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Spectroscopic studies of ionic solvation—XVII. Studies of preferential solvation of the sodium ion in nonaqueous mixed solvents by sodium-23 nuclear magnetic resonance
Spectrcchimica Acta,Voi.31A,pp.697 to705.Pergamon Preen1976.Printed inNorthern Ireland
Spectroscopicstudies of ionic salvation-XVII. Studies of preferential solvation of the sodium ion in nonaqueous mixed solvents by sodium-23 nuclear magnetic resonance M. S. GREENBERG Department
and A. I. POPOV*
of Chemistry, Michigan State University, Michigan 49824, U.S.A.
East Lansing,
(Received 10 June 1974) Abstract-Preferential solvation of the Na+ ion was studied by determining the 2SNa chemical shifts for sodium tetraphenylborate solutions in all possible binary solvent mixtures of nitromethane, acetonitrile, hexamethylphosphoramide, dimethylsulfoxide,pyridineandtetramethylurea. Generally, these studies reflected the relative donicity of each solvent in a given solvent pair where the solvent of higher donicity was preferentially contained in the inner solvation shell of the Na+ ion. The enhanced donicity of tetramethylurea and dimethylsulfoxide in binary mixtures was rationalized in terms of strong solvent-solvent dipole interactions which disrupt, the structure of these solvents. Finally, the results are treated quantitatively to yield a geometric equilibrium constant, K*/“, and the free energy of preferential solvation, AGr .s.In, as outlined by Covington.
INTRODUCTION
metal nuclear magnetic resonance (NMR), and particularly 23Na NMR, have been shown to be a very useful technique for the study of alkali salts in a variety of media [l-4]. The magnitude and direction of the 23Na chemical shifts in various solvents have been related either to the Lewis basicity [Z] of the solvents or to their donor (or solvating) abilities [4]. These studies were extended to binary solvent mixtures where the variation of the 2SNa chemical shift with solvent composition was explained in terms of preferential solvation of the Na+ ion by one of the solvents in the mixture [2, 51 as indicated by the isosolvation point [6]. Generally, the location of the isosolvation point in a given mixture reflected preferential solvation of the Na+ ion by the solvent of greater donicity in that mixture. The isosolvation point has been defined as the chemical shift corresponding to the midpoint between the respective chemical shifts of the studied solvated species in neat solvents A and B. It has been postulated [6] that it corresponds to a solvent composition at which there is equal occupancy of the inner cationic solvation shell by the two solvents of the binary mixture. The purpose of this investigation is to extend our earlier study of preferential solvation of the Na+ ion [5] t,o include mixtures of solvents possessing high, medium ALKALI
[I] C. DEVESELL and R. E. RICHARDS, Mol. Phy8.10,561 (1960). [2] E. G. BLOOR and R. G. KIDD, Can. J. Chem. 46, 3425 (1968). [3] (a) G. J. TEMPLE-N and A. L. VAN GEET, J. Amer. Chem. Sot. 94,657s (1972), (b) A. L. VAN GEET, J. Amer. Chem. Sot. 94, 5583 (1972). [4] M. S. GREENBERG, R. L. BODNER and A. I. POPOV, J. Phye. Chem. 77, 2449 (1973) and references listed therein. [5] R. H. ERLIOH, M. S. GREENBERU and A. I. POPOV, S~ectrochint. Aotu 29A,1927 (1973). (61 (a) L. S. FRIWSEL, T. R. STENULE and C. H. LANOFORD, Chem. Commun. 1965,393; (b) L. S. FBAN~EL, C. H. LAN~FORD and T. R. STENQLE, J. Phye. Chem. 74,1376 (1970). 897
M.
698
S. GREENBERU and A. I. POPOV
and low donicity so as to more fully observe the effect of solvent donicity on the isosolvation point., In recent years GUTMANN and WYCHERA [7] proposed an empirical scale of solvent donor (or solvating) ability. The scale is based on the enthalpy of the reaction S + SbC1, + S*SbCl, in dilute 1,2-dichloroethane solution. The donor number (or the donicity) of the solvent S is defined as D.N.(s)
=
-
AH,.,,,,,
We have previously shown that there is a linear relationship between the solvent donor numbers and the 23Na relative chemical shift in these solvents [4]. EXPERIMENTAL Sodium tetraphenylborate (Baker) was of reagent grade quality and was used without further purification except for drying in VUCZGO at 50°C for 72 hr. After drying, the salt was stored in a vacuum desiccator charged with granulated barium oxide. Hexamethylphosphoramide (HMPA) was vacuum distilled over barium oxide at 20 Torr and 127°C. Acetonitrile (AN) was refluxed over calcium hydride for 48 hr followed by fractional distillation at Sl*S”C in a nitrogen atmosphere. Nitromethane (NM) was percolated through a column of alumina and vacuum distilled over BaO. Dimethylsulfoxide (DMSO) was dried over Linde 4A molecular sieves and then vacuum distilled over freshly activated sieves at 50°C. Pyridine (PY) was refluxed over granular BaO for 24 hr followed by vacuum distillation at 112°C and 60 Torr. Tetramethylurea (TMU) was vacuum distilled over BaO at 38-40%. In all cases, only the middle 60% fraction of solvent was collected. Each mixed solvent solution was prepared by taring a snap-cap vial, adding the desired volume of solvent A, weighing, adding the desired volume of solvent B and weighing again. The mole fraction of each solvent in the solution was then calculated from the known weights. Approximately 0.12 g of sodium tetraphenylborate was weighed into a 1 ml volumetric flask. The desired solvent or solvent mixture was then added up to the mark (N 0.50 M Na+). Sodium-23 NMR studies were performed on a highly modified NMRS-MP-1000 spectrometer operating at 60 MHz at a field strength of 53 kG. The experimental details are outlined in a previous publication [4]. The Wilmad 506-PP, 5 mm OD polished NMR sample tube was fitted with a Wilmad precision coaxial 520-2 NMR tube for the reference solution. The reference for the 23Na measurements however, when the chemical shifts was 3.0 M aqueous sodium chloride solution; were so small that the sample resonance was masked by the reference, a secondary reference of 2.5 M sodium perchlorate in methanol was used. In the latter case, the shifts were corrected so as to apply to the 3.0 M aqueous sodium chloride reference solution. A positive shift from the reference is upfield. The chemical shifts reported are corrected for differences in bulk diamagnetic susceptibility between sample and reference according to the relationship of LIVE [7] (a) V. GUTMANN and E. WYCHERA, Inorg. Nucl. Chew. Lett. 2,257 (1966); (b) V. GTJTMANN, “Coordination
Chemktry
in Nonaqzceowr Solvents”, Springer-Verlag,
Vienna (1968).
