Temperature dependence of Raman spectra in 1,2-dinitrotetrachlorobenzene

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 344 (1995) 183-187

Temperature dependence of Raman spectra in 1,2-dinitrotetrachlorobenzene H . A . K o t o d z i e j a'*, K . O r z e c h o w s k i a, R . S z o s t a k a, S. S o r r i s o b alnstitute of Chemistry, University of Wroctaw, 50-383 Wroetaw, Poland blstituto di Chimica, Universita di Perugia, 06100 Perugia, Italy

Received 30 May 1994

Abstract

The existence of order-disorder type phase transitions around 353 K in 1,2-dinitrotetrachlorobenzene (1,2-DNTCB), found earlier during DSC and dielectric studies, was confirmed on the basis of temperature dependence of selected Raman spectral parameters. It was established that the positions and widths of the studied bands do not vary noticeably near the transition temperature, but the change in their intensity is pronounced and a variation in their shape takes place. As was expected, Raman bands gain gaussian character in the vicinity of the phase transition. The observed properties were consistent with the concept of superstructure formation during phase transition in the 1,2-DNTCB crystal proposed already on the basis of X-ray and dielectric measurements. It was assumed that in the high temperature rotational phase a coupling of molecular movements leads to the collective rearrangement.

1. Introduction

For some time, it has been known that certain hexasubstituted benzene derivatives possess properties that suggest disorder in the crystalline state. This class of compounds includes chlorine and/or methyl derivatives. It is well known from the X-ray diffraction measurements that all these crystals are isostructural with each other [1-6] and in some cases, such as 1,2- and 1,3-dinitrotetrachlorobenzene (DNTCB), are isomorphic at room temperature [7,8]. All these substances crystallize in the centrosymmetric space group. However, the molecules themselves do not possess a centre of symmetry. X-ray diffraction measurements show that the statistical disorder in the case of 1,2-DNTCB * Corresponding author.

leads to random distribution of the chlorine atoms among three crystallographically non-equivalent positions. This orientational disorder has been interpreted in terms of discontinuous molecular reorientation of n'P~3 symmetry as a consequence of the relatively large rigid-body torsional vibrations [9]. The temperature dependence of the disorder has been studied extensively by heat capacity and dielectric techniques [1-6]. The results show pronounced first or second order phase transition. Originally, these transitions were interpreted as representing the onset or the termination of reorientation. In the rotator state, the molecules were assumed to be rotating freely about an axis passing through the centre of the molecule. F r o m this it was concluded that the molecules had to possess an approximate disc-like symmetry in order for rotation to occur. The temperature range in

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H.A. Kolodziej et al./Journal of Molecular Structure 344 (1995) 183-187

which the rotational phase is observed depends on several molecular features and the shape of the molecule seems to be the most important one. Large differences in the van der Waals radii of substituents results in smaller temperature ranges for the rotational phase as in the case when the radii are similar to each other. The appearance of the rotational phase was usually discussed as a consequence of a lowering of the energy barrier for rotation over the pseudosix-fold axis due to a change in the symmetry or dimensions of the unit cell. 1,2-DNTCB crystallizes in the monoclinic system, space group P21/C, with a = 8.11, b = 3.88, c = 16.88 A,/3 = 111 ° [8]. It is also known from our previous paper (see [8]) that in the temperature range 4 1 0 - 4 2 0 K the intensity of the small angle lines of Debye-Sherrer photographs drops dramatically and between 420 and 440K disappears totally. This was discussed in terms of a superstructure formed progressively as the temperature increases. This process seems to be reversible, as was suggested by our dielectric measurements. The dielectric absorption of 1,2-DNTCB in the rotational phase (T > 365K) shows an almost Debeye-like pattern. However, in the vicinity of the transition temperature, a pronounced change in the nature of this dielectric absorption was observed. We believe that the phenomena observed in Debye-Scherer photographs as well as in dielectric absorption have the same causes. The aim of the present work is to answer the following questions: (i) what mechanism leads to the appearance of the rotational phase and (ii) what is responsible for the dielectric relaxation process observed in these crystals. Raman spectra of polycrystalline 1,2-DNTCB measured over a 150 K temperature range up to the melting point can help, at least partially, to answer these questions.

i

40

80

,

i

120

160

~4

Fig. 1. Low frequency Raman spectra of 1,2-DNTCB.

HG2S double monochromator and detected by a thermoelectrically cooled RCA-C 31034A phototube coupled with a photon counting chain. Samples were excited by the 514.5 or 488 nm lines of a Spectra-Physics argon ion laser, model 164-09. The power collimated on the polycrystalline sample was about 200mW. The temperature was stabilized and detected with an accuracy of 0.2 K.

3. Results and discussion The results of our experiments are presented in Figs. 1 and 2. Fig. 2 shows the low frequency, while

2. Experimental The compound 1,2-DNTCB was synthesized by ourselves and purified by fractional recrystallization from chloroform. The Raman spectra were obtained using a J o b i n - Y v o n R A M A N O R

i

200

400

800

1200

L cm-1

Fig. 2. High frequency Raman spectra of 1,2-DNTCB.

H.A. Kotodziej et al./Journal Of Molecular Structure 344 (1995) 183-187 Cm -1

J

i

i

i

,

i

i

i

Tc {tJiet,) 1 1 Tc(DSC)

1356 1355 1354 1353 1352 1351

.=

351

,

350 18 17 16 15

185

Tc'°'e"l

303

323

, l'c', Sc'

343

363

T! K

Fig. 4. Intensity as a function of temperature for the 350cm -I band. Broken lines represent different temperature runs. I

i

I

I

I

253

273

293

313

333

i

353

I

373

I

TI K

Fig. 3. Temperature dependence of the frequency of selected bands: (1) lattice vibration band at about 17cm-l; (2) C - C I stretching vibration band at about 350 cm-l; (3) NO stretching band at about 1353cm -1 .

