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Proton magnetic resonance study of the solvation of aluminum (III) in n-Propanol
JOURNAL
OF MAGNETIC
RESONANCE
6,336-343
(1972)
Proton Magnetic Resonance Study of the Solvation of Aluminum (III) in n-Propanol H. GRASDALEN Institute of Physical Chemistry, The Technical University of Norway, Trondheim, Norway Presented at the Fourth International Symposium on Magnetic Resonance, Israel, August, 1971 Separate OH and OCH2 PMR signals for free and bound propanol molecules have been observed for solutions of AICY3 in n-propanol at reduced temperatures. Qualitatively we find the same results for AlC& in n-propanol as we have earlier found in a similar study of AlCI, in ethanol (I). The solvation number of the A13+ ion is 4 from relative intensity measurements of the OH signals below -20°C. These results contrast with the solvation number 6 for Al’+ in H20, DMSO, DMF, and MeOH. The solvation shell OH signal consists of three lines which differ in their temperature behavior. Two n-propanol ligand exchange processes with different exchange rate constants are present. This is also reflected in the line width and line shift temperature dependence of the bulk OH peak. INTRODUCTION Al(II1) has been shown by NMR to have solvation number 6 in water (2,3), dimethyl sulfoxide (4), dimethylformamide (5,6), and methanol (7). In ethanol, however, we very recently found that A13+ is tetrasolvated (I). In the present study of AlC13 in n-propanol, and in less extensive measurements on n-butanol and isobutanol solutions of AlCls, we also find an A13+ solvation number 4. Hence, with the exception of MeOH, tetrasolvation seems to be typical of Al(III)-alcohol systems. Solubility problems restrict PMR studies to In EtOH and is quite narrow, (8). This implies
the lowest
aliphatic
alcohols.
n-PrOH solutions of AIC13 a single Al-27 NMR line is observed which -70 Hz, and the width is mainly determined by quadrupole interaction a high local symmetry
ion is either tetrahedrally
on the Al nucleus
site and suggests that the A13+
or octahedrally coordinated with the ligand oxygen atoms
with no competition from the Cl- ions for solvation shell sites. The solvation four confirms tetracoordination of A13+ in EtOH and n-PrOH solutions.
number
EXPERIMENTAL
The technique and precision of the PMR measurements at 60 MHz and preparation of the samples were the same as described for ethanol solution (I). Some Al-27 NMR measurements were made at 15 MHz using an AEI RS2 high resolution spectrometer. The salt used was anhydrous Merck zur Analyse. The n-propanol was >99 % Merck zur Analyse containing -0.03 M HzO. Salt and solvent were used as received. 0 1972 by Academic
Press, Inc.
336
SOLVATION
OF
RESULTS
Al(II1) AND
337
IN wPROPANOL
DISCUSSIONS
Figure 1 shows the PMR spectrum of a 0.44 M solution of AlCl, in n-propanol at -25°C. Due to higher viscosity the lines are broader than those in ethanol solution under the same conditions. Separate OH and OCH2 solvation shell signals are observed 0.44
M AICI,
in n-PrOH
-25"~
C@H&
CH,CH,CH,Oc
I CH&CH,Ot
A13’C$H,CH,CH, TMS
/12__ J
I 600
I 400
I 200
\
I 0
Sweep (Hz1 FIG. 1. The PMR spectrum of a 0.44 Msolution of the different peaks is indicated by underlining
of AK& in n-propanol the protons involved. TABLE
PMR
CHEMICAL
Solutions (mole ratios)
Temp. (“C)
1 AlCl, : 20 n-PrOH 1 AlCl, : 20 n-PrOH 1 AlCl, : 20 n-PrOH 1 AlCl, : 20 n-PrOH 1 AlCh : 20 n-PrOH 1 AICla : 40 n-PrOH 1 AlCl, : 59 n-PrOH 1 AlCl, : 118 n-PrOH 1 AlCh : 178 n-PrOH 1 AlCh : 20 n-PrOH:4 1 AlCl3 : 20 n-PrOH: 1 AlCl3 : 20 n-PrOH 1 AlCh : 10 n-PrOH:
-9 -18 -25 -35 -42 -34 -34 -34 -34 -35 -35 -35 -35
Ccl, 8 Ccl, : 16 CCL 8 CC&
3.4 3.8 4.0 4.0 3.8 4.1 4.0 4.1 4.2 4.0 3.9 4.1 3.8
The interpretation
1
SHIFB,SOLVA~ON LINES INTENSITIES,AND CONCENTRATEDSOLUTIONSOFA~C~~INI~-PROPANOL
Ala+ solv. no. (n-PrOH)
at -25°C.
