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NMR STUDIES OF CONCENTRATED LITHIUM-AMMONIA SOLUTIONS
Y. Nakamura, M. Niibe, M. Shimoji
To cite this version:
Y. Nakamura, M. Niibe, M. Shimoji. NMR STUDIES OF CONCENTRATED LITHIUM- AMMONIA SOLUTIONS. Journal de Physique Colloques, 1980, 41 (C8), pp.C8-32-C8-35.
�10.1051/jphyscol:1980808�. �jpa-00220192�
JOURNAL DE PHYSIQUE CoZZoque C8, suppZ6ment aa n o 8, Tome 42, amAt 2980, page C8-32
NMR
STUD1 ES OF CONCENTRATED LITHIUM-AMMONIA SOLUTIONS
Y. Nakamura, M. Niibe and M . S h i m o j iD e p a r h e n t o f Chemistry, FaeuZty o f S c i e n c e , Hokkaido U n i v e r s i t y , 060 Sapporo, Japan
1. INTRODUCTION
Solutions of alkali metals in liquid ammonia show very interesting properties with varying metal concentration; from
electrolytic behaviour in the low metal concentration range to metallic one in the high concentration range. The nuclear
magnetic resonances (NMR) of these metal- ammonia solutions have considerably been studied [I]. However, earlier measurements and analyses were limited mostly to dilute electrolytic solutions. Although Lelieur and Rigny [2] have reported 1 3 3 ~ s and 1 4 ~ NMR in the cesium-ammonia system up to the very concentrated solution range, no com- plete measurements of nuclear relaxation times have so far been made for such a range of concentration in any alkali metal- ammonia systems.
In this paper, we report experimental results of the Knight shift, KLi, and the spin-lattice relaxation tine, T1, of 7 ~ i in the lithium-ammonia system up to the high metal concentrations. The results are discussed in terms of the models proposed in relation to the electrical properties of the system [31.
2. EXPERIMENTAL
The apparatus and procedures for NMR measurements were the same as described
elsewhere 141. Measurements were carried out with a Bruker SXP 4-100 pulse FT spec- trometer operating at 34.98 MHz. Temper- ature of samples was maintained at -57 +_
2OC with a variable temperature accessory.
The Knight shift was determined from the relation:
where Hr and Hs are the magnetic fields for which resonance occurs in the reference and sample solutions, respectively.
Aso- lution of lithium nitrate was taken as a reference. The spin-lattice relaxation time was measured by use of the 180'-T-90"
pulse technique. As the signal intensity was much attenuated due to the skin effect in highly conducting solutions, experi-
mental FID signals were averaged over 5 ~ ~ 2 0 0 pulse sequence repetitions.
3. RESULTS AND DISCUSSION
Fig. 1 shows the concentration depend- ence of the Knight shift of 'Li observed at -57OC. The values of KLi increase sharply around 3 MPM (mole percent metal) with increasing metal concentration. The shifts are less composition dependent at lower metal concentrations; this behaviour is consistent with the previous ,data [5,6
1. Fig. 1 also contains the concentration
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980808
Fig. 1 The Knight shift and the spin-lat- tice relaxation rate in the lithium-ammo- nia system at -57OC.
dependence of l/T1; the rate first de- creases with metal concentration and then increases passing through a broad minimum around 7 MPM. In the concentration range studied the relaxation rate for 7 ~ i
(I=3/2)consists of three terms due to the magnet- ic dipole-dipole interaction between lithi- um nuclei and protons, the quadrupolar in- teraction, and the Fermi contact inter- action with the valence electrons of the dissolved lithium atoms, which is written in order as:
1 1 , 1 1
(-1
= (-1 +(TI +
(-1T1 obs T1 ~ i - H 1 Q T1 e .
( 2 )
The first two terms of Eq. (2) were esti- mated experimentally from measurements of T1 for lithium nitrate-ammonia solutions;
the difference in viscosity of metal-ammo- nia. and salt-ammonia solutions was taken into account [7]. The estimated values of these two terms were not greater than 10%
of the observed rate, (l/Tl) obs, so that approximately
(1 obs
(l/T1 over the concentration range studied.
