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Triplet state in an organic nitroxide
Volume
CHEMICAL
26, number-3
PHYSICS
1 June 1974
LETTERS
TRIPLET STATE IN AN ORGANIC NITROXIDE F.
GENOUD, M.-C. SCHOULER and M. DECORPS
D&artement de Recherche Fondamentale. Section de R&ononce Magnhique, Centre d’Etudes Nucl&ires de Grenoble. 38041 Grenoble-Cedex. France
Received 2 January Revised manuscript received
1974 8 hfarch
1974
The dimerization of the 9-aza-bicycle 13,3,11 nonan-3-one-9-oxyl in the solid state is investigated by use of ESR spectroscopy. Tire tine-structure data identify the spectrum as arising from a thermally accessible triplet state. The singlet-triplet energy gap derived from the observed temperature dependence of the ESR signal intensity isJe = 0.4 1 - 0.04 eV and the zero-tield splitting parameters are D = 0.0705 2 0.0010 cm-’ and E = 0.0042 + 0.0005 cm-‘.
Some of the nitroxide free radicals dimerize in the solid state. For instance this is the case for nitric oxide NO [l] , Fremy’s salt [2,3], 9-aza bicycle 13,3,11 nonan-3-one-9-oxyl [4, S] (radical I), nitroxide I ,5dimethyl S-aza bicycle 13,2,11 octan-3-one-Soxyl [6, 71 (radical II) (fig. I>_ Each dimer is centrosymmetric and the dimerization occurs between the two N-O bonds. The conformation of the N-O groups around the centre of symmetry is shown in fig. 2. The distances and angles are listed in table 1. It appears from these values that the distance Q between two N-O bonds is too great to allow the bond to be a covalent one (as a peroxide link for example). Recently a thermally accessible triplet state has been observed by ESR in the two distinct crystalline forms of Fremy’s salt [S, 91 and by static susceptibility meas-
\N \
b ‘iu
0
‘. \
‘\
a’,
\
\
\
‘. ‘. ‘.
\
6
Fig. 2. DimerS
\
‘\
+
conformation
‘“\ \ around
the centre
of symmetry.
urements in radical II [lo] _We report, in this paper, that a thermally accessible triplet state has also been detected by ESR in radical I. Besides the singlet-triplet energy gap J, we give here the zero-field splitting parameters derived from ESR measurements. At room temperature radical I is pammagnetic in Table 1 Relative
0
0
Flemys
YU
Fig. 1~MoIecularstructures
radica,
I.
of Fremy!,s
r.db.l
salt &of
radicals
spatial
positions
Fremy’s salt (brown form) 131
of the two NQ groups
2.86.
1.28
88
II
I
radical
I [S J
2.278.1.289
89.9
radical
II [7]
3.759.1.274
54.9
:
.
CHEMICAL
Volume 26. number 3
PHYSICS
LmERS
I June 1974
300 Gauss
GAIN
X4
Fig. 3. X band fust derivative ESR spectrum of an arbitrarily oriented crystal of radical I ac323”K. A, Am, = +-2: Xl, Xa. YI, Yz, Am, = + I; B, monomers.
solution and diamagnetic in the solid state. The crystal is monoclinic and there are two dimers per unit cell, each at a centre of symmetry [5] _The ESR singIe crystal spectrum fi band), very anisotropic, is characteristic of a thermally accessible triplet state [I l] . The ESR spectrum, at 323OK, of an arbitrarily oriented crystal is shown in fig. 3. The spectrum consists of two pairs of absorption lines (K, Y) each pair arranged about g = 2 (transitions Am, = f 1) (ascribed to the two sites of the dimers) and a weak anisotropic half-field transition (Am, = 5 2). The intensities of the Am, = + 1 and Am, = + 2 transitions increase with the temperature. According to.the crystal orientation, these lines can exhibit a hyperfme structure; for some crystalline orientations the hyperfme structure of the transitions m, = 0 * + 1 is entirely different from that of m, = -I* Q. This anomaly has been previously obServed and explained by lwasaki et al. in a number of radical pairs [12]. Moreover we can see a very intense structured absorption in the centre of the Am, = f 1 spectrum, which is attributed to monomers (the proportion of monomers with respect to dimers is about 2 %). Because of the decomposition of the radical at high temperatures, the temperature range in which intensities may be’measured is approximately from 280 to 355°K. In this temperature range the exchange interactions are weak, so the positions and the line shapes :
;
:
.. ..y
.; ‘.
