Anomalous temperature dependence of the NQR frequency in NH4ReO4

JOURNAL

OF MAGNETIC

RESONANCE

15,584-589

(19%)

AnomalousTemperatureDependenceof the NQR Frequency in NH,ReO,* R. A. JOHNSON~AND MAX T. ROGERS Department

of Chemistry,

Michigan

State

University,

East

Lansing,

Michigan

48824

ReceivedApril 24, 1974 The temperature dependenceof the ls7Re (+5/2 t) +3/2) nuclear quadrupole resonancetransition frequency has been measuredfrom 387°K down to 245°K. The signal broadensand losesintensityat lower temperaturesand is not observable below 245°K. The temperaturecoefficientof the frequencyis positiveover this range and decreasesfrom an unusually large value at the lowest temperature observable to nearly zero at 387°C. Thesedata, together with the results of separateRamaninfrared studiesof the lattice frequencies,suggestthat the anomalous temperature coefficientis associatedwith temperature dependenceof the reorientation of the ammonium ion. INTRODUCTION In an extensive study of the rhenium nuclear quadrupole resonance (NQR) spectra of perrhenates with the scheelite structure (I, 2), it was found (2) that the temperature dependenceofthe Is53ls7Re frequencies in N&ReO, were anomalous in that they were lower at 257°K than at 300°K. While a number of NQR frequencies have been reported which show positive temperature coefficients, nearly all these have been halogen ligands in heavy metal halides or halometallate ions (3), An increase in the ls7Re (*5/2 t) rt3/2) transition frequency in Re,(CO),, was observed in going from 80°K to about 170”K, but this was found to result from changes in the asymmetry parameter, rather than of the quadrupole coupling constant, with temperature (4). In ammonium perrhenate, the asymmetry parameter is zero and rhenium is bonded to four oxygens in a regular, or slightly distorted, tetrahedral arrangement (5); the electric field gradient, which probably has contributions both from the distortion of the ion and from the charges in the surrounding lattice (I, 2), must therefore be increasing with temperature. In order to investigate the origin of this unusual effect, we have measured the lS7Re (+5/2 * *3/2) transition frequency over the temperature range 387-245”K, below which the signal disappears. EXPERIMENTAL AND RESULTS Ammonium perrhenate was prepared by neutralizing an aqueous solution of Re,07 (Alfa Products, Beverly, Mass.) with aqueous ammonium hydroxide and crystals were grown by slow evaporation of this solution. * This work wassupported in part through a contract with the Atomic Energy Commissionand this is AEC Document No. COO-1385-50. t Presentaddress: The Upjohn Company, Kalamazoo, Michigan 49001. Copyright 0 1974 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

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FIG. 1. Experimental in NH,ReO,.

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temperature dependence of the lS7Re (+5/2 t) +3/2) NQR transition frequency

The frequency of the ls7Re(5/2 f-) 3/2) transition was measured at various temperatures using the NQR spectrometer, variable-temperature bomb, and auxiliary equipment described previously (6). The experimental values are plotted versus temperature in Fig. 1. The temperature coefficient is much larger than observed for the halogen resonances in the metal halides and halometallates as shown by comparison with some typical positive temperature coefficients in Table 1. The signal broadens and loses intensity on cooling and was not observable below 245°K. The temperature coefficient at the highest temperature we could attain (387°K) had dropped to nearly zero (Fig. 1) and it appears that it would become negative, as expected on theoretical grounds, at sufficiently high temperatures. Unfortunately, with our apparatus we were not able to carry the measurements above 387°K. The frequency of the 187Re (+3/2 f-f t-1/2) transition was measured at 257°K and at room temperature previously (2) and it was found that the asymmetry factor was zero within experimental error. DISCUSSION

Positive temperature coefficients may be accounted for in a formal way on the basis of the theory of Kushida, Benedek and Bloembergen (7), as extended by Williams and

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SOMEPOSITIVETEMPERATURECOEFFICIENTSOF NQR FREQUENCIES

Compound NH,Re04 NH& WC16 wm5 CsNbCls CsWCls WUR& KzReCls NUReBr6 a The value for NH4Re04 is at 245°K and is from this work, the value for NH& is at 253°K and is from Ref. (19); the remaining values are from a table in Ref. (3).

Gutowsky (8). In thei(Eqs.