As seen in Fig. 1, the observed results of KLi and l/T1 suggest three different concentration regions: (I) the lower metal concentration region (C (in MPM)
$3), where
the values of l/T1 decrease and those of KLi show a small concentration dependence,
(11) the intermediate region (5&Ck9) ,
where the values of KLi increase, while those of l/T1 are almost concentration in- dependent, and (111) the higher metal con- centration region (C&9), where both the values of KLi and l/T1 increase with metal concentration. This classification rough- ly corresponds to the three different electronic transport regimes, deduced from the measurements of the electrical conduc- tivity, o, and the thermoelectric power
[ 31: i.e., the nonmetallic (semiconducting) regime, the strong scattering regime and the weak scattering regime, respectively.
The metal-nonmetal transition occurs at around 4.5 MPM, between the regions
(I)and (11). Here, an attempt is given to interpret the observed concentration dependence of l/T1 on the basis of the models given above.
The decrease of l/T1 in the region (I)
can be explained by the shortening of the
correlation time, T ~ , of the electron-
nuclear contact interaction due to the
delocalization tendency of solvated (local-
ized) electrons. This correlation time
may be given by
[8]:JOURNAL DE PHYSIQUE
where
Sis the electronic spin, A is the hyperfine coupling constant, wI and we are the angular frequencies of the nucleus and the electron, respectively. The values of
Acan be determined from the Knight shift
data and then we can roughly estimate the values of
L-together with the motional
e
narrowing condition, u2-r;<<1. The results are shown in Fig.
2as a function of metal concentration. The values of
T~decrease from %lo-l2 sec, a. characteristic value for the ionic motion, to %10'15 sec in the transition region.
In the intermediate region (II), there exists no rigorous theory of the NMR relax- ation. Let us use tentatively the expres- sion proposed by Warren
[9 ]for (l/T1), and a in the strong scattering regime which may be applicable to this range:
and
IJ =
e2d2~(EF)/3ke
(5where yLi and ye are the nuclear (Li) and electronic gyromagnetic ratios, respective- ly, N(EF) is the density of states at the Fermi level, d is the jump distance and fi is the atomic volume. From Eqs. (4) and
(5) we have:
The value of (K;~/O) is constant in the strong scattering regime [lo], as confirm-
Fig.
2The correlation time for the elec- tron-nuclear contact interaction in the nonmetallic region in the lithium-ammonia system at -57OC
ed also in this experiment. Thus we may expect that the values of (l/T1), are only weakly dependent on concentration, if the composition dependence of (d2/S2) is small in this region. This is in agreement with the experimental results given in Fig. 3.
In the region (III), both the Knight shift and the relaxation rate data may be described in terms of the Fermi contact interaction of the nuclei with the spins of s-like nearly free electrons. Then, the relaxation rate is given by the well known Korringa relation [9]:
The observed tendency is in agreement with that of the Korrinqa relation. It is noted, however, that the magnitudes of
(l/Tl)e calculated from Eq. (7) are small-
er than the observed rates by an order of high metal concentration range.
magnitude. The consideration of electron
correlation effect, ignored in the ACKNOWLEDGEMENTS
Korringa relation, makes the calculated The authors wish to thank Mr. S. SKimo- values of (l/T1), still smaller. The kawa, Mr. M. Hirasawa and Mr. Y. Kitazawa discrepancy between the observed and calcu- for their valuable assistance during the lated values shown in Fig. 3 may result course of this work.
from the fact that the present system can- not be treated so simply as consisting of
REFERENCES nearly free electrons which do not inter-
[l] e.g., R.Catteral1, "Proceedings of act appreciably with the solvent molecules.
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Hall coefficient, in concentrated metal-
59, 1148 (1973).
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ammonia solutions are successfully treated [3] M.Hirasawa, Y.Nakamura and M.Shimoji, on the basis of the free electron theory Ber. Bunsengesell., 82, 815 (1978).
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