J. Phys. Chem. 1982, 86, 1003-1008
1003
Motion of Methylammonium Ions and the Mechanism of Phase Transitions in Solid Methylammonium Bromide As Studied by Differential Thermal Analysis, Infrared, and Proton Magnetic Resonance Experiments Hlroyukl Ishlda, Ryulchl Ikeda, and Dalyu Nakamura' Bpartment of Chemlsby, Nagoya Unlverslty, Chlkusa. Nagoya 464, Japan (Received: June 15, 1981; In Final Form: October 14, 1981)
The measurement of DTA and the temperature variation study of IR revealed that the reported stable phases a,a', and p of methylammonium bromide (CH3NH3Br)exist above 387 K, between 387 and 281 K, and below 281 K, respectively. The metastable 6 phase prepared by rapidly cooling the sample to 77 K seems to be transformed into a' at 196 K, and, immediately above this temperature, it is transformed again to p. The temperature dependences of the 'H spin-lattice relaxation time Tlat 20 MHz and of the second moment of lH NMR absorptions were determined for the bromide and also ita partially deuterated analogues CD3NH3Br and CH3ND3Brin their respective phases. An analysis of the CH3and NH3 rotational reorientations about the C-N axis occurring in the phase was accomplished by using the T1data of the partially deuterated salts. The onset of both reorientations in the 6 phase occurs at approximately the same temperature which is lower than those found in the phase. The motional parameters of the cation in the and 6 phases could be derived from the present results of 'H NMR.
Introduction From the study of infrared spectroscopy on methylammonium bromide, CH3NH3Br,Sandorfy et a l . ' ~found ~ the existence of four different solid phases in the temperature range 15-470 K. A t room temperature, the salt has a tetragonal structure3s4belonging to the space group P4/nmm, which is almost identical with that of methylammonium chloride in its a p h a ~ e . ~In~ the ~.~ structure, the methylammonium cation having a C3 axis is situated along the fourfold c axis of the crystal. This indicates that the cation must be disordered with respect to the orientation of the methyl and also ammonium groups about the C-N bond axis. On heating the bromide, Cabana and Sandorfyl obtained an infrared spectrum very similar to that of the a-form chloride and named this high-temperature phase and the room-temperature phase as a and a', respectively. Below room temperature, they found the existence of two phases, namely, p and 6. The former is a low-temperature stable phase and gives an infrared spectrum very similar to the one from the &phase chloride, whereas the latter is a metastable phase and is obtained by rapidly cooling the bromide to 77 K. No X-ray data are available for the a-, p-, and &phase bromides. Recently, Jugie and Smith' observed a single 79Brnuclear quadrupole resonance (NQR) line for the bromide in the temperature range 253-350 K, suggesting the occurrence of the rapid reorientation of the cation in the a' phase. Since no resonance could be detected below and above the temperature range, they failed to locate expeded phase transition temperatures (T& The 'H NMR spectra and the 'H spin-lattice relaxation times (T,) of the bromide are of interest in studying the motion of the molecule-like cations in solids. Such in(1) A. Cabana and C. Sandorfy, Spectrochim. Acta, 18, 843 (1962). (2) A. Th€or6t and C. Sandorfy, Spectrochim. Acta, Part A , 23, 519
(1967). (3) S. B. Hendricks, 2.Kristallogr., 67, 106 (1928). (4) E. J. Gabe, Acta Crystallogr., 14, 1296 (1961). (5) E. W. Hughes and W. N. Lipscomb, J. Am. Chem. Soc., 68, 1970 (1946). (6) M. Stammler, J.Inorg. Nucl. Chem., 29, 2203 (1967). (7) G. Jugie and J. A. S. Smith, J. Chem. SOC.,Faraday Trans. 2,74, 994 (1978). 0022-365418212086-1003$01.2510
formation is also of great value in the characterization of each solid phase as well as in the interpretation of the mechanism of phase transitions. For the chloride, various studies7-10have been carried out in an attempt to obtain information about the nature of hydrogen bonds formed between -NH3+ and C1-. The bromide is thought to have weaker hydrogen bonds than the chloride. Therefore, it is also interesting to examine the motion of the cations in relation to the formation of hydrogen bonds. The preliminary results of the DTA and T I measurements of CH3NH3Brhave been reported elsewhere as a letter."
