[14] F. J. Bartoli and T. A. Litovitz, J. chem. Physics 56, 413 (1972). [IS] M. Besnard, J. Devaure, and J. Lascombe, J. Chim. physique. [16] G. Doge, Z. Naturforsch.
Ed 80. Nr. 9 1976
M. Schlaak et al.: Reorientational Motions in Crystalline (CH,),NHCI etc.
[16] A. Abragam and K. Kambe, Physic. Rev. 91, 894 (1953). [17] R. J. Walter and W. T. Chandler, Trans. M.S. AIME 233, 762 (1965). [I81 M. S. Rashid and T. A. Scott, J. Less-Common Metals 30, 399 (1973). (191 H. Asano, Y. Ishino, R. Yamada, and M. Hirabayashi, J. Solid State Chem. 15, 45 (1975). [20] K. Tanaka and T. Hashimoto, J. physic. SOC.Japan 34, 379 (1 973).
881
[21] D. 0. Van Ostenburg, D. J. Lam, M. Shimizu, and A. Katsuki, J. physic. SOC.Japan 18, 1744 (1963). [22] S. Alexander, E. Corenzwit, B. T. Matthias, R. G. Shulman, and B. J. Wyluda, Physic. Rev. 129, 2481 (1963). [23] B. Pedersen and D. Slotfeldt-Ellingsen, J. Less-Common Metals 23, 223 (1971). (Eingegangen am 16. Juni 1976)
E 3438
Reorientational Motions in Crystalline (CH3)3NHCl, Studied by Raman Spectroscopy M. Schlaak, Institut fur Physikalische Chemie, Technische Hochschule, D-6100 Darmstadt
M. Couzi, and P. V. Huong Laboratoire de Spectroscopie Infrarouge, associe au C.N.R.S., Universite de Bordeaux I, F-33405 Talence Kristallstruktur 1 Phasenumwandlungen 1 Ramanspektren 1 Rotation The IR and Raman vibrational spectra of crystalline (CH3),NHtX- (X = CI, Br, I) are recorded. A complete assignment of the measured frequencies to the internal vibrational modes of the (CH,),NH+ cation in the three compounds is obtained. Above the phase transition of (CH,),NH ’ C1- (at 308 K) a reorientational motion of the cation around its threefold axis is deduced from the Raman spectrum. This motion is studied by a line shape analysis of the degenerate vCN(E)vibration. A rotational diffusion constant Dll = (0.38 0.05). 1 O I 2 sec-’ is deduced in agreement with quasielastic neutron scattering results.
Die IR- und Raman-Spektren von kristallinem (CH,),NH+X- (X = Cl, Br, J) sind gemessen worden. Eine vollstandige Zuordnung der beobtichteten Frequenzen zu den internen Schwingungen des (CH,),NH+-Kations in den drei Verbindungen ist erzielt worden. Aus den Raman-Spektren wird auf eine Reorientierung des Kations um seine dreizahlige Symmetrieachse oberhalb des Phaseniibergangs von (CH3),NH C1 (bei 308 K) geschlossen. Diese Bewegung wird untersucht, indem die Linienform der entarteten v,,(E)-Schwingung analysiert wird. In ubereinstimmung mit Ergebnissen der quasielastischen Neutronenstreuung ergibt sich eine rotationelle Diffusionskonstante von Dll = (0.38 & 0.05). 10” sec-‘.