(7) and (8) :

v-1 where v is the NQR frequency, wI are the lattice vibrational frequencies, a = (a V/W),/ V is the coefficient of thermal expansion, and p = -@V/+),/V is the isothermal compressibility coefficient. Since the Bayer term (9), (&/?W),, and the vibration amplitude are always negative while cz/P and (aV/a~?)~ are always term, Z’dW~~l)(~w/~~>, negative, a positive temperature coefficient for an NQR frequency requires that (?~v/iTp)~ be sufficiently negative that the first term on the right of Eq. [I] dominates. This, in turn, requires that (av/aV), be positive and larger than the vibration amplitude term. Since @v/W), becomes more negative at higher temperatures (9), while (&J/+)~ does not seem to depend much on temperature (7), the observed values of @v/U), should become negative at sufficiently high temperatures. This is probably true for NH,ReO, since, at the highest temperature we could attain, the temperature coefficient of v appears to be going through a maximum. Barnes and Engardt (10) found that the 7gBr resonance in TiBr, increased with temperature up to -50°C above which the normal decrease set in. They suggested that the positive value of (av/aV), was due to intermolecular hybridization of the metal-halogen bond caused by the approach of nearest neighbor molecules. Several positive temperature coefficients have since been observed for halogen NQR frequencies in heavy metal halides and in hexahalometallates (3, II, 12) and it has been shown (3) that these can be associated with the extent of metal-halogen z bonding. Also, a theoretical argument has

NQR

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been given for expecting positive (av/i3V), values for certain hexahalometallates (11). Brill and Long (13) found that phase changes occurred on cooling the salts [(C,H&NH,],Bi(or Sb)Br, and that below the phase transition the *lBr quadrupole resonance showed a positive temperature coefficient; they attributed this to an increase in the term (~3VjaT), in going to the low-temperature phase combined with a positive value of (av/aV) in these salts. Mechanisms have been proposed for the positive temperature coefficients of the hexahalometallates based on activation of the metal-halogen bending modes (14) or, more probably, the stretching modes (3). This approach does not provide a basis for the explanation of the temperature coefficient in ammonium perrhenate, which is not only positive but unusually large (Table l), since a number of other perrhenates also having the scheelite structure show normal behavior and the bonding in the ReO; ions in this series of compounds appears to be similar (2). Thus, the temperature dependences of the rhenium NQR frequencies in NaReO,, KReO, and AgReO, (as well as of the lz71 frequency in NaIO, which also has the scheelite structure), have been determined over the approximate range 16-300°K in a separate study and shown to be essentially normal (6). Furthermore, the lattice vibrational modes for these crystals, and their temperature dependences, have also been measured by Raman-infrared spectroscopy (15, 16). While the temperature coefficients calculated by the Bayer-Brown (17) procedure from the vibrational data are somewhat smaller than observed, they are all negative and the differences are about what one might expect from neglecting the effect of volume on the coupling constants. These results also make it unlikely that a soft-mode phase transiton at low temperature, such as suggested by O’Leary (18) to explain the chlorine resonance in K,ReCl,, is operative in ammonium perrhenate. It appears more likely that the positive contributions to the temperature coefficient, and the gradual broadening and disappearance of the NQR signal, observed for NH,ReO, are associated with reorientation of the ammonium ion. A similar explanation was given by Sasena et al. (19) for the very large positive temperature coefficient (Table 1) found for the lz71 resonance in NHJ, while the rubidium and cesium salts showed normal behavior. The Raman-active ammonium ion fundamentals in NH,Re04 and ND,ReO, have been reported, along with the lattice modes (1.5), at room temperature and at 77°K. While the ReO; internal modes show only a small temperature dependence, the ammonium ion fundamentals broaden and lose intensity on warming, leaving only a single broad, weak band for all the ammonium motions at 300°K. Ammonium group rotations have been extensively studied in solids and the best indicator of “free”, or restricted, rotation is the absence or presence, respectively, of the ammonium ion torsional vibration (or of combination bands with this mode) (20,21). Changes in the shape of the ammonium group fundamental bands can also be important (22). The presence of the ammonium ion libration in NH,ReO, at 77°K is, therefore, taken as evidence that the ion undergoes limited torsional motion at that temperature while its disappearance (along with marked changes in the ammonium group fundamentals) at 300°K indicates that reorientation has become rapid, or “free,” at the higher temperatures (15). Unfortunately, while the crystal structure of NH,ReO, has been reported at room temperature (5), the oxygen parameters given are only the “standard” values which lead to a tetrahedral ReO; ion, and a precise determination of oxygen positions has never

