Appl. Magn. Reson. 21, 1-11 (2001)AppliedMagnetic Resonance0 Springer-Verlag 2001Printed in Austria

Caesium and Hydrated Lead ChloratesStudied by 35C1 NQR Spectroscopy

J. N. Latosinska, A. Walczak, and J. Kasprzak

Institute of Physics, Adam Mickiewicz University, Poznan, Poland

Received February 17, 2001

Abstract. Molecular dynamics of caesium and lead chlorates (hydrated form) has been studied by"Cl nuclear quadrupole resonance (NQR) spectroscopy. The occurrence of only one resonance linefor the two compounds in the whole temperature range studied testifies to the crystallographicallyequivalent positions of their molecules in elementary cells. The relaxation processes in the two chlo-rates have been found to be analogous to those in the other chlorates not undergoing phase transi-tions. Analysis of the temperature dependence of the spin-lattice relaxation time has indicated thatat low temperatures the dominant relaxation mechanism consists of torsion vibrations (C1O3 anionlibration), whereas at 260 K the hindered rotation of the chlorate ion about the threefold axis setson and its activation energy is 50.0 kJ/mol. The temperature dependence of the resonance frequencyfor the two chlorates studied is best described by the Brown model extended over the low-tempera-ture range, which indicates large anharmonicity of the vibrations.

1 Introduction

So far a large number of compounds from the groups of chlorates [ 1-4] havebeen studied by 35C1 nuclear quadrupole resonance (NQR) spectroscopy, and forselected compounds the molecular dynamics has been studied as well. The avail-able literature data on 35C1 NQR (the resonance frequency 35Cl NQR at the liq-uid nitrogen temperature or the lowest studied temperature and for the sake ofcomparison at ambient temperature, quadrupole coupling constant, asymmetry pa-rameter and T t relaxation time) are collected in Table 1. As follows from thesedata, the 35C1 NQR spectra of chlorates reveal lines in a relatively narrow rangeof frequencies from 28 to 31 MHz. A narrow range of VQ obtained for thesecompounds [4] confirms the conclusion that the electric field gradient at the chlo-rine atom is determined by the covalent bond Cl-O in the anion C1O3 , whilethe influence of the cation is much less pronounced. However, it is the presenceof the cations which differentiates the 35C1 NQR frequencies of chlorates, as,according to the crystallographic data, the chlorate ion has approximately the samegeometry in all chlorates [4].

J. N. Latosinska et al.

Table 1. Chlorates whose molecular dynamics was studied and the temperature range at which VQ

and T, were studied.

Compound Temperature range (K)VQ Ti

Reference(s)

NaCIO 3 300-450 300-450 4KC1O 3 77-350 — 12RbCl03 77-445 296-445 1AgCl0 3 77-350 290-440 4, 12

Ba(C10 3)2 • H 2O 77-350 290-390 4, 12

Ca(C10 3)2 .2H20 292-345 292-345 2Cu(C103)2 . 6H 20 77-350 — 12La(C10 3 )3 .2H 20 77-298 _a 13

a Studies were not performed.

As follows from Table 1, molecular dynamics has been studied for a rela-tively small group of chlorates, and temperature measurements of quadrupolerelaxation time T, have been performed only for a series of compounds fromthis group: NaC1O 3, AgC1O3, RbC1O 3 , Ba(C10 3)2 • H2 O, and Ca(C10 3 )2 .2HO.

The results of these studies imply that the temperature dependence of the reso-nance frequency 35Cl NQR is determined by the dynamics of the ion C103 , ormore exactly, averaging of the electric field gradient (EFG) at the chlorine atomas a result of the torsion vibrations of the ion C1O3 . The averaging of the EFGtensor components is mainly due to the two reorientation modes presented in Fig.1. It has been established that for the so far studied sodium, barium [1, 2], ru-bidium and caesium [3, 4] chlorates the increase in the relaxation rate is a resultof the onset of the hindered rotation of the chlorate ion about the threefold axis.The aim of this study was to study the dynamics of caesium and lead chloratesand to compare the results with the data for other compounds from this group.