699
Spectroscopic studies of ionic solvation-XVII
and CHAN for high-field
spectrometers 6corr --
Bobs -
[S] 47T/3 (x’v”’ -
xFrn”)
(1)
We assume the contribution of the salt to the susceptibility of the solution to be negligible-a reasonable assumption as shown by TEMPLEMAN and VAN GEET [3a]. Hence, the corrections simply reflect differences between the aqueous reference and the mixed organic sample. By employing Wiedmann’s Law, we may calculate the diamagnetic susceptibility of a given mixture according to the relationship &?: where: ~22 V* VI3 XA
XB
= v,
= volume = volume = volume = volume = volume
V.4 + VB
V, * XA
+
v,
+
v,
(2)
- XB
susceptibility of the solution; (ml) of solvent A; (ml) of solvent B; diamagnetic susceptibility of pure solvent A; diamagnetic susceptibility of pure solvent B.
RESULTS AND DISCUSSION The chemical shifts* of the BSNa nucleus as a function of solvent composition in binary solvent mixtures are illustrated in Figs. l-4. The systems studied in this investigation were, nitromethane with dimethylsulfoxide, pyridine, tetramethylurea, hexamethylphosphoramide and acetonitrile; acetonitrile with pyridine,, dimethylsulfoxide, tetramethylurea and hexamethylphosphoramide; tetramethylurea with pyridine and dimethylsulfoxide; hexamethylphosphoramide with tetramethylurea, dimethylsulfoxide and pyridine, and dimethylsulfoxide with pyridine. In all cases sodium tetraphenylborate was used as the solute. We note a smooth transition of the 23Na resonance as a function of solvent composition as we proceed from one pure solvent to the other. The fact that in these plots we note curves and not straight lines indicates preferential solvation by one of the solvents. The isosolvation points for these solvent systems are summarized in Table 1. It is well known that nitromethane is a solvent of weak donicity as reflected by its donor number of 2.7. As a result, it has been employed as an “inert” medium to study the complexation of Naf and Li+ with biologically active molecules [9]. The data in Table 1 and Fig. 1 seem to justify using nitromethane as an “inert” solvent. Binary solvent mixtures of nitromethane with dimethylsulfoxide, pyridine, tetramethylurea, hexamethylphosphoramide and acetonitrile exhibit isosolvation points of 0.05, 0.12, 0.06, 0.05 and 0.15 mole fraction of the latter respectively (Fig. 1). These data imply that Na+ is, to a large degree, preferentially solvated by these solvents relative to the nitromethane. Hence, compared to * Tabulated A. I. P.
s3Na chemical shift data in these solvent mixtures are available upon writing to
[8] D. H. LJXE and S. I. CHAN, And. Chem. 42,791 (1970). [9] (a) R. L. BODNER, M. S. GREENBERU and A. I. POPOV, h’pectry. Lett. 5, (b) Y. CAHEN, R. F. BIESEL and A. I. POPOV, J. Imorg. NwZ. &em. (in press). 13
489 (1972);
700
M. S. GREENBERG
41 o,o
0.2
Mole
and A. I. POPOV
0.4 fraction
@6
0.6
I.0
solvent
Fig. 1. Variation of the chemical shift of the ssNa resonance as a function of solvent compositionfor binary solvent mixtures of nitromethsnewith acetonitrile (A), tetramethylurea (o), dimethylsulfoxide (m), pyridine (0) and hexamethylphosphoramide (0).
6
-4 0.0
0.2
0.4
Mole fraction
0.6
0.6
I.0
solvent
Fig. 2. Variation of the chemical shift of the 23Na resonance as a function of solvent compositionfor binary solvent mixtures of acetonitrile with tetramethylurea (m), dimethylsulfoxide ( l), pyridine (A) and hexamethylphosphoramide (0).
