J. Phys. Chem. 1985,89,4887-4894 to supplement the single salt Pitzer parameters with parameters 0 and $ to obtain a satisfactory fit. Moreover, the authors found considerable variation of parameters with ionic strength. We found similar behavior in the case of the CaC12-CaBr2 system. We had to introduce 0 and J, to obtain satisfactory convergence between the measured and calculated electromotive force. We also found that these parameters were dependent on ionic strength. These observations are not contradictory to Pitzer's assumptions. As shown by Mayer,26 second and third virial coefficients, A, and fiuk, respectively, are ionic strength dependent for charged species. Thus, Pitzer assumed A, for ions to be a function of ionic strength. Therefore, we should expect 0 dependence on ionic strength, as it is a difference combination of X,'s. In the case of the third virial coefficients, &jk, and consequently $ (which is a combination of p u i s ) Pitzer ignored ionic strength dependence, assuming it to be very small. However, from the Ananthaswamy and Atkinson p a p e P and our work, it appears that these coefficients are distinct functions of ionic strength also. Due to a lack of sufficient literature on the behavior of mixing parameters in several systems, we are unable to discuss this problem further. The mean activity coefficients of CaBr2 and CaCI2 for the CaC12-CaBr2 mixtures at ionic strengths between 0.5 and 7.5 m are summarized in Tables I11 and IV,at different values of YCacl2 (26) Mayer, J. E. J. Chem. Phys. 1950, 18, 1426.
4887
= mCaCl2/(mCaCI2+ mCaBr2) and YCaBr2 = 1 - YCaC12 = mCaBr2/ (mCaBr2+ mCaC12)* The activity coefficients were also fitted to the Hamed equations log
YA
= log
log
YB
= log YB' - ~ B A Y A PBAYA'
YA'
- ~ A B Y-BPABYB'
(11) (12)
where yAoand yBoare the mean activity coefficients of pure CaBr2 and pure CaC12 solutions at the same ionic strength. The plots of log y vs. Yare shown in Figures 2 and 3. The values , along with the root-mean-square deof a A B , PAB, ~ B A and viations are listed in Tables V and VI. The trace activity coefficients of both CaBrz and CaC12 were calculated from the following equations log
Y"CaBr2
= log Y°CaBr2- aAB - PAB
(13)
(14) log ytrcaci2 = log Y°CaC12- ~ B -A PBA which can be obtained from eq 11 and 12 by substituting Y B = 1 and Y A = 1, respectively. The trace activity coefficients for CaBr2 and CaC1, are given in Table VI1 and are plotted with the y's for pure salts as a function of ionic strength in Figure 4.
Acknowledgment. We acknowledge the support of this research by the National Science Foundation under Grant CPE 8017441 and by the Amoco Production Co. of Tulsa, OK. Registry No. CaBr,, 7789-41-5; CaCI,, 10043-52-4.
Vibrational Study of the Dynamics of n-Aikyiammonium Chains in the Perovskite-Type Layer Compounds (C,H,,+,NH,),CdCI, (n = 8, 12, 16) L. Ricard, R. Cavagnat, and M. Rey-Lafon* Laboratoire de Spectroscopie Infrarouge, L.A. 124, Universite de Bordeaux I , 351, cows de la Libzration, 33405 Talence Cedex, France (Received: December 27, 1984; In Final Form: July 8, 1985)
The infrared and Raman spectra of the perovskite-typebidimensional compounds of general formula (CnH2n+lNH3)2CdC14 ( n = 8, 12,16) have been studied as a function of temperature. This study has provided evidence of the Occurrence of structural phase transitions related to the dynamics of the alkylammonium ions. The phases stable at low temperature are ordered and contain the same two types of almost extended chains with only one gauche bond near the ammonium end. Though the phase transition sequence is different for different n, each step of the sequence implies cooperative conformationalchanges in the chains which are probably coupled with reorientational motions of the NH3 polar heads. Kink-type defects (GT2,+,G') appear in the lowest temperature disordered phases. At high temperature, "melting" of the hydrocarbon part does not imply the presence of other types of defects in noticeable quantity; it seems instead to correspond to a decrease of the number of trans bonds between the G and G' defects of the kinks, in relation to an increase of the interlayer distance.
Introduction The tetrahalometallates of bis(n-alkylammonium) of general formula (CnH2,,+lNH3)2MX4( M = Cd, Mn, Cu, ...; X = C1, Br) are known to crystallize in a bidimensional structure of perovskite type.'+ The layers are constituted of corner-sharing MX6 octahedra; the cavities between the octahedra are occupied by the NH3 polar heads of the alkylammonium cations which form weak NH ...X hydrogen bonds with the halogens. Thus, each metallic sheet is sandwiched between two hydrocarbon layers (Figure 1). The layers are bound by van der Waals forces between CH3groups and by long-range Coulomb forces. These compounds have held the attention of scientists as they present interesting magnetic and structural properties.'-" In (1) Peterson, E. R.; Willett, R. D. J . Chem. Phys. 1972, 56, 1879. (2) Chapuis, G.; Arend, H.; Kind, R. Phys. Status Solidi A 1975,31,449. (3) Heger, G.; Mullen, D.; Knorr, K.Phys. Status Solidi A 1975,31,455. (4) Chapuis, G. Phys. Status Solidi A 1977,43, 203. ( 5 ) Depmeier, W. Acta Crystallogr., Sect. B Strucr. Crystallogr. Cryst. Chem. 1977, 33, 3713.
