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Nuclear magnetic resonance experiments on CuCl2·2H2O, K2CuCl4·2H2O and(NH4)2CuCl4· 2H2O
Physica XIX 415-418
Itoh, J. Kusaka, R. Yamagata, Y. Kiriyama, R. Ibamoto, H. 1953
NUCLEAR MAGNETIC RESONANCE EXPERIMENTS ON CuCl,. 2H,O, K,CuCl,. 2H,O AND(NH,),CuCl,. 2H,O by J. ITOH, R. KUSAKA, Dcpnrtment
of Physics.
Y. YAMAGATA, H. IBAMOTO
and Dcpnrtmcnt
of Chemistry,
Osaka
R. KIRIYAMA University,
Osaka,
and Japan
P a k e I) first developed the nuclear magnetic resonance method for the investigation of hydrated single crystals, and since then B 1o e m b e r g e n “) has performed an experiment using a paramagnetic crystal. Recently, Poulis and Hardemana) applied the method of Bloembergen to CuC1,.2H,O and K,CuCl,. .2H,O at very low temperatures and found that the former salt shows a remarkable antiferromagnetic character. We made our nuclear resonance experiments on single crystals of CuC1,.2H,O, K,CuC1,.2H,O and (NH,),CuC1,.2H,O at room temperature and at a quite low field (1800 gauss). Under these conditions the effect of the paramagnetic ions is very small, and can in fact almost be neglected in comparison with the interaction between the two protons in each water molecule. For this reason, p-p direction and p-p separation will be determined more accurately than if the experiment were performed at very low temperatures, where the effect of the paramagnetic ions is very much larger than that of the two-proton interaction. For details of the precise crystal structures of these single crystals, reference should be made to the original papers “), “) . First we shall consider the K- and (NH,)-salts, which have very similar crystal structures. They have tetragonal symmetry as a whole, the c-axis being the axis of tetragonal symmetry, and the unit cell contains two formula units. The configuration of one of these molecules can be derived from the other by displacing (4, 4, +) and rotating by 90” around the c-axis. When the static field is rotated in the plane perpendicular to the c-axis, four resonance peaks are in general observed. However, owing to the broadening effect of the paramagnetic ions, -
415 -
416
J. ITOH,
R. KUSAKA,
Y. YAMAGATA,
R. KIRIYAMA
AND
H. IBAMOTO
each individual peak is considerably broadened, and overlapping of peaks occurs for some ranges of the angle of rotation. P a k e’s theory indicates that in the case of the two-proton system the separation of peaks, AH, from the field strength corresponding to free protons is given by the formula AH=
f~~r-3(3cos~e-l),
(1)
where ,u is the proton moment, r the p--p separation and 8 the angle between p-fi direction and static field. The effect of the paramagnetic copper ion should be added to the expression (l), but this effect is very small in our case less than 0.2 gauss.
0
20
u40
60
Fig. 1. Positions of the resonance peaks for K~CuC14.2H~0. AH is the separation of the peak from the resonance field for liquid water after applying a small correction for the effect of the paramagnetic ions, and 0 is the angle between the static field and the direction [l lo]. Only the peaks for the AH are shown. Circular marks indicate the experimental points, while the curves are drawn according to the theoretical formula (1). Overlapping peaks which could not be well discriminated are denoted by elongated marks.
Fig. 1 shows the experimentally observed AH for the K-salt (after correction for the effect of the paramagnetic ion) when the static field is rotated in a plane perpendicular to the c-axis. Only half the peaks are shown, the others being just the reversed with respect to the abscissa. In addition, the observed structure of peaks at an angle - 0 is just the same as that for +0. Since considerations of crystal structure suggest that the p--fi line in each water molecule is perpendicular to the c-axis, the observed separation may be interpreted directly by (1). The curves in fig. 1 show the values calculated from (l), assuming 6pr -’ = 19.7 gauss and that there are two di-
NUCLEAR
MAGNETIC
RESONANCE
EXPERIMENTS
417
rections of p-p lines each lying along one of the two bisectors of two a-axes. The experimental results obtained for the other orientation of the static field are also in fair coincidence with the calculated curves. Therefore, it is concluded that in the K-salt there are two types of water protons respectively directed along [ 1lo] and [liO], with the same p-p separation of 1.62 A. For the (NH,)-salt, the experimental results are very similar to those for the K-salt, differing only in the existence of an intense central peak due to the protons of the (NH,)-group. In this crystal the p-$ lines are also directed along [ 1IO] and [l-10], with a p-$ separation of 1.59 8.