Cl0 0

0 r,Rx

Cl 1^I0 00

0

Fig. 1. Structural formulae of the chlorates studied.

35C1 NQR of Caesium and Hydrated Lead Chlorates 3

Caesium chlorate crystallizes in the tetragonal system of symmetry R3m (C3v)

with three molecules in the elementary cell of a = 0.6089 nm [5]. The meltingpoint of the compound is not known. The vQ frequencies have been studied atonly five selected temperatures [5-7]. Also no systematic study of the tempera-ture dependence of the T, relaxation time has been made, only in ref. 6 the re-laxation time T, has been estimated as 52 ms at ambient temperature.

Lead chlorate crystallizes in the rhombic system of symmetry Fd2d, witheight molecules in the elementary cell size a = 1.2561 nm, b = 1.1554 nm andc = 0.75877 nm, whereas the hydrated form of lead chlorate crystallizes in themonoclinic system, space group C2/c (C 3h) with four molecules in the elemen-tary cell size a = 0.8811(1) nm, b = 0.7744(1) nm, c = 0.9131(1) nm and /3=93.13(l)° (at 295 K) and a = 0.8755(1) nm, b = 0.7697(1) nm, c = 0.9130(1)nm and /3= 93.45(1)° (at 25 K) [8, 9]. The presence of water significantlyaffects the melting point, which for the unhydrated form is 503 K, while it is383 K for the hydrated.

Temperature dependences of vQ and T, have not been studied for this com-pound by 35C1 NQR and only at two temperatures the resonance frequencies havebeen measured: vQ = 29.712 MHz for T = 77 K and vQ = 29.233 MHz for T =298 K [10].

2 Experimental

The materials studied were standard samples of caesium chlorate and hydratedlead chlorate purchased from ICN Pharmaceuticals Life Science Group (Plainview,N.Y., USA), used without further purification. The producer informs only thatthe lead chlorate is hydrated, but no degree of hydration is specified.

For NQR measurements we used samples of caesium chlorate and lead chlo-rate, about 5.0 ± 0.1 cm 3 and 11.0 ± 0.1 cm3 , respectively, placed in vials. Themeasurements were performed on a pulse NQR spectrometer built at the Insti-tute of Physics, Adam Mickiewicz University (Poznan).

The 35Cl NQR frequencies at 77 K were measured with the classic Hahn se-quence. The x/2 pulse of 13 is duration was applied. The temperature was sta-bilized within ±0.1 K by a cryostat. Signals coming from chlorine nuclei of thecompounds studied were strong with signal-to-noise ratios of 50:1 and 40:1 after400 accumulations for caesium chlorate and potassium chlorate, respectively. Theaccuracy of the frequency determination was ±10 kHz, and that of T, did notexceed 5%.

3 Results and Discussion

The 35C1 NQR spectra at 77 K for caesium and hydrated lead chlorates are com-posed of one line, at 29.248 and 29.712 MHz, respectively consistent with lit-erature data.

29.4

29.2

29.0

28.8

d

28.6

28.4

28.20

29.4

29.2

29.0

N2

. 28.8C

28.6

28.4

IVV LVV .)VV

T (K)

4 J. N. Latosinska et al.

28.20 100 200 300

T (K)

Fig. 2. Experimental results for vQ(T) of caesium chlorate and curves obtained as a result of the fitby particular models: a Bayer, b Brown.

The temperature measurements of the resonance frequencies (Figs. 2 and 3)were performed for caesium chlorate at temperatures from 87.9 to 287 K, at whichthe intensity of the signal considerably decreased, while for hydrated lead chlo-rate they ranged from 98.6 to 334.9 K, which is close to the compound meltingpoint of 383 K. For caesium chlorate and hydrated lead chlorate only one reso-nance line each was recorded in the whole respective temperature range studied,which means that the positions of the molecules of these compounds in elemen-tary cells are crystallographically equivalent. For both compounds a typical courseof temperature dependence of frequency was observed, i.e., the frequency monoto-nously decreased with increasing temperature (Figs. 2 and 3). Temperature depen-dence of T, relaxation time was measured only for hydrated lead chlorate in therange 98.6-334.9 K (Fig. 4).

