2663
ESROF PEROXY RADICALS TRAPPED IN TRIFLUOBOACETAMIDE
Electron Spin Resonance Spectra of Peroxy Radicals Trapped in a ?-Irradiated Single Crystal of Trifluoroacetamide by Kazumi Toriyama and Machio Iwasaki Government Industrial Research Institute, Nagoya, Hirate-machi, Kita-bu, Nagoya, Japan
(Received December BY, 1968)
It was found from the electron spin resonance spectra that the .CF2CONH2 radical trapped in ?-irradiated trifluoroacetamide reacts with atmospheric oxygen to give the peroxy radical, OOCF2CONH2. The esr spectra of the peroxy radicals showed a remarkable change with the observation temperature. The g tensors at 77 and 300°K were determined by analysis of the spectra of single crystals to be 91 = 2.0022, g2 = 2.0074, gs = 2.0384 at 77°K and gl = 2.0210, g~ = 2.0182, ga = 2.0082 at 300°K. The decrease of the anisotropy of the g tensor at 300°K was well interpreted in terms of the partial averaging of the g tensor due to the internal rotation around the C-0 bond. I n addition, from the principal directions of the g tensors at both temperatures, the geometrical structure of the peroxy radical was approximately determined, by comparison with the principal direction of the hyperfine tensor of the a-fluorine atom of the .CF,CONHe radical trapped in the crystal together with the peroxy radicals.
Introduction It is well known that carbon radicals trapped in irradiated polymers react with atmospheric oxygen when air is introduced after irradiation under vacuum.l However, there have been few reports on such a reaction occurring in irradiated organic crystals of simple mole~ u l e s . ~ It J was reported in our previous paper4 that sodium salts and amides of some perfluorinated carboxylic acids gave radicals produced by the detachment of a fluorine atom bonded to the carbon a to the carboxylic group when ?-irradiated under vacuum a t room temperature. During the course of this study it was found that some of these samples gave esr spectra due to peroxy radicals when they were irradiated in air, or air was introduced after irradiation under vacuum. The compounds giving peroxy radicals were CF3COONa, CF3CF2COONa, CF3CF2CF2COONa, and their corresponding amides. It is interesting that these perfluorinated compounds react with atmospheric oxygen in the solid state even in a large single crystal. Furthermore, the spectra of these peroxy radicals showed a reversible change with the temperature at which the observations were made. I n the previous paper of one of the authors (M. 1.) and his c o w ~ r k e rit , ~was reported that esr spectra of peroxy radicals trapped in 7-irradiated polytetrafluoroethylene exhibited a similar temperature change, that is, when measurements were made at room temperature, gll was smaller than ,g, while gli was much larger than g, a t 77°K. This change of the spectrum was reversible and was interpreted in terms of the rapid molecular motion around the chain axis a t room temperature due to the so-called room temperature transition of polytetrafluoroethylene. The spectral changes of peroxy radicals in amides and sodium salts of fluorinated carboxylic acid may be also interpreted as a result of some
molecular motion. I n this case, however, the kind of molecular motion is considered to be internal rotation, because there is no possibility of molecular rotation as a whole. To make this clear, the g tensors were determined both at 77°K and at room temperature for peroxy radicals trepped in single crystals or in powders of trifluoroacetamide. I n addition, an attempt was made to determine the geometrical structure of peroxy radical with reference to the structure of the forerunner radical *CF&OI\THZ.
Experimental Section Single crystals were made by slow evaporation from aqueous solutions at a temperature below 10". The appearance of the single crystal used is shown in Figure 1. From the esr spectra it was found that this crystal has a monoclinic symmetry, and the c axis is parallel to the longest edges of the crystal. The single crystals of trifluoroacetamide studied by Lontz and Gordy,G and by Rogers and Kispert' were also assigned monoclinic symmetry. From our experiment, it was found that the a and b axes assigned by Lontz and Gordy are [hkO]and [h'k'O], respectively. The correct crystalline b axis Iies in between their axes a and b. I n our crystals, the plane, (loo), which was not devel(1) W. B. Ard, H. Shields, and W. Gordy, J. Chem. Phys., 23, 1727 (1955); H. Fisher, K.H. Hellwege, and P. Neudorfl, J . PoZym. Sci., Part A-I, 2109 (1963). (2) R. J. Lontz, J . Chem. Phys., 45, 1339 (1966). (3) A. Fauoitano, A. Perotti, and G. Adler, Ric. Sci., 37, 1149 (1967). (4) M.Iwasaki, K. Toriyama, and B. Eda, J . Chem. Phys., 42, 63 (1965). (5) M. Iwasaki and Y . Sakai, J . PoZym. Sci., Part A-2, 266 (1968). (6) R. J. Lontz and W. Gordy, J . Chem. Phys., 37, 1357 (1962). (7) M. T.Rogers and L. D. Kispert, ibid., 46, 3193 (1967). Volume Y9,Number 8 August 1969
2664
KAZUMI TORIYAMA AND LMACHIO IWASAKI
Figure 1. Sketch of a typical crystal and axes chosen.
