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Proton magnetic resonance in CoHgCl4· 4H2O
O%?23697/82/OlM~&O4SO3.Of~/O Pergamon Press Ltd.
I Phys. Chm. Solids Vol. 43, No. I, pp. ‘D-28. 1982 Printed in Great Britain.
PROTON MAGNETIC RESONANCE IN CoHgCL +4H20 T. F. S. RAJ Department of Physics, S.E. College, Vijayawada-520006, India
and G. SATYANANDAM and C. R. K. MURTY* Departmentof Physics, NagarjunaUniversity, Nagarjunanagar-522 510,India (Received 16 December 1980; accepted in revised form 29 April 1981)
Abstract-Proton magnetic resonance in a new paramagneticcrystalline hydrate, cobalt mercuric chloride tetrahydrate, was observed using a CW wide line NMR spectrometer. The resonance spectra showed appreciable asymmetry caused by the paramagnetic influence of Co*+ions at the proton sites. The angular dependenceof the resonancesplittingsabout each of the three crystallographicaxes was analysedto determinethe p-p vectors in the unit cell of the crystal, and the angular variationof paramagneticshift about the [COl]axis of rotation was studied.
Attempts to grow isomorphous classes of diamagnetic and paramagnetic crystals met with failure. A CW wide-line Varian NMR spectrometer was used to record the proton resonance splittings in CoHgCL,. 4H,O at the Larmor frequency of 15MHz. The modulation frequency and the time constant were set at 40Hz and 3 sec. respectively, the modulation field being 0.75 G. Three different crystals were rotated within the magnetic field through 180” at 10” intervals about the [OOl],[OlO] and [lOO]axes.
1. INTRODUCTION Proton magnetic resonance (PMR) studies of several paramagnetic hydrates reported at room temperature have indicated no asymmetry in the resonance spectra[l-4]. Hence these have been analysed by employing the Pake method [5], which is valid for diamagnetic crystalline hydrates. The influence of the paramagnetic interaction on the proton resonance lines is usually studied at low temperatures[d8] where it is much larger than the dipole-dipole interaction between protons belonging to the same water molecule. Nevertheless, the effect of crystal paramagnetism on proton splittings has also been observed at room temperature in a very few cases[9, lo]. In this paper, results obtained by PMR at room temperature in single crystals of cobalt mercuric chloride tetrahydrate (CoHgCl, .4H,O)are reported. Since the structure of this crystal is not reported in the literature, it is considered that the present PMR study will yield valuable information on the magnitudes and directions of the proton-proton (p-p) vectors in the unit cell and on the paramagnetic interactions.
3. RESULTS ANDDISCUSSION The PMR spectra consisted of four well-resolved lines in most of the orientations about the [OOll axis of rotation. The lines were asymmetrically distributed about a strong narrow line which is due to the trapped water molecules in the crystal. Some typical examples of the spectra are given in Fig. 1. In diamagnetic crystals, the resonance lines will be distributed symmetrically about a central line and the doublet separation between them is given by the Pake equation [5]
2. EXPERIMENTAL. Large single crystals of CoHgCL * 4HsO were grown at 298 K from a saturated aqueous solution containing CoC& and HgCl* in the ratio 1:2.5 by weight [I 11. The crystals were deep pink in colour, and highly hygrostopic and they were given a thin layer of quick-fix for protection. Morphological examination of the crystals indicated dominant (1lo), (liO), (102) and (102) faces with the long axis being thelOOl] axis. A preliminary X-ray analysis of the sample (Spence, private communication) showed that the crystal belongs to the orthorhombic class with four molecules per unit cell. The unit cell dimensions are a = 7.82 A, b = 15.57A, c = 8.17 A, Vc.,, = 994.8 A3 and Dexpt= 3.04gmlcm3.
2(AH) = 213 co? 6 cos* (4 - &,) - l]
(1)
where the parameters have their usual significance. In CoHgCL,*4H20, the paramagnetic interaction is revealed by an asymmetry in the PMR lines even at room temperature (298 K). The observed asymmetry can be explained in terms of a shift in the centre of Pake doublet (with respect to the free line) caused by the crystal paramagnetism[7]. This shift, however, is enough to create a difficulty in identifying the Pake doublets corresponding to different p-p vectors. To overcome this difficulty, spectra at different fields are needed, for the shift is expected to change with the field strength while the pair separation does not change.
*Authorto whom all correspondence should be addressed. 25
T. F. S. RAI et al.
26
(b)
Fig. I. Proton resonance spectra of
CoHgCl, .4H20 at 15MHz for [OOl]rotation at orientations (a) NO’, (b) 110” and (c) 120”.