Line A
SOLVATION
shifts
(Hz)
~0~d~.-d voHbulY A
264 262 258 259 263 266 266 259 261 264 250
B
232 230 230 231 234 236 238 239 230 231 233 216
C
NUMBERDATA
FOR
Relative intensities of OH solvation PMR lines A B C
199 197 194 190
39 40
54 55
7 5
187 186 184 175
48 51 55 60
44 39 32 25
8 10 13 15
338
GRASDALEN
below -+lO”C. The OH solvation peaks are well separated from the bulk OH peak, 190-270 Hz downfield, and the integrated intensity ratio between them reaches a constant value at --20°C consistent with a solvation number 4 as shown in Table 1. This is also the case for solutions diluted with Ccl,+ Addition of CC& results in sharper and better resolved OCH2 signals and their relative intensities confirm a solvation number 4. As the OH and OCHz solvation shell peaks appear at the same time when the temperature is lowered, it is assumed that the exchange is predominantly whole ligand, rather than single proton, exchange.
The Bulk Signals The temperature dependence of the measured bulk OH PMR line width in n-propanol solutions of AICIJ is shown in Fig. 2. The behavior is characteristic of a system in which
loo-
00 ,
,
60 ,
,
40 ,
,
20 ,
0 ,
,
-20 ,
-40 1
I
-60 I
1
*C I
0.66
1
M AU3
P E
2.6
3.2
3.6
4.0 103/T
4.4
4.0
(“K-‘I
FIG. 2. The temperature dependence of the full line width at half-height for the bulk OH PMR line in n-propanol solutions of AlC&. Points represent measured values. The curves are just drawn by hand through the points.
two OH exchange processes with different exchange rates are present. The faster exchange process dominates the line width at T < +13”C. For higher temperatures the slower exchange influences the width. Regarding the chemical exchange, the temperature
SOLVATION
OF
AI(II1)
339
IN II-PROPANOL
variation of the bulk OH line shift given in Fig. 3 agrees completely with that of the line width. The observed downfield shift of the bulk OH peak at high temperatures are about 10% larger than the weighted average shift of the solvation shell and bulk peaks separately observable at low temperature. The solvation shell OH signal shows a slightly smaller upfield temperature shift than the bulk OH peak which shifts in a manner similar to the OH in pure solvent. Accordingly, the shift difference increases 60 IIIIIIII 60
60
40
20
0 I
-20
1
I
I
-40
I
I
-60
1
“C
I
.
50 -
0.66 \
3 I
M AU3
0 \
t
40-
z In G E 0’ : IL
30-
20-
10 -
-.-.-o-o-.
.
-A-ApA-A
A
0’ 2.6
3.2
4.0
3.6 103/T
4.4
4.6
(OK-‘)
FIG. 3. The temperature dependence of the line shift for the bulk OH PMR line in n-propanol solutions of AIQ. Points represent measured values. The curves are just drawn by hand through the points.
with temperature. As the OH line shift in alcohols is strongly influenced by hydrogen bonding, this supports our assumption that the ligand hydrogen bonds are stabilized within the complex and stay more complete and uniform than those between bulk propanol molecules at high temperatures. The shift observed for the bulk OCH2 protons at high temperature is only about l/l 5 of that for OH. No shift is seen for the CH2-CH, signals. Even at the lowest temperature, where the ligand exchange is slowed down, positive temperature-independent shifts are observed for the bulk peaks, most evident for the OH peak. This must be caused by polarization beyond the first solvation shell of the cations, polarization by anions, and structural effects.