Experimental Section Materials. Methylammonium bromide was prepared from an aqueous solution of methylamine by adding hydrobromic acid and purified by repeated recrystallizations from absolute ethanol. The partially deuterated salt CD3NH3Br was obtained from hydrobromic acid and partially deuterated methylamine CD3NH, which was synthesized by adding a concentrated sodium hydroxide solution to a CD3NH3C1aqueous solution. Another partially deuterated salt, CH3ND3Br,was prepared by dissolving CH3NH3Brin heavy water. From the measurements of infrared spectra, the extent of deuteration was estimated to be about 99% and more than 95% for CD3NH3Brand CH3ND3Br,respectively. For NMR and DTA experiments, all of the salts thus prepared were carefully dried and placed in glass tubes. Furthermore, these were evacuated and maintained at about 80 OC for 8 h in order to obtain the samples completely free from water. Then, the glass tubes were sealed off after replacing ambient air with helium or dry nitrogen. Apparatus. The proton TI was determined at 20 MHz by means of a homemade pulsed spectrometer described elsewhere12in combination with a JEOL electromagnet. (8) J. Tegenfeldt and L. Odberg, J.Phys. Chem. Solids, 33,215 (1972). (9) J. Tegenfeldt, T. Keowsim, and C. Saterkvist, Acta Chem. Scand., 26, 3524 (1972). (IO) S. Albert and J. A. Ripmeeater, J. Chem. Phys. 58, 541 (1973). (11)H. Ishida, R. Ikeda, and D. Nakamura, Phys. Status Solidi A , 60, K115 (1980).
0 1982 American Chemical Society
1004
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982
Ishida et ai.
TABLE I: Comparison between Observed and Calculated 2e Values for CH,NH,Br in the a Phase at ca. 403 Ka 2e /deg
b
___ 100
200
T/K
300
400
Flgure 1. DTA curves a and b observed for CH,NH,Br by employing the slow-cooling and rapid-cooling methods, respectively.
A usual 180°-~-900 pulse sequence was employed. The sample temperature between 85 and 400 K was controlled by use of a conventional nitrogen gas flow system.13 A cryostat using liquid or solid nitrogen as a cryogenic medium was employed for the measurements of Tl at various temperatures between 58 and 85 K. A copper vs. constantan thermocouple was used for the determination of temperature. The observed temperature was estimated to be accurate within f l K. Continuous wave (CW) 'H NMR spectra were recorded at 40 MHz by means of a JEOL JNM-MW-40s spectrometer equipped with a temperature controller. The temperature determined in this experiment using a copper vs. constantan thermocouple was estimated to be accurate within f 2 K. The measurements of DTA were performed by using a homemade apparatus already de~cribed.'~ Infrared spectra were recorded in the wavenumber range 4000-700 cm-' by use of a Jasco DS-402G spectrophotometer equipped with a cryostat described e1~ewhere.l~ Samples were run as thin films deposited on KBr plates. The sample temperature determined by a copper vs. constantan thermocouple was estimated to be accurate within f 5 K.
Results and Discussion DTA and IR Measurements. In order to determine the temperature range of the four phases already found from the temperature variation study of IR,'S and also to obtain information on the nature of phase transitions to be detected, we carried out the experiments of DTA in a temperature range 77-420 K. The results obtained for CH,NH3Br are shown in Figure 1. When the sample was gradually warmed from room temperature, an endothermic anomaly appeared at 387 K. However, an exothermic anomaly was detected at about 360 K on cooling. This indicates that the bromide undergoes a first-order phase transition at 387 K. To obtain information about the crystal structure of the a! phase of this salt, we took X-ray powder patterns at ca. 405 K by means of an X-ray powder diffractometer Model D-3F from Rigaku Denki Co. equipped with copper anticathode. (12)L. S. Prabhumirashi, R. Ikeda, and D. Nakamura, Ber. Bunsenges. Phys. Chem., 85,1142 (1981). (13)R.Ikeda, Y.Kume, D. Nakamura, Y. Furukawa, and H. Kiriyama, J.Magn. Reson., 24,9 (1976). (14)Y. Kume, R. Ikeda, and D. Nakamura, J.Magn. Reson., 33, 331 (1979). (15)K. Ichida, Y.Kuroda, D. Nakamura, and M. Kubo, Spectrochirn. Acta, Part A , 28,2433 (1972).