particular, they exhibit structural phase changes related to the dynamics of the alkylammonium ions. A first type implies reorientational motions of the rigid chains about their long axis monitored by flippings of N H 3 heads between two or more potential wells in the cavities. When n 1 3, a second type of transition can occur: it involves cooperative conformational changes in the chains and leads to a "quasi-melting" of the hydrocarbon part of the molecule while the mineral layers play the part of elastic stable matrices. This behavior shows analogies with that of liquid crystals and lipid bilayer membranes. (6) Kind, R. Phys. Status Solidi A 1977, 44, 661. (7) Blmnc, R.; ZiK, B.; Kind, R. Phys. Rev.B: Condens. Matter 1978,17, 3409. (8) Chapuis, G. Acta Crystallogr., Sect. B Struct. Crystallogr. Cryst. Chem. 1978,34, 1506. (9) Kind, R.; Plesko, S.; Arend, H.; Blinc, R. : ZiE, B.; Seliger, J.; L6zar,
B.; Slak, J.; Levstik, A.; Filipic, C.; Tagar, V.; Lahajnar, G.; Milia, F.; Chapuis, G. J. Chem. Phys. 1979, 71, 2118. (10) Needham, G. F.; Willett, R. D.; Franzen, H. F. J . Phys. Chem. 1984, 88, 674. (11) White,
M.A. J . Chem. Phys. 1984, 81, 6100.
0022-3654/85/2089-4887$01.50/0 0 1985 American Chemical Society
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Ricard et al.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
Figure 2. DSC diagrams: (a) C&d, heating and cooling curves; (b) and (c) C&d and C&d, heating curves. TABLE I: Solid-Solid Transition Temperatures, Enthalpies, and Entropies of C,Cd (n = 8, 10,12, 16)
n
phase
ga
LT
T, K 269
L
r
I
10'
t
2
12
308
5.1
308 (308) 313 (313)
" 29.5
i
327
40
i
3
6.5
i
t
0.3
16.7 r 1
)(37.4)
25 95
i
1 0 (15 * 5) 10 (105 r 5 )
3
HT LT IMT 1
(12) Ricard, L.; Rey-Lafon, M.; Biran, C. J . Phys. Chem. 1984,88,5614.
54
HT LT IMT
Among the numerous compounds of this type which are expected to exhibit the two kinds of dynamics, (Cl&12iNH3)2CdC14 (C,,Cd) is the only one which has been extensively studied by using several technique^.^ X ray diffraction shows that the structure of the-low-temper_ature phase (LT) is ordered. Its projections on the ( b , ~and ) (a$) planes are reproduced in Figure 1. Alkylammonium chain axes are tilted by f40° with respect to the normal to the layers ( E axis) and form a zigzag arrangement along it. There are two types of inequivalent chains (A and B) which are packed together. They correspond to almost extended conformations with only a single gauche (G) form between the first and the second carbon atoms for A chains and betweem the second and the third one for B chain^.^ The NH3+groups are linked to the chlorine atoms by three hydrogen bonds with two axial and one equatorial chlorine atoms; this scheme is the only one allowed for steric reasons when n L 2. Let us notice that, in such a configuration, the chains would be approximately perpendicular to the layers if they were all-trans; the G form is responsible for their tilting. This compound was shown to possess one phase transition of each kinda9 A spectroscopic studyi2of the different phases of this solid and three selectively bideuterated derivatives has provided evidence that the first transition implies a partial change of A into B chains and suggests that this rotation around the first two carbon atoms is coupled with the reorientational motion of the NH3polar head between two potential wells in the cavity. Furthermore, it has led to the conclusion that, in the highest temperature phase, the conformational disorder in mainly due to kink structures (GT%+]G'); other types of defects were not observed. In order to investigate the influence of chain length on the dynamics of the cations, we have synthesized three other compounds of the C,Cd series, the tetrachlorocadmates of bis(n-alkylammonium) with n = 8 (CBCd),n = 12 (Ci2Cd),and n = 16 (C16Cd). To determine the number and the temperature of their phase transitions, we have performed preliminary calorimetric and X-ray diffraction measurements which are extensively analyzed elsewhere.I3J4 As the purpose of the present work is the spec-
14.5 r 0.5
AS. J K-' mol-'
IMT
.
Figure 1. Low-temperature phase structure of C&d after ref 9: (a) projection along the d axis; (b) projection along the f axis. The two types of independent chains, A and B, and only the equatorial C1 atoms are shown.
AH,kJ mol-'
-,331
123 f 1 0 3
21
i
10
0.5
93
f
2
23
t
4
IMT 2 345c 16
HT LT 346
32
2
IMT 1 352
82 1
IMT 2 356
26.5
1
74 r 4
HT Values for C,Cd are taken from ref 14. Values for C,,Cd correspond to our measurements; those in parentheses have been published in ref 9. Transition, without any thermal effect, observed only by X-ray diffraction.
troscopic study of these compounds, only a brief summary of the calorimetric and X-ray diffraction results will be given. Then the Raman and infrared spectra of the phases stable at low temperature will be analyzed in order to obtain information on the geometry of the chains. Finally, we shall discuss the type of defects in the chains of the disordered phases.