are constant. Therefore the integrated ESR absorption area I is proportional to the first derivative ESR line intensity. A plot of log IT versus T-1 yields a straight line as predicted by the singlet-triplet model [I 31 (fig. 4). However, if the energy gap Jis linearly dependent on temperature as observed in Fremy’s salt [9], we cannot see the linear term in Tin a relative intensity measurement [14] . Therefore this plot gives a straight line of slope Jo (J = J,(l + (~7)). The value of Jo derived from these data is J,, = 0.41 + 0.04 eV. The hamiltonian of the system is given by log IT
i
24
kl.
3.z
L4
‘S’K!
~,
kg< 4. Temperature dependence of ;he fmt derivative Eg.R ‘lily intensity: Ip;gmwsus 2-l. -. ..’ ., . -. i $!f ‘if -. -, ..;,: ;‘.: :: ‘.’ ,.__..._ .’ ._ _. -.; *. : ..:.: _‘T... I. __ ._ .\.‘. ;,- ,_.,. :.. .‘Y,.. : :_ ,: ,;:: .. : _-:..:-_ ...:,::“. _ :.-_>, -; J .I ;.._-~y_:.-. :.. .:. :. ,_: :_ ‘. __,:. ‘,y _... -.~_,-_~-:‘_-... .: _,, :-~_.X1..~ ( .; _ : :, T.i ., .., ., _.-.;,; ‘. ;..-‘-;.. -.,.-,.
Volume 26,
CHEMICAL PHYSICS LETTERS
number 3
1 June 1974
Table 2 Jo, D and E values for some nitroxide dimers Jo (eV)
DkV)
E(eV)
Fremy’s salt U~rownform) [9] Fremy’s salt (yellow form) [S] radical I (this work)
0.20 + 0.01
0.0692 i 0.0005
0.0037 f 0.0002
0.27 + 0.00
0.0746 + 0.0005
0.0044 * 0.0002
0.41 = 0.04
0.0705 f 0.0010
O-0042 t 0.0005
radical II [ 101
0.011
JC=p,B*g.S+DS;
+E(Sx’-S_;)-$D,
where D and E are the zero-field &tting parameters. These are determined from the experimental results obtained with a powdered sample. All the magnetic field measurements are made by determining the proton magnetic resonance frequency with a nuclear gauss meter. In a polycrystalline sample, the Am, = * 1 resonance occurs only for those molecules with a molecular axis parallel to the magnetic field. From the positions of these lines and with the exact solutions [15] of the hamiltonian (1) we obtain at 323°K:
are greatly indebted to R.M. Dupeyre and A. Rassat for giving us the samples used in our experiments.
References [l] W.J. Dulmage, E.A. Meyers and W.N. Lipscomb, Acta Cryst. 6 (1953) 760. W. Moser and R.A. Howie, J. Cbem. Sot. A (1968) 3039.
;:IA (1968) 3043.
R-A. Howie, L-SD.
Glasser and W. Moser, J. Chem. Sot.
I41 R.M. Dupeyre and A. Rassat, J. Am. Chem. Sot. 88 (1966) 3180.
I51 A. Capiomont, B. Chion and J. Lajzerowicz, Acta Cryst. B 27 (1971) 322.
D=O.O705 t 0.0010 cm-l, E = 0.0042 + 0.0005 cm-l.