35C1 NQR of Caesium and Hydrated Lead Chlorates

30.0

5

29.8

29.6N2

0 29.4

29.2

29.00 100

30.0

29.8

29.6N2

d 29.4

29.2

200 300 400T (K)

29.0 1 ' ' 1 1

0 100 200 300 400T (K)

Fig. 3. Experimental results for vQ(T) of hydrated lead chlorate and curves obtained as a result ofthe fit by particular models: a Bayer, b Brown.

The temperature dependences of the resonance frequencies were analyzedassuming the Bayer and Brown theories [11 -13]:

v T =a—b

Q( exp(c/T) —1 (1)

where a = vQ (0) , b = 6hi v0(0)1(4Iw, — 3h) , and c = hiwilk ;

vQ (T) = a o + a1 T + a 2 T2 + a_,T-' , (2)

where

6 J. N. Latosinska et al.

Table 2. Values of the fitted coefficients for caesium and hydrated lead chlorates.

Compound Eq. a o = a a, • 10 - ' a2 . 10 -6 a 3 • 10 -9 d b c r Sand theory (MHz) (MHz/K) (MHz/K 2) (MHz/K3) (MHz • K) (MHz) (K) (kHz)

CSC1O3

Bayer (1) 29.260 1.007 234.37 0.999 2.585Browne (2) 29.49 —2.811 —2.648 — —3.358 — — 0.999 1.308

Pb(C10 3)2 • mH 2OBayer (1) 29.878 1.226 330.33 0.999 4.140Browns (2) 30.233 —2.605 —1.587 —12.305 0.999 3.840

a Extended to the low-temperature region.

5 3 S M A. 3 S Aao = v, a, = --kvQ^ ( 0 ) 2

= --kv

(w6M o

)

zmei 2

a2 = —3kvQI (w ° ) (0)2(= —3kvQM (C

O )z ' a ^ = __Vs A = — v6 MA'

i=1 t=1 8k

vQ(0) is the NQR frequency at 0 K, I is the moment of inertia, and CO, is thefrequency of librations. A comparison of the obtained correlation coefficients andstandard deviations has shown that they are best described by the Brown theoryextended over low temperatures, which testifies to a considerable anharmonicityof vibrations (Tables 2 and 3, Figs. 2 and 3).

As follows from the results, the Bayer theory is insufficient for a descrip-tion of molecular dynamics since it assumes the occurrence of only one kind ofvibrations. This finding could be expected on the basis of the hitherto studies ofmolecular dynamics of chlorates [1, 4]. The poorest description of the vQ(T) ob-tained for caesium chlorate was provided by the Kushida (KBB) theory. For thechlorates studied the discrepancies between the KBB predictions and experimen-tal results are mainly due to the fact that the assumptions of the quasi-harmonicmodel of interactions are not satisfied. The discrepancy between the KBB theory

Table 3. Physical parameters calculated for caesium and hydrated lead chlorates

Compound vQ(0)and theory (MHz)

vv(MHz)

v, • 106

(MHz)vi

(cm')g • 10 -^

(K-')f• 10 - " I. 10 -47

(K-') (kg • m')

CSC103

Bayer 29.26 — 4.88 162.88 — — 15.24Brown — 29.49 — — 4.66 — 176.84

Pb(C10 3) 2 • mH2OBayer 29.88 — 6.88 229.59 9.097Brown — 30.23 — — 3.04 — 49.47

35C1 NQR of Caesium and Hydrated Lead Chlorates 7

Table 4. Frequencies of vibrations estimated by NQR and measured by RE and IR methods.