Spectroscopicstudies of ionic solvation-XVII
Mole fraction
701
solvent
Fig. 3. Variation of the chemical shift of the 23Na resonance aa a function of solvent compositionfor binary solvent mixtures of hexamethylphosphoramidewith tetramethylurea (e), dimethylsulfoxide (m) and pyridine (A).
nitromethane, the relative order of donor ability is HMPA ‘v DMSO N TMXJ > PY > AN >> NM which, with the exception of pyridine, is the order of the solvent donor numbers. With a donor number of 33.0, however, pyridine should be a stronger donor than DMSO (DN = 29.8) and TMU (DN = 28.9) but weaker than HMPA (DN = 38.0). It is interesting to note that HMPA, DMSO and TMU
Mole fraction solvent Fig. 4. Variation of the chemical shift of the %Na resonance as a function of solvent composition for binary solvent mixtures of tetramethylurea with dimethylsulfoxide (A) and pyridine (0).
M. S. GREENBERG
702
Table 1. Summary Binary solvent system DMSO : NM PY:NM TMU:NM HMPA : NM AN:NM PY:AN DMSO : AN TMU : AN HMPA: AN TMU : HMPA DMSO : HMPA PY : HMPA PY : TMU DMSO : TMU DMSO : PY
Isosolvation 0.05 0.12 0.06 0.05 0.15 0.29 0.10 0.11 0.06 0.23 0.15 0.10 0.16 0.39 0.10
MF MF MF MF MF MF MF MF Ml? MF MF MF MF MF MF
and
A. I. POPOV
of preferential solvation data
point
DMSO PY TMU HMPA AN PY DMSO TMU HMPA HMPA HMPA HMPA TMU DMSO DMSO
@In 14.4 7.44 33.8 13.2 4.48 2.93 12.7 7.01 62.9 0.067 0.166 0.125 0.297 0.92 17.2
-
AGO,.,./n (kJ mole-l)
-
6.61 4.97 8.72 6.39 3.72 2.66 6.30 4.82 10.3 6.70 4.45 5.16 3.01 0.206 7.04
seem to exhibit the same relative donor abilities in nitromethane although the donicity of HMPA is greater than that of DMSO or TMU. Hence the donicity of these solvents in nitromethane is “leveled”. Acetonitrile is a solvent of medium donicity (DN = 15.0) and as such should be more competitive with the preceding solvent series for Na+ ion than was Binary solvent mixtures of acetonitrile with pyridine, dimethylnitromethane. sulfoxide, tetramethylurea, hexamethylphosphoramide and nitromethane exhibit isosolvation points of 0.29, 0.10, 0.11, 0.06 and 0.85 mole fraction of the latter respectively (Fig. 2). Again, the strong donor solvents HMPA, DMSO and TMU are “leveled” but we now see an anomalous position of pyridine. In acetonitrile, the relative order of donor ability is HMPA N DMSO r: TMU > PY > AN > NM. The “repression” of the donicity of pyridine in binary mixed solvents was The data seem to indicate that DMSO and TMU are better solvating surprising. agents for the Na+ ion than pyridine, which is contrary to the 23Na NMR chemical shift data [4] and to the solvents’ donor numbers. To investigate further this unexpected donor repression of pyridine relative to other high donor solvents, solvent mixtures of pyridine with hexamethylphosphoramide, tetramethylurea and dimethylsulfoxide were examined. The 23Na NMR data shown in Figs. 3 and 4 revealed isosolvation points of 0.10, 0.16 and 0.10 mole fraction of the latter indicative of strong preferential solvation by that solvent. Recalling the donor numbers of 38.0, 29.8, 28.9 and 33.0 for HMPA, DMSO, TMU and pyridine respectively, we would expect preferential solvation of the Na+ ion by HMPA with respect to pyridine, but very little or no preferential solvation of the Na+ ion by DMSO or TMU relative to pyridine since these three solvents have similar donor numbers. Instead, strong preferential solvation of the Na+ ion is noted for DMSO and TMU versus PP.
Spectroscopic studies of ionic eolvation-XVII
703
The mixed solvent system, DMSO-PY, was examined in greater detail using vibrational spectroscopy [5] and Brillouin scattering [lo] as previously reported. The data suggest that while pure DMSO is a highly associated solvent due to dipolar interactions through the S-O bond, when mixed with pyridine (p = 2.20), the structure of DMSO is severely altered. Now, it seems reasonable to assume that when the sodium salt is introduced into neat DMSO, a considerable amount of energy must be expended to break up the structure of DMSO before solvation of the cation can occur. Hence, the introduction of even small amounts of pyridine into neat DMSO results in the break up of the polymeric structure of the latter via a dipole interaction, resulting in an enhancement of donicity of DMSO in mixtures where this type of interaction occurs. It seems that the enhanced donicity of TMU relative to pyridine may also be rationalized in terms of structure breaking by pyridine. It has been pointed out [ll] that TMU is a structured solvent due to strong dipole-dipole interactions. The situation, therefore, is similar to that in liquid DMSO. BE~UIN and GUNTHARD [l2] showed the C = 0 stretching frequency to be strongly solvent dependent as is the S-O stretching frequency in DMSO [13]. Hence, a strong TMU-PY dipole-dipole interaction may account for the enhanced donioity of TMU by causing extensive disruption of TMU structure by pyridine. Finally, we wished to differentiate the three solvents that were effectively leveled in nitromethane and acetonitrile. For binary solvent mixtures of hexamethylphosphoramide with tetramethylurea and dimethylsulfoxide, the isosolvation points were 0.23 and 0.15 mole fraction HMPA. An isosolvation point of 0.39 mole fraction DMSO was noted for the dimethylsulfoxide-tetramethylurea system (Fig. 4). These data imply that the relative order of solvating ability is HMPA > DMSO > TMU where DMSO is just slightly stronger than TMU. To this point preferential solvation has been discussed in a qualitative sense as reflected by the isosolvation point. Recently, COVINGTONet al. [ 141 developed a quantitative model for competitive solvation. In the above papers, they present a derivation of preferential solvation that allows the calculation of equilibrium constants and the changes in free energy as the solvation shell of an ion X is progressively changed from n molecules of solvent W to n molecules of solvent P in an isodielectric solvent system. They consider initially ion X is solvated by four molecules of the solvent W. As the second solvent P is introduced into the system, there is a step-wise replacement of W by P in the inner solvation shell. The series of equations can be
[lo]
