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Electron paramagnetic resonance evidence for molecular fluctuations in s-triazine
Volume
127, number
CHEMICAL
2
PHYSICS
LETTERS
6 June 1986
ELECTRON PARAMAGNETIC RESONANCE EVIDENCE FOR MOLECULAR FLUCTUATIONS IN s-TRIAZINE F.J. OWENS Energetic Materials L.aborarory Armament Received
13 November
Research and Development
1985; in final form 18 March
Center, Dover, NJ 07801. USA
1986
A pronounced inward shift of the extrema first-derivative EPR powder spectrum peaks of the nitroxide molecule, 2,2,6,6-tetramethyWoxopiperinooxy (TMOP) and the transition metal complex copper benzoyl acetonate doped into the triazine lattice is observed as the temperature is increased above -66OC. The effects are indicative of the onset of slow tumbling of the paramagnetic species in the crystal. The temperature dependence is Arrhenius, with a barrier to motion of 0.57 eV. Comparison with other measurements suggests the motion of the paramagnetic probes is a result of motion of the intrinsic triazine molecules in the solid.
1. Introduction Electron paramagnetic resonance has been shown to be useful in studying structural phase transitions in solids and the dynamic behavior of the substituents of the lattice during the phase transition. The temperature dependence of the order parameter and critical behavior near the transition temperature have been observed using electron paramagnetic resonance (EPR) [ 11. EPR spectra may be sensitive to order-disorder fluctuations that occur in the lattice. If the correlation time of the fluctuation is in the order of A ,, -A,, where A ,, is the parallel component of the axially symmetric EPR hyperfine tensor and A, the perpendicular component, the motion of the species or its environment will cause a change in the linewidth and hyperfme splitting. Recent theoretical and experimental developments which relate the changes in the spectrum to the correlation time of the motion allow quantitative information about the dynamics to be obtained [2-61. The molecular crystal of symmetric triazine (C3N3H3) undergoes a strain-induced elastic phase transition which has been studied by a number of spectroscopic methods [7-91. The phase transition occurs at 200 K at ambient pressure and the unit cell changes from trigonal to monoclinic. Neutron inelastic scattering indicates the primary displacement is a zone-centered transverse acoustic shearing mode as well as a ro-
tation of the triazine molecule [ 10 1. The absence of central peak scattering in the region of the transition temperature indicated that there are no dynamical fluctuations associated with the phase transition [lo] . In order to further investigate the possibility of molecular fluctuations in s-triazine the material doped with two structurally quite different probes, 2,2,6,6_tetramethyl4-oxopiperinooxy (TMOP) and copper benzoyl acetonate, was studied and the temperature dependence of the EPR spectra of the probes measured through and above the phase transition at 200 K. Although no evidence of motion was found near T, clear indication of a fluctuation of the probes was detected above the phase transition.
2. Experimental EPR measurements were made with a Varian E-9 spectrometer operating at 9.2 GHz. The temperature of the sample is controlled in the microwave cavity with a Varian 4540 temperature controller which controls the temperature to *l”C. The system consists of an evacuated double-walled quartz tube in the cavity through which temperature controlled N2 gas flows. Thus the cavity itself is not heated and no frequency changes occur which could account for the observed line shifts. However to ensure this the frequency of 153
Volume 127, number 2
CHEMJCAL PHYSICS LETTERS
the cavity is constantly monitored through the experiment. The temperature of the sample in the quartz tube is monitored by means of a thermocouple in contact with the sample. The paramagnetic probes were doped into symmetric triazine by dissolving small fractions of them in molten triazine and allo~ng the material to slowly cool. Since the quality of the crystals obtained was not good the crystals were ground into fine powders and studies made on the EPR spectra obtained from the powders.