I. Introduction The crystalline compounds (CH3),NH+X- (X = C1, Br, I) display some structural properties of actual interest in solid state chemistry, such as the existence of a NH+-X- hydrogen bond [l] and of an order-disorder phase transition [2]. In (CH&NH+CI- (TMACl) this transition has been detected by DTA measurements at 308 K [Z]. In the low temperature phase (P-phase) the structure is monoclinic (space group P2,/m) with two formula units in the crystallographic unit cell [3]. Preliminary X-ray diffraction data on the high temperature phase (a-phase) indicate a tetragonal structure [2, 41, the fourfold crystallographic axis being close to or coincident with the NH’. .. C1- hydrogen bond. This is inconsistent with the molecular symmetry, unless the cations are disordered statically or dynamically among at least four equivalent positions about their threefold molecular axis. This arrangement suggests a space group P4/nmm with two “molecules” in the unit cell [4]. In order to obtain further information on the phase transition we performed IR and Raman measurements in the two phases of TMACl. The IR spectrum of TMACl previously reported [ 5 ] is not complete. From our IR and Raman measurements on (CH3),NH+X- (X = C1, Br, I), (C2H&NHtC1-,
and on partially deuterated samples of these compounds, we are able to give a complete assignment of the measured frequencies to the molecular vibrations (see section 2). The frequency spectrum of the lattice vibrations in TMACl is described in [6]. In the a-phase we noticed a broadening of the E-type vibrational resonance lines corresponding to internal vibrations of the (CH3),NHf cation. In view of the structure results discussed above, these broadenings in the vibrational spectrum can be explained in two ways: a) If the cations in the cc-phase of TMACl are disordered around their C3 axis, the observed type-E vibrations can no longer be described by a plane wave in the crystal. A broadening of the observed resonance line may occur [7]. b) Dynamically disordered molecules can produce a broadening of vibrational lines if the reorientational motions occur in the time scale of’ picoseconds. The molecular reorientations in TMACl have been studied by ‘H-NMR wide line measurements [8]. The results could be described by a reorientation of the whole cation and of the individual CH, groups in the low temperature phase. From ‘H Tl-NMR spin-echo measurements [9, 101 a correlation time z, l o - ” sec was obtained for these reorientations in the low temperature phase. But this technique was unable
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to determine the reorientational properties in the high temperature phase of TMAC1. Quasielastic neutron scattering (QNS) experiments have also been performed in order to analyse the reorientational motions in TMACl [9, 111. In the high temperature phase of TMACl a reorientation of the cation is found, having a correlation time T~ = 2.2. IO-"sec at 315 K. Within the measured Q-range (Q is the momentum transfer), the experimental results are in agreement with the model of a rotational diffusion of the cation around the molecular threefold axis. The QNS spectra in the low temperature phase of TMACl could be explained by simultaneous jump-like reorientations of the cation and of the methyl groups. However, the QNS signal in TMACl is mostly due to the methyl protons, thus reflecting the total reorientation of the cation and of the methyl groups. The two motions cannot be observed separately. Due to these results, the Raman vibrational bands can be analysed by assuming that the observed broadening is caused
by "fast" reorientational motions in the a-phase. As far as the approximation of group-vibration holds, the advantage of the Raman technique is that vibrations of specific bonds are observed, thus giving information about specific reorientational motions: for instance, the C- N vibrational bands in TMACl are only affected by reorientations of the cation but not by that of the methyl groups. In section 3 the Raman band shape of the C - N vibrations is analysed in order to study the reorientational motion of the cation in crystalline TMAC1. 2. Vibrational Frequency Spectrum of TMACl 2.1. Experimental The ( C H & N H + X - (X = CI, Br, 1) compounds were prepared by gas phase reaction of (CH,),N with HX. The deuterated samples (CD3)3NH+CI- and (CH3),NDfCI- were obtained by the same method. Single crystals of a- and p-TMACI were grown by slow evaporation of saturated alcoholic solutions over P205, respectively, at 320 and 300K.