J. B. KINSINQER, M. M. TANNAHILL, M. S. GREENBERU and A. I. POPOV, J. Phys. Chem.
2444 (1973).
77,
[Ill A. LUTTRINQHATJS and H. W. DIRKSEN, Agnew Chem. Intern. Ed. 3, 260 (1969). [12] C. BECUIN and H. H. GUNTHARD, HeZw. Chim. Acta 42, 2262 (1969). [13] H. H. SZD~ANT,in “Dimethylaulfoxide”, pp. 1-98, Edited by S. W. JACOB, E. E. ROSENBAUM and D. C. WOOD, Marcel-Dekker, New York, N.Y. (1971). [14] (a) A. K. COVINQTON, T. H. LILLEY, K. E. NEWMAN and G. A. POETHOUSE, J. Chem Sot. Faraday I 69, 963 (1973); (b) A. K. COVINQTON, K. E. NEWMAN and T. H. LILLEY, J. Chem. Sot. Faraday I, 69, 973 (1973).
704
M. S. GREENBERU
expressed
and A. I. POPOV
as, + PP + (w-4)W 2
XW, XW,P,
+ (p4P
+ (p-l)P
XW,P
+ (w-2)W 2 XP,
XWP, + (p-4)P
+ (w-3)W 2
+ (p-3)P
+ (w-1)W 3
+ WW
(3)
In addition to requiring that the solvent mixture is isodielectric and exhibits ideal behavior, Covington assumes that as P replaces W, the observed chemical shift is additive. If BP is the shift in the resonance of X from pure W to pure P, then dxw, = 6; ~XW,P = &fir; dxwtdrI = 38~; ~XWP~ = BP; ~XP, = 8~. Hence, the intrinsic shifts of the various solvated species are proportional to the amount of P which they contain. Another assumption requires that the individual equilibrium constants, KS, are related solely by statistics, that is, when n = 4,
K’ = K1” = (K,K,K,K4)“4 K, = 4K’;
K, = $ K’;
K, = 8 K’;
(4) K, = 2 K’
The final equation in this treatment allows calculation
(5)
of K”” as follows:
(‘3) 6 = observed chemical shift relative to the resonance of X in pure W; BP = the total range of the chemical shifts (i.e. dxw, - dxp,); Kl/” = the geometric equilibrium constant; n = the solvation number; up, aw = the activities of solvents P and W, respectively. By plotting l/S vs. xw/xp, they obtain l/S, from the intercept and Kiln from the slope. It should be noted that in order to substitute mole fractions for activities of solvents, the solution must be ideal. Finally, they calculate the free energy of preferential solvation, AGopJn, as follows:
where :
AG”,.,Jn
= -
RT In KlJn
(7)
We treated our data according to eqns. (6) and (7) and calculated the geometric free energy of preferential equilibrium constant, K”“, and the corresponding solvation, AGo,&. Th e results are shown in Table 1. A plot of l/S vs. zB/xA did in fact yield straight lines in each case, except for the solvent system acetonitrilepyridine, which displayed a small degree of curvature. While studying preferential salvation of the Co3+ ion in binary mixtures of chloroform with acetone and p-dioxane, by 5eCo NMR, FRANKEL etal.[6b],in a, treatment very similar to that of Covington, noted curves in plots of solvation isotherms, not straight lines as predicted. They ascribed this curvature to either a weak solvent-solvent interaction or to nonequal solvation numbers of Co3f in these solvents.
Spectroscopic studies of ionic solvation-XVII
705
0.6.
05'
o.4L
IO ‘pyridine
’ xLXSO
Fig. 6. Covington plot for the binary solvent system dimethylsulfoxid4-pyridine.
The plots were treated by a linear least squares procedure from which we the values of K”“. It is particularly interesting to note that, such a plot for the data in DMSO-pyridine mixtures, where there exists marked solventsolvent interactions, yields a straight line as shown in Fig. 5. The location of the isosolvation point for a given binary solvent system gives a qualitative measure of the salvation abilities of the solvents in the given mixture. In order to extend the comparison to variations in isosolvation values to a series of binary mixtures, more knowledge is needed on the structures of the individual solvents and the changes in this structure on formation of liquid mixtures. These results emphasize the danger of extrapolating the behavior and properties of pure solvents to their properties in solvent mixtures. Covington’s quantitative approach appears to be successful despite the large number of idealized assumptions, some of which may be debatable. However, we agree with his statement that this treatment, while not rigorous, is helpful in understanding preferential solvation phenomenon. obtained
AcknowledgementThe authors gratefully acknowledge support of this work by a grant from the National Science Foundation.