Evidence of Phase Transitions by Calorimetric and Diffraction Measurements A . Calorimetric measurement^.^^ Enthalpic diagrams of CloCd,9 C&d, and C,6Cd show the existence of three or four crystalline phases, depending on the compound (Figure 2). (The phases stable at low, intermediate, and high temperatures are denoted LT, IMT, and HT, respectively.) A previous calorimetric study of CBCdalso shows two phase changes.I4 Transition temperatures and thermodynamic quantities are reported in Table I. Figure 3 displays the dependence of the total transition enthalpies and entropies on the number of carbon atoms. When n is higher than 10, the variation is linear. AH is proportional to the number of C-C-C-C sequences in each cation, i.e., to the (13) Chanh, N. B.; et ai., to be published in J . Phys. Chem. Solids (14) Chanh, N. B.; Haget, Y . ;Hauw, C.; Meresse, A,; Ricard, L.; ReyLafon, M. J . Phys. Chem. Solids 1983, 44, 589.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4889
Dynamics of n-Alkylammonium Chains AH (kJmol")
5
10
15
20
n entropy ( 0 )in
Figure 3. Total transition enthalpy (I) and C,Cd as a function of n. The sums of solidsolid transition and melting entropies (X) are indicated for comparison. of n-paraffins Cn+lH2n+4
number of gauche (G) or trans (T) conformations; AS' follows a variation in 0.7nR per chain ( R = perfect gas constant) which is rather similar to the low A S 0.65R(n - l), observed in the series of the manganese compounds.I5 One can also see in Figure 3 that total entropies of the C,Cd are of the same order of magnitude as the sum of melting and solid-solid transition entropies of corresponding n-alkanes.16 This confirms that phase changes of C,Cd derivatives lead to a dynamical disorder in the cations conformation which can be compared to that of a molten state. The values of transition enthalpy and entropy of C&d are weaker than expected from the above laws (Figure 3): this suggests that the proportion of mobile C-C bonds is less important in this compound than in cations with longer chains. In Table I, one can notice that entropy variation at the lower temperature transition is higher for CsCd, C&d, and C&d than for CloCd; thus, if there is a flipping of the N H 3 heads between two potential wells of the cavities in these three C,Cd, it is accompanied by a partial melting of the chains.I2 8. X-ray Diffraction Measurements. The Guinier-Lennt diffraction pattern of CsCd agrees with the phase transition diagram obtained by calorimetry. The L T and HT diagrams of CBCdhave been indexed,I4 under the assumption of isomorphism of these phases with L T and HT forms of CloCd; the study of the I M T solid is in progress. The low value of the c parameter (which is twice the interlayer distance) in L T (45,34 A) indicates that the longitudinal axis of the cation is tilted with respect to E in nearly the same way as in CloCd. In HT, this parameter increases by 2.5 A, which is not enough to allow the chains to be normal to the layers and have an extended configuration: such a structure would need 1.2 8, more per cation. As a GTG' geometry shortens the alkyl length by 1.26 A,9J7without changing the inclination of the longitudinal axis, the c value in HT can be explained by the presence of one sequence of this type per chain. The phase transition sequence obtained for C&d and CI6Cd from the Guinier-Lennt diagrams confirms the calorimetric results. But the X-ray pattern of C&d exhibits, in addition, a striking modification which does not involve any thermal effect, a t about 345 K.I3 As the c parameter of the unit cell is far greater than the other two, diffraction on layer planes gives rise to (00 r) lines located in the vicinity of the Cu feature. Thus, a rough estimate of the temperature dependence of the interlayer thickness is possible. As in the case of CloCd, the interlayer distance in the L T phases
I
0.05
0.1
'In
Figure 4. Frequency of the accordion mode of C,Cd as a function of 1 / n ( n is the number of carbon and nitrogen atoms) in the different phases: (O), LT; (+), IMT 1; (a), IMT 2; (X), HT; (...), l / n dependence of molten n-alkanes LAM frequency; (-), 1/ n dependence of solid n-alkanes LAM frequency.
of C12Cd (29.5 A) and CI6Cd (36.75 8,)is too short to allow the chains to be in the trans configuration and perpendicular to the metallic sheets. In C12Cd,the interlayer distance strongly increases a t the first transition and then remains about the same (32 A in HT); its value would allow the chains to be perpendicular to the inorganic layers if they have at least one kink, GTG'. In CI6Cd, the interlayer distance increases by about 0.6 8,at the first phase change and then continuously grows until the highest temperature transition where it presents another discontinuity of 2 8, and becomes equal to 40 A in the HT phase; the longitudinal axis of the disordered chains seems to become gradually almost perpendicular to the chlorine layers. Thus, calorimetric and diffraction studies of C,Cd, CI2Cd,and C&d show the existence of phase changes similar to those of CloCd. However, the characteristics of the transitions and the c parameter variations in the various phases are different for each compound.
Spectroscopic Studies A. Experimental Methods. The compounds were synthesized by using the method described by Foster et a1.I8 for the shorter chain alkyl derivatives with the modifications given by Kind et al.19 for (CloH21NH~)2CdCl,.Powder samples were purified by repeated crystallization from methanol. Infrared and Raman spectra have been made on powdered samples. For the infrared experiments, we did not use the usual KBr pellets to avoid any extra effects of pressure or chemical exchange. Our spectra have been performed on Nujol or Fluorolube suspensions squeezed between two CsI windows. Infrared spectra were recorded on a Perkin-Elmer 180 spectrometer with a resolution of 1-2 or 2-4 cm-' depending on the frequency range. Spectra from 100 to 340 K (f0.5 K) were obtained with a cell built in the laboratory. Raman spectra have been recorded with Jobin-Yvon Ramanor and Coderg T 800 spectrometers and Spectra-Physics argon ion lasers (models 165 and 171). The 514.5-nm line was used with a power of 80-700 mW. Spectral slit widths were 1-3 cm-I; spectra of CI2Cd and C&d have been accumulated in a Minc 11.03 computer. B. Study of the Phases Stable at Low Temperature. The vibrational spectra of the LT phases of C&d, C&d, and C&d are studied in order to determine the structure of the alkylammonium chains. Their analysis is necessary to understand the cation dynamics in the disordered phases. Analysis of the Raman Spectra. The longitudinal acoustic mode (LAM-1) frequency is characteristic of both length and
(15) Vacatello, M.;Corradini, P. Gan. Chim. Ital. 1974, 204, 773. (16) Wurflinger, A.; Schneider, G. M.Ber. Bunssen-Ges. Phys. Chem.
1973, 77, 121.
( 1 7 ) Pechold, W. Kolloid-Z. 1967, 228, 1.