a
h k l
obsd
calcd
001 110 101 111 200 201 002 211 220 1 1 2 221 310 202 311 0 0 3 222 302 1 1 3 400 401
17.15 19.74 22.16 26.27 28.06 33.06 34.69 36.05 40.10 40.23 43.88 45.08 45.17 48.53 53.05 54.00 56.11 57.16 58.03 60.90
17.15 19.74 22.14 26.26 28.06 33.07 34.70 36.04 40.10 40.24 43.91 45.08 45.20 48.53 53.10 54.00 56.05 57.19 57.95 60.91
Tetragonal, a = 6.36, c = 5.17 A .
A goniometer was calibrated with a standard sample of silicon. The observed diffraction angles could be well interpreted as arising from a tetragonal lattice with its lattice constant a = 6.36 A and c = 5.17 A. The adequacy of the present analysis is shown in Table I. The X-ray results indicate that the high-temperature a phase of the bromide and the room-temperature a phase of chloride are isomorphous with each other. Below room temperature,, two different methods of sample cooling were employed in the DTA measurements, i.e., slow cooling and rapid cooling, because it was known that the metastable 6 phase was obtained only by rapidly cooling the sample.' In the former method, DTA curves were recorded while the sample was gradually cooled and warmed (ca. 2-4 K min-') as in the usual manner. When the latter method was employed, however, the sample was rapidly cooled at first (ca. 15-20 K min-') from room to liquid-N2 temperature, and then it was warmed at the usual rate. In this method, DTA curves were recorded during both processes of sample cooling and warming. When the slow-cooling method was employed, an endothermic anomaly showing a hysteresis phenomenon appeared at 281 K on warming, indicating the occurrence of a first-order phase transition at this temperature. The cy' phase could be supercooled to ca. 250 K by slow cooling. When the sample was cooled rapidly, on the other hand, the observed DTA curve was quite different from that obtained by the slow-coolingmethod. A small exothermic anomaly having a long tail on the low-temperature side was found at 196 K on rapid cooling. When the sample was warmed after having been rapidly cooled once to 77 K, the DTA curve started to deviate very gradually from the base line to the endothermic direction at ca. 160 K and returned rapidly to the base line at 196 K, immediately above which temperature the next large exothermic anomaly started. It is interesting to note that the appearance of this small endothermic or exothermic anomaly detected on warming or cooling, respectively, bears a strong resemblance to that observed for various methylammonium hexahalometallates(IV).'*J6 Since no hysteresis was observed for this heat anomaly, methylammonium bromide seems to (16)Y.Kume, R.Ikeda, and D. Nakamura, J.Phys. Chern.,82,1926 (1978).
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982
Motion of Methylammonium Ions in Solid CH3NH3Br
1005
TABLE 11: Phase Transition Points Ttr’s Determined from the Measurements of DTA
CH,NH,Br CD,NH,Br CH,ND,Br
281 (-250) 285 (-250) 280 (-250)
387 (-360) 393 (-370) 405 (- 370)
196 196 196
n
800
1200
1000
1600
1400
cm-1
Figure 3. Infrared absorption spectra of CH,NH,Br in its 6 and p phases. The spectra were recorded at various temperatures while the sample was warmed.
*. 30 -
..