Compound T V1Q VRE v,R Reference(s)(K) (cm') (cm-') (cm')

NaC10 3 300 115.7 72 — 1, 483

103122.7131179

KC1O3 300 81 54 — 158298

127145

AgC1O3 300 114 15CsC1O3 — 162.9 this paperSr(C10 3) 2 295 133 17

482514

Ba(C10 3)2 300 — 49 — 1861

122190

Ba(C10 3 )2 • H2 O 300 119 52 96 4, 1970 100

106 102124 106133 126158 134201 138224 148232 165

175207234

Ca(C10 3)2 .2H20 300 44 12Cu(C10 3) 2 •6H20 300 70 12Pb(C10 3)2 • mH 2O 229.6 — this paper

and experimental results for the chlorates studied follow mainly from the factthat this theory takes into account only low-amplitude harmonic vibrations.

According to the crystallographic data, for hydrated lead chlorate tempera-ture changes in volume are relatively small (about 8. 10 -3 nm3 at the transitionfrom 25 to 295 K) and a similar situation can be supposed for caesium chlorate,although the lack of crystallographic data does not allow such an analysis. Thelast conclusion also confirms a good agreement between the predictions of theBrown theory and experiment, as in this theory the influence of thermal expan-sion of the crystal on the resonance frequency is neglected but the anharmonicityof the lattice vibrations is taken into account. On the basis of the parameters of

J. N. Latosinska et al.

the best fit such physical parameters as the resonance frequency at 0 K, meanfrequency of the molecule librations, mean moment of inertia and g and f charac-terizing the anharmonicity of vibrations were calculated.

The frequency of librations for caesium chlorate is 162.89 cm - ', while it isfor hydrated lead chlorate 229.59 cm - '. Unfortunately, hitherto no IR or RE stud-ies have been made for frequencies lower than 300 cm - ', and the effect of theintramolecular vibrations on the averaging of the resonance frequency is insig-nificant. A comparison can be made only with the torsional frequencies knownfor the hitherto studied chlorates (Table 4). As follows from the analysis, themoments of inertia calculated on the basis of the fitted parameters significantlydiffer for different models. Moreover, the calculated moments of inertia also con-siderably differ from the value determined on the basis of the crystallographicdata.

For hydrated lead chlorate the moments of inertia calculated with respect tothe axes x and y on the basis of the crystallographic data are 50.8. 10 -47 and52.8. 10 -47 kg • m2 (at 295 K), and 51.4. 10 -47 and 53.7. 10 -47 kg • m2 (at 25 K)and the mean values are 51.8. 10 -47 and 52.6. 10 -47 kg • m2 . Similar values areobtained only assuming the Brown theory, which is an additional confirmationof the fact that this model gives the best description of the temperature changesof the resonance frequency. Unfortunately, a similar analysis cannot be made forcaesium chlorate because of the lack of crystallographic data.

In order to gain additional information on the dynamics of hydrated leadchlorate and to check the effect of water on the relaxation processes in thiscompound, the temperature dependence of T, quadrupole relaxation time wasstudied. The experimental dependence was fitted with a function in which theinfluence of both librations and hindered rotations of the group containing theresonance nucleus on the relaxation processes was taken into regard:

T-'taT+"+AZexpl—RTI. (3)

The results are collected in Table 5 and presented in Fig. 4. The broken lineillustrates the influence of librations, which permits more accurate determinationof the temperature at which the hindered rotation sets on. Analysis of the valuesand character of the temperature dependence of the fitted parameters indicatesthat in low temperatures the dominant mechanism of relaxation is the librationof the anion C1O3 about two axes perpendicular to its threefold axis, while above

Table 5. Parameters of the fit to T1 (T) by Eq. (3) for hydrated lead chlorate.

a • 10 -' n b • 10 10 (s-') Eo (kJ/mol) r S (s - ')

0.26 2.0 2.34 51.13 0.9978 3.91

1.43 1.68 1.61 50.0 0.9978 3.94

35C1 NQR of Caesium and Hydrated Lead Chlorates 9

6

2

T (K)

EI-C

0

0

1 _1 C1)

E

I-

10 8 6 4 2103 T1 (K1 )

Fig. 4. Experimental results for T1 (T) of hydrated lead chlorate and curves obtained as a result ofthe fit by the model assuming both librations and hindered rotations.