Table 1 Measured Raman and IR frequencies at room temperature. Abbreviations for intensities: w = weak, m = medium, s = strong, 1 = large, sh (CH,),NH+CIRaman IR
3028 s 3011 vs 2974 wsh 2946 ml 2924 wsh
3022 in 3008 vs 2966 w 2938 m 2917 w 2895 in
2872 m 2830 vw
2862 w 2828 w
2816 w
2810 w
2794 w 2664 wsh 2600 ml 2557 wsh 2529 w 2500 vwsh 2477 ml
(CH,),NH+BrRaman IR
3030 w 3019 vs 3012 s 2979 w 2931 ml
3020 s 3008 vs 2954 m
=
shoulder, v
3012 s
2948 m 2936 sl 2918 w
2846 w 2829 w
2859 m
2860 w 2840 w 2829 w 2812 w
2784 w
2793 vw 2785 w
2780 w
2778 w
2775 w
2600 vls
2690 sl
2700 vl
2729 ml
2740 vl
2472 w 2467 m
I
VCH,
YCH, and combinations of &H,* 6 N H *
2505 vw
2514 w 2473 w
Assignements
2 :,>
2863 m 2829 vw
2473 vw
very
(CH,),NH ' 1 Raman IR
2908 w
2516 vw
=
2457 vw
combinations of TCH,
2468 w 2456 w
2415 vw 1494 wsh 1473 m 1464 m 1437 m 1404 w 1251 w 1243 w 1074 w 1065 w 991 s 987 s
2104 w
2118 w
2140 w 1488 s 1473 m 1457 m 1446 w 1436 m 1410 s 1395 w
1487 m 1475 w 1462 w
1488 s 1475 s 1462 m
1480 vw 1467 m 1449 m
1479 m 1470 m 1455 m
1427 w 1399 w
1425 m 1397 w
1416 w 1400 m
1415 s 1399 w 1392 m
1263 s 1244 m 1068 w 1060 w 991 vs 982 wsh
1245 m
1261 s 1244 vw 1065 w 1054 m 986 wsh 983 vs
1071 vw 1061 w 984 s
822 s s15 vw 469 m
822 m 814 vwsh 464 m
818 s 810 wsh 470 m
412 m 400 vw 285 vw
404 m
416 m 408 wsh
819 m 810 w 463 vw
1281 w 1239 w 1054 w 990 vw 978 m 942 m 810 m 752 m 463 m 454 w 411 m 405 m 295 vw
1255 m 1239 vw 1062 vw 1049 m 976 vs 810 m 801 w 469 vw 454 w 400 vw
rCH~
}
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1976
The samples for IR measurements were in the form of emulsions of the compounds in nujol or fluorolub. The IR spectra were recorded in the frequency range 300- 3200cm-' at room temperature and at the temperature of liquid nitrogen on a Perkin Elmer model 180 spectrometer. The results are given in Table 1. The Raman spectra (see Tablel) were obtained with a Coderg T 800 triplemonochromator spectrometer, coupled with a SpectraPhysics 164 argon-ion-laser. Powdered samples were studied at different temperatures between 80 K and 370 K. Polarized Raman spectra of single crystals of w- and P-TMACI were measured at 333 K and at room temperature.
2.2. Assignment of the Vibrations The enumeration of the internal vibrations of the free (CH3),NH+ cation having C3" symmetry is given in Table2. In the P-phase, these cations are in C, sites. The enumeration of the cation internal vibrations in the C$, unit cell of P-TMACl can easily be deduced from the correlation diagram (Table 3). Arguments for the assignment of the observed frequencies are obtained by polarization measurements on P-TMACI single crystal and by comparison with the spectra of (CH3)3NHtX- (X = Br, I), (C2H5),NH+CI-, (CH,),ND'Cl-, and (CD3)3NH+C1-compounds. The results for P-TMACl are given in Table 1, together with those obtained for (CH3)3NH+X- (X = Br, I). Our assignments for the N-H and C-N vibrations in TMACI are different from those given in [5]. Table 2 Enumeration of the vibrational modes of the isolated (CH3)3NH+ cation having C,, symmetry
v9
v131 "14 v15
VlO
v161 "17
Vin v19
"1 I
1'209 v 2 1
V22 "23
v12
v24
Table 3 Correlation diagram for the vibrational modes of the (CH3),NHt cation in P-TMACl Isolated cation C3"
Site CS
Unit cell Cth
For a-TMAC1, the theoretical enumerations of the cation internal vibrations cannot be made rigorously, because the cations are disordered and thus occupy C4" sites in the tetragonal unit cell while their maximum symmetry is C3". In a first approximation, the enumeration for the free CSvcation can be applied (Table 2). In the frequency range of special interest, i.e. that of the C3N group vibrations, only a very weak Ag -Bg splitting of the E modes (see Table 3) is observed by polarization measurements on a b-TMACl single crystal. This is observed for most of the E-type vibrations of the cation (Table 1) and means that the influence of the site symmetry on the vibrational properties of the cation is very weak. On the other hand, examination of Table 1 also reveals that the observed difference between the IR and Raman frequencies generally lies within the limit of experimental errors. Thus it is concluded,that vibrationalinteractionsbetween the two (CH3),NH+
883
cations in the unit cell are almost negligible. Consequently the internal vibrations of the cations in P-TMACl are describable in a good approximation by those of the free C,, (CH3),NH+ cation (Table 2). In a-TMACl no splitting of the E modes can be detected, indicating a complete C3" symmetry for the cation. Furthermore it is observed that all the E bands are broadened in a-TMAC1 in contrast with the A l bands which have approximately the same width as in P-TMAC1.