CH,NH,Er
I
Figure 2. Infrared absorption spectra of CH3NH3Br. The spectra were recorded at various temperatures in the a’ and 6 phases while the sample was rapidly cooled.
undergo a second-order or higher-order phase transition at 196 K. When the sample is warmed further, the large exothermic anomaly appearing successively is really unusual and suggests that the salt in its unstable phase is transformed into the @ phase which is stable at the lower temperatures. In fact, on further warming, a sharp endothermic anomaly appears at the same temperature as that observed for the sample obtained by slow cooling, indicating the presence of the phase transition from @ to a’. The unstable phase from which the salt is transforme4 into the @ phase seems to be a’,because the room-temperature stable d phase is transformed into the metastable 6 phase through the second-order phase transition on rapid cooling. The a’ phase is unstable at such a low temperature and, therefore, it may be immediately transformed into @justabove Ttr (6 a’). From the foregoing experimental results, the following conclusions can be obtained. First, there exist three stable solid phases, a, a’,and @,for the bromide above 387 K, between 387 and 281 K, and below 281 K, respectively. The phase transitions found at Tt, (a e a’)and Tt,(a’e @)are of first order. Second, the a‘ phase can be supercooled to 196 K on rapid cooling, below which temperature the metastable 6 phase appears. For the deuterated analogues CH3ND3Br and CD3NH3Br, almost the same DTA curves were obtained and the transition temperatures were determined as given in Table 11. To obtain detailed information about the phase transition taking place between 6 and @, we recorded infrared spectra at various temperatures between ca. 100 and 250 K. The temperature dependence of the infrared spectra recorded for CH3NH3Br in the wavenumber region of 700-1600 cm-’ is depicted in Figures 2 and 3. According to Cabana and Sandorfy,’ the methylammonium cation in each solid phase gives the most characteristic spectra in this wavenumber region.
-
I
Figure 4. Temperature variation of the second moment M2 observed for CH,NH,Br. Circles and dots indicate the values measured in the 6 phase and in the other phases of the salt, respectively.
When infrared spectra were taken with the sample while it was cooled rapidly, the bands at 1567 and 1443 cm-’, attributable to asymmetric NH3 bending, vg, and asymmetric CH, bending, vl0, respectively, became fairly strong below about 240 K. However, the spectra taken at about 210 K were essentially similar to those of the a’ phase. When the sample was cooled further, each shoulder peak of vg and vl0 gradually grew to a separate peak and the spectra of this wavenumber region taken at 120 K were characteristic of 6. With gradually increasing temperature from 120 K, the typical spectra of the 6 phase were gradually varied to those bearing a characteristic featlbre of the a’ phase. At about 200 K, however, various bands characteristic of the @ phase appeared and the spectra completely peculiar to @ were recorded at 220 K. From these infrared results along with those of DTA, it becomes clear that the a’phase is directly transformed into 6 on rapid cooling and, when the sample thus obtained is heated, the metastable 6 phase is transformed into the stable @ phase at the temperature immediately above 196 K passing through another intermediate unstable phase which is possibly a’. Continuous Wave ‘H NMR. Figure 4 shows the temperature dependence of the second moment of ‘HNMR absorptions observed for CH3NH3Br. The ‘H NMR absorptions recorded for the bromide above room temperature were easily saturated. The second-moment values obtained in this temperature region were smaller than 10
1008
The Journal of Physical Chemistry, Vol. 86, No. 6, 7982
TABLE 111: Observed and Calculated Second Moments of Broad-Line 'HNMR Absorptions in Methylammonium Bromide
Ishida et al. 300
200
I
I
100
t
T/K
loo
80
I
I
60
1
a .
second moment/G2 calcd obsd
77 K
Me-
C,-
(phase)
(phase)
rigid
rot
rot
CH,NH,Br
37 ( 0 )
8 (a')
37.5
22.5
8.9
CH,ND,Br
11 ( 6 ) 9.4 ( a )
7.0 (O)=
25.9 33.4
6.1
6.1
33.4
8.0
compd
CD,NH,Br a
ionic
300 K
9.1 (6)
I
I
I *
I
I
I
I
Obtained at 266 K.
.