These values are compared with those available for other nitroxide dimers in table 2. Work is in progress to determine the directions of the axes of the fine and hyperfine tensors. The presence of a triplet statejs characteristic of the four centre bond describing the dimerization of Fremy’s salt, radical I and radical II. This mechanism explains probably the dimerization of some other solid nitroxlde free radicals (for instance bis (trifluoromethyl) nitroxide 1161, tetraphenylpyrrole nitroxide [17], nortropane N-oxyl [Iii]). If this is the case, they must exhibit a triplet state ESR spectrum (if the triplet state is thermally accessible, i.e., Js 20 k7’). Radical I has been synthesized in the Laboratory: of “Chimie Organique Physique of C.E.N.G.” arid we.. .416
:
161 J. Ronzaud and A. Rassat. Brevet Fraqais EN 700 1865 (1970).
I71 A. Capiomont, Acta Cryst- B 29 (1973) 1720. ISI B.D. Perlson and D.B. Russell, J. Chem. Sot. Chem. Commun. (1972) 69.
t91 M. Decorps. F. Genoud and M.C. Schouler, Mol. Phys. 26 (1973) 237.
1101 C. Veyret and A. Blaise, Mol. E’hys. 25 (1973) 873. 1111 D-B. Chesnut and W.D. Phillips, J. Chem. Phys. 35 (1961) 1002.
IL21 M. Iwasaki, K. Minakata and K. Toriyama, J. Chem. Phys. 54 (1971) 3225. Innes, J. Chem. Phys. 30 (1959) 765. Ii41 R-G. Kepler, J. Chem. Phys. 39 (1963) 3528. I151 E. Wasserman, .L.C. Snyder and W.A. Yager, J..Chem. Phys. 41 (1964) 1743. IL61 W.D. Blackley &td R.R. Reinhart, J. Am. Chem. Sot. 87 (1965) 802. I171 R_ Bamasseul. A. Rassat, G. Rio and M.J. Scholl, Bull. Sot. Chim. France (1971) 215. 1181 G.D. Mendenhall and K.U. Ingold, J. Am: Chem. Sot. 95 (1973)63?0..
I131 D. Bijl, H. Kainer and A.C. Rose
j
CHEMICAL
26, number-3
PHYSICS
1 June 1974
LETTERS
TRIPLET STATE IN AN ORGANIC NITROXIDE F.
GENOUD, M.-C. SCHOULER and M. DECORPS
D&artement de Recherche Fondamentale. Section de R&ononce Magnhique, Centre d’Etudes Nucl&ires de Grenoble. 38041 Grenoble-Cedex. France
Received 2 January Revised manuscript received
1974 8 hfarch
1974
The dimerization of the 9-aza-bicycle 13,3,11 nonan-3-one-9-oxyl in the solid state is investigated by use of ESR spectroscopy. Tire tine-structure data identify the spectrum as arising from a thermally accessible triplet state. The singlet-triplet energy gap derived from the observed temperature dependence of the ESR signal intensity isJe = 0.4 1 - 0.04 eV and the zero-tield splitting parameters are D = 0.0705 2 0.0010 cm-’ and E = 0.0042 + 0.0005 cm-‘.
Some of the nitroxide free radicals dimerize in the solid state. For instance this is the case for nitric oxide NO [l] , Fremy’s salt [2,3], 9-aza bicycle 13,3,11 nonan-3-one-9-oxyl [4, S] (radical I), nitroxide I ,5dimethyl S-aza bicycle 13,2,11 octan-3-one-Soxyl [6, 71 (radical II) (fig. I>_ Each dimer is centrosymmetric and the dimerization occurs between the two N-O bonds. The conformation of the N-O groups around the centre of symmetry is shown in fig. 2. The distances and angles are listed in table 1. It appears from these values that the distance Q between two N-O bonds is too great to allow the bond to be a covalent one (as a peroxide link for example). Recently a thermally accessible triplet state has been observed by ESR in the two distinct crystalline forms of Fremy’s salt [S, 91 and by static susceptibility meas-
\N \
b ‘iu
0
‘. \
‘\
a’,
\
\
\
‘. ‘. ‘.