3. Analysis of the Raman Band Shapes for the Study of Reorientational Motions 3.1. Theoretical Background The theory of Raman and IR band shapes of molecules performing reorientational motions in inert liquid solutions has been developed by various authors (see for instance [12-141). The theory, in general, is based on the following assumptions [131: a) There is no phase relation between the vibrational motions of the different active molecules. b) There is no induced polarizability in the active molecule. c) The vibrations are- describable by quantum mechanics and the reorientations by classical mechanics. The theory has also been used for the study of pure liquids, as for instance liquid (CH3),N [IS]. There is no special theory dealing with the rotational broadening of Raman lines in solids. We shall apply the above mentioned theory, developed for the liquid state, for the analysis of the band shape of Raman lines of crystalline TMACl. No argument will be given to prove that the basic assumptions made for molecular motions in liquids are also reasonable in a crystal such as TMACl, but the results obtained from the analysis of the Raman band shape using this theory will be compared with those obtained by the QNS technique [9, 131. Under the assumptions given above, the depolarized scattered intensity of a rotational-vibrational band can be described as a Fourier transform of the product of a vibrational and of a reorientational relaxation function 1131:
1 Idcp(w)= -J
2n
G,(t)G,,(t)e-'"'dt.
w = 0 corresponds to the center of the band, G,(t) and GZR(t) are the Raman monomolecular correlation functions for the vibrational and for the reorientational relaxation. We emphasize that collective effects, which may become important in crystals, are neglected. It is assumed that the intermolecular interactions in a-TMACl are similar to those in a liquid; this is in part justified by the fact that crystal effects are only weak perturbations of the internal force field of the molecular cation (see section 2.2).
3.2. The Reorientational Correlation Function for the Motion of the Cation in a-TMACI As mentioned above, the C3N group vibrations are affected by reorientation of the whole cation but not by that of the methyl groups. Furthermore, a reorientation of the cation about its molecular C3axis should influence only the degenerate vCN(E) and 6,,(E) vibrations, while reorientation of this
M. Schlaak et al.: Reorientational Motions in Crystalline (CH3)3NHC1etc.
884
molecular axis should affect both the symmetric A, and the degenerate E vibrations. But structure data [3] as well as hydrogen bonding studies [l] give no hint for a reorientation of the cation about an axis perpendicular to the molecular axis for temperatures up to 370K. Spectra of the CN vibrations were recorded using an instrumental resolution between 1 and 2 m-' (FWHM). Only the high frequency side of the bands is used for the analysis of the band shape in order to avoid the discussion on the influence of hot bands and in order to restrict the discussion on the high frequency component of the E-type vibration, splitted into a Bg and an Ag component in the P-phase. The temperature dependence of the linewidths of the vCN(E), vCN(Al), 6cN(E), and SCN(AI) vibrational bands between 200 K and 360 K is shown in Fig. 1. There is a drastic change in the linewidth of the type E vibrations at the phase transition (T, = 308 K). The linewidths of the symmetric A, vibrations are nearly unchanged by the phase transition.
I
I -50
0
linewidths of the symmetric A, vibrations, which are not influenced by reorientations of the cation about the molecular axis, are nearly unchanged by the phase transition (Fig. 1). Thus, we come to the conclusion that the broadening of the E bands, observed when passing through the phase transition, is not due to a change of G,(t) but may be interpreted by a reorientation of the cation, starting to rotate at T, with a frequency of the order of magnitude of the Raman frequency. This hypothesis is supported by QNS results [9, 111. It leads to 1 IT,+ 6T(w)= -jG,(t)G,,(t)e-'"'dt. 2x
(3)
Consequently, the orientational correlation function can be obtained from (4)
The measured frequency spectrum is the convolution of the true band shape by the instrumental resolution function R(w).If the resolution function R(w) is the same for I T p - G T ( ~ ) and for ITp+6T(w) then GZR(t)obtained from the division of the Fourier-transformed measured frequency spectra does not depend on R(w). It was shown [9,11] that a model of diffusional reorientations describes quite well the QNS results obtained for a-TMACl. For such a model, the correlation function G2R(t) of the E vibrations in a C3" molecule has, in general, two components [141 : G2,(t) = Ae-(5D.t +DII)' + ~ e - ( 2 +4010f 4 (5)
I
0
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50
-
TPC
Fig. 1 The linewidth (Full Width at Half Maximum) of the vibrationalrotational bands vCN(E),vcN(Al), ScN(E), and SCN(A,)as function of temperature. The phase transition is at 308 K (35"C).Instrumental resolution: 1.5 cm-' (FWHM)
Dll is the diffusion coefficient for the reorientation of the molecule about the molecular axis. DI describes the reorientation of the molecule about an axis perpendicular to the C3 symmetry axis, which is not observed in the Raman spectra: D , = 0. The corresponding frequency spectrum is a superposition of two Lorentzians with linewidths equal to Dll and 4 D I 1 Nothing . is known about the intensity relation A / B exept that A + B = 1 since GZR(0)= 1.