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been carried out. At room temperature the lattice parameters a, = 5.883 A and co = 12.979 A, along with the nominal oxygen positions x = 0.25, y = 0.11, z = 0.075, correspond to tetrahedral ReO; (Re-0 = 1.87 A) with each ammonium ion surrounded by eight oxygen% four in a regular tetrahedron at 2.72 A and four in a flattened tetrahedron (LO-N-O = 142”) at 2.82 A. In the ammonium halides, which have been extensively studied by NMR and other methods (23), the hydrogen atoms of the ammonium ions point toward four of eight equivalent halide ions at the corners of the unit cube and there is a lambda point above which the ammonium ion orientations are random. On the basis of these various experimental observations we suggest the following possible explanation for the NQR results. At very low temperatures the ammonium ion would presumably be oriented with the hydrogens pointing toward the four closest oxygen neighbors at 2.72 A (configuration I). On warming, ammonium ion reorientation to a second configuration (II) of somewhat higher energy, with the hydrogens directed approximately toward the four oxygens at 2.82 A, would become energetically allowable and a transition of the order-disorder type could occur. Since the magnitudes and directions of the field gradient tensor components would differ in configurations I and PI, reorientations between them would produce a fluctuating field gradient at the nucleus and the resonance would be unobservable, as is the case below 245°K in NH,ReO,. At some still higher temperature transition to a “free” rotation on the NQR time scale could occur and the ammonium ions would appear as simple cations to the rhenium nucleus so the NQR signal would again be observable. The positive temperature coefficient above 245°K could result from changes in covalent bonding for the Re-0 bonds as the weak N-H . . . 0 bonds are increasingly broken and the mean positive charge seen by a given oxygen becomes smaller. Although no sharp phase transitions can be identified in the Raman-infrared work in the range 77-300”K, the spectra are consistent with the above model (1.5). ACKNOWLEDGMENTS We are indebted to Dr. K. V. S. Rama Rao and Dr. Dinesh for assistance with the experimental work. REFERENCES 1. M. T. ROGERS AND K. V. S. RAMA RAO, J. Chem. Phys. 49,1229 (1968). 2. M. T. ROGERS AND K. V. S. RAMA RAO, J. Chem. Phys. 58, 3233 (1973). 3. T. L. BROWN AND L. G. KENT, J. Phys. Chem. 74,3572 (1970). 4. S. L. SEGEL AND L. A. ANDERSON, J. Chem. Phys. 49,1407 (1968). 5. J. BEINTEMA, 2. Krist. 97A, 300 (1937). 6. R. A. JOHNSON AND M. T. ROGERS, “Proceedings of the International Conference on Nuclear Quadrupole Resonance” (J. A. S. Smith, Ed.) Heyden and Sons, London, pp. 297-313,1974. 7. T. KUSHIDA, G. B. BENEDEK, AND N. BLOEMBERGEN, Phys. Rev. 104, 1364 (1956). 8. H. S. GUTOWSKY AND G. A. WILLIAMS, Phys. Rev. 105,464 (1957). 9. H. BAYER, 2. Phys. 130,227 (1951); T. KUSHIDA, J. Sci. Hiroshima Univ. A19,327 (1955). 10. R. G. BARNES AND R. D. ENGARDT, J. Chem. Phys. 29,248 (1958). 11. R. IKEDA, D. NAKAMURA AND M. KUBO, J. Phys. Chem. 69, 2101 (1965); M. KUBO AND D. NAKAMCJRA, Advan. Inorg. Chem. Radiochem. 6,257 (1966). 12. K. R. JEFFREY AND R. L. ARMSTRONG, Phys. Rev. 174, 359 (1969) and earlier articles. 13. T. B. BRILLAND G. G. LONG, J. Phys. Chem. 75,189s (1971). 14. T. E. HAAS AND E. P. MARRAM, J. Chem. Phys. 43,3985 (1965).

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19. 20.

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22. 23.

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AND G. E. LEROI, J. Chem. Phys. 56,189 (1972). AND G. E. LEROI, unpublished results; D. A. HATZENBUHLER, Ph.D. thesis, Michigan State University, 1970. R. J. C. BROWN, J. Chem. Phys. 32,116 (1960). G. P. O’LEARY, Phys. Rev. Letters 23,182 (1969); G. P. O’LEARY AND R. G. WHEELER, Phys. Rev. Bl, 4409 (1970). A. SASANE, D. NAKAMIJRA, AND M. KUBO, J. Phys. Chem. 71,3249 (1967). G. PIMENTEL AND A. L. MCCLELLAN, “The Hydrogen Bond”, W. H. Freeman, San Franciso, 1960. J. R. DURIG AND D. J. ANTION, J. Chem. Phys. 51,3639 (1969); C. H. PERRY AND R. P. LOWNDES, J. Chem. Phys. 51, 3648 (1969); D. R. CLUTTER AND W. E. THOMPSON, J. Chem. Phys. 51,153 (1969); C. J. H. SCH~TTE AND A. M. HEYNS, J. Chem. Phys. 52,864 (1970). B. H. TORRIE, C. C. LIN, 0. S. BINBRECK, AND A. ANDERSON, J. Phys. Chem. Solids, 33,697 (1972); C. H. WANG AND R. B. WRIGHT, J. Chem. Phys. 57,4401(1972). See, for example, D. E. WOESSNER AND B. S. SNOWDEN, JR., J. Chem. Phys. 47,378 (1967).

R. A. JOHNSON,

M. T. ROGERS,

16. R. A. JOHNSON, D. A. HATZENBUHLER, 18.

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