- Email: [email protected]
ESR study of coupled pairs of molecules in copper diethyldithiocarbamate
Volume 8. number 1
E-SR
CREMICAL PHYSICS LETTERS
STUDY
OF
COPPER
COUPLED
PAIRS
OF
1 January 1971
MOLECULES
IN
-DTETHYLDJTHIOCARBAMATE
R. K COWSIK, G. RkNGARAJ_4N and R. SRINSJASAN D~:~arfmsntof Pl~ysics, Indian Institzcle of Science, Bangalore-12, India
Received 12 October 1970
Single crystal ESR study of copper diethyldithiocarbamate has shown evidence for the presence of -coupled pairs of Cu++ Ions. . While the g-values for the triplet state remain the same as for the doublet (uncoupled ions), the hyperfine splitting is halved. The mro field splitting parameter D = 276 X 10-a cm-l. The results have been correlated with crystal structure and susceptibility data.
1. INTRODUCTION The single crystal ESR study of copper (n!
bis (N, N-diethyldithiocarbamate) was carried out by Reddy and Srinivasan [l]. With the isomorphous zinc compound as diluent, they were able to fully resolve the hyperfine structure and to evaluate the spin hamiltonian parameters. We have now observed spectra from mixed crystals of Cu, Zn(detc)2, in which, along with the II&II four line hyperfine structure of the Cu* ion, there are additional weak lines. The observed intensity pattern of these lines indicated equal coupling to more than one nucleus. A more careful examination showed that these lines occurred in groups of seven. This could arise only from an equal coupling to two copper nuclei because of exchange coupling between the unpaired electron& This exchange coupling would then give rise to a diamagnetic singlet state and a paramagnetic triplet state. ESR transitions are observed between the states of b the triplet. .The zero fieid splitting of the triplet levels produces a fine structure of two lines: Each fine structure component is further split into seven hyperfine lines. due to equal coupiing to the two coppei nuclei of the exchange coupled pair. This aeven lirmhyperfine p&tern has an Men-sity dJst.ribution of 1 : 2 : 3 : 4 : 3 : 2 : 1 (fig. 1). In general a.ll the hyperfine liaes coiild not .be seen because of overlap with the stronger lines of the .:. .doublet spectrum..
2. RESULTS The ESR measurements were made at X-band 100 kHz field modulation. A proton resonance meter was used for magnetic field measurements. DPPH was used as the g-marker. Experiments were done at room temperature and at 90% For a general orientation of the crystal, two sets of these extra lines were obtained. If we note that the crystal belongs to the monoclinic using
Fig. i. ESR speck-urn of copper-zinc diethyldithidcarbamate showing lines due to isolated copper ions. .,anacu-cu pairs.
Volume 8, number 1
CHEMICAL PHYSICS LETTERS
space group, unit cell,
P2l/c, with four molecules in the this indicates that it is the centro-
symmetrically
related copper ions which are
coupled, giving rise to two inequivalent pairs. The crystal was first mouted with the b sxis vertical. Since the two pairs are equivalent in the ac plane, only one set of lines was observed. In this mouting the central hyperfine lines (ml = 0) could be seen and followed as the magnet was rotated. The fine structure splitting was measured as the separation between these lines. The direction in the ac plane for which the fine structure splitting is maximum was located. The plane containing this direction and the b axis was then made horizontal by mounting
the crystal on a suitably cut wedge. The angular variation of the pair spectrum was then studied in this plane. The g-values for the triplet spectrum were roughly the same as those for the doublet. However, there were small deviations in certain directions and these are being investigated. It was observed that in all directions the triplet hyperfine splitting is half that of the doublet within experimental error. Hence the principal axes of the g-tensor and the hyperfine tensor of the coupled pair are the same as those of the uncoupled ion. The angular variation of the spectrum should be appropriate to a spin hamiltonian: Q = B k,, Hz% + g_!_(% S, +
+I+,2 -f c AS,I,
q&)1
S(S+l)]+E@-$1
+ B&I,
+ S,,I,,) ,
with S = 1, I = 3, and where xyz are the principal axes of the g and the hyperfine tensors and kVylzl those of the I) tensor. A value of 276 x 10s4 cm-l was obtained for D. The angle between the b axis and the principal axis of the D tensor was found to be 68O.. Study of the angular variation of the spectrum showed that the tensor was essentially axial. By following the ml = 0 lines it was found that the fine structure splitting collapsed at an angle of about 55’ from the principal axis direction. The relative intensities of the doublet and triplet lines were examined at room temperature and at 9O’K. No significant changes could be detected. To study this further, experiments at lower temperatures are planned. 3. DISCUSSION ’
The X-ray
structure analyses of copper and
zinc (detc)2 were performed by Bonamico et al. [2,3]. It was found that the copper ion is coordi-
nated to five sulphur atoms, four of them (S), at an average distance of 2.31 A, lie in square planar arrangement, and the fifth, (S’) at a distance of 2.85 A, belongs to the centrosymmetricalIy related molecule. This means that each pair of centrosymmetrically related copper atoms share sulphur atoms thus forming a type of bimolecular unit and suggests superexchange through these sulphur atoms as a possible mechanism .f”r the coupling. The Cu-Cu distance of 3.59 A in this case appears to be rather long [4-t?]. The Cu-S-Cu’ angle was calculated to be 87O. Susceptibility measurements carried out by Gregson and Mitra [7] have shown evidence for an exchange coupling between copper ions in pure copper (detc)2, with J = 8cmDf_ This smaI1 value of Jand the long Cu-Cu distance would lead one to expect the coupling between the ions to be weak. This is borne out by our ESR observations, viz. , the g-values are not appreciabLy affected; there are no significant changes in the relative intensities down to gOoK; and the zero field splitting parameter
has a small value.