Fig. 2. Positions of the resonance peaks for CuC12.2H20. AH is the separation of the peak from the resonance field for liquid water, after applying a small correction for the effect of the paramagnetic ions, and 0 is the angle between the static field and the u-axis. Only the peaks for the AH are shown. Circular marks indicate the experimental points, while the curves are drawn according to the theoretical formula (1). Overlapping peaks which could not be well discriminated are denoted by elongated marks.
Next, CuC1,.2H,O will be discussed. It has rhombic symmetry, as a whole, and the unit cell contains two formula units, one of which is derived from the other by displacing (4, 4, 0) and reflecting with respect to the a- or c-plane. Since considerations of crystal structure suggest that the p--$ line in each water molecule is perpendicular to the b-axis, we performed the experiment for the case when the static field is rotated in the b-plane. Fig. 2 shows the experimental results. The observed peaks are very similar to those found for the K-salt. The curves in the figure are drawn according to eq. (l), taking Physica
XIX
27
418
NUCLEAR
MAGNETIC
RESONANCE
EXPERIMENTS
6,~-~=20.6 gauss and assuming that the two +-$ directions lie at angles f 37.5” from the a-axis. The experimental results for the other orientation of the crystal also coincide with the calculated curves. Therefore it is concluded that the +-$ lines of the water molecules in this crystal lie at angles f (37.5” f 1.5”) from the a-axis in the plane perpendicular to the b-axis, with a p+ separation of 1.60 A. Thus in all these crystals, the p - p separation is very nearly that for a free water molecule, actually a little larger. Our conclusion for the case of CuC1,.2H,O differs from that of P o u 1 i s and H a r d e m a n 3). It is possible that the structure of the water molecules changes at low temperatures; but for the NH,-salt the value and the direction of maximum separation did not alter at liquid air temperatures. A similar experiment on CuC1,.2H,O at liquid air temperatures has not yet been performed. Received
2-l-53.
REFERENCES 1) 2) 3) 4) 5)
I’ n k e, G. E., J. them. Phys. Iti (1948) 327. B 1 o e 111b c r g c II, S., Physica 14; (1950) 95. P o u I is, I\‘. J. and H a r d e 111n II, G. E. G., Physica 1-I a r k e r, D., Z. Krist. !):) (1936) 136. C h r CI II a b, J., Z. Iirist. 88 (1934) 35.
IIJ (1952)
201.
Itoh, J. Kusaka, R. Yamagata, Y. Kiriyama, R. Ibamoto, H. 1953
NUCLEAR MAGNETIC RESONANCE EXPERIMENTS ON CuCl,. 2H,O, K,CuCl,. 2H,O AND(NH,),CuCl,. 2H,O by J. ITOH, R. KUSAKA, Dcpnrtment
of Physics.
Y. YAMAGATA, H. IBAMOTO
and Dcpnrtmcnt
of Chemistry,
Osaka
R. KIRIYAMA University,
Osaka,
and Japan
P a k e I) first developed the nuclear magnetic resonance method for the investigation of hydrated single crystals, and since then B 1o e m b e r g e n “) has performed an experiment using a paramagnetic crystal. Recently, Poulis and Hardemana) applied the method of Bloembergen to CuC1,.2H,O and K,CuCl,. .2H,O at very low temperatures and found that the former salt shows a remarkable antiferromagnetic character. We made our nuclear resonance experiments on single crystals of CuC1,.2H,O, K,CuC1,.2H,O and (NH,),CuC1,.2H,O at room temperature and at a quite low field (1800 gauss). Under these conditions the effect of the paramagnetic ions is very small, and can in fact almost be neglected in comparison with the interaction between the two protons in each water molecule. For this reason, p-p direction and p-p separation will be determined more accurately than if the experiment were performed at very low temperatures, where the effect of the paramagnetic ions is very much larger than that of the two-proton interaction. For details of the precise crystal structures of these single crystals, reference should be made to the original papers “), “) . First we shall consider the K- and (NH,)-salts, which have very similar crystal structures. They have tetragonal symmetry as a whole, the c-axis being the axis of tetragonal symmetry, and the unit cell contains two formula units. The configuration of one of these molecules can be derived from the other by displacing (4, 4, +) and rotating by 90” around the c-axis. When the static field is rotated in the plane perpendicular to the c-axis, four resonance peaks are in general observed. However, owing to the broadening effect of the paramagnetic ions, -
415 -
416
J. ITOH,
R. KUSAKA,
Y. YAMAGATA,
R. KIRIYAMA
AND
H. IBAMOTO
each individual peak is considerably broadened, and overlapping of peaks occurs for some ranges of the angle of rotation. P a k e’s theory indicates that in the case of the two-proton system the separation of peaks, AH, from the field strength corresponding to free protons is given by the formula AH=
f~~r-3(3cos~e-l),
(1)
where ,u is the proton moment, r the p--p separation and 8 the angle between p-fi direction and static field. The effect of the paramagnetic copper ion should be added to the expression (l), but this effect is very small in our case less than 0.2 gauss.