G2.With decreasing temperature, a constant value of 11 G2was obtained below room temperature down to about 200 K. Below this temperature, the second moment increased with decreasing temperature and a value of 37 G2 was reached at 77 K. When the sample was gradually warmed after having been cooled rapidly, on the other hand, almost the constant second-moment value of 12 G2 was observed from 77 K to about 200 K. Above this temperature, exactly the same values were obtained as those of the slowly cooled sample. On the basis of the experimental results of the NMR line width, it is evident that the temperature dependence of the second-moment values determined below 275 K for the slowly cooled sample arises from the /3 phase of CH3NH3Brand also that the motion of the cation in the 6 phase is reflected in the temperature dependence determined between 77 and 180 K for the rapidly cooled sample. The second-moment values determined at room and liquid-N2 temperatures for CH3NH3Br, CD3NH3Br,and CH3ND3Brin their a', p, and 6 phases are given in Table 111. The calculated second-moment values for the cation in various motional states in solids are also included in Table 111. In the calculation, the bond distances of C-H, N-H, and C-N were taken to be 1.096,1.045, and 1.47 A, respectively, and all of the bond angles in the cation were assumed to be tetrahedral.17~18The interionic contribution to the second moment was roughly estimated by using the lattice parameters determined at room temperature for both bromide and ~ h l o r i d e . ~ ~ ~ From the foregoing experimental results of the second moment, it is concluded that the cation in the @ phase of the bromide is fixed rigidly at 77 K, whereas both CH3and NH3 groups of the cation in the 6 phase reorient around the C-N axis even a t liquid-nitrogen temperature. This indicates that the structure of the former phase is more closely packed in which the cation is fixed firmly, whereas that of the latter phase is rather loosely packed. The cations i9 a' and cy reorient fast or nearly freely rotate about the respective C3 axis. Spin-Lattice Relaxation Time TI. The temperature dependence of T I obserbed for CH3NH3Br is shown in Figure 5 . The T 1 curve in the cy and a' phases has a maximum at ca. 310 K, where the value of Tl reached ca. 48 s. At T,, ( a z a'), the Tl value is varied continuously and the same is true for ita gradient. This indicates that the motional state of the cation does not change appreciably at T,, ( a e a') and suggests that this phase transition can be interpreted in terms of a structural one mainly involving the rearrangement of the bromide-ion lattice. When the sample was cooled gradually from room temperature, the TIvalue decreased discontinuously and (17)J. Tsau and D. F. R. Cilson, Can. J. Chem., 48,717 (1970). (18)E.A. Andrew and P. C. Canepa, J.Magn. Reson., 7 , 429 (1972).
-*##.&A'
Figure 5. Temperature dependence of T , determined at 20 MHz for CH,NHSBr. Triangles and dots represent T , values obtained for the 6 phase and the other phases of the salt, respectlvely.
'r
300
2
T/K
80
loo
60
I
I
-C..
1
,*
CHjND38r
Figure 8. Temperature dependence of T , observed at 20 MHz for CH3ND3Br. Triangles and dots represent T , values obtained for the 6 phase and the other phases of the salt, respectively. The solid line is the theoretical curve calculated by use of eq 1 and 2 along with the parameters given in Table I V .
-
also by a large amount at T,, (a' P), ca. 250 K. Below this temperature, the Tl curve had a minimum and a shoulder at 232 and 185 K, respectively. In order to assign the minimum and the shoulder to the motional modes of the cation in solids, we determined the temperature dependence of T l for the partially deuterated salts CH3ND3Br and CD3NH3Br. The results are shown in Figures 6 and 7, respectively. In all of these salts studied, the height of free induction decay (fid) signals varied somewhat nonexponentially depending on the spacing time T between the .rr and ~ / 2 pulses. The nonexponential behavior of fid signals was marked near the Tl minimum temperature. Therefore, the observed nonexponential decay can be attributed mainly to the cross correlation of intragroup dipolar relaxation in Tl values given in Figures a CH3 or NH3 g r o ~ p . ' The ~~~ 5-7 were determined from the initial linear portion of fid signals plotted against T . (19)R. L. Hilt and P. S. Hubbard, Phys. Reo. A, 134, 392 (1964). (20)S.Emid, R.J. Baarda, J. Smidt, and R. A. Wind, Physica B+C (Amsterdam), 93, 327 (1978).