Spectroscopicstudies of ionic salvation-XVII. Studies of preferential solvation of the sodium ion in nonaqueous mixed solvents by sodium-23 nuclear magnetic resonance M. S. GREENBERG Department
and A. I. POPOV*
of Chemistry, Michigan State University, Michigan 49824, U.S.A.
East Lansing,
(Received 10 June 1974) Abstract-Preferential solvation of the Na+ ion was studied by determining the 2SNa chemical shifts for sodium tetraphenylborate solutions in all possible binary solvent mixtures of nitromethane, acetonitrile, hexamethylphosphoramide, dimethylsulfoxide,pyridineandtetramethylurea. Generally, these studies reflected the relative donicity of each solvent in a given solvent pair where the solvent of higher donicity was preferentially contained in the inner solvation shell of the Na+ ion. The enhanced donicity of tetramethylurea and dimethylsulfoxide in binary mixtures was rationalized in terms of strong solvent-solvent dipole interactions which disrupt, the structure of these solvents. Finally, the results are treated quantitatively to yield a geometric equilibrium constant, K*/“, and the free energy of preferential solvation, AGr .s.In, as outlined by Covington.
INTRODUCTION
metal nuclear magnetic resonance (NMR), and particularly 23Na NMR, have been shown to be a very useful technique for the study of alkali salts in a variety of media [l-4]. The magnitude and direction of the 23Na chemical shifts in various solvents have been related either to the Lewis basicity [Z] of the solvents or to their donor (or solvating) abilities [4]. These studies were extended to binary solvent mixtures where the variation of the 2SNa chemical shift with solvent composition was explained in terms of preferential solvation of the Na+ ion by one of the solvents in the mixture [2, 51 as indicated by the isosolvation point [6]. Generally, the location of the isosolvation point in a given mixture reflected preferential solvation of the Na+ ion by the solvent of greater donicity in that mixture. The isosolvation point has been defined as the chemical shift corresponding to the midpoint between the respective chemical shifts of the studied solvated species in neat solvents A and B. It has been postulated [6] that it corresponds to a solvent composition at which there is equal occupancy of the inner cationic solvation shell by the two solvents of the binary mixture. The purpose of this investigation is to extend our earlier study of preferential solvation of the Na+ ion [5] t,o include mixtures of solvents possessing high, medium ALKALI
[I] C. DEVESELL and R. E. RICHARDS, Mol. Phy8.10,561 (1960). [2] E. G. BLOOR and R. G. KIDD, Can. J. Chem. 46, 3425 (1968). [3] (a) G. J. TEMPLE-N and A. L. VAN GEET, J. Amer. Chem. Sot. 94,657s (1972), (b) A. L. VAN GEET, J. Amer. Chem. Sot. 94, 5583 (1972). [4] M. S. GREENBERG, R. L. BODNER and A. I. POPOV, J. Phye. Chem. 77, 2449 (1973) and references listed therein. [5] R. H. ERLIOH, M. S. GREENBERU and A. I. POPOV, S~ectrochint. Aotu 29A,1927 (1973). (61 (a) L. S. FRIWSEL, T. R. STENULE and C. H. LANOFORD, Chem. Commun. 1965,393; (b) L. S. FBAN~EL, C. H. LAN~FORD and T. R. STENQLE, J. Phye. Chem. 74,1376 (1970). 897
M.
698
S. GREENBERU and A. I. POPOV
and low donicity so as to more fully observe the effect of solvent donicity on the isosolvation point., In recent years GUTMANN and WYCHERA [7] proposed an empirical scale of solvent donor (or solvating) ability. The scale is based on the enthalpy of the reaction S + SbC1, + S*SbCl, in dilute 1,2-dichloroethane solution. The donor number (or the donicity) of the solvent S is defined as D.N.(s)
=
-
AH,.,,,,,
We have previously shown that there is a linear relationship between the solvent donor numbers and the 23Na relative chemical shift in these solvents [4]. EXPERIMENTAL Sodium tetraphenylborate (Baker) was of reagent grade quality and was used without further purification except for drying in VUCZGO at 50°C for 72 hr. After drying, the salt was stored in a vacuum desiccator charged with granulated barium oxide. Hexamethylphosphoramide (HMPA) was vacuum distilled over barium oxide at 20 Torr and 127°C. Acetonitrile (AN) was refluxed over calcium hydride for 48 hr followed by fractional distillation at Sl*S”C in a nitrogen atmosphere. Nitromethane (NM) was percolated through a column of alumina and vacuum distilled over BaO. Dimethylsulfoxide (DMSO) was dried over Linde 4A molecular sieves and then vacuum distilled over freshly activated sieves at 50°C. Pyridine (PY) was refluxed over granular BaO for 24 hr followed by vacuum distillation at 112°C and 60 Torr. Tetramethylurea (TMU) was vacuum distilled over BaO at 38-40%. In all cases, only the middle 60% fraction of solvent was collected. Each mixed solvent solution was prepared by taring a snap-cap vial, adding the desired volume of solvent A, weighing, adding the desired volume of solvent B and weighing again. The mole fraction of each solvent in the solution was then calculated from the known weights. Approximately 0.12 g of sodium tetraphenylborate was weighed into a 1 ml volumetric flask. The desired solvent or solvent mixture was then added up to the mark (N 0.50 M Na+). Sodium-23 NMR studies were performed on a highly modified NMRS-MP-1000 spectrometer operating at 60 MHz at a field strength of 53 kG. The experimental details are outlined in a previous publication [4]. The Wilmad 506-PP, 5 mm OD polished NMR sample tube was fitted with a Wilmad precision coaxial 520-2 NMR tube for the reference solution. The reference for the 23Na measurements however, when the chemical shifts was 3.0 M aqueous sodium chloride solution; were so small that the sample resonance was masked by the reference, a secondary reference of 2.5 M sodium perchlorate in methanol was used. In the latter case, the shifts were corrected so as to apply to the 3.0 M aqueous sodium chloride reference solution. A positive shift from the reference is upfield. The chemical shifts reported are corrected for differences in bulk diamagnetic susceptibility between sample and reference according to the relationship of LIVE [7] (a) V. GUTMANN and E. WYCHERA, Inorg. Nucl. Chew. Lett. 2,257 (1966); (b) V. GTJTMANN, “Coordination
Chemktry
in Nonaqzceowr Solvents”, Springer-Verlag,
Vienna (1968).
