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Electron spin resonance studies of a mixed phenoxy - nitroxide biradical
Volume 2. number 5
September 1968
CHElkIICALPHYSICS LETTERS
ELECTRON OF
A
MIXED
SPIN
RESONANCE
PHENOXY-NITROXIDE
STUDIES BIRADICAL
R. W. KREILICK Department of Chemistry,, University of Rochester, Rochester. NW York 14627, USA
Received 1 July 1968
The ESR spectrum of a mixed phenoxy-nitroxide biradical has been investigated. Spectra were observed from both strongly and weakly coupled conformers. The biradical was found to be in equilibrium with a paramagnetic dimer which contained two nitroxide ends.
. 6% r
1. INTRODUCTION
The electron spin resonance spectra of a number of nitroxide and phenoxy biradicals have been reported El]. These biradicals were made by bonding together either two nitroxide or two phenoxy ends with a suitable linking group, The ESR spectra of these biradicals depends on the rate of spin exchange between the two halves. The spin Hamiltonian which has been used to account for the hyperfine and exchange interactions in a biradical containing two nuclei can be written as [Z]: H = a&Z1
i- S2Z2) + JS1’S2.
0
-0
cl I 0.
COMPOUND
(1)
in this expression a is the hyperfine splitting constant and Sl,S2 and Zl,Z2 are the electron and nuclear spin operators, respectively_ The magnitude of J governs the electron exchange interaction. In instances in which J is small compared to a, the molecule acts as two separate monoradicals and one observes 21.~ 1 lines sqarated by a. When J is greater than a one obr,erves 4Z+ 1 lines separated by a/2. Both the case of strong and weak exchange have been observed from the nitroxide biradicals. The magnitude of J appears to depend on the relative conformations of these biradicals. We have been interested in the magnitude of the exchange interaction in biradicals containing two different monoradical halves. In order to obtain information about this problem we have investigated a biradical containing a phenoxy and a nitroxide end connected by an ester linkage (compound I).
I
(2)
2. EXPERIMENTAL a) Compounds: Biradical I was made by oxidizing the corresponding nitroxide-phenol with either PbO2 or a basic solution of E3PeCN6. The phenol was made in 55% yield by the reaction_ _of 3,5-dit-butyl-4-hydroxy-benzoyl chloride 13J with 2,2,6,6-tetramethyl-4-hydroxy-piperidine-loxide 143, melting point 174’. Analysis: calculated for C24H37N04: C, i’f -5; H, 9.2; found: c, 71.5; H, 9.5. b) Instruments: The ESR spectra were taken on a JEOLCO 3BSX X-band ESB spectrometer. Samples for the ESR experiments were made in sealed degassed apparatus. AH of the ESR experiments were run with toluene as :he salvent. 3. RESULTS AND DISCUSSION The ESR spectrum of the nitroxide-phenol monoradical exhibited 3 groups of 10 Ltnes (CN = 277
Volume 2, number 5
CHEMICAL PHYSICS LETTERS
.
I
1 0
September 1968
I
3.4
3.6
I
3.8
4.0
4.2
4.4
- 24-C +x
IO3
Fig. 2. A plot of Iog K versus the reciprocal of temperature.
Fig. 1. ESR spectra of biradical I at a series of tempemtures. 2i8
= 15.4 G). The spIittings from the protons was identical to that observed from the 2,2,6,5- tetramethyipiperidine-4-acetate-l-oxide radical 151. When the-monoradical was oxidized, the ESR spectrum shown in fig. 2A was observed. This spectrum can be analyzed by assuming it results ~from a superposition of spectra from three different paramagnetic species. Each of the three species has a slightly different g-value, The spectrum can be interpreted in terms of a spectrum with three lines of equal intensity (spectrum 1, a = 15.4 G), a second three-line spectrum with a smaller splitting (spectrum 2, (z = 7.7 G), and a spectrum of three lines of relative intensity 1 : 2 : 1 (spectrum 3, a = 2.14 G). The center of spectrum 1 is 0.85 G to low field of the center of spectrum 2 and the center of spectrum 3 is 1.42 G to high field of spectrum 2. The area of the lines in spectrum 2 are a factor of ten greater than the area of the peaks in spectrum 1 at 22OC and a factor of ‘75 greater than the area of the outer line of spectrum 3. The reIative areas of the lines show a sharp temperature dependence (fig. lj. As the temperature is lowered the areas of the lines in spectra 2 and 3 decrease while the area of spectrum 1 increases. At low enough temperature, one observes a single three-line spectrum. In the limit of slow spin exchange (o > J) the two halves of the biradical are independent and one should observe two superimposed spectra. The nitroxide end-of the biradical should show three lines of equal~intensity while the phenoxy
Volume 2. number 5
CHEMICAL PHYSICS LETTERS
September
1968