(18) Foster, J. J.; Gill, N. S.J . Chem. SOC.A 1968, 26, 29. (19) Kind, R.;Roos, J. Phys. Rev. B Solid Stale 1976, 13, 45.
4890
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
[m-'
1150
Ricard et al.
1050
Figure 6. Temperature dependence of the Raman spectra between 1400 and 1500 cm-I of (a) C&d, (b) C12Cd,and (c) C16Cd.
0 cm-'
115 0
1050 -&I
0 ~~
,-
IMT
347u
293 K
3000
2900
2950
2850
r
Figure 5. Temperature dependence of the Raman spectra between 1000 and 1180 cm-' of (a) C&d, (b) C12Cd,and (c) C&d.
geometry of the chair^.^^,^^ Figure 4 shows the dependence of this frequency on l/n. Experimental points of C,Cd compounds follow the same law as the LAM frequencies of solid n-alkane~,,~ but they are always higher. The difference is larger than expected for chains of the same length engaged in hydrogen bonds, as has been pointed out in the case of CloCd.12 These values suggest that the chain structure is not completely extended; they are consistent with the existence of a gauche conformation in the vicinity of one end of the alkylammonium. chain. The Raman spectra below 1400 cm-I are very similar to those of corresponding n-paraffins; the most intense bands correspond to the limiting cases 9k= 0 or a which are the only ones active in polyethylene (Figure 5): one can observe u(CC) (a)at 1063 cm-I, v(CC) (0) near 1139 cm-', and methylene twisting, t (P), a t about 1300 cm-I. The line near 1076 cm-I, whose intensity is related to the presence of gauche defects, is also observed: when n increases, it weakens with respect to intense modes, in particular v(CC) (0), characteristic of trans sequences in the chain. In LT, the ratio Z1139/Z1076is roughly proportional to n - 3 which is the number of trans conformations in the CloCd cation; though the intrinsic intensity of v(CC) (0) is not linearly dependent on the number of trans sequence~,2~ such a variation suggests a similarity of the chain geometry in the L T phases of the four compounds: sequences of n - 3 trans structures and one gauche bond in the vicinity of one end. The spectral domains corresponding to CH, bending vibrations (1400-1 500 cm-l) and to CH, stretching vibrations (2800-3100 (20) Hsu, S. L.; Krimm, S.; Ford, G. J . Polym. Sci., Polym. Phys. Ed. 1977, IS, 1769. (21) Strobl, G.; Eckel, R. J . Polym. Sci., Polym. Phys. Ed. 1976, 14,913. (22) Kobayashi, M.; Sakagami, K.; Tadokoro, H. J. Chem. Phys. 1983, 78, 6391. (23) Minoni, G.; Zerbi, G. J. Phys. Chem. 1982, 86, 4791. (24) Scherer, J. R.; Snyder, R. G. J. Chem. Phys. 1980, 72, 5798. Snyder, R. G. J . Chem. Phys. 1982, 76, 3921. (25) Pink, D. A,; Green, T. J.; Chapman, D. Biochemistry 1980, 29, 349. Wunder, S . L.; Macromolecules 1981, 14,1024.
Figure 7. Temperature dependence of the Raman spectra in the CH stretching domain of (a)C&d, (b) CI2Cd,and (c) C16Cd.
cm-') also show analogies with those of solid n-paraffins. A splitting due to the crystalline field effect is observed on CH, bending frequencies (Figure 6). The frequency of the most symmetric component (near 1420 cm-') perturbed by Fermi resonance decreases in a continuous way with increasing n, that is to say, with increasing the number of coupled methylenes. The other component (at 1442 cm-I) is accompanied, a t higher wavenumbers, by several shoulders and bands whose intensity is due to Fermi remnance with overtones and combinations of r(CH,) transitions as shown by Abbate et al. for polyethylene.26 CH, stretching spectra show a well-structured pattern characteristic of almost extended polymethylene chains (Figure 7). The most important feature is the antisymmetric CHz stretching mode (v,(CH2)). Its frequency (2882-2885 cm-I) has the same value as in corresponding solid n-alkanes. As it is known that both conformational disorderz7and dynamic factors2* shift this frequency upward, the low value measured in the LT phase is consistent with the existence of long trans sequences in the chains. ( 2 6 ) Abbate, S.; Zerbi, G.; Wunder, S. L. J. Phys. Chem. 1982,86,3140. (27) Ricard, L.; Abbate, S.; Zerbi, G., to be published in J . Phys. Chem.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4891
Dynamics of n-Alkylammonium Chains
TABLE 11: NH, Deformation Frequencies (cm-') Observed in the Infrared Spectra of the Different Phases of C,Cd, C,,Cd, C,,Cd, and C,,Cd C,Cd LT
6,
6s
IMTl
6.
IMT2
6, 6, 6, 6,
HT
1588
{i::
1578
1578 1483
C,,Cd(2)
C,,Cd
1589 1492 i1488 1583
1588
1578 1484
1"s
nbo
r1bc
12oc
1000
a00
960
7 2 .