The angular dependence of the resonance splittings obtained at two different fields corresponding to Larmor frequencies 15 MHz and 4 MHz is shown in Fig. 2. The spectra were also recorded at closer intervals of orientations near crossover points. Having separated out the intra-proton interaction from the paramagnetic interaction by utilising the previously mentioned differential property, the experimental pair separations were fitted to
eqn (1). The Pake curves thus obtained are shown in Fig, 3, and the orientations of the p-p vectors deduced are given in Table 1. The error involved in the determination of the p - p direction is estimated to be less than 3”,while that in the determination of the p-p distance is less than 0.015 A. The results given in Table 1 indicate that the two vectors derived are symmetry related. The angular variation of the paramagnetic shift, the
Fig. 2. Angular variation of the resonance peak positions at IS MHz and 4 MHz in CoHgC& . 4H20. [OOl] is the crystal rotation axis. Circles and triangles represent experimental data for high and low fields respectively.
Proton magneticresonance in CoHgCl, .4H20
21
[1003
Lo101 t
eAH(GAUSS) t -@(DEGREES)
Fig. 3. Pake curves in CoHgC&. 4H20 for the [Ool] axis of rotation. Circles and squares represent experimental pair separations for high and low fields respectively.
Angular variation of the paramagnetic shift at 15MHz in CoHgCl,. 4Hz0 about the [OOl]axis of rotation. Circles and squares represent the experimental points for the two vectors (Table 1).
Table 1. The magnitudes and directions of the observed p - p vectors in CoHgCI.,. 4H20
-_-------p-p Veotor
4 (%I
I
II
42
136
s
*y
(W)
(
32*69
r
1*699
339 I3
-Pa
Direction
1
1*606
a
=
1.603
separation between the center of gravity of the Pake doublet from the usual central free proton line at 15 MHz follows a dipolar (3 cos* 0- 1) type variation and is shown in Fig. 4. However, the fit is not too satisfactory partly because the (3 cos* l3- 1) dependence is only a first order approximation. For a field of 3523G corresponding to 15 MHz the maximum shifts were found to be 1.45G and 1.39G for vectors I and II respectively. To confirm the number of p-p vectors obtained, room temperature spectra about the [IOO] and [OlO] axes of rotation were also recorded at ISMHz. Four lines were obtained at a very few orientations about the [lOO]axis of rotation. In all other orientations, the discrete structure of the spectra could not be observed, which might be due to high g-anisotropy. Theretical Pake curves were drawn about this axis of rotation from the data obtained for the [OOl]axis. It was found that these Pake curves passed through the experimental points, indicating that
1001
[
0101
0.6a40
0.5619
@lo)
(56’)
0*&X4
0.58
(53’)
( 126’1
Cosines .pO31
0*5430
(57O) 17
O*MQ (57O)
P
there are only two p-p vectors in the unit cell. For the [OlO] axis of rotation, only a pair of asymmetrically distributed lines was recorded, in concurrence with the data of Table 1. Further detailed discussion cannot be carried out without a knowledge of the crystal structure. To get a complete understanding of the paramagnetic interactions and the crystal structure, low temperature PMR and deuteron resonance (DMR) work is needed. Ackhowlegements-Thanks are due to Prof. R. Vijayaraghavan for extending the facilities of the Varian spectrometer, TIFR, Bombay and to Dr. V. Nagarajan, TIFR, Bombay for helpful discussions. The authors are grateful to Prof. R. D. Spence for his private communication on preliminary X-ray data taken on the crystal. Financial assistance from C.S.I.R., Delhi and N. S. F. (Grant No. GF 36748)U.S.A., is gratefully acknowledged. REFERENCES 1. Pedersen B., Thesis, Central Institute for Industrial Research, Oslo, Norway (1964).
28
T. F. S. RAI et al.
2. Reeves L. W., Prog. NMR Spectrosc. 4 193(1969). 3. El Saffar Z. M., J. Chem. Phys. 45 4643 (1%6). 4. Vizia N. C., Murty P. N. and Murty C. R. K., J. Mag. Rex 22 439 (1976). 5. Pake G. E., J. Chem. Phys. 16 327 (1948). 6. Abe H. and Matsura M., J. Phys. Sot., Japan. 19 1867(1964). 7. Bloembergen N., Physica 16,96 (1950).
8. Poulis N. J., Physica 17 392 (1951). 9. Padmanabhan A. C., Rangarajan G. and Srinivasan R., Proc. Indian Nat. Sci Acad., Part A40 241 (1974). 10. Bose M., Ghosh-Ray A. and Basu A., Proc. Nucl. Phys. Solid-St. Phys. Symp. 19C 363 (1976). 11. Linke W. F., Solubilities of Inorganic and Metal Organic Compounds, Vol. I. Van Nostrand, New Jersey (1958).