end should give three lines of intensity 1: 2 : 1. When spin exchange is rapid (J > a) one predicts three groups of triplets with nitrogen and phenoxy ring splittings one half the magnitude of those observed for the weakly coupled case. The lines in spectrum 3 and spectrum 1 correspond to the peaks predicted for the weakly coupled CaSe (a~ = 15.4 G, “(phenoxy rin ) = 2.14 G). Spectrum 2 corresponds to the s9 rongly coupled case (QN = 7.7 G). The lines in spectrum 2 are somewhat broader than those of spectrum 1 or 3 and we were -unable to resolve the splitting from the meta-phenoxy protons. These two spectra are probably a result of different conformations of the biradical. Conformations favoring spin exchange appear to be preferred as the area of the lines from the strongly coupled molecules is much greater than that from the weakly coupled molecules. The temperature dependence of the spectra can be explained by a monomer-dimer equilibrium. Previous work has shown that phenoxy esters of this type are in rapid equilibrium with a dimer [6]. The dimerization reaction can be written as:
DIMER
MONOMER K_
bONOMEfl* CDIMER]
The dimer is a nitroxide biradical which could exhibit two different spectra. If the nitroxide ends are weakly coupled one predicts a threeline spectrum while if they are strongly coupIed one predicts five lines of intensity 1 : 2 : 3 : 2 : 1 with one half the splitting of the weakly coupled case. The observed spectra correspond to the weakly coupled case. The lines from the dimer are superimposed on the three lines from the nitroxide end of the weakly coupled monomer. At low enough temperature (ca. -6O’C) the equilibrium is shifted almost completely to the side of the dimer and one observes a three-line spectrum. The equilibrium constant for the dimerization reaction can be estimated from the relative areas
ESR spectrum of the center Fig. 3. A) High-resolution group of lines of biradical I. B) High-resolution ESR spectrum of the low-field group of lines of biradicaL 1.
lines from the strongly coupled monomer and the dimer. A plot of log K versus the reciprocal of temperature is shown in fig. 2. The value of Ap for the dimerization reaction determined from this plot is 6.65 kc?!. Different samples of ihe biradical decomposed between 10 and 20 percent during the prolonged periods necessary to carry out the variable temperature experi. ments. This decomposition produces scme inaccuracy in our value of AB”. The value obtained is close to the value determined for the analogous cyclohexyl phenoxy ester in chloroform (A@ = ‘I,2 kcal). The lines from the weakly coupled monomer and the dimer could be resolved farther in dilute of the
279
vohme
2, number
5
CHEMICAL
PHYSICS
solutions. The center group of lines and the low -field group are shown in fig_ 3. The phenoq-lines are partially resolved into a doublet with a splitting of 0.31 G. This splitting probably results from the interaction of the phenoxy electron with the y proton on the nitroxide ring. The nitrogen lines could be resolved into ten peaks with a splitting of 0.41 G. Most of the intensity in these lines comes from the dimer. The ten Iine spectrum is identical with that observed from the 2,2,6,6tetramethylpiperidine-4-acetate-l-oxide radical. The proton couplings in this radical have been determined from NMR spectra 151, (a methyl) = = 0.41 G, a,g = 0.29 G, a,, .= 0.09 6). $ one assumes that the nitroxide end of the weakly coupled monomer gives the same splittings then both the nitroxide and phenoxy electrons interact with the y proton on the nitroxide ring.
ACKNOWLEDGEMENT This wash was supported in part by NationaI Science Foundation Grant GP 5482.
LETTERS
September
1968
REFERENCES J_ Chem.Phys. 47 S-1. Glarum and J. H. Marshall, (1967) 1374; G. R. Luckhurst. Mol. Phys. 10 (1966) 543: R. Briere, R. m. Dupeyre. H. Lema&e, C. Morat, A. Rassat and P. Rey. Bull. Sot. Chim. France 11 (1965) 3290; E. A. Chnndross, J. Am. Chem. Sot. 86 (1964) 1263; E. A. Chandross and R.Kreilick, J. Am. Chem. Sot. 86 (1964) 117; J. Am. Chem. Sot. 85 (1963) 2530. [2] C.P.SLichter. Phys.Rev. 99 (1955) 479; D.C. Reitz and S.I.Weissman, J. Chem. Phys. 33 (1960) 700. [3 J R. M&Her, A. Ricker, R. Mayer and K. Scheffer. Arm. Chem. 645 (1961) 36. [4] R. Briere. H. Lemaire and A. Rassat, Bull. Sot. Chim. France 11 (1965) 3273. [5] R. W.Kreilick, J. Chem. Phys. 46 (1967) 4260. [6] D. J. Williams and R.Kreilick, J.Am. Chem.Soc. 90 (1968) 2775. [l]
September 1968
CHElkIICALPHYSICS LETTERS
ELECTRON OF
A
MIXED
SPIN
RESONANCE
PHENOXY-NITROXIDE
STUDIES BIRADICAL
R. W. KREILICK Department of Chemistry,, University of Rochester, Rochester. NW York 14627, USA
Received 1 July 1968
The ESR spectrum of a mixed phenoxy-nitroxide biradical has been investigated. Spectra were observed from both strongly and weakly coupled conformers. The biradical was found to be in equilibrium with a paramagnetic dimer which contained two nitroxide ends.