' 500
C,,Cd 1589
i:1584 ::: {: 91582 8: 1578 1483 1578 1483
1579 1484 1578 1483
The symmetric CHI stretching mode (q(CH,)) a t 2850 cm-' strongly interacts with combinations and overtones of CH2bending giving rise to the broad background from 2880 to 3000 cm-1.26,2&30 The shoulders near 2872, 2930, and 2961 cm-' belong to methyl vibrations. Analysis of the Raman spectra of the LT phases demonstrates that the RNH3+ chains have an almost extended configuration with only one gauche structure at one end. It also suggests that the LT phases of C,Cd are ordered, since the spectroscopicfeatures due to intermolecular forces are similar in all the compounds. Analysis of the Infrared Spectra. Some spectral domains are nearly similar for all the compounds, namely those corresponding to the stretching and deformation vibrations of methylene and ammonium groups. Table I1 displays the symmetric and antisymmetric deformation frequencies of N H 3 in C&d, C&d, CI2Cdrand C16Cd;in the L T phase, they are nearly the same, which indicates that the strength of the hydrogen bonds is similar in these compounds: as only the monoclinic bonding scheme can be r e a l i ~ e dthis , ~ result suggests that the chains are immobile in the LT phases as demonstrated for CloCd. The NH3 symmetric deformation is split into two components owing to the crystalline field effect in the four C,Cd spectra, as for CH2 bending and rocking vibrations. It is well-known that the frequency and the intensity of the infrared absorptions observed in the range 300-1400 cm-I are dependent on chain length and conformation and on the presence of the N H 3 polar head; the assignment of these bands needs a normal-mode calculation. Analysis of the Raman spectra shows that all the cations are in an almost extended configuration with a single gauche bond near a chain end. Structural data (rough estimation of the c parameter after Guinier-LennE diagrams) give evidence of an inclination of the chains with respect to c which can be realized only if the gauche conformation is located in the vicinity of the N H 3 group, since the bonding scheme of the N H 3 must be the same for all the c o m p o ~ n d s .So, ~ by analogy with the structure of C&d, the cg, CI2,and cl6 cations have been assumed to be in the A or B configuration, i.e., with the G bond between the first and the second carbon atoms or between the second and the third ones. We have performed normal-coordinate treatment of A and B alkylammonium chains for the three compounds,using geometrical parameters identical with those of CloCd and the same force field.I2 The agreement between calculated frequencies and experimental results is rather good. (Calculated frequencies and potential energy distributions (PED) of c8, c12,and cl6 alkylammonium cations in A and B configurations are contained in the supplementary material in Tables I, 111, and IV, respectively) Moreover, assuming the existence of two types of chains is necessary to account for all the frequencies of the progression bands of the skeletal deformations, rocking, and wagging-twisting vibrations (Figures 8-10). Thus, it seems reasonable to think that the structure of the cations is the same in the L T phases of the four C,Cd. (28) Snyder, R. G.; Scherer, J. R.; Gaber, B. P.Biochim. Biophys. Acta 1980, 60, 47. (29) Snyder, R. G . ; Hsu, S. L.; Krimm, S . Spectrochim. Acta, Part A 1978, 34, 395. (30) Snyder, R. G.; Scherer, J. R. J . Chem. Phys. 1979, 71, 3221.
Figure 8. Temperature dependence of the infrared spectrum of CsCd between 400 and 1380 cm-I: (-) Nujol mull; (- - -), Fluorolube mull.
1300
,
1100
,
TO
900
,
qo
Figure 9. Temperature dependence of the infrared spectrum of C&d between 300 and 1380 cm-I: (-), Nujol mull; (---), Fluorolube mull. ) < d I&
I
'
' 1100
'
' 900
700
H I
500
'
300
I
I
Figure 10. Temperature dependence of the infrared spectrum of C&d between 300 and 1380 cm-': (-), Nujol mull; (---), Fluorolube mull.
C. Study of the Disordered Phases. If the correlation times governing the dynamics of the alkylammonium ions in the disordered phases are similar to those of the decylammonium chains in CloCd (7, 2 s ) , ~each cation position and conformer can be observed by vibrational spectroscopy. So the average molecular structure of the hydrocarbon chains can be explored. In the case of C8Cd, CI2Cd,and C&d, the only features which may give an indication on the onset of a reorientational motion of the N H 3 polar heads between potential wells, as reported in CloCd disordered phases, are observed in the infrared spectrum of the N H 3 group. As can be noticed in Table 11, the N H 3 deformation frequencies are lower in the first IMT phases of each compound than in LT, which generally indicates a weakening of the hydrogen bonds. Moreover, the crystalline field splitting of 6, disappears at the first transition in the case of C&d and CI2Cd and at the second one for C&d: this corresponds to a weakening of the correlations between the positions of the ammonium groups on each side of a metallic sheet which can be due to the onset of a flipping of these groups. Analysis of the enthalpic diagrams and of Guinier-Lend X-ray diffraction patterns shows that when the temperature is raised, the behavior of each compound is different; so we separately consider each of them. D. n-Octylammonium Tetrachlorocadmate. IMT Phase. The vibrational spectra indicate a high degree of disorder in IMT.
4892 The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
Figure 11. Temperature dependence of the Raman spectra of the LAM and Cd-CI stretching domain of (a) CaCd, (b) C12Cdr and (C) C16Cd. Asterisks denote bands assigned to octahedral modes.