oped in the crystal used by Lontz and Gordy appeared as shown in Figure 1. So, the orthogonal axes a', b, and c were chosen as indicated by the arrows in Figure 1. Our experimental coordinate system a'bc was the same as that used by Rogers and Kispert,' although they have stated that they chose the same coordinate system used by Lontz and Gordy. Samples were irradiated by s°Co y rays at room temperature under vacuum. The total dose was about 5 X 106 R at a dose rate of 2 X 106 R/hr. After irradiation under vacuum, oxygen was introduced into the sample tubes, and the samples were kept at -20" for about 1 month in order to get a strong enough signal due to the peroxy radicals to make an accurate measurement, since the peroxy radical in trifluoroacetamide is rather unstable, and it decays even a t room temperature. Esr spectra were measured both at 300 and at 77°K with a Japan Electron Optics Model 3BS spectrometer operated at 24 and 9.4 GHz. The spectra of single crystals were recorded as second derivative curves with the double modulation of 100 KHz and 80 Hz, while those of powder were recorded as first derivative curves with 100-KHz modulation. The signal of Mn2+in ZnS was used as a marker for the magnetic field. The resonance position and the hyperfine splittings of Mn2+were calibrated using the signal of DPPH and a side-band technique of the proton magnetic resonance.
Experimental Results Powder Spectra. Figure 2a shows the powder spectrum of CFICONHz y-irradiated a t room temperature under vacuum. The origin of this spectrum is the radical .CFzCONH2,as already r e p ~ r t e d . ~ , When ~" air was introduced into this sample, an asymmetric spectrum appeared gradually at a position of slightly lower field than the strong peak at the center as shown in Figure 2b. When the sample was allowed to stand at - 20" for about 1 month, the signal of CFzCONHz disappeared leaving the asymmetric spectrum alone shown in Figure 3. The spectrum showed a remarkable change of line shape depending not only on the microThe Journal of Physical Chemistry
II
v
w 50 G
Figure 2. The esr spectra of y-irradiated powders of trifluoroacetamide: (a) observed under vacuum; (b) observed in air. Measurements were made at 300°K using a 9.4-GHz spectrometer.
wave frequency but also on the observation temperature as indicated in Figures 3 and 4. All these changes were perfectly reversible. It is evident that the asymmetry of the spectum is due to the g anisotropy. The principal values of the g tensor estimated from the line shape were gl = 2.0022, gz = 2.0075, and g3 = 2.0384 at 77°K and were g'l = 2.0210, gtZ = 2.0182, and gta = 2.0082 a t 300°K. The g tensors at both temperatures are nearly axially symmetric, that is, g3 = glI, gl = gz = gl. It should be noted that a t 77°K gll is larger than gL, while a t 300°K 9/11is smaller than gIL, and the g anisotropy is remarkably reduced. However, the averaged g values were 2.0158 and 2.0160 a t 300 and 77°K) respectively. The reversibility of the temperature change and the invariance of the averaged g values strongly indicate that the change in the g tensor is caused by partial averaging due to some molecular motion. To confirm this interpretation, the principal directions of the g tensors both at 77 and at 300°K were determined from the angular dependence of the spectra using single crystals. Spectra of Single Crystals. Experiments were carried out using the single crystal in which the mother radical .CFZCONHZ coexists. A typical example of the spectra of a single crystal is shown in Figure 5. Ob-
2665
ESR OF PEROXY RADICALS TRAPPED IN TRIFLUOROACETAMIDE
b) 24
GHz
2.4 GHz c.------------j
50 G
Figure 3. The esr spectra of powders of trifluoroacetamide kept a t -20” for about 1 month in air after y irradiation at room temperature under vacuum: (a) measured a t 9.4 GHz; (b) a t 24 GHz. The arrows indicate the position of the resonance field of DPPH.
servations were made at a microwave frequency of 24 GHz. The three lines having the intensity ratios 1:2 : 1 are the signal due to -CF2CONH2,and the remaining single line exhibiting the large g anisotropy corresponds to the asymmetric line of the powder spectrum. Since the crystal has monoclinic symmetry, two sets of these signals were observed when the external magnetic field was applied in the (a’b) or in the (bc) plane. At room temperature, the two sites accidentally coincided with each other in the (bc) plane. The angular dependences of the g values of the single line observed both at 77 and 300°K are shown in Figures 6, 7, and 8. From these angular dependences the g tensors were determined a t both temperatures and are listed in Table I. Further, the calculated g values are indicated by solid (77°K) and dotted (300°K) lines in Figures 6, 7, and 8. The poor agreement with the observed values in the (a’b) plane is due to overlapping of the spectrum with that of the coexisting radical CF2CONHzwhich made it difficult to measure the position of the single line accurately.
