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Paramagnetic resonance of the reactionproducts of the free radical tri-p-nitrophenylmethyl and oxygen
Schimmel, F.M. Heineken, F.W. 1957
Physica XXlII 781-784
PARAMAGNETIC RESONANCE OF THE REACTIONPRODUCTS OF T H E F R E E RADICAL T R I - p - N I T R O P H E N Y L M E T H Y L AND OXYGEN by F. M. SCHIMMEL and F. W. H E I N E K E N Zeeman Laboratorium, Universiteit van Amsterdam, Nederland.
Synopsis Paramagnetic resonance of the free radical tri-p-nitrophenylmethyl is measured at a frequency of 9000 Mc s-1. By admitting air to the sample placed in the cavity in the magnet gap, a second resonance absorption is observed. A. possible explanation, is given and some suggestions for future research are made.
1. Introduction. Paramagnetic resonance of tri-p-nitrophenylmethyl, in the following called TpNPM, has already been detected by several authors 1)2)3). Their value of the g-factor and of the half-value width of this free radical are confirmed by our measurements. TpNPM is a free radical with chemical formula (CsHcNO2)3C', molecular weight 378.33 and stable in nitrogen but not stable in air. The crystals are dark-blue. The molecular structureformula is shown in fig. 2. The sample was prepared at the Laboratory for organic Chemistry of our University, following the instructions of L e f f l e r 4). 2. Description o/ apparatus. Our set-up is very similar to the one used by U e b e r s f e l d 5)6). The main magnetic field is provided by an electromagnet giving a field of 2.5 105 A m-1 in a gap of 1.5 cm. This field is sine-wave modulated at a frequency of 50 cycles s-1 and at any value between 5 102 A m -1 and 20 102 A m -1. The field could be measured by means of proton resonance. A 723A/B klystron (9000 Mc s-1) with electronic stabilized power supply feeds power to a rectangular TE01 cavity, containing the sample. The cavity is placed in the gap of the magnet and coupled to one of the arms of a balanced microwave bridge made by a magic T. When the magnetic field is swept through the resonance region of the sample, this bridge is no longer in balance because of the paramagnetic absorption in the cavity. The out-of-balance signal is fed to a low frequency detector and amplifying system. The resulting resonance curve is displayed on an oscilloscope having a sine-wave timebase. With this set-up 5.1016 paramagnetic centra of c¢~-diphenyl-#-picryl hydrazyl can be observed. -
-
781
-
-
782
F. M. SCHIMMELAND F. W. HEINEKEN
W i t h a 35 m m camera pictures of the oscilloscope p a t t e r n are m a d e at a scale of 1 : 1. The resulting pictures are enlarged. The horizontal axis (magnetic field strength) is calibrated with the aid of paramagnetic resonance of ~-diphenyl-fl-picryl hydrazyl.
3. Results and discussion. All measurements are carried out on a polycrystalline sample at room temperature. T p N P M in a nitrogen atmosphere gives a single line with a g-factor of 2.0036 and a half-value width AH½ of 103 A m -1. On a d m i t t i n g air to the sample a second resonance line appears (fig. 1) with a g-factor of 2.006 and a half-value width AH½ of
AEMLK)RPTION
TpNPM
peroxide of
TpNPM H. 10-5 (AMP M-~) 2.4196
2.4199
Fig. 1. Resonance lines half an hour after admitting air to the sample. 72 A m -1. Each half hour a picture of the oscilloscope screen was m a d e till after about 48 hours the concentration of the free radicals was too low to be observed a n y longer (fig. 2). The colour of the sample h a d t h e n changed from dark-blue to yellow. The original resonance in pure T p N P M is due to the unpaired electron, which is localized near the m e t h y l carbon atom. We assume t h a t we can ascribe the second resonance line to the formation of the free radical (C6H4N02)3CO0" according to the reaction equations
(C6H4N02)3C" + 02 -+ (C6H4N02)3CO0" (C6H4NO2)zCO0" + (C6H4N02)3C"-+ (C6H4NO2)3COOC(C6H4N02)3 This is confirmed by the disappearance of the second line after evacuating the sample. In this case the free radical (C6H4N02)3CO0", which is formed,
PARAMAGNETIC
RESONANCE
OF T p N P M A N D ITS P E R O X I D E
783
reacts with TpNPM and only paramagnetic resonance is observed of the remaining TpNPM. There is a difference between the half-value widths of the two free radicals, the resonance line of TpNPM being broader than the one of the intermediate reaction product (C6H4NO2)3COO'. An explanation is, that the resonance line of TpNPM is broadened b y the dipole-dipole interaction between the spin of the free electron and the spins of the protons of the phenyl-groups. In a dilute solution of tri-phenyl methyl the hyperfine structure, caused b y this dipole-dipole interaction, has been observed 7).