. 6% r
1. INTRODUCTION
The electron spin resonance spectra of a number of nitroxide and phenoxy biradicals have been reported El]. These biradicals were made by bonding together either two nitroxide or two phenoxy ends with a suitable linking group, The ESR spectra of these biradicals depends on the rate of spin exchange between the two halves. The spin Hamiltonian which has been used to account for the hyperfine and exchange interactions in a biradical containing two nuclei can be written as [Z]: H = a&Z1
i- S2Z2) + JS1’S2.
0
-0
cl I 0.
COMPOUND
(1)
in this expression a is the hyperfine splitting constant and Sl,S2 and Zl,Z2 are the electron and nuclear spin operators, respectively_ The magnitude of J governs the electron exchange interaction. In instances in which J is small compared to a, the molecule acts as two separate monoradicals and one observes 21.~ 1 lines sqarated by a. When J is greater than a one obr,erves 4Z+ 1 lines separated by a/2. Both the case of strong and weak exchange have been observed from the nitroxide biradicals. The magnitude of J appears to depend on the relative conformations of these biradicals. We have been interested in the magnitude of the exchange interaction in biradicals containing two different monoradical halves. In order to obtain information about this problem we have investigated a biradical containing a phenoxy and a nitroxide end connected by an ester linkage (compound I).
I
(2)
2. EXPERIMENTAL a) Compounds: Biradical I was made by oxidizing the corresponding nitroxide-phenol with either PbO2 or a basic solution of E3PeCN6. The phenol was made in 55% yield by the reaction_ _of 3,5-dit-butyl-4-hydroxy-benzoyl chloride 13J with 2,2,6,6-tetramethyl-4-hydroxy-piperidine-loxide 143, melting point 174’. Analysis: calculated for C24H37N04: C, i’f -5; H, 9.2; found: c, 71.5; H, 9.5. b) Instruments: The ESR spectra were taken on a JEOLCO 3BSX X-band ESB spectrometer. Samples for the ESR experiments were made in sealed degassed apparatus. AH of the ESR experiments were run with toluene as :he salvent. 3. RESULTS AND DISCUSSION The ESR spectrum of the nitroxide-phenol monoradical exhibited 3 groups of 10 Ltnes (CN = 277
Volume 2, number 5
CHEMICAL PHYSICS LETTERS
.
I
1 0
September 1968
I
3.4
3.6
I
3.8
4.0
4.2
4.4
- 24-C +x
IO3
Fig. 2. A plot of Iog K versus the reciprocal of temperature.
Fig. 1. ESR spectra of biradical I at a series of tempemtures. 2i8
= 15.4 G). The spIittings from the protons was identical to that observed from the 2,2,6,5- tetramethyipiperidine-4-acetate-l-oxide radical 151. When the-monoradical was oxidized, the ESR spectrum shown in fig. 2A was observed. This spectrum can be analyzed by assuming it results ~from a superposition of spectra from three different paramagnetic species. Each of the three species has a slightly different g-value, The spectrum can be interpreted in terms of a spectrum with three lines of equal intensity (spectrum 1, a = 15.4 G), a second three-line spectrum with a smaller splitting (spectrum 2, (z = 7.7 G), and a spectrum of three lines of relative intensity 1 : 2 : 1 (spectrum 3, a = 2.14 G). The center of spectrum 1 is 0.85 G to low field of the center of spectrum 2 and the center of spectrum 3 is 1.42 G to high field of spectrum 2. The area of the lines in spectrum 2 are a factor of ten greater than the area of the peaks in spectrum 1 at 22OC and a factor of ‘75 greater than the area of the outer line of spectrum 3. The reIative areas of the lines show a sharp temperature dependence (fig. lj. As the temperature is lowered the areas of the lines in spectra 2 and 3 decrease while the area of spectrum 1 increases. At low enough temperature, one observes a single three-line spectrum. In the limit of slow spin exchange (o > J) the two halves of the biradical are independent and one should observe two superimposed spectra. The nitroxide end-of the biradical should show three lines of equal~intensity while the phenoxy
Volume 2. number 5
CHEMICAL PHYSICS LETTERS
September
1968