340
GRASDALEN
The Solvation Shell Signals Figure 4 shows the temperature dependence of the three solvation shell OH PMR lines denoted by A, B, and C. The intermediate line B appears at a lower temperature than lines A and C, and must be associated with the faster exchange process. The shifts relative to the bulk OH line and the relative intensities listed in Table 1 are much the
0.66 M AU3
in
n-PrOH
N- 50 Hz +
NJ-
FIG. 4. The temperature dependence of the solvation shell OH PMR lines of a 0.66 M solution of AICls in n-propanol.
same as those reported earlier for ethanol solutions of AlCl, (I). Addition of CC& causes a marked reduction in the intermediate line intensity, while the two other lines are correspondingly increased as shown in Fig. 5 and Table 1. The assignment is based upon the expectation that the four ligand hydroxyl groups may form stabilizing hydrogen bonds within the complex. Scaled molecular models show that steric hindrance would be severe in a system with more than four hydrogen bonded n-PrOH ligands and would probably appear to limit the solvation number to a maximum of four. Recently Breivogel(9) suggested steric crowding around the larger Fe3+ ion in ethanol assuming
SOLVATION OF AI(II1) IN ?Z-PROPANOL
341
hexacoordination. Steric problems in DMF and DMSO are not comparable since the oxygen through which the solvent coordinates to the A13+ ion is bonded to only one atom in the solvent molecule. Our previously proposed complexes in ethanol solutions of AlC13 (I) also fit in with the results for AIC13 in n-propanol and n-butanol which show A13+- OCH,CH2CH3 ii T=- 35OC
J\
A
% I.-
50
5. The solvent composition solutions of AICI, at -35°C. FIG.
Mole
ratios
1:20:16
Hz --.I
HO----dependence of the solvation shell OH PMR lines in n-PrOH/CCl,
a similar solvation shell OH line pattern. In isobutanol, however, only one broad complexed OH line was observed. Here the branching leads to greater steric hindering effects and probably impedes the formation of complexes of the types in linear alcohols. It should be emphasized that the proposal of complex structures based on the present data are hypothetical. However, the data seem to indicate the presence of polymeric species in n-PrOH solutions of AlC&, and that hydrogen bonding may be a factor in stabilizing these polymers. Hydrogen bonding permits the mutually repelling Al)+ ions to be fairly far apart, and will cause little perturbation at the Al atoms. It seems reasonable to assume a rather weak polymer bonding and that the polymer forming ligands exchange between the solvation sites and the free solvent faster than do the rest of the
342
GRASDALEN
ligands. Hence, we assign the intermediate line B to the OH protons associated with the polymerization process. Dilution with CCL, reduces the possibility for polymer formation and leads to a weaker intermediate line as shown in Fig. 5. The measured shifts for the complexed OH lines relative to the bulk solvent OH lines are identical for AlC13 solutions of EtOH, n-PrOH, and n-BuOH to within the experimental error, while a slightly smaller shift is observed in the isobutanol solution. This indicates that the geometry of the tetrasolvated ligands probably is similar in the normal alcohols, while the additional steric requirements of isobutanol with the more screened OH groups probably reduces formation of hydrogen bonds within the complexes. Exchange Rates The bulk OH peak of solutions containing AlC13 exhibits no triplet structure down to -70°C which means that the OH proton exchange between bulk n-PrOH molecules goes faster than about 10 set-’ at this temperature. Possible triplet structure of the solvation shell peaks is masked by broadening effects. Very broad and poorly resolved solvation shell signal pattern prevents us from evaluating accurate ligand exchange rate parameters for AlC13 in n-propanol. However, since the complexed OH line pattern and the temperature behavior of the bulk OH line in the n-PrOH solutions so nearly coincide with that for EtOH solutions (I), it appears likely that the parameters for ligand exchange are of the same order of magnitude in the two systems. As already mentioned, two ligand exchange processes with different rate constants are simultaneously present in these systems. The values of the two rate constants we earlier derived for AlC13 in ethanol at 25°C are 5 x IO2 set-’ and 8 x IO3 set-‘, respectively. They are from two to four orders of magnitude less than the rate constants for solvent exchange at 25°C for Fe3” (9), Ni2+ (9), and Mg2+ (20) in ethanol. Qualitatively this agrees with corresponding observations for ligand exchange in water solutions (11-14). In general, the lifetimes of the solvated molecules increase with decrease in ionic size and with increase in ionic charge. CONCLUSIONS
In n-propanol solutions of AlC13 the A13+ ion is solvated by four solvent molecules. Comparisons of the solvation shell OH and OCH2 PMR signals support that ligand exchange is via whole n-propanol molecules. Two exchange processes between the solvation sites and the free solvent are simultaneously present. The results indicate the presence of polymeric species. ACKNOWLEDGMENTS The author is very grateful to Dr. I. Svare for his criticism of the manuscript and for valuable discussions on the subject. The financial support of The Norwegian Council for Science and the Humanities is greatly appreciated. REFERENCES 1. H. GRASDALEN, J. Magn. Resonance 5,84 (1971). 2. R. E. C~NNICK AND D. FIAT, J. Chem. Phys. 39,1349 (1963). 3. R.E.SCHU~RANDA.FRATIELLO, J. Chem.Phys.47(1967).