As discussed before (section 3.1) we assume that Equation (1) may be applied to describe the frequency spectrum of the rotationally broadened vCN(E) and 6,,(E) bands in the a-phase of TMACl. In order to separate GZR(t)form G,(t) we assume that : a ) T h e Raman bands in the low temperature P-phase of TMACl are not influenced by reorientational motions: the reorientation about the molecular axis is too slow [9, 101 to produce a detectable band broadening in this phase. Thus, for a temperature just below the phase transition (T, - 6 T )the frequency spectrum may be described by:
-4.01
Fig. 2
b) Passing through the phase transition, G,(t) does not change essentially. This assumption seems reasonable, since the
Reorientationalcorrelation function G,,(t) of the vCN(E)vibrationalrotational band in the high temperature phase of TMACl, as derived from the experimental data
Bd 80 Nr 9
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1976
Fig. 2 gives h[GZR(t)] as a function oft, obtained from the band shape analysis of vCN(E) using Equation (4). Within the experimental accuracy, In [GzR(t)] can be described by one straight line for t between 0.5 and 2.5 psec (Fig. 2). That means, G,R(t) is determined by one exponential only: by e-D"' or by e-4D11 '. Since the broadening of &N(E) for T = T, + FT is less than half the broadening of vCN(E), one may conclude that the more broadened vCN(E)is related to e-4D11tand not to e-"']'. From the logarithmic plot of G Z R ( t ) against t (Fig. 2), a correlation time
is deduced. A further value is obtained from
G2~(t)df= 0.76. 10-'2SeC.
5, = 0
The mean value Tc
- (0.7 & 0.1)~10-12sec,Dil = (0.36k 0.05).10'2sec
-7
4011
is estimated to be a reliable value for the reorientation of the cation.
3.3. Analysis of the Linewidths of the C - N Vibrations in Crystalline TMACl If the correlation function consists of a simple exponential (as for vCN(E)in TMACl), we may interpret the change of the spectral linewidth at the phase transition A\',,,,,
=
(6)
AVT,,+ST- AVT,-ST
to be due to the reorientation of the cation and relate it to the correlation time. The substraction in Equation (6) is only correct if G,(t) has also an exponential form which, in general, is not true [16]. However the error introduced by approximating G,(t) by a simple exponential is negligible for the estimation of Av,,,, by Equation (6). For the evaluation of the linewidth of Z(w), the instrumental resolution has to be taken into account. This is done by folding a Lorentzian with the triangular instrumental resolution function, in order to reproduce the measured linewidth. For vc.,(E) one obtains as reorientation broadening Av,,,, = (8.0 & 0.5)cm-' (HWHM), which corresponds to a correlation time (0.65 t 0.05). 10-
T c = - =
4011
'' sec,
in agreement with the value obtained from the analysis of G L R ( t ) As . diffusion constant, we deduce from the different experimental methods a mean value of Di! = (0.38 +_ 0.05). 10" sec-' at about 312 K. This value agrees within the experimental accuracy with that obtained from QNS: Di, = (0.45 & 0.08). 10" secT~ =
1 = (0.55 f 0.07). 10- l 2 sec
4011
at 315 K.