Motion of Methylammonium Ions in Solid CH,NH3Br
300
,A
ALA
r/n
200 AI .A.
'
100
00
I
I
The Journal of Physical Chemistry, Vol. 86, No. 6, 1982
60
.
TABLE IV: Activation Parameters of Methylammonium Cations in Methylammonium Bromide ~~
reorientaE,/ tional (kJ mode mol-')
k
compd
CH,NH,Br
phase 4 4
CH, 3"
6 (below
CH,ND,Br
160 K) 6 (160196K) D 6 (below
6
CD,NH,Br
2
4
6
8 ~ K / T IO
12
14
16
Figure 7. Temperature dependence of T I determined at 20 MHz for CD,NH,Br. Dots and triangles indicate T I values obtained for the @ phase and the other phases of the salt, respectively. When the bphase salt was warmed, two T I curves were obtained in the temperature range between 196 and 280 K (see text). The solid line is the theoretical curve calculated by use of eq 1 and 2 along with the parameters glven in Table I V .
For a CH3or NH3 group which is regarded as an isolated three-spin system reorienting about the respective C3axis, the spin-lattice relaxation time T1 can be expressed by
T1-l = (9/20)r%(r1)
(1)
Here, r denotes the interproton distance in the CH3or NH3 group and h ( ~is ~given ) by h(71) =
1007
y4h2[71(1+ wo2712)-1 4- 471(1 +
(2)
where y, 71,and wo stand for the gyromagnetic ratio of a proton, the correlation time for the reorientational motion, and an angular resonance frequency, respectively. The resultant Tl value of the cation which includes both a CH3 and an NH3 group can be written as an approximation
T1-' = (l/2)(Tl(CH3)-I + T1(NH3)-l)
(3)
where T1(CH3)and T1(NH3)are given by eq 1. On slow cooling, the deuterated salt CD3NH3Brshowed a T1 minimum of 8.4 ms at 235 K, while another deuterated salt, CH3ND3Br,yielded a minimum of 14.5 ms at 192 K. The former temperature agreed well with the T1 minimum temperature of CH3NH3Br. Both of the part i d y deuterated salts showed a single Tl minimum having no shoulder in their 0phase. The theoretical Tl minimum value was evaluated for the reorienting CH3 or NH3 by using eq 1 and 2, where the interproton distance was obtained by assuming the same geometry of the cation as that used in the calculation of the second moment. The calculated Tl minimum values were 8.49 and 11.3 ms for the NH3 and CH3 reorientations, respectively, in fairly good agreement with the observed values. In the calculation, the effect of the cross correlation of dipolar interaction was not taken into account. This is a possible reason that small differences arise between the observed and calculated values. From these experimental and theoretical results, the TI minimum and the shoulder obtained in the @ phase of CH3NH3Brare unambiguously attributed to the NH3 and CHBreorientational modes, respectively, in referring also to the results of the second moment mentioned before. When the sample was rapidly cooled to 77 K and then the measurements of T1were carried out with increasing
4
160 K) (160196 K)
6
(below
6
160 K) (160196 K)
16.4 20.4
CH, and NH,
8.2
CH.and
11.6
NH, CH, CH,
16.5 8.6
CH,
14.0
3"
NH,
24.5 9.5
NH,
11.2
7.h
1.7 X 10'13 1.2 x 4.0 x
1.7 x 1.9 X
1.9 x 10-14 9.0 x 10-14
temperature, a large Tl minimum was observed at 82,85, and 82 K for CH3NH,Br, CD3NH3Br,and CH3ND3Br, respectively. The T1 minimum values were 10, 8.2, and 14.5 ms, respectively. By considering the results of the second moment obtained in the 6 phase and of the theoretical analysis of Tl in the 0phase described above, one may attribute these minima to the reorientational motion of the CH3 and NH3 groups simultaneously taking place in CH,NH,Br and to that of the NH3 or CH, group for the partially deuterated salts. When the sample was warmed further, the gradient of each temperature dependence curve of Tl became slightly steeper in the temperature range of 160-200 K, where the DTA curve recorded for each salt shows