\
6
Fig. 2. DimerS
\
‘\
+
conformation
‘“\ \ around
the centre
of symmetry.
urements in radical II [lo] _We report, in this paper, that a thermally accessible triplet state has also been detected by ESR in radical I. Besides the singlet-triplet energy gap J, we give here the zero-field splitting parameters derived from ESR measurements. At room temperature radical I is pammagnetic in Table 1 Relative
0
0
Flemys
YU
Fig. 1~MoIecularstructures
radica,
I.
of Fremy!,s
r.db.l
salt &of
radicals
spatial
positions
Fremy’s salt (brown form) 131
of the two NQ groups
2.86.
1.28
88
II
I
radical
I [S J
2.278.1.289
89.9
radical
II [7]
3.759.1.274
54.9
:
.
CHEMICAL
Volume 26. number 3
PHYSICS
LmERS
I June 1974
300 Gauss
GAIN
X4
Fig. 3. X band fust derivative ESR spectrum of an arbitrarily oriented crystal of radical I ac323”K. A, Am, = +-2: Xl, Xa. YI, Yz, Am, = + I; B, monomers.
solution and diamagnetic in the solid state. The crystal is monoclinic and there are two dimers per unit cell, each at a centre of symmetry [5] _The ESR singIe crystal spectrum fi band), very anisotropic, is characteristic of a thermally accessible triplet state [I l] . The ESR spectrum, at 323OK, of an arbitrarily oriented crystal is shown in fig. 3. The spectrum consists of two pairs of absorption lines (K, Y) each pair arranged about g = 2 (transitions Am, = f 1) (ascribed to the two sites of the dimers) and a weak anisotropic half-field transition (Am, = 5 2). The intensities of the Am, = + 1 and Am, = + 2 transitions increase with the temperature. According to.the crystal orientation, these lines can exhibit a hyperfme structure; for some crystalline orientations the hyperfme structure of the transitions m, = 0 * + 1 is entirely different from that of m, = -I* Q. This anomaly has been previously obServed and explained by lwasaki et al. in a number of radical pairs [12]. Moreover we can see a very intense structured absorption in the centre of the Am, = f 1 spectrum, which is attributed to monomers (the proportion of monomers with respect to dimers is about 2 %). Because of the decomposition of the radical at high temperatures, the temperature range in which intensities may be’measured is approximately from 280 to 355°K. In this temperature range the exchange interactions are weak, so the positions and the line shapes :
;
:
.. ..y
.; ‘.
are constant. Therefore the integrated ESR absorption area I is proportional to the first derivative ESR line intensity. A plot of log IT versus T-1 yields a straight line as predicted by the singlet-triplet model [I 31 (fig. 4). However, if the energy gap Jis linearly dependent on temperature as observed in Fremy’s salt [9], we cannot see the linear term in Tin a relative intensity measurement [14] . Therefore this plot gives a straight line of slope Jo (J = J,(l + (~7)). The value of Jo derived from these data is J,, = 0.41 + 0.04 eV. The hamiltonian of the system is given by log IT
i
24
kl.
3.z
L4
‘S’K!
~,
kg< 4. Temperature dependence of ;he fmt derivative Eg.R ‘lily intensity: Ip;gmwsus 2-l. -. ..’ ., . -. i $!f ‘if -. -, ..;,: ;‘.: :: ‘.’ ,.__..._ .’ ._ _. -.; *. : ..:.: _‘T... I. __ ._ .\.‘. ;,- ,_.,. :.. .‘Y,.. : :_ ,: ,;:: .. : _-:..:-_ ...:,::“. _ :.-_>, -; J .I ;.._-~y_:.-. :.. .:. :. ,_: :_ ‘. __,:. ‘,y _... -.~_,-_~-:‘_-... .: _,, :-~_.X1..~ ( .; _ : :, T.i ., .., ., _.-.;,; ‘. ;..-‘-;.. -.,.-,.
