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.