The exchange frequency is yet high enough in comparison to the hyperfine frequency. The hyperfine splitting for the coupled pair of crystallographicaiIy equivalent Cu* ions is therefore found to be half that for the uncoupled ion [83. The principal direction for D is expected to coincide with the Cu-Cu line. The angIe between the b axis and the Cu-Cu line was calculated to be 79O in the pure copper detc crystal. In the isomorphous zinc compound the corresponding angle is 65’. Our experimentai value of 6B” is closer to the latter. This is probably because the relative orientation of the two copper detc molecules in the mixed crystal is influenced mainly by packing considerations. Hence the angle is closer to the value obtained for the host
zinc lattice. The coupling between the copper ions in this case may arise from (a) Superexchange through sulphur atoms and (b) direct dipolar interaction_ The formula given by Bleouey and Bowers (41 leads to a value Of about 6.002 cm-1 for D car_ responding to J x 8 cm-l. The dipolar contribution to D was estimated to be = 6.05 cm-1 car responding to a Cu-Cu distance of 3.59 A. Compariug these prith the observed value for D, we may conclude that while the weak exchange interaction ensures equal coupling to the nuclear spins, the main contribution to the zero field splihg comes from the bipolar interaction_ 137
E-SR
CREMICAL PHYSICS LETTERS
STUDY
OF
COPPER
COUPLED
PAIRS
OF
1 January 1971
MOLECULES
IN
-DTETHYLDJTHIOCARBAMATE
R. K COWSIK, G. RkNGARAJ_4N and R. SRINSJASAN D~:~arfmsntof Pl~ysics, Indian Institzcle of Science, Bangalore-12, India
Received 12 October 1970
Single crystal ESR study of copper diethyldithiocarbamate has shown evidence for the presence of -coupled pairs of Cu++ Ions. . While the g-values for the triplet state remain the same as for the doublet (uncoupled ions), the hyperfine splitting is halved. The mro field splitting parameter D = 276 X 10-a cm-l. The results have been correlated with crystal structure and susceptibility data.
1. INTRODUCTION The single crystal ESR study of copper (n!
bis (N, N-diethyldithiocarbamate) was carried out by Reddy and Srinivasan [l]. With the isomorphous zinc compound as diluent, they were able to fully resolve the hyperfine structure and to evaluate the spin hamiltonian parameters. We have now observed spectra from mixed crystals of Cu, Zn(detc)2, in which, along with the II&II four line hyperfine structure of the Cu* ion, there are additional weak lines. The observed intensity pattern of these lines indicated equal coupling to more than one nucleus. A more careful examination showed that these lines occurred in groups of seven. This could arise only from an equal coupling to two copper nuclei because of exchange coupling between the unpaired electron& This exchange coupling would then give rise to a diamagnetic singlet state and a paramagnetic triplet state. ESR transitions are observed between the states of b the triplet. .The zero fieid splitting of the triplet levels produces a fine structure of two lines: Each fine structure component is further split into seven hyperfine lines. due to equal coupiing to the two coppei nuclei of the exchange coupled pair. This aeven lirmhyperfine p&tern has an Men-sity dJst.ribution of 1 : 2 : 3 : 4 : 3 : 2 : 1 (fig. 1). In general a.ll the hyperfine liaes coiild not .be seen because of overlap with the stronger lines of the .:. .doublet spectrum..