699
Spectroscopic studies of ionic solvation-XVII
and CHAN for high-field
spectrometers 6corr --
Bobs -
[S] 47T/3 (x’v”’ -
xFrn”)
(1)
We assume the contribution of the salt to the susceptibility of the solution to be negligible-a reasonable assumption as shown by TEMPLEMAN and VAN GEET [3a]. Hence, the corrections simply reflect differences between the aqueous reference and the mixed organic sample. By employing Wiedmann’s Law, we may calculate the diamagnetic susceptibility of a given mixture according to the relationship &?: where: ~22 V* VI3 XA
XB
= v,
= volume = volume = volume = volume = volume
V.4 + VB
V, * XA
+
v,
+
v,
(2)
- XB
susceptibility of the solution; (ml) of solvent A; (ml) of solvent B; diamagnetic susceptibility of pure solvent A; diamagnetic susceptibility of pure solvent B.
RESULTS AND DISCUSSION The chemical shifts* of the BSNa nucleus as a function of solvent composition in binary solvent mixtures are illustrated in Figs. l-4. The systems studied in this investigation were, nitromethane with dimethylsulfoxide, pyridine, tetramethylurea, hexamethylphosphoramide and acetonitrile; acetonitrile with pyridine,, dimethylsulfoxide, tetramethylurea and hexamethylphosphoramide; tetramethylurea with pyridine and dimethylsulfoxide; hexamethylphosphoramide with tetramethylurea, dimethylsulfoxide and pyridine, and dimethylsulfoxide with pyridine. In all cases sodium tetraphenylborate was used as the solute. We note a smooth transition of the 23Na resonance as a function of solvent composition as we proceed from one pure solvent to the other. The fact that in these plots we note curves and not straight lines indicates preferential solvation by one of the solvents. The isosolvation points for these solvent systems are summarized in Table 1. It is well known that nitromethane is a solvent of weak donicity as reflected by its donor number of 2.7. As a result, it has been employed as an “inert” medium to study the complexation of Naf and Li+ with biologically active molecules [9]. The data in Table 1 and Fig. 1 seem to justify using nitromethane as an “inert” solvent. Binary solvent mixtures of nitromethane with dimethylsulfoxide, pyridine, tetramethylurea, hexamethylphosphoramide and acetonitrile exhibit isosolvation points of 0.05, 0.12, 0.06, 0.05 and 0.15 mole fraction of the latter respectively (Fig. 1). These data imply that Na+ is, to a large degree, preferentially solvated by these solvents relative to the nitromethane. Hence, compared to * Tabulated A. I. P.
s3Na chemical shift data in these solvent mixtures are available upon writing to
[8] D. H. LJXE and S. I. CHAN, And. Chem. 42,791 (1970). [9] (a) R. L. BODNER, M. S. GREENBERU and A. I. POPOV, h’pectry. Lett. 5, (b) Y. CAHEN, R. F. BIESEL and A. I. POPOV, J. Imorg. NwZ. &em. (in press). 13
489 (1972);
700
M. S. GREENBERG
41 o,o
0.2
Mole
and A. I. POPOV
0.4 fraction
@6
0.6
I.0
solvent
Fig. 1. Variation of the chemical shift of the ssNa resonance as a function of solvent compositionfor binary solvent mixtures of nitromethsnewith acetonitrile (A), tetramethylurea (o), dimethylsulfoxide (m), pyridine (0) and hexamethylphosphoramide (0).
6
-4 0.0
0.2
0.4
Mole fraction
0.6
0.6
I.0
solvent
Fig. 2. Variation of the chemical shift of the 23Na resonance as a function of solvent compositionfor binary solvent mixtures of acetonitrile with tetramethylurea (m), dimethylsulfoxide ( l), pyridine (A) and hexamethylphosphoramide (0).