885
Avreor for &N(E) obtained by the same method is (3.5 & 0.5)cm-'. Since this value is neither equal to Av,,,, nor to $AvreOrof vCN(E), one might conclude that GzR(t) of &(E) is determined by two exponentials (see Equation (6)). 4. Concluding Remarks
We analysed the line shape of the type E Raman lines corresponding to the C - N vibrations of (CH,),NH+ cations in a-TMACI in terms of a dynamical broadening due to a "fast" reorientational motion of the cation about its threefold axis. For this interpretation, a theory developed for molecular motions in liquids was applied, assuming that the vibrational relaxation is not changed by the phase transition and that the intermolecular interactions in the solid influence the Raman bands in the same way as they do in a liquid. A reorientation of the cation about the molecular C, axis is found and can be characterized by a rotational diffusion constant Dll = (0.38 0.05). 10" sec-'. This result is in agreement with the value obtained from quasielastic neutron scattering and shows the above mentioned assumptions to be valid for solid (CH3)3NH+C1- within experimental accuracy. The activation energy for the cation reorientation in the (rphase of TMACl is rather low. From the temperature dependence of the linewidth of the vCN(E)Raman bands and from QNS [11] it can be estimated to be in the order of (4-8). lo3J/mol. In the p-phase the activation energy for the cation reorientation is about 36. lo3 J/mol [lo] allowing "slow" reorientational motions of the cation with a correlation time in the order of IO-'Osec [lo, 111. In the a-phase of CH,NH;X- (X = C1, Br) compounds, it is known [17] that the cations are disordered about their molecular axis in a similar way as in cr-TMACI. The measured widths of the degenerate E infrared bands were tentatively discussed in terms of dynamical broadening [17]. This interpretation was not valided by Whalley [7], who argued that in such uniaxial crystals, with orientational disorder about the molecular symmetry axis, the degenerate E vibrations may be broadened in a spectrum resambling the frequency distribution of different vibrational states. In other words, the widths of the type E vibrational bands could also be interpreted by static disorder. But the present study of the Raman line shape in (r-TMACl,when compared with NMR and QNS results, proves that in this case the vibrational broadening of the vibrational E bands is due to a dynamical disorder of the cations around their threefold axis. We gratefully acknowledge many helpful discussions with Prof. J. Lascombe. M. Schlaak thankfully remembers the friendly hospi-
tality of the "Laboratoire de Spectroscopie Infrarouge" during his stay in Bordeaux.
References [I] P. V. Huong and M. Schlaak, Chem. Physics Letters 27, 113 (1974). [2] M. Stammler, J. inorg. nucl. Chem. 29, 2203 (1967). [3] J. Lindgren and J. Olovsson, Acta crystallogr. B 24, 554 (1968). [4] J. Lindgren, private communication. [5] J. Bellanato and J. R. Barcelo, An. Fis. Quim. B 52,469 (1956). [6] M. Couzi, D. Larroque, M. Schlaak, and P. V. Huong, in: Molecular Spectroscopy of Dense Phases, p. 327, Proc. of the
886
W. Spratte and G. M. Schneider: Differential Thermal Analysis (DTA) under High Pressure, VI., etc.
XI1 European Congress on Molecular Spectroscopy, Strasbourg 1975, Ed.: M. Grosmann, S. G. Elkomoss, and J. Ringeissen, Elsevier Publ. Co., Amsterdam 1975. [7] E. Whalley, J. chem. Physics 51, 4040 (1949). [8] R. Sjoblom and J. Tegenfeldt, Acta chem. scand. 26, 3075 (1972). 191 A. Heidemann. J. C. Lassegues, R. Lechner, and M. Schlaak, in: Molecular Spectroscopy of Dense Phases, p. 327. Proc. of the XI1 European Congress on Molecular Spectroscopy, Strasbourg 1975, Ed.: M. Grosmann, S. G. Elkimoss, and J. Ringeissen, Elsevier Publ. Co., Amsterdam 1975.
~~~~~~~$:el,schaft
[lo] R. Sjoblom and M. Punkkinen, J. Magn. Res. 20, 491 (1975). [ l l ] M. Schlaak, J. C. Lassegues, A. Heidemann, and R. Lechner, to appear. [12] R. G. Gordon, Adv. Magnetic Res. 3, 1 (1968). 1131 S. Bratos and E. Marechal, Physic. Rev. A 4, 1078 (1971). [14] F. J. Bartoli and T. A. Litovitz, J. chem. Physics 56, 413 (1972). [IS] M. Besnard, J. Devaure, and J. Lascombe, J. Chim. physique 72, 453 (1975). [16] G. Doge, Z. Naturforsch. 28a, 919 (1972). [17] A. Theoret and C. Sandorfy, Spectrochim.Acta23 A , 519 (1967).