2. RESULTS The ESR measurements were made at X-band 100 kHz field modulation. A proton resonance meter was used for magnetic field measurements. DPPH was used as the g-marker. Experiments were done at room temperature and at 90% For a general orientation of the crystal, two sets of these extra lines were obtained. If we note that the crystal belongs to the monoclinic using
Fig. i. ESR speck-urn of copper-zinc diethyldithidcarbamate showing lines due to isolated copper ions. .,anacu-cu pairs.
Volume 8, number 1
CHEMICAL PHYSICS LETTERS
space group, unit cell,
P2l/c, with four molecules in the this indicates that it is the centro-
symmetrically
related copper ions which are
coupled, giving rise to two inequivalent pairs. The crystal was first mouted with the b sxis vertical. Since the two pairs are equivalent in the ac plane, only one set of lines was observed. In this mouting the central hyperfine lines (ml = 0) could be seen and followed as the magnet was rotated. The fine structure splitting was measured as the separation between these lines. The direction in the ac plane for which the fine structure splitting is maximum was located. The plane containing this direction and the b axis was then made horizontal by mounting
the crystal on a suitably cut wedge. The angular variation of the pair spectrum was then studied in this plane. The g-values for the triplet spectrum were roughly the same as those for the doublet. However, there were small deviations in certain directions and these are being investigated. It was observed that in all directions the triplet hyperfine splitting is half that of the doublet within experimental error. Hence the principal axes of the g-tensor and the hyperfine tensor of the coupled pair are the same as those of the uncoupled ion. The angular variation of the spectrum should be appropriate to a spin hamiltonian: Q = B k,, Hz% + g_!_(% S, +
+I+,2 -f c AS,I,
q&)1
S(S+l)]+E@-$1
+ B&I,
+ S,,I,,) ,
with S = 1, I = 3, and where xyz are the principal axes of the g and the hyperfine tensors and kVylzl those of the I) tensor. A value of 276 x 10s4 cm-l was obtained for D. The angle between the b axis and the principal axis of the D tensor was found to be 68O.. Study of the angular variation of the spectrum showed that the tensor was essentially axial. By following the ml = 0 lines it was found that the fine structure splitting collapsed at an angle of about 55’ from the principal axis direction. The relative intensities of the doublet and triplet lines were examined at room temperature and at 9O’K. No significant changes could be detected. To study this further, experiments at lower temperatures are planned. 3. DISCUSSION ’
The X-ray
structure analyses of copper and
zinc (detc)2 were performed by Bonamico et al. [2,3]. It was found that the copper ion is coordi-
nated to five sulphur atoms, four of them (S), at an average distance of 2.31 A, lie in square planar arrangement, and the fifth, (S’) at a distance of 2.85 A, belongs to the centrosymmetricalIy related molecule. This means that each pair of centrosymmetrically related copper atoms share sulphur atoms thus forming a type of bimolecular unit and suggests superexchange through these sulphur atoms as a possible mechanism .f”r the coupling. The Cu-Cu distance of 3.59 A in this case appears to be rather long [4-t?]. The Cu-S-Cu’ angle was calculated to be 87O. Susceptibility measurements carried out by Gregson and Mitra [7] have shown evidence for an exchange coupling between copper ions in pure copper (detc)2, with J = 8cmDf_ This smaI1 value of Jand the long Cu-Cu distance would lead one to expect the coupling between the ions to be weak. This is borne out by our ESR observations, viz. , the g-values are not appreciabLy affected; there are no significant changes in the relative intensities down to gOoK; and the zero field splitting parameter
has a small value.
The exchange frequency is yet high enough in comparison to the hyperfine frequency. The hyperfine splitting for the coupled pair of crystallographicaiIy equivalent Cu* ions is therefore found to be half that for the uncoupled ion [83. The principal direction for D is expected to coincide with the Cu-Cu line. The angIe between the b axis and the Cu-Cu line was calculated to be 79O in the pure copper detc crystal. In the isomorphous zinc compound the corresponding angle is 65’. Our experimentai value of 6B” is closer to the latter. This is probably because the relative orientation of the two copper detc molecules in the mixed crystal is influenced mainly by packing considerations. Hence the angle is closer to the value obtained for the host
zinc lattice. The coupling between the copper ions in this case may arise from (a) Superexchange through sulphur atoms and (b) direct dipolar interaction_ The formula given by Bleouey and Bowers (41 leads to a value Of about 6.002 cm-1 for D car_ responding to J x 8 cm-l. The dipolar contribution to D was estimated to be = 6.05 cm-1 car responding to a Cu-Cu distance of 3.59 A. Compariug these prith the observed value for D, we may conclude that while the weak exchange interaction ensures equal coupling to the nuclear spins, the main contribution to the zero field splihg comes from the bipolar interaction_ 137