'LOG. I NTENSITY 40
2C
(orbitrQry units)
o
y
10_ 8_ 6_ 4
peroxide of
[~
~ t rt - p - nt trophenylmethyl ° -
o o o
1_ O.8 0~5 C~4
NO2
o~
1
2
3
4
5
7
1
1
12
13
14
15,
H5
1
18
TIME IN HOURS
Fig. 2. Decrease of t h e c o n c e n t r a t i o n of t h e free radicals as a f u n c t i o n of t h e t i m e .
The hyperfine structure of (C6HaNO~)3COO" probably will be less predominant, as the free electron, being localized at the oxygen atom without nuclear moment, is more isolated from the phenyl-groups. In addition the lower g-factor of TpNPM suggests that the free electron is more delocalized than in the intermediate free radical. This could be in accordance with the structural formula of the latter. It seems, that we have found a possibility of measuring certain reaction velocities and of indicating the intermediate products, whereas up to now no other methods as sensitive as this one are known to do this. Of course, if possible, measurements like this should be done in a homogeneous sohition rather than on a polycrystalline substance.
784
PARAMAGNETIC RESONANCE OF TpNPM AND ITS PEROXIDE
This work is part of the research program of the Foundation for fundamental Research of M a t t e r (F.O.M.). Financial support by the Organization for pure scientific Research (ZWO) is gratefully acknowledged. The auth6rs w i s h t o express their sincere thanks to Prof. dr F. L. J. S i x m a and dr F. B r u i n for the stimulating and valuable discussions and to Prof. dr G. W. R a t h e n a u for his interest in this investigation. Received 12-6-57.
REFERENCES 1) t h u , T. L., Pake, G. E., Paul, D. E., T o w n s e n d , J., W e i s s m a n , S. E., J. phys. Chem. 57 (1953) 504. 2) L l o y d , J. P., Pake, G. E., Phys. Rev. 92 (1953) 1576L. 3) Singer, L. S., S p e n c e r , E. G., J. chem. Phys. 21 (1953) 939L. 4) L e f f l e r , J. E., J. Am. chem. Soc. 75 (1953) 3598. 5) U e b e r s f e l d , J., Onde 61ect. 35 (1955) 492. 6) U e b e r s f e l d , J., Thesis, Paris (1955). 7) J a r r e t , H. S., Sloan, G. J., J. chem. Phys. 22 (1954) 1783.
Physica XXlII 781-784
PARAMAGNETIC RESONANCE OF THE REACTIONPRODUCTS OF T H E F R E E RADICAL T R I - p - N I T R O P H E N Y L M E T H Y L AND OXYGEN by F. M. SCHIMMEL and F. W. H E I N E K E N Zeeman Laboratorium, Universiteit van Amsterdam, Nederland.
Synopsis Paramagnetic resonance of the free radical tri-p-nitrophenylmethyl is measured at a frequency of 9000 Mc s-1. By admitting air to the sample placed in the cavity in the magnet gap, a second resonance absorption is observed. A. possible explanation, is given and some suggestions for future research are made.
1. Introduction. Paramagnetic resonance of tri-p-nitrophenylmethyl, in the following called TpNPM, has already been detected by several authors 1)2)3). Their value of the g-factor and of the half-value width of this free radical are confirmed by our measurements. TpNPM is a free radical with chemical formula (CsHcNO2)3C', molecular weight 378.33 and stable in nitrogen but not stable in air. The crystals are dark-blue. The molecular structureformula is shown in fig. 2. The sample was prepared at the Laboratory for organic Chemistry of our University, following the instructions of L e f f l e r 4). 2. Description o/ apparatus. Our set-up is very similar to the one used by U e b e r s f e l d 5)6). The main magnetic field is provided by an electromagnet giving a field of 2.5 105 A m-1 in a gap of 1.5 cm. This field is sine-wave modulated at a frequency of 50 cycles s-1 and at any value between 5 102 A m -1 and 20 102 A m -1. The field could be measured by means of proton resonance. A 723A/B klystron (9000 Mc s-1) with electronic stabilized power supply feeds power to a rectangular TE01 cavity, containing the sample. The cavity is placed in the gap of the magnet and coupled to one of the arms of a balanced microwave bridge made by a magic T. When the magnetic field is swept through the resonance region of the sample, this bridge is no longer in balance because of the paramagnetic absorption in the cavity. The out-of-balance signal is fed to a low frequency detector and amplifying system. The resulting resonance curve is displayed on an oscilloscope having a sine-wave timebase. With this set-up 5.1016 paramagnetic centra of c¢~-diphenyl-#-picryl hydrazyl can be observed. -
-
781
-
-
782
F. M. SCHIMMELAND F. W. HEINEKEN
W i t h a 35 m m camera pictures of the oscilloscope p a t t e r n are m a d e at a scale of 1 : 1. The resulting pictures are enlarged. The horizontal axis (magnetic field strength) is calibrated with the aid of paramagnetic resonance of ~-diphenyl-fl-picryl hydrazyl.
