J. Phys. Chem. 1981, 85, 1595-1599
TABLE VII: Dissociation Constants in SO,, TMS, and Me,SO 1 0 4 ~ M~ TMS,a so,, 298.15 K 303.15 K LiBr LiI NaI KI
NaSCN Et4NI DCC d d e b ye a
Reference 54.
,
Me,SO,a
298.15 K
0.10 36 diss 0.60 1786 0.83 2128 1.05 1538 0.08 212 diss 16.1 2174 3846b 43.3 46.I 12 4.81 3.96 1.62 Reference 55. Dielectric constant.
for the halides, C1-, Br-, and I-, 104Kd= 0.74,1.43, and 3.27 M, respectively, at 273.15 K, and Tokura reported lo4& = 0.19,0.44, and 1.05 M, respectively, at 298.15 K.21This regularity is demonstrated among the R4N+ions as well since we have reportedI3 for the halides of Me4N+values of lo4& = 10.0,11.4, and 14.1 M, respectively, for C1-, Br-,
1595
and I- at 273.15 K while Tokura reported values of 5.52, 5.95, and 7.63 M at 298.15 K. For the two salts containing planar cations which were studied, MePyrI and Ph3CC104,dissociation constants are quite large, indicative of diminished ion-ion interaction, as would be expected from the charge delocalization. The salt with the greater delocalization, Ph3CC104,has the greater dissociation constants. Table VI1 contains dissociation constants for the salts in the three sulfur-containing solvents. Dielectric constants and dipole moments of the solvents are also included. The small difference in dielectric constants of TMS and M e a 0 does not explain the difference in association behavior in these two solvents, but the greater cation solvating ability of Me2S0 could reduce the cation charge density sufficiently to prevent ion pairing. Extension of conductance measurements in these solvents to other temperatures should contribute to further elucidation of ion-solvent and ion-ion interactions in these solvents. Acknowledgment. This work was supported by the National Science Foundation under Grant No. GP4023.
Dielectric-Permittivity Analysis of Likely Self-Association of N,N-Disubstituted Amldes M. M. Kopecnl,+R. J. Laub,” and Dj. M. Petkovld Chemical Dynamics Laboratory, The Boris Kidric Institute of Nuclear Sciences-VINCA, 1 100 I-Beograd, Yugoslavia, and the Department of Chsmistw, The Ohlo State Univeristy, Columbus, Ohio 43210 (Received: October 6, 1980; In Final Form: February 16, 7981)
Dielectric properties of solutions of N,N-disubstituted amides with n-hexane and with carbon tetrachloride solvents are examined. Variation of the solution permittivity CAS with the notional molar concentration of amide is used to assess apparent intermolecularassociation constants Kd of the latter, where the simplifying assumption is made that complexes of order higher than dimerization are unimportant. The values of Kd obtained are as expected considerablysmaller than those generally observed for unsubstituted or N-monosubstitutedspecies, which is discussed in light of the assumptions made and with regard to the general applicability of the approach employed.