Volume 26,
CHEMICAL PHYSICS LETTERS
number 3
1 June 1974
Table 2 Jo, D and E values for some nitroxide dimers Jo (eV)
DkV)
E(eV)
Fremy’s salt U~rownform) [9] Fremy’s salt (yellow form) [S] radical I (this work)
0.20 + 0.01
0.0692 i 0.0005
0.0037 f 0.0002
0.27 + 0.00
0.0746 + 0.0005
0.0044 * 0.0002
0.41 = 0.04
0.0705 f 0.0010
O-0042 t 0.0005
radical II [ 101
0.011
JC=p,B*g.S+DS;
+E(Sx’-S_;)-$D,
where D and E are the zero-field &tting parameters. These are determined from the experimental results obtained with a powdered sample. All the magnetic field measurements are made by determining the proton magnetic resonance frequency with a nuclear gauss meter. In a polycrystalline sample, the Am, = * 1 resonance occurs only for those molecules with a molecular axis parallel to the magnetic field. From the positions of these lines and with the exact solutions [15] of the hamiltonian (1) we obtain at 323°K:
are greatly indebted to R.M. Dupeyre and A. Rassat for giving us the samples used in our experiments.
References [l] W.J. Dulmage, E.A. Meyers and W.N. Lipscomb, Acta Cryst. 6 (1953) 760. W. Moser and R.A. Howie, J. Cbem. Sot. A (1968) 3039.
;:IA (1968) 3043.
R-A. Howie, L-SD.
Glasser and W. Moser, J. Chem. Sot.
I41 R.M. Dupeyre and A. Rassat, J. Am. Chem. Sot. 88 (1966) 3180.
I51 A. Capiomont, B. Chion and J. Lajzerowicz, Acta Cryst. B 27 (1971) 322.
D=O.O705 t 0.0010 cm-l, E = 0.0042 + 0.0005 cm-l.
These values are compared with those available for other nitroxide dimers in table 2. Work is in progress to determine the directions of the axes of the fine and hyperfine tensors. The presence of a triplet statejs characteristic of the four centre bond describing the dimerization of Fremy’s salt, radical I and radical II. This mechanism explains probably the dimerization of some other solid nitroxlde free radicals (for instance bis (trifluoromethyl) nitroxide 1161, tetraphenylpyrrole nitroxide [17], nortropane N-oxyl [Iii]). If this is the case, they must exhibit a triplet state ESR spectrum (if the triplet state is thermally accessible, i.e., Js 20 k7’). Radical I has been synthesized in the Laboratory: of “Chimie Organique Physique of C.E.N.G.” arid we.. .416
:
161 J. Ronzaud and A. Rassat. Brevet Fraqais EN 700 1865 (1970).
I71 A. Capiomont, Acta Cryst- B 29 (1973) 1720. ISI B.D. Perlson and D.B. Russell, J. Chem. Sot. Chem. Commun. (1972) 69.
t91 M. Decorps. F. Genoud and M.C. Schouler, Mol. Phys. 26 (1973) 237.
1101 C. Veyret and A. Blaise, Mol. E’hys. 25 (1973) 873. 1111 D-B. Chesnut and W.D. Phillips, J. Chem. Phys. 35 (1961) 1002.
IL21 M. Iwasaki, K. Minakata and K. Toriyama, J. Chem. Phys. 54 (1971) 3225. Innes, J. Chem. Phys. 30 (1959) 765. Ii41 R-G. Kepler, J. Chem. Phys. 39 (1963) 3528. I151 E. Wasserman, .L.C. Snyder and W.A. Yager, J..Chem. Phys. 41 (1964) 1743. IL61 W.D. Blackley &td R.R. Reinhart, J. Am. Chem. Sot. 87 (1965) 802. I171 R_ Bamasseul. A. Rassat, G. Rio and M.J. Scholl, Bull. Sot. Chim. France (1971) 215. 1181 G.D. Mendenhall and K.U. Ingold, J. Am: Chem. Sot. 95 (1973)63?0..
I131 D. Bijl, H. Kainer and A.C. Rose
j