I Phys. Chm. Solids Vol. 43, No. I, pp. ‘D-28. 1982 Printed in Great Britain.
PROTON MAGNETIC RESONANCE IN CoHgCL +4H20 T. F. S. RAJ Department of Physics, S.E. College, Vijayawada-520006, India
and G. SATYANANDAM and C. R. K. MURTY* Departmentof Physics, NagarjunaUniversity, Nagarjunanagar-522 510,India (Received 16 December 1980; accepted in revised form 29 April 1981)
Abstract-Proton magnetic resonance in a new paramagneticcrystalline hydrate, cobalt mercuric chloride tetrahydrate, was observed using a CW wide line NMR spectrometer. The resonance spectra showed appreciable asymmetry caused by the paramagnetic influence of Co*+ions at the proton sites. The angular dependenceof the resonancesplittingsabout each of the three crystallographicaxes was analysedto determinethe p-p vectors in the unit cell of the crystal, and the angular variationof paramagneticshift about the [COl]axis of rotation was studied.
Attempts to grow isomorphous classes of diamagnetic and paramagnetic crystals met with failure. A CW wide-line Varian NMR spectrometer was used to record the proton resonance splittings in CoHgCL,. 4H,O at the Larmor frequency of 15MHz. The modulation frequency and the time constant were set at 40Hz and 3 sec. respectively, the modulation field being 0.75 G. Three different crystals were rotated within the magnetic field through 180” at 10” intervals about the [OOl],[OlO] and [lOO]axes.
1. INTRODUCTION Proton magnetic resonance (PMR) studies of several paramagnetic hydrates reported at room temperature have indicated no asymmetry in the resonance spectra[l-4]. Hence these have been analysed by employing the Pake method [5], which is valid for diamagnetic crystalline hydrates. The influence of the paramagnetic interaction on the proton resonance lines is usually studied at low temperatures[d8] where it is much larger than the dipole-dipole interaction between protons belonging to the same water molecule. Nevertheless, the effect of crystal paramagnetism on proton splittings has also been observed at room temperature in a very few cases[9, lo]. In this paper, results obtained by PMR at room temperature in single crystals of cobalt mercuric chloride tetrahydrate (CoHgCl, .4H,O)are reported. Since the structure of this crystal is not reported in the literature, it is considered that the present PMR study will yield valuable information on the magnitudes and directions of the proton-proton (p-p) vectors in the unit cell and on the paramagnetic interactions.
3. RESULTS ANDDISCUSSION The PMR spectra consisted of four well-resolved lines in most of the orientations about the [OOll axis of rotation. The lines were asymmetrically distributed about a strong narrow line which is due to the trapped water molecules in the crystal. Some typical examples of the spectra are given in Fig. 1. In diamagnetic crystals, the resonance lines will be distributed symmetrically about a central line and the doublet separation between them is given by the Pake equation [5]
2. EXPERIMENTAL. Large single crystals of CoHgCL * 4HsO were grown at 298 K from a saturated aqueous solution containing CoC& and HgCl* in the ratio 1:2.5 by weight [I 11. The crystals were deep pink in colour, and highly hygrostopic and they were given a thin layer of quick-fix for protection. Morphological examination of the crystals indicated dominant (1lo), (liO), (102) and (102) faces with the long axis being thelOOl] axis. A preliminary X-ray analysis of the sample (Spence, private communication) showed that the crystal belongs to the orthorhombic class with four molecules per unit cell. The unit cell dimensions are a = 7.82 A, b = 15.57A, c = 8.17 A, Vc.,, = 994.8 A3 and Dexpt= 3.04gmlcm3.
2(AH) = 213 co? 6 cos* (4 - &,) - l]
(1)
where the parameters have their usual significance. In CoHgCL,*4H20, the paramagnetic interaction is revealed by an asymmetry in the PMR lines even at room temperature (298 K). The observed asymmetry can be explained in terms of a shift in the centre of Pake doublet (with respect to the free line) caused by the crystal paramagnetism[7]. This shift, however, is enough to create a difficulty in identifying the Pake doublets corresponding to different p-p vectors. To overcome this difficulty, spectra at different fields are needed, for the shift is expected to change with the field strength while the pair separation does not change.
*Authorto whom all correspondence should be addressed. 25
T. F. S. RAI et al.
26
(b)
Fig. I. Proton resonance spectra of
CoHgCl, .4H20 at 15MHz for [OOl]rotation at orientations (a) NO’, (b) 110” and (c) 120”.