It is generally agreedl that dielectric properties of e.g., alcohols and amides can in part be ascribed to intermolecular association of one kind or another with resultant formation of dimer or multimer s p e ~ i e s . ~Thus, - ~ N-monosubstituted amides yield dielectric permittivities which are considerably higher than those of unsubstituted speciesH which is consistent with intermolecular hydrogen bonding of the former in chains and the latter in paris with concomitant ring formation. N,N-Disubstituted amides presumably cannot associate in either mode since they lack an active proton. Nevertheless, it can in fact be inferred from previous ~ 0 r k l O -that l ~ disubstituted amides form predominantly cyclic dimers as a result of dipole-dipole interactions. IR and NMR spectroscopic measurem e n t ~ of~ N,N-dimethylformamide ~ - ~ ~ (DMF) and N,Ndimethylacetamide (DMA), for example, provide supportive evidence of association of these species. To our knowledge, however, no attempts have been made to measure the degree of dimerization of N,N-disubstituted amides via assessment of dielectric permitChemical Dynamics Laborat.ory, T h e Boris Kidric Institute of Nuclear Sciences. *Author to whom correspondence should be addressed at T h e Ohio State University. 0022-365418112085-1595$01.2510
tivities of mixtures of these compounds with notionally inert hydrocarbon diluents. We therefore report in this work permittivity measurements of solutions of DMA and NJV-dibutyl-2-ethylhexylamide(DBEHA) in n-hexane and (1)Kulevsky, N. In “Molecular Association”;Foster, R., Ed.; Academic Press: London, 1975;Vol. 1, Chapter 2. (2)Brown, A. C.; Ives, D. J. G. J. Chem. SOC. 1962,1608. 1966.20. 1. (3)Franks. F: Ives. D. J. G. 9.Ref. Chem. SOC. (4)Lawrence,’A. S.’C.; McDonald,’M. P.; Stevens, J. V. Trans. Faraday SOC.1969,65,3231. (5)Leader, G.R.; Gormley, J. F. J. Am. Chem. SOC.1951, 73,5731. (6)Worsham, J. E.;Hobbs, M. E. J. Am. Chem. SOC.1954,76,206. (7)Vaughin, J. W.; Sears, P. G. J.Phys. Chem. 1958,62,183. (8)Lin, R.-Y.; Dannhauser, W. J. Phys. Chem. 1963,67,1805. (9)Dannhauser, W.; Johari, G. P. Can. J . Chem. 1968,46,3143. (10)Hobbs, M. E.;Bates, W. W. J . Am. Chem. SOC.1952,74, 746. (11)Pohl, H. A,; Hobbs, M. E.; Gross, P. M. J. Chem. Phys. 1941,9, 408. (12)Meigham, R. M.; Cole, R. H. J. Phys. Chem. 1964,68,503. 1955,77,2012. (13)Cole, R. H. J.Am. Chem. SOC. (14)Davies, M.; Thomas, D. K. J. Phys. Chem. 1956, 60,767. (15)Hatton, J. V.; Richards, R. E. Mol. Phys. 1960,3,253. 1962,84,13. (16)Woodbrey, J. C.; Rogers, M. T. J. Am. Chern. SOC. (17)Newman, Jr., R. C.; Snider, W.; Jonas, V. J. Phys. Chem. 1968, 72,2469. (18)Rabinowitz, M.; Pines, A. J. Chem. SOC.B 1968,1110. 1969, 91, 1585. (19)Rabinowitz, M.; Pines, A. J. Am. Chem. SOC. (20)Bittrich, H.-J.; Kirsch, D. 2.Phys. Chern. (Leipzig) 1975,256,808. (21)Bittrich, H.-J.; Kietz, E. Wiss. 2. THLeunna-Mers. 1979,21,205. (22)Bittrich, H.-J.; Kietz, E. 2.Phys. Chem. (Leipzig) 1980,267,17.
0 1981 American Chemical Society
1596
The Journal of Physical ChemMty, Vol. 85, No. 11. 198 1
Kopecni et ai.
in carbon tetrachloride and DMF in carbon tetrachloride, since in preliminary experiments we found that their IR spectra exhibit a concentration-dependent shift of the amide band at 1650 cm-l which is consistent with that occasionally observed for weak intermolecular complexation (e.g., ref 25). In addition, Laub and Wellingtonz6have shown that the hypothesis of self-association of one or the other of the components of mixed-solvent systems provides a basis for rationalization of the curvature found in plots of (infinite-dilution)solute liquid-gas partition coefficients as a function of solvent composition. This report is thus intended in addition to provide further and independent evidence regarding the analytical and physico~hemical~’-~~ consequences of data such as those reportedm recently for systems wherein self-associationmay be supposed to occur.