end should give three lines of intensity 1: 2 : 1. When spin exchange is rapid (J > a) one predicts three groups of triplets with nitrogen and phenoxy ring splittings one half the magnitude of those observed for the weakly coupled case. The lines in spectrum 3 and spectrum 1 correspond to the peaks predicted for the weakly coupled CaSe (a~ = 15.4 G, “(phenoxy rin ) = 2.14 G). Spectrum 2 corresponds to the s9 rongly coupled case (QN = 7.7 G). The lines in spectrum 2 are somewhat broader than those of spectrum 1 or 3 and we were -unable to resolve the splitting from the meta-phenoxy protons. These two spectra are probably a result of different conformations of the biradical. Conformations favoring spin exchange appear to be preferred as the area of the lines from the strongly coupled molecules is much greater than that from the weakly coupled molecules. The temperature dependence of the spectra can be explained by a monomer-dimer equilibrium. Previous work has shown that phenoxy esters of this type are in rapid equilibrium with a dimer [6]. The dimerization reaction can be written as:
DIMER
MONOMER K_
bONOMEfl* CDIMER]
The dimer is a nitroxide biradical which could exhibit two different spectra. If the nitroxide ends are weakly coupled one predicts a threeline spectrum while if they are strongly coupIed one predicts five lines of intensity 1 : 2 : 3 : 2 : 1 with one half the splitting of the weakly coupled case. The observed spectra correspond to the weakly coupled case. The lines from the dimer are superimposed on the three lines from the nitroxide end of the weakly coupled monomer. At low enough temperature (ca. -6O’C) the equilibrium is shifted almost completely to the side of the dimer and one observes a three-line spectrum. The equilibrium constant for the dimerization reaction can be estimated from the relative areas
ESR spectrum of the center Fig. 3. A) High-resolution group of lines of biradical I. B) High-resolution ESR spectrum of the low-field group of lines of biradicaL 1.
lines from the strongly coupled monomer and the dimer. A plot of log K versus the reciprocal of temperature is shown in fig. 2. The value of Ap for the dimerization reaction determined from this plot is 6.65 kc?!. Different samples of ihe biradical decomposed between 10 and 20 percent during the prolonged periods necessary to carry out the variable temperature experi. ments. This decomposition produces scme inaccuracy in our value of AB”. The value obtained is close to the value determined for the analogous cyclohexyl phenoxy ester in chloroform (A@ = ‘I,2 kcal). The lines from the weakly coupled monomer and the dimer could be resolved farther in dilute of the
279
vohme
2, number
5
CHEMICAL
PHYSICS
solutions. The center group of lines and the low -field group are shown in fig_ 3. The phenoq-lines are partially resolved into a doublet with a splitting of 0.31 G. This splitting probably results from the interaction of the phenoxy electron with the y proton on the nitroxide ring. The nitrogen lines could be resolved into ten peaks with a splitting of 0.41 G. Most of the intensity in these lines comes from the dimer. The ten Iine spectrum is identical with that observed from the 2,2,6,6tetramethylpiperidine-4-acetate-l-oxide radical. The proton couplings in this radical have been determined from NMR spectra 151, (a methyl) = = 0.41 G, a,g = 0.29 G, a,, .= 0.09 6). $ one assumes that the nitroxide end of the weakly coupled monomer gives the same splittings then both the nitroxide and phenoxy electrons interact with the y proton on the nitroxide ring.
ACKNOWLEDGEMENT This wash was supported in part by NationaI Science Foundation Grant GP 5482.
LETTERS
September
1968
REFERENCES J_ Chem.Phys. 47 S-1. Glarum and J. H. Marshall, (1967) 1374; G. R. Luckhurst. Mol. Phys. 10 (1966) 543: R. Briere, R. m. Dupeyre. H. Lema&e, C. Morat, A. Rassat and P. Rey. Bull. Sot. Chim. France 11 (1965) 3290; E. A. Chnndross, J. Am. Chem. Sot. 86 (1964) 1263; E. A. Chandross and R.Kreilick, J. Am. Chem. Sot. 86 (1964) 117; J. Am. Chem. Sot. 85 (1963) 2530. [2] C.P.SLichter. Phys.Rev. 99 (1955) 479; D.C. Reitz and S.I.Weissman, J. Chem. Phys. 33 (1960) 700. [3 J R. M&Her, A. Ricker, R. Mayer and K. Scheffer. Arm. Chem. 645 (1961) 36. [4] R. Briere. H. Lemaire and A. Rassat, Bull. Sot. Chim. France 11 (1965) 3273. [5] R. W.Kreilick, J. Chem. Phys. 46 (1967) 4260. [6] D. J. Williams and R.Kreilick, J.Am. Chem.Soc. 90 (1968) 2775. [l]