SOLVATION
4. 5. 6. 7. 8.
9. 10. II. 12. 13.
14.
OF
AI(II1)
IN EPROPANOL
S. THOMAS AND W. L. REYNOLDS, J. Chem. Phys. 44,314s (1966). W. G. MOVIUS AND N. A. MATWNOFF, Znorg. Chem. 6,847 (1967). A. FRATIELLO AND R. E. SCHUSTER, J. Phys. Chem. 71,1948 (1967). S. NAKAMURA AND S. MEIBOOM, Tenth International Conference on Coordination Nikko, Japan, p. 42,1967. H. HARAGUCHI AND S. FUIIWARA, J. Phys. Chem. 73,3467 (1969). F. W. BREIVOGEL, JR., J. Phys. Chem. 73,4203 (1969). T. D. ALGER, J. Amer. Chem. Sot. 91,222O (1969). D. FIAT AND R. E. CONNICK, J. Amer. Chem. Sot. 90,608 (1968). M. R. JUDKINS, Ph.D. Thesis, University of California, Berkeley, CA, 1967. T. J. SWIFT AND R. E. CONNICK, J. Chem. Phys. 37, (1962). M. EIGEN, Pure Appl. Chem. 6,97 (1963).
343
Chemistry,
OF MAGNETIC
RESONANCE
6,336-343
(1972)
Proton Magnetic Resonance Study of the Solvation of Aluminum (III) in n-Propanol H. GRASDALEN Institute of Physical Chemistry, The Technical University of Norway, Trondheim, Norway Presented at the Fourth International Symposium on Magnetic Resonance, Israel, August, 1971 Separate OH and OCH2 PMR signals for free and bound propanol molecules have been observed for solutions of AICY3 in n-propanol at reduced temperatures. Qualitatively we find the same results for AlC& in n-propanol as we have earlier found in a similar study of AlCI, in ethanol (I). The solvation number of the A13+ ion is 4 from relative intensity measurements of the OH signals below -20°C. These results contrast with the solvation number 6 for Al’+ in H20, DMSO, DMF, and MeOH. The solvation shell OH signal consists of three lines which differ in their temperature behavior. Two n-propanol ligand exchange processes with different exchange rate constants are present. This is also reflected in the line width and line shift temperature dependence of the bulk OH peak. INTRODUCTION Al(II1) has been shown by NMR to have solvation number 6 in water (2,3), dimethyl sulfoxide (4), dimethylformamide (5,6), and methanol (7). In ethanol, however, we very recently found that A13+ is tetrasolvated (I). In the present study of AlC13 in n-propanol, and in less extensive measurements on n-butanol and isobutanol solutions of AlCls, we also find an A13+ solvation number 4. Hence, with the exception of MeOH, tetrasolvation seems to be typical of Al(III)-alcohol systems. Solubility problems restrict PMR studies to In EtOH and is quite narrow, (8). This implies
the lowest
aliphatic
alcohols.
n-PrOH solutions of AIC13 a single Al-27 NMR line is observed which -70 Hz, and the width is mainly determined by quadrupole interaction a high local symmetry
ion is either tetrahedrally
on the Al nucleus
site and suggests that the A13+
or octahedrally coordinated with the ligand oxygen atoms
with no competition from the Cl- ions for solvation shell sites. The solvation four confirms tetracoordination of A13+ in EtOH and n-PrOH solutions.
number
EXPERIMENTAL
The technique and precision of the PMR measurements at 60 MHz and preparation of the samples were the same as described for ethanol solution (I). Some Al-27 NMR measurements were made at 15 MHz using an AEI RS2 high resolution spectrometer. The salt used was anhydrous Merck zur Analyse. The n-propanol was >99 % Merck zur Analyse containing -0.03 M HzO. Salt and solvent were used as received. 0 1972 by Academic
Press, Inc.