Spectroscopicstudies of ionic solvation-XVII
Mole fraction
701
solvent
Fig. 3. Variation of the chemical shift of the 23Na resonance aa a function of solvent compositionfor binary solvent mixtures of hexamethylphosphoramidewith tetramethylurea (e), dimethylsulfoxide (m) and pyridine (A).
nitromethane, the relative order of donor ability is HMPA ‘v DMSO N TMXJ > PY > AN >> NM which, with the exception of pyridine, is the order of the solvent donor numbers. With a donor number of 33.0, however, pyridine should be a stronger donor than DMSO (DN = 29.8) and TMU (DN = 28.9) but weaker than HMPA (DN = 38.0). It is interesting to note that HMPA, DMSO and TMU
Mole fraction solvent Fig. 4. Variation of the chemical shift of the %Na resonance as a function of solvent composition for binary solvent mixtures of tetramethylurea with dimethylsulfoxide (A) and pyridine (0).
M. S. GREENBERG
702
Table 1. Summary Binary solvent system DMSO : NM PY:NM TMU:NM HMPA : NM AN:NM PY:AN DMSO : AN TMU : AN HMPA: AN TMU : HMPA DMSO : HMPA PY : HMPA PY : TMU DMSO : TMU DMSO : PY
Isosolvation 0.05 0.12 0.06 0.05 0.15 0.29 0.10 0.11 0.06 0.23 0.15 0.10 0.16 0.39 0.10
MF MF MF MF MF MF MF MF Ml? MF MF MF MF MF MF
and
A. I. POPOV
of preferential solvation data
point
DMSO PY TMU HMPA AN PY DMSO TMU HMPA HMPA HMPA HMPA TMU DMSO DMSO
@In 14.4 7.44 33.8 13.2 4.48 2.93 12.7 7.01 62.9 0.067 0.166 0.125 0.297 0.92 17.2
-
AGO,.,./n (kJ mole-l)
-
6.61 4.97 8.72 6.39 3.72 2.66 6.30 4.82 10.3 6.70 4.45 5.16 3.01 0.206 7.04
seem to exhibit the same relative donor abilities in nitromethane although the donicity of HMPA is greater than that of DMSO or TMU. Hence the donicity of these solvents in nitromethane is “leveled”. Acetonitrile is a solvent of medium donicity (DN = 15.0) and as such should be more competitive with the preceding solvent series for Na+ ion than was Binary solvent mixtures of acetonitrile with pyridine, dimethylnitromethane. sulfoxide, tetramethylurea, hexamethylphosphoramide and nitromethane exhibit isosolvation points of 0.29, 0.10, 0.11, 0.06 and 0.85 mole fraction of the latter respectively (Fig. 2). Again, the strong donor solvents HMPA, DMSO and TMU are “leveled” but we now see an anomalous position of pyridine. In acetonitrile, the relative order of donor ability is HMPA N DMSO r: TMU > PY > AN > NM. The “repression” of the donicity of pyridine in binary mixed solvents was The data seem to indicate that DMSO and TMU are better solvating surprising. agents for the Na+ ion than pyridine, which is contrary to the 23Na NMR chemical shift data [4] and to the solvents’ donor numbers. To investigate further this unexpected donor repression of pyridine relative to other high donor solvents, solvent mixtures of pyridine with hexamethylphosphoramide, tetramethylurea and dimethylsulfoxide were examined. The 23Na NMR data shown in Figs. 3 and 4 revealed isosolvation points of 0.10, 0.16 and 0.10 mole fraction of the latter indicative of strong preferential solvation by that solvent. Recalling the donor numbers of 38.0, 29.8, 28.9 and 33.0 for HMPA, DMSO, TMU and pyridine respectively, we would expect preferential solvation of the Na+ ion by HMPA with respect to pyridine, but very little or no preferential solvation of the Na+ ion by DMSO or TMU relative to pyridine since these three solvents have similar donor numbers. Instead, strong preferential solvation of the Na+ ion is noted for DMSO and TMU versus PP.
Spectroscopic studies of ionic eolvation-XVII
703
The mixed solvent system, DMSO-PY, was examined in greater detail using vibrational spectroscopy [5] and Brillouin scattering [lo] as previously reported. The data suggest that while pure DMSO is a highly associated solvent due to dipolar interactions through the S-O bond, when mixed with pyridine (p = 2.20), the structure of DMSO is severely altered. Now, it seems reasonable to assume that when the sodium salt is introduced into neat DMSO, a considerable amount of energy must be expended to break up the structure of DMSO before solvation of the cation can occur. Hence, the introduction of even small amounts of pyridine into neat DMSO results in the break up of the polymeric structure of the latter via a dipole interaction, resulting in an enhancement of donicity of DMSO in mixtures where this type of interaction occurs. It seems that the enhanced donicity of TMU relative to pyridine may also be rationalized in terms of structure breaking by pyridine. It has been pointed out [ll] that TMU is a structured solvent due to strong dipole-dipole interactions. The situation, therefore, is similar to that in liquid DMSO. BE~UIN and GUNTHARD [l2] showed the C = 0 stretching frequency to be strongly solvent dependent as is the S-O stretching frequency in DMSO [13]. Hence, a strong TMU-PY dipole-dipole interaction may account for the enhanced donioity of TMU by causing extensive disruption of TMU structure by pyridine. Finally, we wished to differentiate the three solvents that were effectively leveled in nitromethane and acetonitrile. For binary solvent mixtures of hexamethylphosphoramide with tetramethylurea and dimethylsulfoxide, the isosolvation points were 0.23 and 0.15 mole fraction HMPA. An isosolvation point of 0.39 mole fraction DMSO was noted for the dimethylsulfoxide-tetramethylurea system (Fig. 4). These data imply that the relative order of solvating ability is HMPA > DMSO > TMU where DMSO is just slightly stronger than TMU. To this point preferential solvation has been discussed in a qualitative sense as reflected by the isosolvation point. Recently, COVINGTONet al. [ 141 developed a quantitative model for competitive solvation. In the above papers, they present a derivation of preferential solvation that allows the calculation of equilibrium constants and the changes in free energy as the solvation shell of an ion X is progressively changed from n molecules of solvent W to n molecules of solvent P in an isodielectric solvent system. They consider initially ion X is solvated by four molecules of the solvent W. As the second solvent P is introduced into the system, there is a step-wise replacement of W by P in the inner solvation shell. The series of equations can be
[lo]