The crystalline field effects on methylene vibrations, namely on the r(CH2) infrared fundamental (Figure 8) and 6(CH2) scattering band (Figure 6a), disappear due to a modification of the intermolecular forces. The features characteristic of almost extended chains are lost. The frequency of the accordion mode (LAM), which is close to that of liquid nonadecane in LT, is only slightly shifted. This band corresponds to the LAM-like skeleton deformation modes of the disordered chains (DLAM);% its peak height strongly decreases while its width at half-height increases (Figure 1 la). A line at about 204 cm-’ assigned, through vibrational calculations, to a transverse mode of an A cation has vanished; this indicates that the probability of configuration A becomes very weak, while other conformations occur. The intensity of the Raman bands corresponding to the limiting cases ipk = 0 or ?r of the extended chain vibrations decreases while their width increases owing to the occurrence of lines at close frequencies (Figure 5a). The 1076-cm-’ maximum due to defects becomes more intense than those which correspond to v(CC) ( T ) at 1063 cm-l and u(CC) (0) at 1141 cm-’. The intensity of the high-frequency Raman shoulder at 1468 cm-l weakens (Figure 6a), which also indicates an increased proportion of gauche bonds.26 The C H stretching region also shows drastic changes analogous to those observed in paraffins in going from the solid to the liquid phase.26 The peak intensity of the 2885-cm-I line collapses, and the band melts into the background. The frequency of the 2850-cm-’ line shifts upward by 4 cm-I as the size of all-trans sequences dim in is he^.^' Then, the peak intensity near 2940 cm-I increases with regard to that of the 2850-cm-I line. All these changes are characteristic of an increase of gauche content. The types of defects occurring in this phase can be specified from both an analysis of the infrared spectrum (Figure 8) and packing considerations. Within the accuracy of our infrared measurements we were unable to detect gauche defects in the vicinity of the methyl group (absorption near 1342 cm-I) or GG sequences (near 1355 cm-I), but we observed a wide absorption at 1308 cm-’ which has been shown in alkanes to be characteristic of GTG’ or GTG defect^.'^,^^.^^ Because of the low value of the c parameter determined by X-ray diffraction in this phase, the longitudinal axis of the chains must be tilted in a way which does not allow the presence of kinks of type GTG’. On the basis of packing considerations, it also seems reasonable to exclude the presence of the GTG sequence. Thus, the 1308-cm-I absorption which shows the asymmetrical shape of that measured in liquid n-alkanes has to be assigned to other types of defects. Owing to packing considerations, only kinks (GT2,,+’G’ sequences) or G or G G defects near the methyl end of the chains can exist. As the latter are not observed, the 1308-cm-’ band must be assigned to a chain conformer which contains kinks. The only ones compatible with the low value of (31) Snyder, R. G . J . Chem. Phys. 1967, 47, 1316. S.i, P.; Straw, H. L.; Snyder, R. G. J . Am. Chem. (32) Maroncelli, M.; Q SOC.1982, 104, 6231.
Ricard et al. the c parameter are of the type GTTTG’. Indeed, in this configuration, the TTT sequence is tilted by 40° with respect to E, due to the first G defect; the second G’ defect causes the end part of the chain (which is much shorter than the TTT sequence) to be perpendicular to the layers. Two types of chain configurations possess this type of kink: GTTT G’T and TGTTTG’. As we did not observe gauche defects near the CH3, the probability of existence of the second configuration is negligible. The normal modes of the f i t conformer have been calculated by using the same force field as for CloCd. (Calculated frequencies and PED of three different conformers of CBCd(GTG’TTT, TGTG’TT, G T I T G T ) are contained in the supplementary material, in Table 11.) The results of this calculation may help to explain he vanishing of some bands due to A conformers and the occurrence of other modes: (1) The r,(NH3) vibration which is certainly responsible for the intensity of the 1350-cm-’ band of A chains does not contribute to the PED of the mode calculated at the same frequency for the GTTTG’T geometry; the corresponding line disappears at the transition LT-IMT. (2) The band a t 1318 cm-’ due to w(CH,) rl,(NH,) of the A forms is not observed in the I M T spectrum. (3) A wagging mode containing a contribution of rllNH3is calculated a t 1308 cm-’ for the kink configuration GTTTG’T. This band effectively exists in the I M T spectrum. (4) Vanishing of the 812-cm-I rocking and 400-cm-I skeletal deformation bands and Occurrence of a line at 469 cm-I can also be explained by the transformation of A chains into GTTTG’T structures in IMT. In contrast, all the progression bands of rocking and skeletal deformation modes of B chains persist. HT Phase. Only slight differences are observed between the spectra of the HT and I M T phases. The half-width of the DLAM band again increases, and two maxima at 263 and 272 cm-I can be distinguished; this implies that the conformers are more numerous or different from those of IMT. The Raman spectra near 1050-1 150, 1420-1470, and 2850-3000 cm-’ are more “liquidlike”. In the infrared spectrum, only a shoulder at 845 cm-’ and a weak absorption near 478 cm-I reveal the existence of new conformers (Figure 8). Due to the increasing of the c parameter of the unit cell, the longitudinal axis may become normal to the inorganic sheets and the conformers are allowed to possess kinks of the type GTG’. A normal-mode calculation of the forms G T G m and T G T G T T and the comparison with experimental results indicate that the presence of these two conformers may account for all the bands of the spectrum except the one a t 830 cm-I; other conformers probably also exist in this phase. Let us notice that the 1228-cm-’ t(CH,) absorption characteristic of the B chains vanishes; the probability of this conformer in the H T phase is negligible. To sum up, spectrbscopic analysis of the disordered phases of C&d shows that melting occurs, a t least in A chains of the LT phase, at the same time as the onset of the reorientational motion of the polar N H 3 head between two potential wells. If the space group, proposed for the IMT phase, A m ~ a is ? ~confirmed by X-ray diffraction studies on single crystals, the cations must be symmetry equivalent; then, the melting of the chains can be described by a change from conformers B to GTTTG’T (and maybe TTGTTT), and vice versa. This partial melting is different from that observed in the IMT phase of CloCd since the G structure between the first and second carbon atoms does not seem less stable than the other one: maybe it is stabilized by the occurrence of G’ between the fifth and sixth carbon atoms. In the high-temperature phase, X-ray diffraction and vibrational spectroscopy establish that the defects which travel along the cations are mainly kinks which are allowed to be of the type GTG’, due to the increasing of the interlayer distance; analysis of the progression bands of the methylene groups shows that the kinks must have at least two different positions in the chain.
+
(33) Chanh, N. B.; Haget, Y., private communication. (34) Zerbi, G., private communication.
