped in beta-
ine, betaine hydrochloride, betaine monohydrate and in
[(CH3)3NH]2SnCl6. In the former compounds in addition
2. Experimental
[(CH3)4N]2SiF6 single crystals were prepared by slow
evaporation at ambient temperature from an aqueous solu-
tion containing stoichiometric amounts of H2SiF6 and
(CH3)4NOH (1:1 proportion). The X-ray study of the
[(CH3)4N]2SiF6 single crystal at ambient temperature shows
that this compound is face centred cubic. The space group
to methyl group reorientations around the N–C bonds
ꢀ
and the reorientation of the ðCH3Þ3Nþ around its C3m-axis,
*
Corresponding author.
E-mail address: esatb@omu.edu.tr (E. Bozkurt).
0022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.09.034

E. Bozkurt et al. / Journal of Molecular Structure 834–836 (2007) 308–310
309
˚
Fm3m, of antifluorite type, has a = 11.172 A parameter [5].
The unit cell contains four molecules (Z = 4). Suitably
sized crystals were irradiated with 60Co c-ray source at
ambient temperature to a dose of about 158 kGy. The
dimensions of single crystal are about 1 · 2 · 3 mm3.
The EPR spectra were recorded with a Varian E-109C
model, X-band EPR spectrometer using 63 mW micro-
wave power and the modulation frequency of the magnetic
field was 100 kHz. The modulation amplitude was around
16 G. The single crystals were rotated on a Lucit pillar
about three mutually perpendicular planes (ab, ac and
bc) and the spectra were recorded at 10ꢁ steps. The g
values were obtained by comparison with a diph-
enylpicrylhydrazyl(dpph) sample of g = 2.0036.
3. Results and discussion
Fig. 1a and b show the recorded EPR spectra of c-irra-
diated [(CH3)4N]2SiF6 single crystals at room temperature,
when the magnetic field is in the (bc)-plane and 140ꢁ away
from the b-axis and along the c-axis, respectively. The
recorded EPR spectra at all orientations of the magnetic
field show many lines at room temperature, which are
ꢀ
attributed to ðCH3Þ3Nþ radicals. The spectrum in Fig. 1a
exhibits an intensity distribution as 1:1:9:1:9:36:
9:36:84:36:84:126:84:126:126:126:126:84:126:84:126:84:36:-
84:36:9:1:9:1:1 when the magnetic field is along the b-axis.
ꢀ
This spectrum belongs to the ðCH3Þ3Nþ radical since the
hyperfine interactions of the free electron with nine equiv-
alent methyl protons and 14N nucleus of nuclear spin I = 1
ꢀ
with the hole ðCH3Þ3Nþ radical. At this orientation the
hyperfine value of protons of the methyl groups is almost
isotropic and aCH ¼ 29 G. In Fig. 1b, this spectrum was
3
recorded when the magnetic field was along the c-axis. At
this orientation the hyperfine value of the protons is
aCH ¼ 29 G and AN = 14.5 G. The hyperfine coupling
3
constant of protons is isotropic and methyl groups are
equivalent. The hyperfine coupling constant of the N
ꢀ
nucleus with the hole in ðCH3Þ3Nþ in [(CH3)4N]2SiF6
was found to be axially symmetric. The principal values
and their direction cosines of EPR parameters are given
in Table 1. ꢀThe principal values and the averages of AN
for ðCH3Þ3Nþ radical are given in Table 2 in various
substances with the results of the present work. Each
methyl groups rotates around their C3m-axes and they
ꢀ
also rotate around the C3m-axis of ðCH3Þ3Nþ radical.
In addition to these motions, the C3m-axis of
ꢀ
ðCH3Þ3Nþ radical seem to rotate with an angle around
the c-axis of the crystal, since Az < Ax = Ay is in Table
2 and the signal is isotropic in the (ab) plane. The iso-
tropic g value of the radical in [(CH3)4N]2SiF6 is
g = 2.004 and is in agreement with the previously
obtained values for this radical. The powder spectrum
Fig. 1. (a) The EPR spectrum of c-irradiated [(CH3)4N]2SiF6 single crystal
(a) when the magnetic field is in the (bc) plane and 140ꢁ away from the
b-axis. (b) The EPR spectrum of c-irradiated [(CH3)4N]2SiF6 single crystal
at room temperature. The magnetic field is along the c-axis.