336
SOLVATION
OF
RESULTS
Al(II1) AND
337
IN wPROPANOL
DISCUSSIONS
Figure 1 shows the PMR spectrum of a 0.44 M solution of AlCl, in n-propanol at -25°C. Due to higher viscosity the lines are broader than those in ethanol solution under the same conditions. Separate OH and OCH2 solvation shell signals are observed 0.44
M AICI,
in n-PrOH
-25"~
C@H&
CH,CH,CH,Oc
I CH&CH,Ot
A13’C$H,CH,CH, TMS
/12__ J
I 600
I 400
I 200
\
I 0
Sweep (Hz1 FIG. 1. The PMR spectrum of a 0.44 Msolution of the different peaks is indicated by underlining
of AK& in n-propanol the protons involved. TABLE
PMR
CHEMICAL
Solutions (mole ratios)
Temp. (“C)
1 AlCl, : 20 n-PrOH 1 AlCl, : 20 n-PrOH 1 AlCl, : 20 n-PrOH 1 AlCl, : 20 n-PrOH 1 AlCh : 20 n-PrOH 1 AICla : 40 n-PrOH 1 AlCl, : 59 n-PrOH 1 AlCl, : 118 n-PrOH 1 AlCh : 178 n-PrOH 1 AlCh : 20 n-PrOH:4 1 AlCl3 : 20 n-PrOH: 1 AlCl3 : 20 n-PrOH 1 AlCh : 10 n-PrOH:
-9 -18 -25 -35 -42 -34 -34 -34 -34 -35 -35 -35 -35
Ccl, 8 Ccl, : 16 CCL 8 CC&
3.4 3.8 4.0 4.0 3.8 4.1 4.0 4.1 4.2 4.0 3.9 4.1 3.8
The interpretation
1
SHIFB,SOLVA~ON LINES INTENSITIES,AND CONCENTRATEDSOLUTIONSOFA~C~~INI~-PROPANOL
Ala+ solv. no. (n-PrOH)
at -25°C.
Line A
SOLVATION
shifts
(Hz)
~0~d~.-d voHbulY A
264 262 258 259 263 266 266 259 261 264 250
B
232 230 230 231 234 236 238 239 230 231 233 216
C
NUMBERDATA
FOR
Relative intensities of OH solvation PMR lines A B C
199 197 194 190
39 40
54 55
7 5
187 186 184 175
48 51 55 60
44 39 32 25
8 10 13 15
338
GRASDALEN
below -+lO”C. The OH solvation peaks are well separated from the bulk OH peak, 190-270 Hz downfield, and the integrated intensity ratio between them reaches a constant value at --20°C consistent with a solvation number 4 as shown in Table 1. This is also the case for solutions diluted with Ccl,+ Addition of CC& results in sharper and better resolved OCH2 signals and their relative intensities confirm a solvation number 4. As the OH and OCHz solvation shell peaks appear at the same time when the temperature is lowered, it is assumed that the exchange is predominantly whole ligand, rather than single proton, exchange.
The Bulk Signals The temperature dependence of the measured bulk OH PMR line width in n-propanol solutions of AICIJ is shown in Fig. 2. The behavior is characteristic of a system in which
loo-
00 ,
,
60 ,
,
40 ,
,
20 ,
0 ,
,
-20 ,
-40 1
I
-60 I
1
*C I
0.66
1
M AU3
P E
2.6
3.2
3.6
4.0 103/T
4.4
4.0
(“K-‘I
FIG. 2. The temperature dependence of the full line width at half-height for the bulk OH PMR line in n-propanol solutions of AlC&. Points represent measured values. The curves are just drawn by hand through the points.