260 K it is the hindered rotation of the chlorate ion about its threefold axis, whichis manifested as a reduction of the T, relaxation time. The estimated activationenergy of the hindered rotation in hydrated lead chlorate is about 50.0 kJ/mol.For the sake of comparison, Table 6 presents Ea values obtained for other chlo-rates and the value for Pb(C10 3)2 • mH2O is close to them. The difference is thatfor other hydrated chlorates the activation energies of both hindered rotation andwater diffusion were obtained, whereas for hydrated lead chlorate no additionalactivation process was observed. Perhaps in the temperature range studied thediffusion is small enough not to affect the value of T,. Another possibility is a

Table 6. Comparison of the activation energies determined for different chlorates.

Compound E. (kJ/mol) Reference

NaCIO 3 27.2a 14RbCIO 3 44.3e 13Ba(CI0 3 )2 • H2 O 48.2e 14

59.9aCa(C10 3)2 .2H 20 38.0a 14

40.4a.óPb(C10 3)2 • mH 2O 50.08 this paper

a Hindered rotation.e Water diffusion.

10 J. N. Latosinska et al.

small content of the crystalline water in the compound or its loss on heating athigh temperatures. The temperature changes of T l above 260 K can be producedby a simultaneous effect of water diffusion and hindered rotation of the ion CO 3 ,

as it happens in Ba(C10 3)2 • H2 O [4]. The temperature changes of the hydratedlead chlorate NQR line width reveal its significant increase above 280 K. Ananalysis of 4v(T) performed in an analogous way as in ref. 2 gave an activa-tion energy of the hindered rotation twice lower than the value obtained fromT,(T), which means that some other factors apart from the hindered rotation musthave affected the line width. Most probably the increase in the line width is aresult of dehydration of hydrated lead chlorate. A similar effect was observed forparadichlorobenzene admixtured with paradibromobenzene [14] and hydrated bariumchlorate [4]. For caesium chlorate no significant changes in the 35C1 NQR line widthwere noted and it monotonously decreased with increasing temperature.

4 Conclusions

Only one NQR resonance line each was observed for caesium chlorate and hy-drated lead chlorate in the whole temperature range studied, which means thattheir molecules occupy crystallographically equivalent positions.

The experimental dependences of the resonance frequency obtained for thetwo chlorates is best described by the Brown model extended over low tempera-tures, which indicates large anharmonicity of molecular vibrations.

The analysis of the temperature dependence of T, spin-lattice relaxation timeshows that at low temperatures the dominant relaxation mechanism consists ofthe torsional vibrations (C1O3 anion librations) and at 260 K the hindered rota-tion of the chlorate ion about the threefold axis characterized by the activationenergy —50.0 kJ/mol is set on.

Results of the studies of caesium chlorate and hydrated lead chlorate per-formed have shown that the relaxation processes in them are similar to those inother chlorates not undergoing phase transitions.

References

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35C1 NQR of Caesium and Hydrated Lead Chlorates 11

13. Brown R.I.C.: J. Chem. Phys. 42, 116 (1960)14. Woessner D.E., Gutowsky H.S.: J. Chem. Phys. 27, 1072 (1957)15. Suhara M.: Bull. Chem. Soc. Jpn. 46, 1053 (1973)16. Smirnov N.I., Volkov A.F.: Dokl. Akad. Nauk SSSR 217, 639 (1974)17. Vargas H., Pelz! J., Laugier J., Dautreeppe D., Mattieu J.P.: J. Magn. Reson. 20, 269 (1975)18. Miller F.A., Wilkins Ch.M.: Anal. Chem. 24, 1253 (1952)19. Bertie J.E., Heyns A.M., Oehler 0.: Can. J. Chem. 51, 2275 (1973)

Authors' address: Jolanta N. Latosinska, Institute of Physics, Adam Mickiewicz University, Umul-towska 85, 61-614 Poznan, Poland