Figure 4. Temperature change of the esr spectra shown in Figure 3: (a) observed a t 300°K; (b) observed at 77°K. The microwave frequency used is 24 GHz. The arrows indicate the position of the resonance field of DPPH.
The principal ua&es agree with those obtained from the line shapes of the powder spectra within the experimental error. The principal values and their directions at 300°K are quite different from those at 77°K. Table I: Observed Principal Values and Their Direction Cosines for the g Tensors of .OOCF&ONHZ at 77 and 300°K Principal valuas
Direction cosines with respect t o a’, bt c-
7 -
a’
b
C
2.0022 2.0074 2.0384 2,0160
-0.290 $0.944 +0.155
10.943 h0.254 10.215
-0.163 -0.208 $0.964
2.0210 2.0182 2.0082 2.0158
-0.151 -0.736 $0.660
10.089 10.655 zIz0.750
-0.985 $0.172 -0.033
There are no data on the crystal structure and the molecular geometry for this compound. Therefore, in order to define the principal directions of the g tensor to Volume 78,Number 8 August 1960
2868
KAZUMI TORIYAMA AND
R/IAcHIo
IWASAKI
g1
2.04
n
b)
l
at 77'K
J
Figure 5 . The esr spectra of a single crystal of trifluoroacetamide exposed to air after y irradiation under vacuum: (a) measured a t 300°K; (b) at 77°K. The microwave frequency used is 24 GHz. The magnetic field was applied along the a' axis. The arrows indicate the position of the resonance field of DPPH.
2-00I 0"
90" [ bl
60"
30"
(Cl
120"
150"
180'
Figure 7. Angular dependence of the observed and calculated the peroxy radical in trifluoroacetamide. The magnetic field was applied in the (bc) plane. The dots and the lines are used in the same manner AS in Figure 6. g values of
gT
2.02
gl
2.04
2*03
"\\
2.00
I
2.01
2.00 30'
' I!I
60'
( b)
90'
120'
150'
I&
(a')
Figure 6. Angular dependence of the observed and calculated g values of the peroxy radical in trifluoroacetamide. The magnetic field was applied in the (a'b) plane. The white and black dots are observed Q values at 77 and 300"K, respectively. The solid and dotted lines show the calculated angular dependence of g values for 77 and 300°K, respectively.
the structure of the radical, the a-fluorine hyperfine tensor of CF2COYHZ coexisting with this single line was also determined, because its principal directions were well defined t o the radical geometry. The results are tabulated in Table 11. Although Rogers and Kispert' suggested that the hyperfine tensor of the a-fluorine nucleus deviates slightly from axial symmetry, we could not find the evidence at any field directions a t 300°K. The tensor elements obtained me in agreement with those reported by Lontz and Gordy,B although the maximum principal value and its direction for the a-fluorine tensor are very close to those determined by Rogers and Kispert.7
0' (C)
-
Analysis and Discussion Averaging of the g Tensor by Molecular Motion. T o interpret the experimental results in terms of the molecThe Journal of Physical Chemistry
60"
30"
90"
126
156
l&
(a')
Figure 8. Angular dependence of the observed and calculated g values of the peroxy radical in trifluoroacetamide. The magnetic field was applied in (ea') plane. The dots and the
lines are used in the same manner as in Figure 6.
Table I1 : Observed Principal Values and Their Direction Cosines for the or-Fluorine Hyperfine Tensor of .CF2CONH2 a t 300'K Principal values
(a)
811
Al.