Experimental Section The best available grades of Merck n-hexane and carbon tetrachloride were used without further purification. N,N-Dibutyl-2-ethylhexylamide(DBEHA) was purified by molecular distillation (389-390 K; 0.01 torr) and exhibited dielectric permittivities of 15.92,15.46,15.01, 14.60, and 14.18 over the temperature region 288.2-308.2 K in increments of 5 K. N,N-Dimethylacetamide (DMA) was distilled at room temperature and at 0.1 torr. However, we were unable to determine the permittivity of pure DMA or of N,N-dimethylformamide (DMF) with the standard cells and so capacitance cells of the shielded type were employed. DMF was 99.4% pure and was not subjected to further purification. All pure components were stored over molecular sieves and all solutions were prepared in a drybox. The permittivity measurements were carried out with a Weihein Obermayern WTW DM-01 dipolmeter at 2 MHz with appropriate cells which were thermostated to k0.05 K with a Lauda circulating water bath. The usual dipolmeter calibrations and experimental precautions were observed.
Results The dielectric permittivity cA,S of a two-component mixture comprising nonpolar and nonreacting constituents can be described in terms of the permittivities eAo and €so of pure components A and S via a simple additive relationB EA,S
=~
+ 4seso
A ~ A O
+
= f A C A + fScS
..
e
t
11 0
’
I
I
I
I
0.4
0.2
*
8
.
3t
e
e
I
I
I
*
0.8
0.6
1
I
1.0
I
1.2
=4
Figure 1. Plot of against molar concentration for solutions of N ,Ndibutyl-2-ethylhexylamide (DBEHA) with n-hexane at 293.2 K.
sition next of the existence of both monomeric (M) and dimeric (D)forms of amide in solution results in expansion of eq 2 to the form: cA,S
= fMcM +fDcD
+ fScS
(3)
Mass balance of the total concentration of amide, C A = CM
+ 2CD, combined with eq 3, thus provides
2fM)CD (4) Further, in regions where CD can be neglected, Le., dilute solutions for which CD 0, eq 3 reduces to CA,S - f S c S - f M c A
=
(fD -
-+
Plots of cA,S/CS against CA/Cs in the limit CD 0 thus provide fs directly as the intercept and fM as the slope of a straight line. We now require a means of calculation of CD Expressing the putative association constant of amide as Kd = CD/CM2,followed by substitution for CM, and solving the resultant quadratic function yields +
(1)
where 4 denotes a volume fraction of the indicated species. Since cbA & = 1,deviations from linearity of the functions c A , S ( ~ A )or ~ ~ , ~can ( hence 4 ~ ) be ascribed to interactional and/or association processes of various types.23 We seek first to express the amounts of the components in eq 1 in units of molar concentration, denoted as C: cA,S
-I
.
e e
with which CD may be evaulated at given values of CAfor assumed values of Kd. Finally, upon substitution of eq 6 into eq 4 we obtain
(2)
where, since 4c = CVc, we have defined f as an intensitivity factor, f = Vc, where V denotes a molar volume. Suppo(23) Bottcher, C. J. F. “Theory of Electric Polarization”; Elsevier: Amsterdam, 1973; Vol. I, 2nd ed., section 34. (24) Petkovic, Dj. M. In “Solvent Extraction Research’; Kertes, A. S., Ed.; Wiley: New York, 1979; p 93. (25) Yarwood, J.; Arndt, R. In “Molecular Association”; Foster, R., Ed.; Academic Press: London, 1979; Vol. 2, chapter 4. (26) Laub, R. J.; Wellington, C. A. In “Molecular Association”;Foster, R., Ed., Academic Press: London, 1979; Vol. 2, Chapter 3. 1975, (27) Purnell, J. H.; Vargas de Andrade, J. M. J.Am. Chem. SOC. 97, 3585,3590. (28) Laub, R. J.; Purnell, J. H. J. Am. Chem. SOC.1976, 98, 30, 35. (29) Harbison, M. W. P.; Laub, R. J.; Martire, D. E.; Purnell, J. H.; Williams, P. S. J.Phys. Chem. 1979,83, 1879. (30) Acree, W. E.; Bertrand, G. L. J. Phys. Chem. 1979, 83, 2355.