Table 1
ꢀ
The principal values and direction cosines with respect to a, b and c axes of g and the AN hyperfine tensors of the ðCH3Þ3Nþ radical observed in c-
irradiated [(CH3)4N]2SiF6 single crystal at room temperature
gN
Direction cosines
Hyperfine AN (G)
Direction cosines
c
a
b
c
a
b
gz = 2.0041
gx = 2.0036
gy = 2.0032
0.805
ꢁ0.046
ꢁ0.592
0.291
0.900
0.325
0.518
Az = 15
Ax = 23
Ay = 23
0.997
ꢁ0.003
0.068
ꢁ0.025
0.914
0.405
ꢁ0.063
ꢁ0.406
0.912
ꢁ0.433
0.738
AN are in G. Dg = 0.0005 and DA = 0.5G.

310
E. Bozkurt et al. / Journal of Molecular Structure 834–836 (2007) 308–310
Table 2
ꢀ
14
The principal values and the averages of the N nucleus hyperfine coupling constants (in G) of ðCH3Þ3Nþ radicals in various substances at room
temperature
Substance
gizo
aCH
Ax
Ay
Az
AN
Reference
3
[(CH3)4N]2SiF6
[(CH3)4N]2ZnCl4
(CH3)3NHBF4
(CH3)4NPF6
[(CH3)4N]2CdCl4
[(CH3)4N]InCl4
[(CH3)4N]CdCl3
[(CH3)3NH]2SnCl6
(CH3)4NCl
2.0043
2.0044
2.0024
2.0046
2.0032
2.003
2.0036
2.002
2.0044
–
28.7
28.7
28.5
28.8
29
28.6
29.2
29.4
26.7
28.9
28.2
28.8
23
14.5
4.3
22.8
–
14
12.8
7.2
20.4
24.6
7.9
23
19
10
22.8
–
16.2
12.8
7.2
20.4
25
15
20.3
20.7
20.2
20.6
20
19.8
19.8
20.3
18
Present work
28.7
46.2
16.2
–
29.2
33.1
46.4
13.2
13.6
50.8
16.2
[1]
[1]
[6]
[2]
[2]
[3]
[3]
[4]
[7]
[7]
[6]
(CH3)4NCl
Betaine
(CH3)4NBF4
21.1
21.3
20.3
–
5.3
22.5
2.0051
22.5
Dg = 0.0005 and DA = 0.5 G.
4. Conclusions
The EPR results presented in this study indicate the rad-
icals induced by c-irradiation in [(CH3)4N]2SiF6 is
ꢀ
ðCH3Þ3Nþ. The EPR parameters of the radicals are found
to be consistent with the previous results in the literature.
In this single crystal, radical was found to be in various
motions. As a result of thiꢀs work, it can be concluded that
the C3m-axis of the ðCH3Þ3Nþ radical seem to rotate around
the c-axis of the crystal since the EPR spectra did not chan-
ged in the (ab) plane.
References
_
[1] F. Ko¨ksal, I. Kartal, Radiat. Phys. Chem. 53 (1998) 501–504.
[2] F. Ko¨ksal, Y. Yerli, J. Phys. Chem. Solids 64 (8) (2003) 1349–1352.
[3] J.T. Yu, K.T. Liu, Y.H. Jeng, Solid State Commun. 89 (6) (1994)
543–547.
Fig. 2. The EPR spectrum of c-irradiated [(CH3)4N]2SiF6 powder crystals
at room temperature.
[4] A.J. Tench, J. Chem. Phys. 38 (1963) 593–596.
[5] A. Ouasri, A. Rhandour, M.C. Dhamelincourt, P. Dhamelincourt, A.
Mazzah, Spectrochim. Acta Part A 58 (2002) 2779–2788.
[6] F. Ko¨ksal, F. C¸ elik, O. C¸ akır, Radiat. Phys. Chem. Part C 33 (2)
(1989) 135–138.
is shown in Fig. 2, with its g value is g = 2.0043. The
hyperfine coupling constant of protons is aCH
¼
3
28:7 G and the hyperfine coupling constant of nitrogen
nucleus is not measured exactly.
[7] J.P. Michaut, J. Roncin, Can. J. Chem. 55 (1977) 3554.