(Eingegangen am 18. Mai 1976)
E 3409
Differential Thermal Analysis (DTA) under High Pressure VI :*) Phase Transitions of Some Liquid Crystals up to 3 kbar W. Spratte and G . M. Schneider Institute of Physical Chemistry, University of Bochum, German Federal Republic
Fliissige Kristalle 1 Hohe Drucke 1 Phasenumwandlungen 1 Stojfeigenschaften
Therniodvnamik
With a high pressure DTA apparatus previously described the phase transition temperatures solid/nematic (melting temperatures) and nematic/liquid isotropic (clearing temperatures) of the liquid crystals MBBA, EBBA, PBBA, PEBAB, PAA, and PAP were measured as a function of pressure up to 3 kbar and in the temperature range 273 K to 550 K. For PBBA the pressure dependence of the transition temperature smectic/nematic was additionally determined. For all compounds rising pressure causes an increase of the transition temperatures as well as an extension of the nematic range (especially for PAA and PEBAB). According to the thermal pretreatment two different melting curves were found for MBBA, EBBA and PEBAB separating the nematic liquid from a stable and a metastable solid phase respectively. The melting curves diverge slightly for MBBA and EBBA; for PEBAB, however, an intersection point can be extrapolated at about 2600 bar and 450 K. From the peak areas at atmospheric pressure the transition enthalpies AH and from these the transition entropies AS were determined. Using these values and the slopes of the experimental U p ) transition curves the volume changes A V accompanying the transitions solid/ nematic, smectic/nematic and nematic/liquid isotropic respectively were calculated from the Clausius-Clapeyron equation. The AH, AS and A V values thus obtained are discussed and compared with the literature data available. Mit einer friiher beschriebenen Hochdruck-DTA-Apparatur wurden die Schmelz- und Klartemperaturen der Flussigen Kristalle MBBA, EBBA, PBBA, PEBAB, PAA und PAP in Abhangigkeit vom Druck bis 3 kbar und im Temperaturbereich 273 bis 550 K gemessen; bei PBBA wurde zusatzlich die Druckabhangigkeit der Umwandlungstemperatur smektisch/nematisch bestimmt. Bei allen Substanzen bewirkt steigender Druck ein Ansteigen der Umwandlungstemperaturen sowie eine Vergroaerung des nematischen Zustandsbereiches (besonders bei PAA und PEBAB). Je nach thermischer Vorbehandlung wurden bei MBBA, EBBA und PEBAB zwei verschiedene Schmelzdruckkurven gefunden, die jeweils den nematisch-flussigen Bereich von einer stabilen bzw. metastabilen festen Phase trennen. Die Schmelzdruckkurven von MBBA und EBBA divergieren leicht zu hoheren Drucken, wahrend sich bei PEBAB ein Schnittpunkt bei ca. 2600 bar und ca. 450 K extrapolieren laat. Aus den Peakflachen der DTA-Thermogramme bei Normaldruck wurden die Umwandlungsenthalpien AH und die Umwandlungsentropien AS bestimmt. Mit Hilfe dieser Werte und aus den Steigungen der T(p)-Vmwandlungskurvenwurden unter Benutzung der Clausius-ClapeyronGleichung die Volumenanderung A V bei den Phasenumwandlungen berechnet. Die so bestimmten AH-, AS- und AV-Werte werden diskutiert und mit Literaturdaten (soweit vorhanden) verglichen.
I. Introduction
N-(4-pentoxybenzylidene)-4-n-butylaniline(PBBA)
During the last years DTA has proved to be a convenient method to study phase transitions of pure compounds and mixtures at high pressures [I -41. In the present work these investigations are extended to the liquid crystals
4-ethoxybenzylidene-4-aminobenzonitrile (PEBAB) C,,H,,N20 ( M
N-(4-methoxybenzylidene)-4-n-butylaniline (MBBA),
4,4-dimethoxyazoxybenzene (PAA) C,,H,,N,O,
C,,H,,NO ( M
C,H,NO
(M = 323) =
250)
( M = 258)
= 267)
4,4'-diethoxyazoxybenzene(PAP), CI6H,,N,O, ( M = 286).
N-(4-ethoxybenzylidene)-4-n-butylaniline(EBBA), C,,H,,NO ( M = 281)
Whereas a lot of experimental work has been carried out for the investigation of liquid crystals at atmospheric pressure high pressure studies are still rather scarce. Up to now the most important high pressure investigations on these sub-
*)
V (see [4]); IV (see [3]); 1 bar
=
lo5 Pa, 1 kbar = lo3 bar.