Linear least-squares regression correlation coefficients of plots of the left-hand side of eq 4 against CD (the latter being calculated from eq 6 with assumed values of &) may now be evaluated and a “best fit” Kd determined. The procedure is clearly tedious although straightforward, and has been employed previously in studies of the dimerization of tri-n-butyl phosphate in n - h e ~ a n e . ~ ~ Dielectric permittivities of solutions of DMA and DBEHA in n-hexane and in carbon tetrachloride and DMF in carbon tetrachloride at various temperatures are provided in Table I and Table 11, which are a portion of the data determined in this work. Deviations from the linearity predicted by eq 1 are clearly evident as shown in Figure 1 for the system DBEHA/n-hexane.
The Journal of Physical Chemistry, Vol. 85, No. 11, 198 1
Self-Association of N,N-Disubstituted Amides
1597
TABLE I: Dielectric Permittivities (293.2 K ) of N,N-Dialkylamide/Solvent Systems as a Function of Solution Composition DMA/CCl, DBEHAln-hexane DBEHA/CCl, DM F /CC1 DMA/n-hexane
CDMA 0 0.0046 0.0091 0.0143 0.0191 0.0235 0.0280 0.0323 0.0409 0.1163 0.2120 0.3184 0.4292 0.5341 0.6227 0.7420 0.8617 0.9673 1.0717
€AS
CDMA
€A.S
1.8837 1.8902 1.8973 1.9046 1.9113 1.9195 1.9261 1.9329 1.9458 2.0671 2.2212 2.3937 2.6439 2.8826 3.0234 3.2277 3.4437 3.8051 3.9352
0 0.0045 0.0091 0.0178 0.2224 0.0322 0.0376 0.0419 0.0517 0.0892 0.1701 0.3611 0.5701 0.6499 0.7367 0.8249 0.9037 1.0779 1.2574 1.4004 1.5652 1.6251 1.7599 1.8517 1.9645 2.1199 2.2601
2.2357 2.2440 2.2527 2.2699 2.2788 2.2978 2.3085 2.3514 2.3367 2.559 2.726 3.146 3.623 3.811 4.009 4.224 4.417 4.842 5.306 5.655 6.065 6.228 6.569 6.798 7.089 7.489 7.869
CDBEHA 0 0.0034 0.0036 0.0094 0.0138 0.0178 0.0182 0.0224 0.0276 0.0280 0.0356 0.1633 0.2517 0.3199 0.3971 0.4683 0.5455 0.6203 0.7188 0.8427 0.9349 1.0528 1.1689 1.2133 1.3230 1.4452 1.5620
CDBEHA 0 0.0042 0.0054 0.0082 0.0129 0.0249 0.0304 0.0311 0.0395 0.0408 0.0512 0.1242 0.1843 0.3377 0.4780 0.5657 0.6387 0.7432 0.8443 0.9149 1.0038 1.1500 1.2451 1.3306 1.3971 1.5025 1.6457
288.2 K
I 02551
'A3
EA.S
1.8879 1.8928 1.8931 1.9015 1.9082 1.9141 1.9146 1.9211 1.9285 1.9293 1.9407 2.2477 2.4217 2.5872 2.7341 2.9033 3.1227 3.3178 3.5483 3.9087 4.2084 4.5862 4.9595 5.1088 5.4815 5.0066 6.3496
CDMF
EA.S
2.24 14 2.2443 2.2469 2.2514 2.2598 2.2816 2.2918 2.2928 2.3081 2.3106 2.3293 2.6322 2.7635 3.1105 3.4545 3.6876 3.8876 4.3635 4.4569 4.6648 4.9343 5.3634 5.6721 5.9428 6.1671 6.4525 6.8503
0 0.0129 0.0260 0.0382 0.0528 0.0643 0.0772 0.0908 0.1316 0.2118 0.3070 0.4929 0.6630 0.8496 1.0265 1.1800 1.3642 1.5140 1.7629 1.8793 2.1890 2.3966 2.4817 2.6057
298.2 K
CDBEHA
€A,S
CDBEHA
0.5056 0.5283 0.5535 0.5790 0.6109 0.6394 0.6666 0.6918 0.7384 0.7569 0.7934 0.8325 0.8847 0.9234 0.9828 1.0402 1.0879 1.1528 1.2041 1.2712 1.3492 1.4946
3.4151 3.4490 3.5375 3.5806 3.6743 3.7367 3.8317 3.9226 4.0296 4.0777 4.2096 4.2769 4.4609 4.5376 4.8432 4.9533 5.0947 5.4326 5.5237 5.9505 6.0599 6.6346