two OH exchange processes with different exchange rates are present. The faster exchange process dominates the line width at T < +13”C. For higher temperatures the slower exchange influences the width. Regarding the chemical exchange, the temperature
SOLVATION
OF
AI(II1)
339
IN II-PROPANOL
variation of the bulk OH line shift given in Fig. 3 agrees completely with that of the line width. The observed downfield shift of the bulk OH peak at high temperatures are about 10% larger than the weighted average shift of the solvation shell and bulk peaks separately observable at low temperature. The solvation shell OH signal shows a slightly smaller upfield temperature shift than the bulk OH peak which shifts in a manner similar to the OH in pure solvent. Accordingly, the shift difference increases 60 IIIIIIII 60
60
40
20
0 I
-20
1
I
I
-40
I
I
-60
1
“C
I
.
50 -
0.66 \
3 I
M AU3
0 \
t
40-
z In G E 0’ : IL
30-
20-
10 -
-.-.-o-o-.
.
-A-ApA-A
A
0’ 2.6
3.2
4.0
3.6 103/T
4.4
4.6
(OK-‘)
FIG. 3. The temperature dependence of the line shift for the bulk OH PMR line in n-propanol solutions of AIQ. Points represent measured values. The curves are just drawn by hand through the points.
with temperature. As the OH line shift in alcohols is strongly influenced by hydrogen bonding, this supports our assumption that the ligand hydrogen bonds are stabilized within the complex and stay more complete and uniform than those between bulk propanol molecules at high temperatures. The shift observed for the bulk OCH2 protons at high temperature is only about l/l 5 of that for OH. No shift is seen for the CH2-CH, signals. Even at the lowest temperature, where the ligand exchange is slowed down, positive temperature-independent shifts are observed for the bulk peaks, most evident for the OH peak. This must be caused by polarization beyond the first solvation shell of the cations, polarization by anions, and structural effects.
340
GRASDALEN
The Solvation Shell Signals Figure 4 shows the temperature dependence of the three solvation shell OH PMR lines denoted by A, B, and C. The intermediate line B appears at a lower temperature than lines A and C, and must be associated with the faster exchange process. The shifts relative to the bulk OH line and the relative intensities listed in Table 1 are much the
0.66 M AU3
in
n-PrOH
N- 50 Hz +
NJ-
FIG. 4. The temperature dependence of the solvation shell OH PMR lines of a 0.66 M solution of AICls in n-propanol.
same as those reported earlier for ethanol solutions of AlCl, (I). Addition of CC& causes a marked reduction in the intermediate line intensity, while the two other lines are correspondingly increased as shown in Fig. 5 and Table 1. The assignment is based upon the expectation that the four ligand hydroxyl groups may form stabilizing hydrogen bonds within the complex. Scaled molecular models show that steric hindrance would be severe in a system with more than four hydrogen bonded n-PrOH ligands and would probably appear to limit the solvation number to a maximum of four. Recently Breivogel(9) suggested steric crowding around the larger Fe3+ ion in ethanol assuming
SOLVATION OF AI(II1) IN ?Z-PROPANOL
341
hexacoordination. Steric problems in DMF and DMSO are not comparable since the oxygen through which the solvent coordinates to the A13+ ion is bonded to only one atom in the solvent molecule. Our previously proposed complexes in ethanol solutions of AlC13 (I) also fit in with the results for AIC13 in n-propanol and n-butanol which show A13+- OCH,CH2CH3 ii T=- 35OC