3. Results and discussion. All measurements are carried out on a polycrystalline sample at room temperature. T p N P M in a nitrogen atmosphere gives a single line with a g-factor of 2.0036 and a half-value width AH½ of 103 A m -1. On a d m i t t i n g air to the sample a second resonance line appears (fig. 1) with a g-factor of 2.006 and a half-value width AH½ of
AEMLK)RPTION
TpNPM
peroxide of
TpNPM H. 10-5 (AMP M-~) 2.4196
2.4199
Fig. 1. Resonance lines half an hour after admitting air to the sample. 72 A m -1. Each half hour a picture of the oscilloscope screen was m a d e till after about 48 hours the concentration of the free radicals was too low to be observed a n y longer (fig. 2). The colour of the sample h a d t h e n changed from dark-blue to yellow. The original resonance in pure T p N P M is due to the unpaired electron, which is localized near the m e t h y l carbon atom. We assume t h a t we can ascribe the second resonance line to the formation of the free radical (C6H4N02)3CO0" according to the reaction equations
(C6H4N02)3C" + 02 -+ (C6H4N02)3CO0" (C6H4NO2)zCO0" + (C6H4N02)3C"-+ (C6H4NO2)3COOC(C6H4N02)3 This is confirmed by the disappearance of the second line after evacuating the sample. In this case the free radical (C6H4N02)3CO0", which is formed,
PARAMAGNETIC
RESONANCE
OF T p N P M A N D ITS P E R O X I D E
783
reacts with TpNPM and only paramagnetic resonance is observed of the remaining TpNPM. There is a difference between the half-value widths of the two free radicals, the resonance line of TpNPM being broader than the one of the intermediate reaction product (C6H4NO2)3COO'. An explanation is, that the resonance line of TpNPM is broadened b y the dipole-dipole interaction between the spin of the free electron and the spins of the protons of the phenyl-groups. In a dilute solution of tri-phenyl methyl the hyperfine structure, caused b y this dipole-dipole interaction, has been observed 7).
'LOG. I NTENSITY 40
2C
(orbitrQry units)
o
y
10_ 8_ 6_ 4
peroxide of
[~
~ t rt - p - nt trophenylmethyl ° -
o o o
1_ O.8 0~5 C~4
NO2
o~
1
2
3
4
5
7
1
1
12
13
14
15,
H5
1
18
TIME IN HOURS
Fig. 2. Decrease of t h e c o n c e n t r a t i o n of t h e free radicals as a f u n c t i o n of t h e t i m e .
The hyperfine structure of (C6HaNO~)3COO" probably will be less predominant, as the free electron, being localized at the oxygen atom without nuclear moment, is more isolated from the phenyl-groups. In addition the lower g-factor of TpNPM suggests that the free electron is more delocalized than in the intermediate free radical. This could be in accordance with the structural formula of the latter. It seems, that we have found a possibility of measuring certain reaction velocities and of indicating the intermediate products, whereas up to now no other methods as sensitive as this one are known to do this. Of course, if possible, measurements like this should be done in a homogeneous sohition rather than on a polycrystalline substance.
784
PARAMAGNETIC RESONANCE OF TpNPM AND ITS PEROXIDE
This work is part of the research program of the Foundation for fundamental Research of M a t t e r (F.O.M.). Financial support by the Organization for pure scientific Research (ZWO) is gratefully acknowledged. The auth6rs w i s h t o express their sincere thanks to Prof. dr F. L. J. S i x m a and dr F. B r u i n for the stimulating and valuable discussions and to Prof. dr G. W. R a t h e n a u for his interest in this investigation. Received 12-6-57.
REFERENCES 1) t h u , T. L., Pake, G. E., Paul, D. E., T o w n s e n d , J., W e i s s m a n , S. E., J. phys. Chem. 57 (1953) 504. 2) L l o y d , J. P., Pake, G. E., Phys. Rev. 92 (1953) 1576L. 3) Singer, L. S., S p e n c e r , E. G., J. chem. Phys. 21 (1953) 939L. 4) L e f f l e r , J. E., J. Am. chem. Soc. 75 (1953) 3598. 5) U e b e r s f e l d , J., Onde 61ect. 35 (1955) 492. 6) U e b e r s f e l d , J., Thesis, Paris (1955). 7) J a r r e t , H. S., Sloan, G. J., J. chem. Phys. 22 (1954) 1783.