0.4990 0.5209 0.5457 0.5709 0.6023 0.6305 0.6593 0.6822 0.7282 0.7559 0.7828 0.8215 0.8728 0.9113 0.9697 1.0263 1.0734 1.1377 1.1881 1.2547 1.3320 1.4760
EA,S
EA.S
2.2355 2.2595 2.2848 2.3089 2.3341 2.3591 2.3840 2.4103 2.4481 2.822 3.016 3.395 3.732 4.114 4.471 4.803 5.706 5.544 6.097 6.386 7.165 7.775 8.007 8.286
308.2 K
CDBEHA EA,S
~
1
02451
510-~
1 10'~
1 5 10"
c, /cs Flgure 2. Plots of e,,JCS against C A / C s(cf. text and eq 5) for dilute solutions of N,Ndibutyl-2-ethylhexylamide(DBEHA) with n-hexane at 308.2 (top line) to 288.2 K in increments of 5 K.
Shown in Figure 2 are examples of plots of tA,s/Cs against CA/Cs for dilute solutions of DBEHAln-hexane from which f s and f M were determined (eq 5 ) . Table I11 lists these values and the linear regression coefficients for all systems studied. Dimerization constants of the amides were next determined by fitting reiteratively & values to the function CA,S - fsCs - f M c A = f ( C D ) . Two criteria can be applied in searching for the best-fit &, namely, evaluation of the regression coefficient r and the zero intercept of the least-squares line of eq 4. The former was employed here since there a number of reasons which can be envisaged for nonzero intercepts (such as formation of multimer or strongly solvated species). An example of the variation of the left-hand side of eq 4 with dimer concentration is presented graphically in Figure 3 for various assumed values of & for the system DBEHAln-hexane, which illustrates the concave or convex dependence of the function tA,S - fsCs - f&A = f(C,). Acceptable linearity is achieved as shown only for a dis-
3.3054 0.4919 3.3520 0.5135 3.4208 0.5379 3.4721 0.5627 3.5426 0.5937 3.6182 0.6214 3.6829 0.6500 3.7585 0.6725 3.8872 0.7180 3.9444 0.7447 4.0249 0.7719 4.1293 0.7771 4.2628 0.8608 4.3680 0.8996 4.5972 0.9566 4.7480 1.0124 4.8671 1.0590 5.1411 1.1224 5.2677 1.1730 5.5078 1.2381 5.7489 1.3146 6.2960 1.4570
3.2143 3.2692 3.3233 3.3730 3.4351 3.5052 3.5753 3.6190 3.7595 3.8033 3.8774 4.0213 4.0927 4.2068 4.3631 4.4256 4.6706 4.8814 5.0446 5.2485 5.4794 5.9855
Crete value of the dimerization constant, selected as that corresponding to the maximum regression coefficient. Table I11 lists the Kd values determined in this way for the systems indicated together with the available literature data; the interactions are as expected weak. We were unable to determine the dimerization constant of DMF in n-hexane due to poor solubility of the former in the latter. Furthermore, even though plots of the dielectric permittivity of mixtures of DMF/carbon tetrachloride against amide concentration exhibit curvature as predicted by eq 3, our fit of the experimental data indicates that the dimerization constant tends to zero. The Kirkwood parameterz3for DMF is in fact unity (in contrast to
1598
The Journal of Physical Chemistry, Vol. 85, No. 11, 198 1
Kopecni et al.