-
Direction cosines with respeot ot-
a', b , ca'
177 25
$0.842
b
C
jz0.539
0.000
It o Ail
2667
ESROF PEROXY RADICALS TRAPPED IN TRIFLUOROACETAMIDE uIar motion the partial averaging of the g tensor, which is expected when rotation around some axis takes place, was calculated, Suppose that g l , g2, and g3 are the principal values of the g tensor, 4, 7, and { their principal directions, and X , Y , and 2 the Cartesian coordinate system, where 2 is the axis of molecular rotation. When rotations around more than one axis are to be taken into consideration, the axis of the invariance for the entire molecular motion should be taken as the 2 axis. The coordinate systems t ( i , q , t ) and X(X,Y,Z) are related by the transformation matrix L.
t
= LX
g\\rot=
glLl3'
+
g2L23'
+
(7)
g3L3B2
This is evident from the fact that (g& , is unchanged with the rotational motion since g,, is independent of angle 6. Therefore, gllrot is easily obtained from the tensor component to the rotational axis. Equation 5 indicates the invariance of the trace of the tensor from which glrot is obtainable using the value of g,lrot. If the oscillational amplitude is smaller than one revolution, one may have small off-diagonal elements of the gx tensor, resulting in a slight discrepancy of the tensor from axial symmetry.
(1)
The elements of the L matrix are easily expressed by the Eulerian angles 0, cp, and x, shown in Figure 9. The g tensor in the new coordinate system X is expressed by the following equations.
f
TZ
gx = Zg,L
where
L
6
= [Lij] =
-
e COS cp COS x sin cp sin x -cos 0 cos cp sin x sin cp cos x sin 8 cos cp COS
[
-
Figure 9. The relation between the space-fixed coordinate X(X, Y , 2 ) and the molecular fixed coordinate ((67, f ) which are connected by the Eulerian angles e, +, and x.
+ +
e sin cp COS x cos cp sin x -cos 0 sin cp sin x cos cp cos x sin 6 sin cp
COS
-
-sin 6 cos x sin e sin x
](3)
cos 0
If rotation around the 2 axis takes place, the coordinate system 4 rotates with respect to the coordinate system X which is fixed in space. Therefore, the elements of g x should be averaged with respect to angle 4. Then the g x tensor becomes the (gx)+ tensor, the elements of which are (gzz)+ = g1 sin2 e cos2 x
+
g2
sin2 0 sin2 x
(gxx)+ = ( g w > + = '/z[(gi
+ g3 cos26
(4)
+ g2 + gal - (~2z)cI ( 5 ) 0
(6) Consequently, the coordinate axes X , Y,and Z become the principal axes for such a system, and the tensor has axial symmetry around the rotational axis, that is, (~zz)+ = glirot and (gx& = (gyy)+ = gLrot. Taking into account that sin 8 cos x, sin B sin x, and cos 6 in eq 4 are the direction cosines L13, L23, and La3of the rotational axis 2 with respect to the principal axes of g x , gilrot may be expressed as (gxy)+ = bY&
= (gzxjcb =
Temperature Change of the g Tensor. The origin of the single line is attributable to peroxy radical OOCF2CONH2,from the fact that the spectrum has no hyperfine structure but has a large g anisotropy with nearly axial symmetry, and that this spectrum appears a t the expense of the signal due to .CF&ONH2 when air or oxygen gas is introduced into the sample tube. I n fact, the principal values of the g tensor obtained a t 77°K are in good agreement with those of HOO. in a single crystal of an Hz02-urea addition compound8 or ROO. in polytetrafluoroethylene previously reported by one of the authors (M. I.) and his coworker. The values of the former are g l = 2.0018, g~ = 2.0081, and g3 = 2.0495, and those of the latter are gl = 2.0026, g2 = 2.0071, and 93 = 2.0384. The averaged g values for these cases are 2.0160, 2.0198, and 2.0160, respectively. It is well known from theoretical considerations that in peroxy radicals the direction of the largest g value should be parallel to the 0-0 bond, and this was verified by analysis of the esr spectra of HOO. in a single crystal of an Hz02-urea addition compound.8 Hence, if the system is rigid enough at 77"K, the direction of g3 is considered to be parallel to the 0-0 bond. Suppose that the rotation around some axis in a molecule takes place at room temperature; the new principal axes of the (8)
T. Iohikawa, M. Iwasaki, and K. Kuwata, J. Chem. Phys., 44,
2979 (1966).
volume 78, Number 8 Auguat 1959
2668
KAZUMI TORIYAMA AND MACHIO IWASAKI
g tensor should be formed as a result of the partial averaging by molecular motion, resulting in an axially symmetric tensor as described in the foregoing section. Actually, the observed g tensor at room temperature has reduced principal values with an approximately axial symmetry. Therefore, one may be able to assume g'z) = gLrot, and that the that 9'3 E= gllrot and '/2 (g'l direction of 9'3 E= gllrot is along the rotational axis. If these assumptions are allowed, the angle between the 0-0 bond and the rotational axis is directly determined to be 76" from the directions of ga and gf3 (= grot). Since the complementary angle 104" is reasonable as the bond angle