A 10 ~
2 A A
3. Results The hype&e and Zeeman interactions for a fixed paramagnetic molecule depend on the orientation of the dc magnetic field with respect to the EPR symmetry axis of the molecule. In a powder, the EPR spectrum is a superposition of spectra from all orientations of the dc magnetic field with respect to the symmetry axis of the molecule weighted by the probab~ty for a given direction. Fig. 1 shows the powder spectrum of the S = l/2, I = 1 TMOP radical in powders of triazine at -77’C. This powder spectrum is well understood and the spin Ham~tonian parameters can be readily obtained [ 1I]. The separation between the highest and the lowest lines of the spectrum measures 2A,,. As the temperature is raised to about 20’ above the phase transition at -76’C the spectrum begins to show marked changes as shown in fig. 1. In particular there is a coales-
6 June 1986
G
P
AA
I
I
I
I
I
-70
I
-50
I
-30
DEGREES(C)
Fig. 2. Plot of the shift of the low-field extrema of the EPR powder spectrum of TMOP in s-triazine as a function of increasing temperature.
cing of the extrema lines of the spectrum. The lowfield extrema shifts to higher magnetic field values and the high-field extrema shifts to lower values, meaning
2730 1
2690 -
I
32306
154
I
-60
0
Fig. 1. EPR spectrum of nitroxide molecule, TMOP, in striazine at a number of temperatures showing the effects of slow tumbling of the radical on the spectrum.
I
I
I
-20
I
I
26
DEGREES(C)
3. Plot of the shift of the low-field extrema of the EPR powder spectrum of copper benzoyl acetonate doped in striazine as a function of increasing temperature. Pig.
I
Volume 127, number 2
6 June 1986
CHEMICAL PHYSICS LETTERS
in effect that A ,, is decreasing. Figs. 2 and 3 are plots of the shift of the low-field extrema of TMOP and CUB in triazine, respectively, as a function of temperature.
l
10-l TMOP
l
4. Analysis
A
l l
10-Z -
The hype&e splittings of a molecule or molecular complex may display a temperature dependence due to a coupling of the hyperfine interaction to the normal modes of vibration. This occurs via the temperaturedependent changes of the population of the vibrational states and is referred to as the explicit temperature dependence. Generally this temperature dependence is only observed in systems with hyperfme interactions much larger than observed here and depends continuously on temperature from zero kelvin and at higher temperature above 77 K is linear [ 121. Since below 226 K the extrema lines show no shift with temperature it is concluded that an explicit temperature dependence is not responsible for the shifts of the resonances. The shifts are also not due to changes of the microwave frequency with temperature which was monitored through the measurements and did not change with temperature. The changes, the coalescing and the broadening, are however characteristic of the effects of slow tumbling of the paramagnetic probes on their EPR spectra. Recent thepretical treatments of the effect of slow tumbling on EPR spectra, allow quantitative information to be obtained about the dynamics of the motion [2-6]. It has been shown that the inward shift of the parallel edge lines of the powder spectrum depend on the correlation time, T, describing the motion as ~~~1~. Fig. 4 is a plot of the log of 1/A2 where A is the inward shift of the parallel edge lines of the powder spectrum, versus the reciprocal of the absolute temperature for CUB and TMOP. The data for both probes yield straight lines on the plot indicating the kinetics of tumbling is governed by thermally activated Arrhenius kinetics. A fit of the CUB data to the Arrhenius expression yields an activation energy of 0.57 eV. Note that the TMOP probes starts tumbling at a slightly lower temperature than CUB but has approximately the same barrier to motion. The theory also predicts that the net motional linewidth should depend on the correlation time as 7-U2. The net motional linewidth is obtained from the difference in widths of the highest and the lowest hyper-
I
I 3.2
I
I
I
3.6
I
I
4.0
1000
K Fig. 4. Plot of the log of the reciprocal of the square of the low-field shift for the TMOP and CUB paramagnetic species in s-triazine versus the reciprocal of the absolute temperature.