The angular dependence of the resonance splittings obtained at two different fields corresponding to Larmor frequencies 15 MHz and 4 MHz is shown in Fig. 2. The spectra were also recorded at closer intervals of orientations near crossover points. Having separated out the intra-proton interaction from the paramagnetic interaction by utilising the previously mentioned differential property, the experimental pair separations were fitted to
eqn (1). The Pake curves thus obtained are shown in Fig, 3, and the orientations of the p-p vectors deduced are given in Table 1. The error involved in the determination of the p - p direction is estimated to be less than 3”,while that in the determination of the p-p distance is less than 0.015 A. The results given in Table 1 indicate that the two vectors derived are symmetry related. The angular variation of the paramagnetic shift, the
Fig. 2. Angular variation of the resonance peak positions at IS MHz and 4 MHz in CoHgC& . 4H20. [OOl] is the crystal rotation axis. Circles and triangles represent experimental data for high and low fields respectively.
Proton magneticresonance in CoHgCl, .4H20
21
[1003
Lo101 t
eAH(GAUSS) t -@(DEGREES)
Fig. 3. Pake curves in CoHgC&. 4H20 for the [Ool] axis of rotation. Circles and squares represent experimental pair separations for high and low fields respectively.
Angular variation of the paramagnetic shift at 15MHz in CoHgCl,. 4Hz0 about the [OOl]axis of rotation. Circles and squares represent the experimental points for the two vectors (Table 1).
Table 1. The magnitudes and directions of the observed p - p vectors in CoHgCI.,. 4H20
-_-------p-p Veotor
4 (%I
I
II
42
136
s
*y
(W)
(
32*69
r
1*699
339 I3
-Pa
Direction
1
1*606
a
=
1.603
separation between the center of gravity of the Pake doublet from the usual central free proton line at 15 MHz follows a dipolar (3 cos* 0- 1) type variation and is shown in Fig. 4. However, the fit is not too satisfactory partly because the (3 cos* l3- 1) dependence is only a first order approximation. For a field of 3523G corresponding to 15 MHz the maximum shifts were found to be 1.45G and 1.39G for vectors I and II respectively. To confirm the number of p-p vectors obtained, room temperature spectra about the [IOO] and [OlO] axes of rotation were also recorded at ISMHz. Four lines were obtained at a very few orientations about the [lOO]axis of rotation. In all other orientations, the discrete structure of the spectra could not be observed, which might be due to high g-anisotropy. Theretical Pake curves were drawn about this axis of rotation from the data obtained for the [OOl]axis. It was found that these Pake curves passed through the experimental points, indicating that
1001
[
0101
0.6a40
0.5619
@lo)
(56’)
0*&X4
0.58
(53’)
( 126’1
Cosines .pO31
0*5430
(57O) 17
O*MQ (57O)
P
there are only two p-p vectors in the unit cell. For the [OlO] axis of rotation, only a pair of asymmetrically distributed lines was recorded, in concurrence with the data of Table 1. Further detailed discussion cannot be carried out without a knowledge of the crystal structure. To get a complete understanding of the paramagnetic interactions and the crystal structure, low temperature PMR and deuteron resonance (DMR) work is needed. Ackhowlegements-Thanks are due to Prof. R. Vijayaraghavan for extending the facilities of the Varian spectrometer, TIFR, Bombay and to Dr. V. Nagarajan, TIFR, Bombay for helpful discussions. The authors are grateful to Prof. R. D. Spence for his private communication on preliminary X-ray data taken on the crystal. Financial assistance from C.S.I.R., Delhi and N. S. F. (Grant No. GF 36748)U.S.A., is gratefully acknowledged. REFERENCES 1. Pedersen B., Thesis, Central Institute for Industrial Research, Oslo, Norway (1964).
28
T. F. S. RAI et al.
2. Reeves L. W., Prog. NMR Spectrosc. 4 193(1969). 3. El Saffar Z. M., J. Chem. Phys. 45 4643 (1%6). 4. Vizia N. C., Murty P. N. and Murty C. R. K., J. Mag. Rex 22 439 (1976). 5. Pake G. E., J. Chem. Phys. 16 327 (1948). 6. Abe H. and Matsura M., J. Phys. Sot., Japan. 19 1867(1964). 7. Bloembergen N., Physica 16,96 (1950).
8. Poulis N. J., Physica 17 392 (1951). 9. Padmanabhan A. C., Rangarajan G. and Srinivasan R., Proc. Indian Nat. Sci Acad., Part A40 241 (1974). 10. Bose M., Ghosh-Ray A. and Basu A., Proc. Nucl. Phys. Solid-St. Phys. Symp. 19C 363 (1976). 11. Linke W. F., Solubilities of Inorganic and Metal Organic Compounds, Vol. I. Van Nostrand, New Jersey (1958).