Fig. 2 shows the high frequency Raman spectra of 1,2-DNTCB. Figs. 1 and 2 demonstrate that no pronounced change in the spectra before and after phase transition is observed. It may be interpreted that there is no change in the symmetry of the unit cell during the phase transition, otherwise we should observe significant changes in the spectra before and after phase transition and/or conspicuous shifts of some absorption bands. For comparison, we have chosen well formed bands at about 17 cm-I (lattice vibration), 350 cm-I (C-CI stretching) and 1353 cm -1 (NO stretching) shown in Fig. 3 as 1, 2 and 3 respectively. The frequencies of the vibrations change only slightly as the temperature increases from 290 up to 395 K. It was found also, that there is no discontinuity in the temperature dependence of the halfwidth of the bands over the investigated temperature range. Nevertheless, if we look carefully at the intensity of the selected bands (shown for the 350 cm -1 band in Fig. 4), we notice that the intensity changes abruptly in the vicinity of the phase transition temperature. At temperatures close to that of the phase transition the behaviour of the band intensity depends on the thermal history of the sample and on the rate of heating or cooling (broken lines in Fig. 4). The pre-transitional range

is even larger than used to be observed by other methods. Although an analysis of the band parameters in terms of molecular behaviour is not simple, looking for an explanation for the phase transition mechanism, we have decided to approximate selected bands by the Lorentz-Gauss product function [10] al

y = l+a2(x_a3)2exp[-a2(x-a3)

2]

(1)

where al is the band intensity, a 3 is the band maximum, a2 and a 4 are shape parameters. In Figs. 5 and 6 the change in the parameter X L -- -

a2 -

(2)

X 100

a2 + a4

which is equal to 100 for a Lorentzian curve and zero for a Gaussian one, is presented for the analysed bands over the studied temperature

.5 60

000000000

g ~20 0 323

I

I

I

I

333

343

353

363

T/OK

Fig. 5. The shape parameter as a function of temperature for the C-C1 stretching vibration band at about 350 cm -1 .

186

H.A. Kotodziej et al./Journal of Molecular Structure 344 (1995) 183-187

.~ 90

-~ 80 E

0

0

0

0

0

0

0

000 O0

0

0

0

0

0

0 0

~_ 70 I:i

60

I

323

333

I

3~3

I

353

I

363

T/°K

Fig. 6. The shape parameter as a functionoftemperature for the NO stretching band at about 1353cm -1 .

range. Because the band at 350cm appears to consist of two closely positioned bands (intensity ration 1 : 8), these were separated numerically and their shape was then analysed. Fig. 5 presents the results for the stronger component only. The results of a similar analysis for the complex contour above 1350cm -1 are presented in Fig. 6. The complex band was numerically separated into three Lorentz-Gauss bands and for the strongest component the XL parameter versus temperature is presented. In each case analysed, the value of XL is far from that for the Lorentzian band. The inhomogeneous broadening in the non-polar phase (T < T~) is caused by a mixture of different isotopic forms and partly by the presence of statistical disorder in the crystal. In the vicinity of the phase transition the parameter has approximately constant value. In the rotator phase the parameter XL seems to exhibit a minimum between 303 and 313K. Analysis of the band shape in terms of Lorentzlike and Gauss-like character is based on the Kubo theory [11]. The Lorentz-like character describes the band shape when a fast modulation process could be supposed. The Gauss function represents the limiting case of slow modulation such as that caused by the transition between local structures due to rotation around a pseudo C3 axis. The results presented in Fig. 6 demonstrate that the discussed phase transition does not lead to pronounced changes in the band shape parameter in spite of a significant increase in the rotational freedom of the rotational phase deduced from dielectric measurements. This observation is not

astonishing if one assumes that in the rotational phase a coupling of molecular movements leads to a collective rearrangement. Such dipole-dipole type coupling is possible only if the molecular vibrations are sufficiently large. The large dipole moment of the 1,2-DNTCB molecule and the pronounced librational disorder demonstrated by Xray experiments leads to the observed change in the shape parameter. The appearance of the additional small angle lines in the Debye-Scherer photograph discussed above and the relatively small activation energy obtained from dielectric relaxation measurements seem to verify the proposed model. Our results are in agreement with the model of Gavezzoti and Simonetta [12]. These authors demonstrated that the small activation energy obtained from dielectric experiments could be explained by at least 25 molecules forming a cluster and assuming cooperational movement. It seems that this model is in agreement with the so called superstructure formation during the phase transition of the 1,2-DNTCB crystal. Taking into consideration the above model it is much easier to understand the nature of the dielectric absorption observed in both low and high temperature phases.

Acknowledgement The authors H.A.K., K.O. and R.S. are indebted to the Polish Committee of Scientific Research for grant No. 22676 91 02 in support of this scientific project.

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H.A. Kotodziej et al./Journal of Molecular Structure 344 (1995) 183-187

[8] H.A. Kolodziej, T. G~owiak, S. Sorriso, V. Caia and M. Braghetti, Chem. Phys., 145 (1990) 359. [9] D.E. Perry and G. Williams, Chem. Phys. Lett., 58 (1978) 586. [10] J. Pitha and R.N. Jones, Can. J. Chem., 45 (1967) 2347.

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[11] R. Kubo, in D. ter Haar (Ed.), Fluctuations, Relaxation and Resonance in Magnetic Systems, Oliver and Boyd, Edinburgh, 1996, p. 23. [12] A. Gavezzoti and M. Simonetta, Acta Crystallogr., Sect. A, 3 (1976) 997.