tine resonances of the parallel edge spectrum. In the case of CUB the highest-field hyperfme line of the parallel edge spectrum is not clearly resolved because it overlaps the perpendicular edge spectrum. The shifts of the lines and thus the onset of the motion do not occur until the temperature is well above the phase transition temperature in agreement with lack of evidence for molecular fluctuations at the phase transition [lo]. The observed slow tumbling of the two probes is believed to be a result of the intrinsic fluctuations of the C,N,H, molecules of the lattice. This conclusion is supported from other work such as nuclear quadrupole resonance studies which indicate a fluctuation of the intrinsic molecules of the lattice. Measurements of the NQR relaxation time as a function of temperature indicate that the triazine molecules undergo reorientation above -66’C and in fact the barrier to motion obtained from these measurements was in good agreement with that obtained here from the EPR data [9]. The barrier obtained from NQR measurements was 0.60 eV. The observation of rotational fluctuations in triazine well above the -76’C phase transition suggests the possibility of a higher temperature phase transition to a rotator-librator phase (plastic phase). 15.5
Volume 127, number 2
CHEMICAL PHYSICS LETTERS
References [l] F.J. Owens, in: Magnetic resonance of phase transitions, eds. F.J. Owens, C.P. Poole and H.A. Farach (Academic Press, New York, 1979). [2] S. Lee and D.P. Ames, J. Chem. Phys. 80 (1984) 1766. [3] S. Lee, I.M. Brown and D.P. Ames, J. Chem. Phys. 80 (1984) 3984. [4] A. Baram, Mol. Phys. 44 (1981) 1009. [5] D. Kivelson and S. Lee, J. Chem. Phys. 76 (1982) 5746.
156
6 June 1986
[6] S. Lee and S.Z. Tang, Phys. Rev. B31 (1985) 1308. [7] G.R. EIliot and Z. Iqbal, J. Chem. Phys. 63 (1975) 1916. [8] S. Daunt, H.F. ShurueIl and L. Pazdernick, J. Raman Spectry. 4 (1975) 205. [9] A. Zussman and M. Oron, J. Chem. Phys. 66 (1977) 743. [lo] I.U. Hellman,W. Ellenson and J. Eckert, J. Phys. Cl 1 (1978) 185. [ 1 l] O.H. Griffith, D.W. Cornell and H.M. McConnel, J. Chem. Phys. 43 (1965) 2909. [12] W. Dreybrodt, Phys. Stat. Sol. 21 (1976) 99.
127, number
CHEMICAL
2
PHYSICS
LETTERS
6 June 1986
ELECTRON PARAMAGNETIC RESONANCE EVIDENCE FOR MOLECULAR FLUCTUATIONS IN s-TRIAZINE F.J. OWENS Energetic Materials L.aborarory Armament Received
13 November
Research and Development
1985; in final form 18 March
Center, Dover, NJ 07801. USA
1986
A pronounced inward shift of the extrema first-derivative EPR powder spectrum peaks of the nitroxide molecule, 2,2,6,6-tetramethyWoxopiperinooxy (TMOP) and the transition metal complex copper benzoyl acetonate doped into the triazine lattice is observed as the temperature is increased above -66OC. The effects are indicative of the onset of slow tumbling of the paramagnetic species in the crystal. The temperature dependence is Arrhenius, with a barrier to motion of 0.57 eV. Comparison with other measurements suggests the motion of the paramagnetic probes is a result of motion of the intrinsic triazine molecules in the solid.
1. Introduction Electron paramagnetic resonance has been shown to be useful in studying structural phase transitions in solids and the dynamic behavior of the substituents of the lattice during the phase transition. The temperature dependence of the order parameter and critical behavior near the transition temperature have been observed using electron paramagnetic resonance (EPR) [ 11. EPR spectra may be sensitive to order-disorder fluctuations that occur in the lattice. If the correlation time of the fluctuation is in the order of A ,, -A,, where A ,, is the parallel component of the axially symmetric EPR hyperfine tensor and A, the perpendicular component, the motion of the species or its environment will cause a change in the linewidth and hyperfme splitting. Recent theoretical and experimental developments which relate the changes in the spectrum to the correlation time of the motion allow quantitative information about the dynamics to be obtained [2-61. The molecular crystal of symmetric triazine (C3N3H3) undergoes a strain-induced elastic phase transition which has been studied by a number of spectroscopic methods [7-91. The phase transition occurs at 200 K at ambient pressure and the unit cell changes from trigonal to monoclinic. Neutron inelastic scattering indicates the primary displacement is a zone-centered transverse acoustic shearing mode as well as a ro-
tation of the triazine molecule [ 10 1. The absence of central peak scattering in the region of the transition temperature indicated that there are no dynamical fluctuations associated with the phase transition [lo] . In order to further investigate the possibility of molecular fluctuations in s-triazine the material doped with two structurally quite different probes, 2,2,6,6_tetramethyl4-oxopiperinooxy (TMOP) and copper benzoyl acetonate, was studied and the temperature dependence of the EPR spectra of the probes measured through and above the phase transition at 200 K. Although no evidence of motion was found near T, clear indication of a fluctuation of the probes was detected above the phase transition.