an anomaly. Since the TI value is inversely proportional to r1on the high-temperature side of the TI minimum, the above Tl results indicate that the reorientational correlation time in this temperature range may be shortened, with increasing temperature, more rapidly than below the range of temperature. This is probably because the cation in the crystal can move easily in the temperature range where the thermal anomaly of the phase transition was detected by DTA. When the sample of the 6 phase was warmed, the observed Tl value of these salta suddenly became short at 196 K, which agrees well with TJ6 a') determined by DTA. Above ca. 200 K, the sample of CH3NH3Brprepared by rapid cooling followed exactly the same curve of that prepared by slow cooling. In the measurements of T1 of CD3NH3Br,an fid consisting of two components having different time constants was observed above Tt, ( b 0). One component gave rise to shorter Tl values which agreed well with those of the 0phase. The other component, however, yielded longer Tl values which fell along the extended T1 curve of that obtained below Tt, (6 0). The extended curve connected with that of the a' phase without discontinuity. These experimental facts indicate that the 0phase of CD3NH3Br produced on warming it after it is once rapidly cooled is a mixed phase of the 0and probably a' phases. When the sample was allowed to stand in its mixed state at 200 K, fid signals resulting from the 0 phase became gradually stronger and, after a few days, fid signals arising from the single 0 phase were observed. From the Tl data of the present salts, we can obtain the activation energy E, for the reorientational motion of the CH, and NH3 groups of the cation. If one assumes the Arrhenius relationship, the correlation time T~ appearing in eq 2 can be expressed at a given temperature T as 71 = 7, exp(E,/k'T? (4)
-
-
-
J. Phys. Chem. 1902, 86, 1008-1012
1000
where k represents the Boltzmann constant and 7, is the correlation time at the limit of infinite temperature. The results obtained are listed in Table IV. The activation energies for the reorientations of the CH3 and NH3 groups in the 0phase are larger than those determined for the groups in the 6 phase. This confirms the preceding conclusion derived from the result of CW NMR that the 0-phase salt forms rather more closely packed crystals than the &phase one. For 0-phase methylammonium chloride, the activation energy values for the CH3 and NH3 reorientations have been reportedlo to be 18.6 and 23.8 kJ mol-', respectively. These values are only slightly larger than the corresponding values of the bromide, indicating that the cations in each salt have very similar surroundings. The greater value of the activation energy for the NH3 reorientation can be interpreted as the N-H.. .Br type hydrogen bonds being stronger than those of the C-H...Br type.
For the 6 phase, almost the same activation energy value of ca. 8 kJ mol-' has obtained for both the CH3 and NH3 reorientations. Previously, we studied the motion of methylammonium (MA) cation in (MA)2SnC16 and (MA),PtC16 and found that the cation performs uncorrelated reorientation with an activation energy of ca. 8 kJ mol-'; i.e., both groups of the cation are activated to reorient individually over the internal barrier of reorientation. Accordingly, the present results indicate that the correlated reorientation of the cation in the 6 phase of the bromide has approximately the same correlation time as the uncorrelated reorientation of the cation. Here, the correlated reorientation means the rotational reorientation of the cation as a whole about its C-N bond axis. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research No. 543006 from the Japanese Ministry of Education.