J. B. KINSINQER, M. M. TANNAHILL, M. S. GREENBERU and A. I. POPOV, J. Phys. Chem.
2444 (1973).
77,
[Ill A. LUTTRINQHATJS and H. W. DIRKSEN, Agnew Chem. Intern. Ed. 3, 260 (1969). [12] C. BECUIN and H. H. GUNTHARD, HeZw. Chim. Acta 42, 2262 (1969). [13] H. H. SZD~ANT,in “Dimethylaulfoxide”, pp. 1-98, Edited by S. W. JACOB, E. E. ROSENBAUM and D. C. WOOD, Marcel-Dekker, New York, N.Y. (1971). [14] (a) A. K. COVINQTON, T. H. LILLEY, K. E. NEWMAN and G. A. POETHOUSE, J. Chem Sot. Faraday I 69, 963 (1973); (b) A. K. COVINQTON, K. E. NEWMAN and T. H. LILLEY, J. Chem. Sot. Faraday I, 69, 973 (1973).
704
M. S. GREENBERU
expressed
and A. I. POPOV
as, + PP + (w-4)W 2
XW, XW,P,
+ (p4P
+ (p-l)P
XW,P
+ (w-2)W 2 XP,
XWP, + (p-4)P
+ (w-3)W 2
+ (p-3)P
+ (w-1)W 3
+ WW
(3)
In addition to requiring that the solvent mixture is isodielectric and exhibits ideal behavior, Covington assumes that as P replaces W, the observed chemical shift is additive. If BP is the shift in the resonance of X from pure W to pure P, then dxw, = 6; ~XW,P = &fir; dxwtdrI = 38~; ~XWP~ = BP; ~XP, = 8~. Hence, the intrinsic shifts of the various solvated species are proportional to the amount of P which they contain. Another assumption requires that the individual equilibrium constants, KS, are related solely by statistics, that is, when n = 4,
K’ = K1” = (K,K,K,K4)“4 K, = 4K’;
K, = $ K’;
K, = 8 K’;
(4) K, = 2 K’
The final equation in this treatment allows calculation
(5)
of K”” as follows:
(‘3) 6 = observed chemical shift relative to the resonance of X in pure W; BP = the total range of the chemical shifts (i.e. dxw, - dxp,); Kl/” = the geometric equilibrium constant; n = the solvation number; up, aw = the activities of solvents P and W, respectively. By plotting l/S vs. xw/xp, they obtain l/S, from the intercept and Kiln from the slope. It should be noted that in order to substitute mole fractions for activities of solvents, the solution must be ideal. Finally, they calculate the free energy of preferential solvation, AGopJn, as follows:
where :
AG”,.,Jn
= -
RT In KlJn
(7)
We treated our data according to eqns. (6) and (7) and calculated the geometric free energy of preferential equilibrium constant, K”“, and the corresponding solvation, AGo,&. Th e results are shown in Table 1. A plot of l/S vs. zB/xA did in fact yield straight lines in each case, except for the solvent system acetonitrilepyridine, which displayed a small degree of curvature. While studying preferential salvation of the Co3+ ion in binary mixtures of chloroform with acetone and p-dioxane, by 5eCo NMR, FRANKEL etal.[6b],in a, treatment very similar to that of Covington, noted curves in plots of solvation isotherms, not straight lines as predicted. They ascribed this curvature to either a weak solvent-solvent interaction or to nonequal solvation numbers of Co3f in these solvents.
Spectroscopic studies of ionic solvation-XVII
705
0.6.
05'
o.4L
IO ‘pyridine
’ xLXSO
Fig. 6. Covington plot for the binary solvent system dimethylsulfoxid4-pyridine.
The plots were treated by a linear least squares procedure from which we the values of K”“. It is particularly interesting to note that, such a plot for the data in DMSO-pyridine mixtures, where there exists marked solventsolvent interactions, yields a straight line as shown in Fig. 5. The location of the isosolvation point for a given binary solvent system gives a qualitative measure of the salvation abilities of the solvents in the given mixture. In order to extend the comparison to variations in isosolvation values to a series of binary mixtures, more knowledge is needed on the structures of the individual solvents and the changes in this structure on formation of liquid mixtures. These results emphasize the danger of extrapolating the behavior and properties of pure solvents to their properties in solvent mixtures. Covington’s quantitative approach appears to be successful despite the large number of idealized assumptions, some of which may be debatable. However, we agree with his statement that this treatment, while not rigorous, is helpful in understanding preferential solvation phenomenon. obtained
AcknowledgementThe authors gratefully acknowledge support of this work by a grant from the National Science Foundation.