Dynamics of n-Alkylammonium Chains
E. n-Dodecylammonium Tetrachlorocadmate. IMT 1 Phase.
As for the other compounds, vanishing of the factor group splitting on the methylene bending indicates a modification of the intermolecular interactions. Calorimetric and diffraction measurements suggest that the “melting” of the cations is important in the I M T 1 phase. This is confirmed in the Raman spectrum by the increase of the 1082-cm-’ line intensity more or less related to the number of gauche defects (Figure 5b) and by the modifications in the C H stretching domain; the line at 2885 cm-’ melts into the broad background, the frequency of the band near 2850 cm-’ shifts upward, and the peak intensity near 2930 cm-’ increases. In the frequency range 150-250 cm-’ of the Raman spectrum, two shoulders of the CdCl stretching band can be assigned to the DLAM (Figure l l b ) . The highest frequency one, at 230 cm-’, indicates a beginning of melting of the chains. As far as the skeletal deformation frequencies of n-alkanes and n-alkylammonium can be compared, the frequency of the other shoulder, at 195 cm-’, corresponds to a conformer which still possesses a long sequence of trans bonds.24 This result is confirmed by the observation of the methylene rocking fundamental r(r), near 720 cm-’, in the infrared spectrum; the crystalline field splitting on this vibration weakens but is still observed (Figure 9), which indicates that trans sequences whose movements are correlated persist. The study of the infrared spectra in the wagging-twisting domain points out the existence of GTG’ defects and the weak probability of GG (or GG’) sequences and of gauche forms in the vicinity of the methyl group ( G C H , c H , ) . The estimated value of the c parameter after X-ray data allows the presence of GTG’ forms in I M T 1. IMT 2 Phase. Tire weak transition entropy when going from I M T 1 to I M T 2 corresponds to a further melting of the chains. In the I M T 2 phase, the shoulder at 195 cm-’ is no longer observed; the size of trans sequences in a chain becomes less important as shown by the frequency of the DLAM (230 cm-1) and the intensity decrease of the progression absorption bands. The scattering near 1450 cm-’ due to uncoupled also increases in I M T 2 (Figure 6b). Let us notice that the half-width of the DLAM band (-30 cm-l) is less than in the liquid phase of the corresponding alkane (-70 cm-I) which indicates a less important variety of defects. As expected from the enthalpic diagram, the difference is not very important between the two intermediate phases of C&d. Chains are highly distorted by kinks which may have the form GTG’. In I M T 1 a small number of relatively extended conformers still remain and then completely disappear in IMT 2. In this phase the number of progression bands decreases, but we are unable to decide if some kink conegurations are favored, as in CloCd,unless studying selectively bideuterated derivatives. HT Phase. As the phase transition observed at 345 K on the X-ray diagram involves no thermal effect, this structural change is not expected to imply any additional conformational disorder. The identity of the infrared spectra of the HT and IMT 2 phases, at frequencies higher than 300 cm-I, supports this idea. Consequently, the Raman spectrum has not been studied. Thus, the high-temperature transition does not seem to be related to internal rotations of the chains but probably to a weak motion of the inorganic part. Infrared and Raman study of the low-frequency spectra of a single crystal, if available, and determination of the crystalline structures would be necessary in order to obtain more accurate information on this latter point. F. n-Hexadecylammonium Tetrachlorocadmate. After calorimetric and X-ray diffraction studies as a function of temperature, the three phase transitions of C16Cd lead to a gradual “melting” of the cation. IMT I Phase. Similarities between IMT 1 and the intermediate phase of CIOCdl2are deduced from the study of the accordion mode scattering. The LAM frequency does not shift in an appreciable way at the first transition (Figure 1IC), which indicates that the alkylammonium ions remain in an almost extended configuration. The intensity of the progression bands in the infrared spectrum weakens. But the frequency of the skeletal
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4893 deformations is not modified: the structures of the chains remain similar to those of the L T phase, and the number of possible conformers is small. However, conformational defects are evidenced. In particular, the intensity of the scattering line near 1085 cm-’, due to gauche forms, increases with respect to that of the 1138-cm-’ band (v(CC) (0)) (Figure 5c). The CH stretching domain spectrum is also characteristic of a diminution of the trans sequences size in the chains conformers: the bands near 2850 and 2885 cm-’ shift upward by 2 cm-1.27937 The peak intensity of the 2885-cm-’ line decreases, and the ratio 12930/12850 which is thought to be indicative of the number of G forms in n - p a r a f f i n ~slightly ~ ~ - ~ ~increasas (it is reasonable to think that, for this compound, the bands due to methyl vibrations are very weak). However, the C H stretching spectrum still keeps well-structured features; in this phase, the number of G defects is not very important and the chains conformers still remain rather extended. The analysis of the infrared spectrum shows that, as for the other compounds, no gauche form in the vicinity of the methyl group or G G sequences are observed. Thus, due to packing considerations, the only defects which can occur in the chains are very long kinks or G defects alone situated in the first part of the cation near the NH3. The existence of very long kinks seems to be evidenced by the occurrence of a very weak absorption at 1308 cm-’ in the infrared spectrum. Indeed, our calculations of the two A and B conformers show that no localized wagging mode coupled with a motion of the NH3 polar head exists in this frequency range. If the G defect is still farther from the N H 3 group, there is no reason for the existence of such a coupling. But when kinks are present in the chain, our calculations on some conformers of CsCd show that it may exist as a coupling between the waggings of the methylenes situated between the two G defects and the motions of the NH3 group. This contribution would enhance the intensity of the absorption near 1308 cm-’. In the C16Cdcompound, the weakness of this band in the IMT 1 phase may originate from the delocalization of the wagging motions in large kinks. Since the DLAM progression frequencies are not modified at the LT-IMT 1 phase transition, these kinks seem to be located in such a way that their first G defect is between the first and the second carbon atoms (as in A conformer) or between the second and the third ones (as in B conformer). Apparently none of these positions is favored with regard to the other as in CloCd. Melting of C&d in IMT 1 is much less important than that of CsCd in IMT, but it is similar: gauche forms near the chain ends seem to stabilize the two kink positions. IMT 2 Phase. The chains are no longer in an extended configuration, as indicated by the strong decrease of the progression band intensities and the vanishing of 6(CCC) peaks. Moreover, the Raman line near 155 cm-’ is not observed; the DLAM band is maybe overlapped by the Cd-Cl stretching line near 213 cm-’. This frequency is slightly lower than that of the corresponding liquid alkane. The crystal effect on 6(CH2) completely vanishes while the number of uncoupled methylenes increases as shown by the evolution of the scattering 1400-1 500- and 2800-3 100-cm-l spectral domains. The absorption near 1308 cm-I which we assign to kink defects becomes more intense. HT Phase. Analysis of the Raman spectrum shows that the (35) Snyder, R. G.; Straws, H. 2;Elliger, C. A. J . Phys. Chem. 1982,
86, 5145.