Assessment of Intermolecular Association of Haloalkanes with N,N-Dlbutyl-2-ethylhexanamide and Pyridine by Dielectric Permittivity and Polarization M. M. KopeEnl,+ R. J. Laub,' 01. M. PetkovlC,+ and C. A. Smlth Chemical Dynamics Laboratory, The Boris KMric Institute of Nuclear Sciences -VINCA, 1100 1 Beograd, Yugoslavia, and the Department of Chemlstry, The Ohio State Unlversw, Columbus, Ohio 43210 (Received: June 18, 1981: In Final Form: September 8, 198 1)
Notional stability constants K , for an N,N-disubstituted amide donor with chloroform additive (n-hexaneand n-hexadecane solvents) are deduced from novel methods utilizing dielectric permittivity and are then contrasted with those found by dielectric polarization. Data arising from GLC, NMR, and IR for the same systems are presented for comparison. These range from 0.23 dm3mol-' (IR)to 2.34 dm3mol-' (NMR). The five permittivity methods are internally consistent and give on average 1.13 dm3mol-'. The average dipole moment of the amide with media other than chloroform (4.32 D)is 3.70 D. Solvent effects and the permittivity methods are also examined with pyridine donor (benzene reference solvent) with carbon tetrachloride and carbon tetrabromide additives, where the former gives K1 on average of 0.06 dm3mol-' while the latter exhibits K , of 0.20 dm3mol-'. The dipole moment of pyridine is 2.19 D in benzene and 2.33 D in carbon tetrachloride; based upon the K1 data it is expected to be 2.6-2.8 D in carbon tetrabromide.
Hypothesis of weak yet discrete donor (D)-additive (A) intermolecular association seemingly accounts for many of the thermodynamic and spectroscopic properties of a wide range of nonelectrolyte solutions. There remain nevertheless well-known and considerable discrepancies in measured stoichiometric equilibrium constants K1 for the reaction D A F= DA in supposed inert solvent S, whether assessed by chromatography, by spectroscopy, or by other techniques (e.g., ref 1 and 2). The situation has been compounded in recent years by Purnell and Vargas de Andrade3 and by Laub and P ~ r n e l lwho , ~ found that a simple additive function described the infinite-dilution liquid-gas partition coefficients of a wide range of solutes with an equally diverse set of binary solvents without reference to terms in K1. Studies contemporary with these have cast some doubt upon the relations thereby derived, however, and, indeed, on the experimental evidence upon
+
'Chemical Dynamics Laboratory, The Boris Kidric Institute of Nuclear Sciences. * Address correspondence to this author at the Department of Chemistry, The Ohio State University, Columbus, OH 43210. 0022-3654/82l2086-1008$01.25/0
which they were f ~ u n d e d .Reassessment ~ of the situation with systems comprising n-alkanes6 to those exhibiting consolute temperature7 has moreover failed to resolve the matter, since it has been demonstrated that multiple equilibria8(particularly solvent self-association)might well mitigate the additivity hypothesis. In the course of experimental study of systems wherein one or the other of the solvent components appears to d i m e r i ~ enotional ,~ constants Kd have recently been de(1) Laub, R. J.; Pecsok, R. J . J. Chromatogr. 1975, 113, 47. (2) Laub, R. J.; Wellinpton, C. A. In "MolecularAssociation";Foster, R., Ed.; Academic Press: London, 1979; Vol. 2, Chapter 3. (3) Purnell, J. H.; Vargas de Andrade, J. M. J . Am. Chem. SOC.1975, 97, 3585, 3590. (4) Laub, R. J.; Purnell, J. H. J. Am. Chem. SOC.1976, 98, 30, 35. (5) Harbison, M. W. P.; Laub, R. J.; Martire, D. E.; Purnell, J. H.; Williams, P. S. J . Phys. Chem. 1979, 83, 1262 and references therein. (6) Laub, R. J.; Martire, D. E.; Purnell, J. H. J.Chem. Soc., Faraday Trans. I 1977,73, 1686; J. Chem. SOC., Faraday Trans. 2, 1978, 74, 213. (7) Laub, R. J.; Purnell, J. H.; Summers,D. M. J. Chem. Soc., Faraday Trans. 1 1980, 76, 362. (8) Williams, P. S. Ph.D. Thesis, University College of Swansea, Swansea, Wales, 1980. Laub, R. J.; Purnell, J. H., unpublished work. See ref 2.
0 1982 American Chemical Society