2. Experimental EPR measurements were made with a Varian E-9 spectrometer operating at 9.2 GHz. The temperature of the sample is controlled in the microwave cavity with a Varian 4540 temperature controller which controls the temperature to *l”C. The system consists of an evacuated double-walled quartz tube in the cavity through which temperature controlled N2 gas flows. Thus the cavity itself is not heated and no frequency changes occur which could account for the observed line shifts. However to ensure this the frequency of 153
Volume 127, number 2
CHEMJCAL PHYSICS LETTERS
the cavity is constantly monitored through the experiment. The temperature of the sample in the quartz tube is monitored by means of a thermocouple in contact with the sample. The paramagnetic probes were doped into symmetric triazine by dissolving small fractions of them in molten triazine and allo~ng the material to slowly cool. Since the quality of the crystals obtained was not good the crystals were ground into fine powders and studies made on the EPR spectra obtained from the powders.
A 10 ~
2 A A
3. Results The hype&e and Zeeman interactions for a fixed paramagnetic molecule depend on the orientation of the dc magnetic field with respect to the EPR symmetry axis of the molecule. In a powder, the EPR spectrum is a superposition of spectra from all orientations of the dc magnetic field with respect to the symmetry axis of the molecule weighted by the probab~ty for a given direction. Fig. 1 shows the powder spectrum of the S = l/2, I = 1 TMOP radical in powders of triazine at -77’C. This powder spectrum is well understood and the spin Ham~tonian parameters can be readily obtained [ 1I]. The separation between the highest and the lowest lines of the spectrum measures 2A,,. As the temperature is raised to about 20’ above the phase transition at -76’C the spectrum begins to show marked changes as shown in fig. 1. In particular there is a coales-
6 June 1986
G
P
AA
I
I
I
I
I
-70
I
-50
I
-30
DEGREES(C)
Fig. 2. Plot of the shift of the low-field extrema of the EPR powder spectrum of TMOP in s-triazine as a function of increasing temperature.
cing of the extrema lines of the spectrum. The lowfield extrema shifts to higher magnetic field values and the high-field extrema shifts to lower values, meaning
2730 1
2690 -
I
32306
154
I
-60
0
Fig. 1. EPR spectrum of nitroxide molecule, TMOP, in striazine at a number of temperatures showing the effects of slow tumbling of the radical on the spectrum.
I
I
I
-20
I
I
26
DEGREES(C)
3. Plot of the shift of the low-field extrema of the EPR powder spectrum of copper benzoyl acetonate doped in striazine as a function of increasing temperature. Pig.
I
Volume 127, number 2
6 June 1986
CHEMICAL PHYSICS LETTERS
in effect that A ,, is decreasing. Figs. 2 and 3 are plots of the shift of the low-field extrema of TMOP and CUB in triazine, respectively, as a function of temperature.