0
20
u40
60
Fig. 1. Positions of the resonance peaks for K~CuC14.2H~0. AH is the separation of the peak from the resonance field for liquid water after applying a small correction for the effect of the paramagnetic ions, and 0 is the angle between the static field and the direction [l lo]. Only the peaks for the AH are shown. Circular marks indicate the experimental points, while the curves are drawn according to the theoretical formula (1). Overlapping peaks which could not be well discriminated are denoted by elongated marks.
Fig. 1 shows the experimentally observed AH for the K-salt (after correction for the effect of the paramagnetic ion) when the static field is rotated in a plane perpendicular to the c-axis. Only half the peaks are shown, the others being just the reversed with respect to the abscissa. In addition, the observed structure of peaks at an angle - 0 is just the same as that for +0. Since considerations of crystal structure suggest that the p--fi line in each water molecule is perpendicular to the c-axis, the observed separation may be interpreted directly by (1). The curves in fig. 1 show the values calculated from (l), assuming 6pr -’ = 19.7 gauss and that there are two di-
NUCLEAR
MAGNETIC
RESONANCE
EXPERIMENTS
417
rections of p-p lines each lying along one of the two bisectors of two a-axes. The experimental results obtained for the other orientation of the static field are also in fair coincidence with the calculated curves. Therefore, it is concluded that in the K-salt there are two types of water protons respectively directed along [ 1lo] and [liO], with the same p-p separation of 1.62 A. For the (NH,)-salt, the experimental results are very similar to those for the K-salt, differing only in the existence of an intense central peak due to the protons of the (NH,)-group. In this crystal the p-$ lines are also directed along [ 1IO] and [l-10], with a p-$ separation of 1.59 8.
Fig. 2. Positions of the resonance peaks for CuC12.2H20. AH is the separation of the peak from the resonance field for liquid water, after applying a small correction for the effect of the paramagnetic ions, and 0 is the angle between the static field and the u-axis. Only the peaks for the AH are shown. Circular marks indicate the experimental points, while the curves are drawn according to the theoretical formula (1). Overlapping peaks which could not be well discriminated are denoted by elongated marks.
Next, CuC1,.2H,O will be discussed. It has rhombic symmetry, as a whole, and the unit cell contains two formula units, one of which is derived from the other by displacing (4, 4, 0) and reflecting with respect to the a- or c-plane. Since considerations of crystal structure suggest that the p--$ line in each water molecule is perpendicular to the b-axis, we performed the experiment for the case when the static field is rotated in the b-plane. Fig. 2 shows the experimental results. The observed peaks are very similar to those found for the K-salt. The curves in the figure are drawn according to eq. (l), taking Physica
XIX
27
418
NUCLEAR
MAGNETIC
RESONANCE
EXPERIMENTS
6,~-~=20.6 gauss and assuming that the two +-$ directions lie at angles f 37.5” from the a-axis. The experimental results for the other orientation of the crystal also coincide with the calculated curves. Therefore it is concluded that the +-$ lines of the water molecules in this crystal lie at angles f (37.5” f 1.5”) from the a-axis in the plane perpendicular to the b-axis, with a p+ separation of 1.60 A. Thus in all these crystals, the p - p separation is very nearly that for a free water molecule, actually a little larger. Our conclusion for the case of CuC1,.2H,O differs from that of P o u 1 i s and H a r d e m a n 3). It is possible that the structure of the water molecules changes at low temperatures; but for the NH,-salt the value and the direction of maximum separation did not alter at liquid air temperatures. A similar experiment on CuC1,.2H,O at liquid air temperatures has not yet been performed. Received
2-l-53.
REFERENCES 1) 2) 3) 4) 5)
I’ n k e, G. E., J. them. Phys. Iti (1948) 327. B 1 o e 111b c r g c II, S., Physica 14; (1950) 95. P o u I is, I\‘. J. and H a r d e 111n II, G. E. G., Physica 1-I a r k e r, D., Z. Krist. !):) (1936) 136. C h r CI II a b, J., Z. Iirist. 88 (1934) 35.
IIJ (1952)
201.