TABLE 111: Intensivity Factors and Regression Coefficients (Eq 5), and Best-Fit Dimerization Constants for Indicated Systems system DMA/n-hexane DMA/CCl, DM A/cyclohexane DBEHAln-hexane DBEHA/CCI, DMF/CCl, DMF/cyclohexane a
T/K 293.2 293.2 301.5 (room) 301.5 288.2 293.2 298.2 293.2 293.2 301.5 (room) 301.5
fs
fM
r
Kd/dm3mol-’
ref
0.2464 0.2159
1.7291 2.1460
0.9998 0.9999
0.2460 0.2465 0.2483 0.2160 0.21 59
2.0746 2.0096 1.9648 2.4249 2.0886
0.9996 0.9995 0.9999 0.9991 1.0000
0.3 ? 0.1 0.16 2 0.05 0.51 la 0.37a O.56Oa 0.21 f 0.02 0.20 f 0.02 0.19 ? 0.02 0.03 t 0.02 0.00 c 0.03 0.618a 0.60a 0.495’”
this work this work 20 17 22 this work this work this work this work this work 20 17 22
NMR.
extant in solution, the latter referred to as “sociation” by I I G ~ g g e n h e i m . Further, ~~ spectroscopic methods are said that to provide values of K pertaining only to the is, complexation in excess of that due to collision-induced pairing. Heats of formation of contact pairs derived from spectroscopic measurements are predicted42to approximate -2RT, Le., -5 kJ mol-’ at room temperature. For-example, Janini, King, and Martire%obtained values of AHf of form -4.5 f 0.3 to -6.7 f 0.3 kJ mol-l for a variety of aromatic hydrocarbons with carbon tetrabromide. In contrast, Tamres and G r ~ n d n e assert s ~ ~ that values of K derived for contact pairing only may well exhibit ARf 0 since the concentration of contact pairs varies as the product of collision frequency (0: W2)with distance of approach CD (0: W2). Virtually the same conclusion can be drawn from Figure 3. Plots of - fsCs- f&A against C D(cf. text and eq 4, consideration of the variation of the slopes of repulsive 6, and 7) for solutions of N ,Ndibutyl-2-ethylhexylamlde(DBEHA) with pairwise interaction potential^.^^ The temperature den-hexane at 293.2 K. Values of C Dwere generated with assumed pendence of the absorption spectra of systems such as O2 0.01 M-’; (+) values of Kd: (0) 10 M-’; (A)1.0 M-’; (0)0.1 M-‘; (0) 0.001 M-’. with N,N-dimethylaniline and with triethylamine& and I2 with alkanes44thus lends weight to the view that true higher homologues which exhibit g much greater than 1) random collisions, however promiscuous, can in principle which reinforces the contention that the degree of selfbe distinguished from those instances wherein “sticky” association of this species is negligible. Solvation by carbon collisions* persist even though neither can be regarded as t e t r a c h 1 0 r i d e ~ ~therefore J~J~~~~ appears to dominate to such isolated events of well-defined stoichiometry. It seems, an extent that eq 7 becomes inappropriate, and some other however, that any degree of distinction can be best be or extended form hence is required. viewed only as highly qualitative since it hardly seems likely that molecular dynamics calculations, for example, Discussion can, at least at present, provide models of solutions with The small constants (presumed here to pertain to amide which the two effects can be separated with any measure dimerization) observed for DMA and DBEHA, and a Kd of certainty. There is in addition the likelihood that the of zero for DMF, are reminiscent of random “contact” two types of interactions comprise merely one or another for which a K of -0.1-0.2 dm3 mol-l can be expected on the basis of normal forces of d i ~ p e r s i o n . ~ ~ manifestations of an energetically continuous spectrum of intermolecular encounters. The matter is of course Thus, and in addition to difficulties associated with the further exacerbated by practical difficulties associated both spectroscopicsis methods of measurement commonly emwith vapor-phase and with condensed-phase spectroscopic p l ~ y e dequilibrium , ~ ~ ~ ~ ~contants of this magnitude [e.g., measurements, regarding which the GLC m e t h ~ d ~ ~ ~ ~ ~ 0.009 dm3 mol-’ for benzene with carbon tetrachloride in continues to offer considerable advantage. n - h e ~ a n e0.22 ; ~ ~dm3 mol-l for benzene with carbon tetAssessment of weak intermolecular interactions via r a ~ h l o r i d e ~must ~ ] inevitably be regarded with some analysis of the dielectric properties of solutions, in contrast skepticism. Alternatively, it has been argued that a range to the above, would seem to offer several advantages. The of interactions encompassing both random collisions as well most noteworthy of these is that at low to moderate (MHz) as well-defined and stoichiometric e q ~ i l i b r i a ~ pmay ~l,~ be~ frequencies and in contrast to measurements at high (GHz)
-
(31)Whittaker, G. A,; Siegel, S. J. Chem. Phys. 1965,42,3320. (32)Mulliken, R.S. Red. Trau. Chim. Pays-Bas 1956,75,845. (33)Orgel, L. E.;Mulliken, R. S. J. Am. Chem. SOC.1957,79,4839. (34)Prue, J. E. J. Chem. SOC.1965,7534. 1949,71,2703. (35)Benesi, H. A.; Hildebrand,J. H. J.Am. Chem. SOC. (36)Scott, R. L.Red. Trau. Chim. Pays-Bas 1956,75,787. (37)Person, W. B.J. Am. Chem. SOC.1965,87,167. 1974, (38)Janini, G. M.; King, J. W.; Martire, D. E. J.Am. Chem. SOC. 96,5368. (39)Anderson, R.;Prausnitz, J. M. J. Chem. Phys. 1963,39, 1225. (40)McGlashan, M. L.;Stubley, D.; Watts, H. J. Chem. SOC.A 1969, 673.
(41)Carter, S.;Murrell, J. N.; Rosch, E. J. J. Chem. SOC.1965,2048. (42)Scott, R. L.J . Phys. Chem. 1971,75,3843. 1960,56,1159. (43)Guggenheim, E.A. Trans. Faraday SOC. (44)Tamres, M.;Grundnes, J. J. Am. Chem. SOC.1971,93,801.See also Tamres, M.; Yarwood, J. In “Spectroscopy and Structure of Molecular Complexes”;Yarwood, J.; Ed., Plenum Press: New York, 1973;p 217. 1971,67,2234. (45)North, A. M.; Parker, T. G. Trans. Faraday SOC. (46)Tsubomura,H.; Mulliken, R. S. J. Am. Chem. SOC.1960,82,5966. (47)Laub, R. J.;Pecsok, R. L. “Physicochemical Applications of Gas Chromatography”;Wiley-Interscience: New York, 1978.
Self-Association of N,N-Disubstituted Amides
the contribution of short-lived s or so) specific short-ranged interactions to t must be averaged In the absence of long-lived complexes, the Martin-Bell-Kirkwood (MBK) relation (eq 1)23would then be expected to hold, while deviations from it, as in Figure 1, are supportive49of discrete and persistent association.24 Further evidence of the weak interaction of amines, ethers, and sulfides with solvents has also been deduced from the differences between the dipole moments of each of these species in solution with carbon tetrachloride and with benzene, AD of -0.04 D (Dccq > Dbenzene) being taken as confirmation of solute-solvent pairing.60 The dipole moments (25 “C) of phenol with n-heptane, benzene, and carbon tetrachloride, for example, are 1.44,1.47, and 1.47 D, respectively, while for pyridine solute the values are 2.21,2.20, and 2.34 D.6l The dielectric evidence thus infers weak association of pyridine with carbon tetrachloride,@’Is2 in agreement with the calorimetric studies reported by Morcom and Travems3 The dipole moments of DBEHAs4in n-hexane (3.67 D), cyclohexane (3.67 D), benzene (3.69 D), and carbon tetrachloride (3.75 D) within experimental error are virtually solvent independent; those for DMF [3.92 D in benzene, 3.90 D in n-heptane;= 3.80 D in n-heptaneM]and for DMA [3.72 D in ben~ene,~’ 3.70 D in cyclohexane,663.79 D in n - h e ~ t a n e follow ~ ~ ] a similar pattern. The same cannot be said, however, of the data presented in Table 111; the values of K for DBEHA, DMA, and DMF without exception diminish from hexane to carbon tetrachloride solvent (0.2 to 0.03 dm3 mol-l for DBEHA; 0.3 to 0.16 dm3 mol-l (48) C p m p , R. A,; Price, A. H. Trans. Faraday SOC. 1969, 65, 3195. (49) Since a Kirkwood g parameter of unity forms the basis of the