J\
A
% I.-
50
5. The solvent composition solutions of AICI, at -35°C. FIG.
Mole
ratios
1:20:16
Hz --.I
HO----dependence of the solvation shell OH PMR lines in n-PrOH/CCl,
a similar solvation shell OH line pattern. In isobutanol, however, only one broad complexed OH line was observed. Here the branching leads to greater steric hindering effects and probably impedes the formation of complexes of the types in linear alcohols. It should be emphasized that the proposal of complex structures based on the present data are hypothetical. However, the data seem to indicate the presence of polymeric species in n-PrOH solutions of AlC&, and that hydrogen bonding may be a factor in stabilizing these polymers. Hydrogen bonding permits the mutually repelling Al)+ ions to be fairly far apart, and will cause little perturbation at the Al atoms. It seems reasonable to assume a rather weak polymer bonding and that the polymer forming ligands exchange between the solvation sites and the free solvent faster than do the rest of the
342
GRASDALEN
ligands. Hence, we assign the intermediate line B to the OH protons associated with the polymerization process. Dilution with CCL, reduces the possibility for polymer formation and leads to a weaker intermediate line as shown in Fig. 5. The measured shifts for the complexed OH lines relative to the bulk solvent OH lines are identical for AlC13 solutions of EtOH, n-PrOH, and n-BuOH to within the experimental error, while a slightly smaller shift is observed in the isobutanol solution. This indicates that the geometry of the tetrasolvated ligands probably is similar in the normal alcohols, while the additional steric requirements of isobutanol with the more screened OH groups probably reduces formation of hydrogen bonds within the complexes. Exchange Rates The bulk OH peak of solutions containing AlC13 exhibits no triplet structure down to -70°C which means that the OH proton exchange between bulk n-PrOH molecules goes faster than about 10 set-’ at this temperature. Possible triplet structure of the solvation shell peaks is masked by broadening effects. Very broad and poorly resolved solvation shell signal pattern prevents us from evaluating accurate ligand exchange rate parameters for AlC13 in n-propanol. However, since the complexed OH line pattern and the temperature behavior of the bulk OH line in the n-PrOH solutions so nearly coincide with that for EtOH solutions (I), it appears likely that the parameters for ligand exchange are of the same order of magnitude in the two systems. As already mentioned, two ligand exchange processes with different rate constants are simultaneously present in these systems. The values of the two rate constants we earlier derived for AlC13 in ethanol at 25°C are 5 x IO2 set-’ and 8 x IO3 set-‘, respectively. They are from two to four orders of magnitude less than the rate constants for solvent exchange at 25°C for Fe3” (9), Ni2+ (9), and Mg2+ (20) in ethanol. Qualitatively this agrees with corresponding observations for ligand exchange in water solutions (11-14). In general, the lifetimes of the solvated molecules increase with decrease in ionic size and with increase in ionic charge. CONCLUSIONS
In n-propanol solutions of AlC13 the A13+ ion is solvated by four solvent molecules. Comparisons of the solvation shell OH and OCH2 PMR signals support that ligand exchange is via whole n-propanol molecules. Two exchange processes between the solvation sites and the free solvent are simultaneously present. The results indicate the presence of polymeric species. ACKNOWLEDGMENTS The author is very grateful to Dr. I. Svare for his criticism of the manuscript and for valuable discussions on the subject. The financial support of The Norwegian Council for Science and the Humanities is greatly appreciated. REFERENCES 1. H. GRASDALEN, J. Magn. Resonance 5,84 (1971). 2. R. E. C~NNICK AND D. FIAT, J. Chem. Phys. 39,1349 (1963). 3. R.E.SCHU~RANDA.FRATIELLO, J. Chem.Phys.47(1967).
SOLVATION
4. 5. 6. 7. 8.
9. 10. II. 12. 13.
14.
OF
AI(II1)
IN EPROPANOL
S. THOMAS AND W. L. REYNOLDS, J. Chem. Phys. 44,314s (1966). W. G. MOVIUS AND N. A. MATWNOFF, Znorg. Chem. 6,847 (1967). A. FRATIELLO AND R. E. SCHUSTER, J. Phys. Chem. 71,1948 (1967). S. NAKAMURA AND S. MEIBOOM, Tenth International Conference on Coordination Nikko, Japan, p. 42,1967. H. HARAGUCHI AND S. FUIIWARA, J. Phys. Chem. 73,3467 (1969). F. W. BREIVOGEL, JR., J. Phys. Chem. 73,4203 (1969). T. D. ALGER, J. Amer. Chem. Sot. 91,222O (1969). D. FIAT AND R. E. CONNICK, J. Amer. Chem. Sot. 90,608 (1968). M. R. JUDKINS, Ph.D. Thesis, University of California, Berkeley, CA, 1967. T. J. SWIFT AND R. E. CONNICK, J. Chem. Phys. 37, (1962). M. EIGEN, Pure Appl. Chem. 6,97 (1963).
343
Chemistry,