(36) These measurements have been performed in the Laboratoire de Cristallographie de I’Universitb de Bordeaux I. (37) A frequency shift of the antisymmetric mode has also been observed in urea clathrate of n - a h ” where there is no conformational disorder. But this shift has been observed over a large range of temperatures and is generally explained by rotational-twisting motions of the chain through a coupling with torsions.34 In the disordered phases of C,Cd, the motions of the chains, as measured in C&d, are far too slow to induce such effects. As the shifts are induced by the transitions, it seems much more reasonable to explain them by conformational disorder.2’
4894
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
number of gauche defects is a little more important in this phase. Infrared features are not strongly different from those of I M T 2. Increase of the c parameter allows the presence of shorter kinks which would keep the longitudinal axis of the chain approximately perpendicular to the layers. Thus, gauche defects occur at the first phase change; the almost extended configuration of the chain in the L T phase is kept in IMT 1 and lost at the transition IMT 1-IMT 2. Conclusion New information on the structure and the reorientational dynamics of alkylammonium cations in C,Cd compounds has been obtained from the above study. First, the analysis of Raman and infrared spectra suggests that the LT phases of C&d, C&d, and CI6Cdare ordered. Alkylammonium cations seem to be packed in the same way (intermolecular forces are of the same order of magnitude) and to possess an almost extended configuration. Interlayer distance measurements as well as the comparison of experimental frequencies with normal-mode calculations indicate that the alkylammonium cations can take two different configurations which correspond to the A and B forms of the cations in the L T phase of CloCd: they are almost extended with only one gauche bond near the NH3 group between the first and the second carbon atoms for the A conformer and between the second and the third ones for the B conformer. Such a similarity between the configurations of the alkylammonium cations of the C,Cd compounds with n = 8, 10, 12, 16 suggests that the chain conformations and packing are closely related to the chlorine matrix structure, at least in the ordered phases. The order of magnitude of the total transition enthalpies of each compound shows that the phase changes are governed by conformational modifications leading to a more and more important disorder and to a partial ”melting” of the chains. Because of the relatively low times of the cation dynamics, vibrational spectroscopy cannot give direct evidence of an onset of a flipping of the NH, polar heads between several potential wells as was observed in the disordered phases of CloCd.9 But as far as the
Additions and Corrections weakening of hydrogen bonds N H ...Cl can be considered as an indirect “proof‘, if this motion exists, it appears at the first phase transition in all the compounds. The first transitions are also connected with a partial melting. Its degree strongly depends on the compound. Because of the narrow tempeature domain of IMT 1 in C&d, it is very difficult to observe the onset of the conformational dynamics of the cations for this compound. The analysis of the disordered phases of C8Cd and C16Cdsuggests that, at first, the ends are the most flexible parts of the chains. When the temperature is raised, the interlayer spacing becomes greater, allowing a motion of defects along the chain. Let us notice that the only defects we have been able to observe are kinks, which is not surprising because they keep the longitudinal axis of the chain and decrease its overall length. Comparison between the interlayer spacing and spectroscopic results leads to the conclusion that in the first I M T phases these kinks are rather long (that is to say, of the form GTwlG’ with n I1). When the temperature is increased, the forms GTG’ begin to occur. Thus, the conformational dynamics of the alkylammonium cations is very different from that of n-alkanes or lipid biomembranes where the tails are more flexible and the possible conformers more numerous. We can conclude that the dynamics of the hydrocarbon part is strongly dependent on the interaction between the cations and the chlorine matrix. Acknowledgment. We thank Dr. N. B. Chanh and Dr. Y . Haget for their valuable help during the calorimetric and X-ray diffraction study. We are also greatly indebted to J. C. Cornut for obtaining the low-temperature infrared spectra. Registry NO.(C8H17NH3)2CdC14, 98394-08-2; (Cl2H25NH3)2CdCI,, 79001-08-4; (C16H33NH3)2CdCl,, 53188-91-3.
98394-09-3; (CloH,INH3)2CdCl,,
Supplementary Material Available: Comparison between experimental infrared (and Raman when different) frequencies of C&d, C&d, and CI6Cd below 2000 cm-I and the results of normal-mode calculations for some conformers of these compounds using the force field presented in ref 12 (Tables I-IV) (8 pages). Ordering information is given on any current masthead page.
ADDITIONS AND CORRECTIONS 1985, Volume 89 1. Gonzalo* and T. Montoro: Interpretation of the Fluorescence Decay of 1-Methylindole in Polar Solvents by Reorientational Effects. Pages 1608-1612. Replace 288 nm with 347 nm and replace 336 nm with 297 nm in the following places: Figures 3 and 4 and their respective captions; Figure 5; Table I; page 161 1, second column, paragraph (iii), third line and twelfth line.