MBK model, the latter may well be regarded as inappropriate for mixtures of substances such as water, alcohols, amides, and so forth for which g is substantially greater than unity. As a result, the MBK model has on occasion been criticized as only a rough approximation at best to molecular orientation in nonelectrolyte solutions, e.g., Bayliss, N. S. J . Chem. Phys. 1950, 18, 292. Bellamy, L. J.; Williams, R. L. h o c . R. SOC. London, Ser. A 1960,255,22. A. D. E. Pullin, A. D. E. Ibid. 1960,255, 39. Coulson, C. A. Ibid. 1960,255, 69. (50) Sharpe, A. N.; Walker; S. J. Chem. SOC.1961, 2974; 1962, 157; 1964, 2340. (51) Bishop, R. J.; Sutton, W. E. J . Chem. SOC.1964, 6100. (52) Crossley, J.; Hassell, W. F.; Walker, S. Can. J . Chem. 1968, 46, 2181. (53) Morcom, K. W.; Travers, D. N. Trans. Faraday SOC. 1963, 62, 2063. (54) Kopecni, M. M.; Laub, R. J.; Petkovic, Dj. M.; Smith, C. A., work
to be published. (55) Lee, C. M.; Kumler, W. D. J. Am. Chem. SOC.1962, 84, 571. (56) Steffen, M. Ber. Bunsenges. Phys. Chem. 1970, 74, 505. (57) Thompson, H. B.; LaPlanche, L. A. J. Phys. Chem. 1963,67,2230.
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for DMA). We are therefore led at this point to conclude that whatever form of complexation exists in solution cannot arise from solute-solvent interactions, since it is difficult to imagine that amide/alkane complexation could exceed that of amidelcarbon tetrachloride. We instead postulate that the amides studied here are in fact weakly self-associated in solution and that the resultant dimer (or multimer) species are destabilized in the latter solvent. Further, since the dielectric permittivities were determined only at moderate frequency, any short-lived solute-solvent contact pairing was not detected; hence, Kd tends to zero in each case in CCl@ The same trend can also be observed in the data (25 “C) of Bishop and SuttonPl who found that K1for phenol with pyridine decreased from 80 dm3 mol-’ in n-heptane to 45 dm3 mol-l in carbon tetrachloride [thence to 36 dm3 mol-l in benzene, a result of the strong interaction of benzene with phen01,~f’~ that is, even further destabilization of the phenol-pyridine moiety]. The widely recognized solvent dependence of spectroscopic K1data, in accordance with the reasoning of Carter, Murrell, and Rosch,ll may thereby also be explained on the basis that neither the UV/visible nor the NMR methods (as here) are sensitive to contact pairing. Only the “sociation” interaction is measured directly with these techniques while, at the same time, destabilization (or, conversely, apparent enhancement) is reflected indirectly by the change in K1 from one solvent to the next. The relations detailed here therefore appear to offer an alternative and supplemental method to the study of weak intermolecular interactions, for which only a well-defined stoichiometry is in principle required and where the basic equations need only straightforward extension in order to encompass other types of reaction or interaction. Further, and despite difficulties associated with experimental study by UV and NMR, combination of spectroscopic data with dielectric measurements seems likely also to provide complementary insight into the matter of weak complexation in nonelectrolyte media. Acknowledgment. Support provided by the National Science Foundation and by the Graduate School of The Ohio State University is gratefully acknowledged. We thank D. E. Martire and W. J. Taylor for helpful discussions during the course of this work. (58) West, R.; Powell, D. L.; Whatley, L. S.; Lee, M. K. T.; Schleyer, P. von R. J. Am. Chem. SOC.1962,84, 3221. (59) Allerhand, A,; Schleyer, P. von R. J. Am. Chem. SOC. 1963,85, 371.