l
10-l TMOP
l
4. Analysis
A
l l
10-Z -
The hype&e splittings of a molecule or molecular complex may display a temperature dependence due to a coupling of the hyperfine interaction to the normal modes of vibration. This occurs via the temperaturedependent changes of the population of the vibrational states and is referred to as the explicit temperature dependence. Generally this temperature dependence is only observed in systems with hyperfme interactions much larger than observed here and depends continuously on temperature from zero kelvin and at higher temperature above 77 K is linear [ 121. Since below 226 K the extrema lines show no shift with temperature it is concluded that an explicit temperature dependence is not responsible for the shifts of the resonances. The shifts are also not due to changes of the microwave frequency with temperature which was monitored through the measurements and did not change with temperature. The changes, the coalescing and the broadening, are however characteristic of the effects of slow tumbling of the paramagnetic probes on their EPR spectra. Recent thepretical treatments of the effect of slow tumbling on EPR spectra, allow quantitative information to be obtained about the dynamics of the motion [2-6]. It has been shown that the inward shift of the parallel edge lines of the powder spectrum depend on the correlation time, T, describing the motion as ~~~1~. Fig. 4 is a plot of the log of 1/A2 where A is the inward shift of the parallel edge lines of the powder spectrum, versus the reciprocal of the absolute temperature for CUB and TMOP. The data for both probes yield straight lines on the plot indicating the kinetics of tumbling is governed by thermally activated Arrhenius kinetics. A fit of the CUB data to the Arrhenius expression yields an activation energy of 0.57 eV. Note that the TMOP probes starts tumbling at a slightly lower temperature than CUB but has approximately the same barrier to motion. The theory also predicts that the net motional linewidth should depend on the correlation time as 7-U2. The net motional linewidth is obtained from the difference in widths of the highest and the lowest hyper-
I
I 3.2
I
I
I
3.6
I
I
4.0
1000
K Fig. 4. Plot of the log of the reciprocal of the square of the low-field shift for the TMOP and CUB paramagnetic species in s-triazine versus the reciprocal of the absolute temperature.
tine resonances of the parallel edge spectrum. In the case of CUB the highest-field hyperfme line of the parallel edge spectrum is not clearly resolved because it overlaps the perpendicular edge spectrum. The shifts of the lines and thus the onset of the motion do not occur until the temperature is well above the phase transition temperature in agreement with lack of evidence for molecular fluctuations at the phase transition [lo]. The observed slow tumbling of the two probes is believed to be a result of the intrinsic fluctuations of the C,N,H, molecules of the lattice. This conclusion is supported from other work such as nuclear quadrupole resonance studies which indicate a fluctuation of the intrinsic molecules of the lattice. Measurements of the NQR relaxation time as a function of temperature indicate that the triazine molecules undergo reorientation above -66’C and in fact the barrier to motion obtained from these measurements was in good agreement with that obtained here from the EPR data [9]. The barrier obtained from NQR measurements was 0.60 eV. The observation of rotational fluctuations in triazine well above the -76’C phase transition suggests the possibility of a higher temperature phase transition to a rotator-librator phase (plastic phase). 15.5
Volume 127, number 2
CHEMICAL PHYSICS LETTERS
References [l] F.J. Owens, in: Magnetic resonance of phase transitions, eds. F.J. Owens, C.P. Poole and H.A. Farach (Academic Press, New York, 1979). [2] S. Lee and D.P. Ames, J. Chem. Phys. 80 (1984) 1766. [3] S. Lee, I.M. Brown and D.P. Ames, J. Chem. Phys. 80 (1984) 3984. [4] A. Baram, Mol. Phys. 44 (1981) 1009. [5] D. Kivelson and S. Lee, J. Chem. Phys. 76 (1982) 5746.
156
6 June 1986
[6] S. Lee and S.Z. Tang, Phys. Rev. B31 (1985) 1308. [7] G.R. EIliot and Z. Iqbal, J. Chem. Phys. 63 (1975) 1916. [8] S. Daunt, H.F. ShurueIl and L. Pazdernick, J. Raman Spectry. 4 (1975) 205. [9] A. Zussman and M. Oron, J. Chem. Phys. 66 (1977) 743. [lo] I.U. Hellman,W. Ellenson and J. Eckert, J. Phys. Cl 1 (1978) 185. [ 1 l] O.H. Griffith, D.W. Cornell and H.M. McConnel, J. Chem. Phys. 43 (1965) 2909. [12] W. Dreybrodt, Phys. Stat. Sol. 21 (1976) 99.