1640
J . Phys. Chem. 1984,88, 1640-1642
the structure of the hydrogen form was probably due to its having a cell dimension of 15 A, close to the expected transition between Im3m and I43m (Figure 3 ) . The small value of A suggests that the sample studied was probably not completely dehydrated, Removal of Cs from the channels of Rho the elliptical distortion to decrease, A relaxing from a value of 1.60 A for Cs-exchanged Rho to 1.52 8, for dD-Rho.
Conclusion Upon dehydration, the deammoniated form of zeolite Rho undergoes a symmetry change from space group Im3m to I43m.
This change is predicted on the basis of DLS structural modeling.
Acknowledgment. This work would not have been possible without the Of DrsL. Abrams in preparing the for the diffraction experiments. Thanks are also due to Ms. Y . King for the preparation of this manuscript and to Drs. A. W. Sleight, and G. D. Stucky for encouragement and many enlightening discussions. Supplementary Material Available: Observed and calculated profiles corresponding to Figure 5 (4 pages). Ordering information is given on any current masthead.
Carbon-I3 NMR Study of the Barrier to Internal Rotation of N,N-Dimethylacetamide in the Adduct with Antimony( II I)Chloride Fred Y. Fujiwara* and Claudio Airoldi Instituto de Quimica, Universidade Estadual de Campinas, 13.100 Campinas, SP, Brazil (Received: May 1 1 , 1982; In Final Form: September 12, 1983)
The barrier to internal rotation of N,N-dimethylacetamide(DMA) when coordinated to the Lewis acid SbC13was determined from total line-shape analyses of the "C NMR spectra of the neat liquid adduct SbC13*DMA.The Arrenhius energy of activation for the hindered rotation about the CN bond was found to be 73.8 1.1 kJ mol-', and the enthalpy and entropy of activation, 71.0 f 1.2 kJ mol-' and -5 i 4 J K-'mol-', respectively. The rotational barrier in DMA decreases by a small amount on formation of this adduct in contrast with the increase normally observed for similar complexes. The decrease of the rotational barrier is discussed in terms of relative stabilizing effects of adduct formation on the ground and transition states.
*
Introduction The barrier to internal rotation in amides has been extensively studied by nuclear magnetic resonance (NMR) spectroscopy,',* however, there are few studies of the effect of adduct formation with Lewis acids on the rotational barrier about the C-N bond in a m i d e P and in ureas.&* These studies have shown, with some exceptions, that adduct formation at the carbonyl oxygen increases the barrier to internal rotation. However, in almost all of the studies reported only approximate methods could be used to determine the activation parameters' The study Of the rotation in these adducts is often complicated by the donor-acceptor exchange process in solution, which may be slow or fast relative to the rate of internal roation, and by the presence of multiple equilibria. Also, a total line-shape analysis of the 'H spectra of most amides requires a knowledge of the small spinspin coupling constants, which are difficult to resolve in the spectra of the adducts. In the present paper a '" NMR study Of the adduct between antimony(r11) and N,N-dimethylacetamide (DMA) is (1) W. E. Stewart and T. H. Siddal, 111, Chem. Rev., 70, 517 (1970). (2) L. M. Jackman in 'Dynamic Magnetic Resonance Spectroscopy", L. M. Jackman and F. A. Cotton, Eds., Academic Press, New York, 1975, p 203. (3) E. S . Gore, D. J. Blears, and S . S. Danyluk, Can. J . Chem., 43, 2135 (1965). (4) G. Matsubayshi and T. Tanaka, J . Inorg. Nucl. Chem., 31, 1963 (1969). (5) P. Stilbs, Tetrahedron, 29, 2269 (1973). (6) G. Olofsson, P. Stilbs, T. Drakenburg, and S . ForsBn, Tetrahedron, 27, 4583 (1971). (7) J. S. Hartmann and G. J. Schrobilgen, Can. J. Chem., 51, 99 (1973). (8) M. L. Martin, M. L. Filleux-Blanchard,G. J. Martin, and G. A. Webb, Org. Magn. Reson., 13, 396 (1980).
0022-3654/84/2088-1640$01.50/0
presented. The adduct, SbC13.DMA, has been isolated, and spectroscopic studies have shown that coordination occurs through the oxygen atom of the carbonyl g r ~ u p . ~ - 'In ~ solution, the association constants for the formation of the 1:l adduct between SbCI3 and DMA are not exceptionally large;"^'^,'^ however, in the NMR spectra of the neat adduct, SbC13.DMA, which is a viscous liquid, no apparent dissociation of the adduct was observed. A total line-shape analysis of the 'H spectra of the neat adduct was not possible since the small spin-spin coupling constants and the line widths in the absence of the exchange could not be resolved. However, the proton~decoupled13C N M R spectra of the adduct can be analyzed as a simple "two-site'' exchange process using a total line-shape analysis.
Experimental Section N,N-Dimethylacetamide (E. Merck) was carefully dried over Ba0,15 distilled through an efficient column, and stored Over molecular sieves. Antimony(II1) chloride (E. Merck) was sublimed in vacuo and transferred to a 10-mm N M R tube under a dry nitrogen atmosphere. The N M R tube was connected to a vacuum line, an excess of DMA distilled into the tube, and then (9) V. Gutmann and H. Czuba, Monatsh. Chem., 100, 708 (1968). (10) A. Kiennemann, G. Levy, and C. Tanielian, J . Organomet. Chem., 46, 305 (1972). (11) M. Van Cauteren and T. Zeegers-Huyskens, Inorg. Nucf. Chem. Lett., 12, 323 (1976). (12) M. Van Cauteren-Thevissen and T. Zeegers-Huysken, Inorg. Chim. Acta, 32, 33 (1979). (13) C. Airoldi, Inorg. Chem., 20, 998 (1981). (14) C. Airoldi, P. L. 0. Volpe, and J. M. M. de M. Lira, Polyhedron, in
press. (15) C. D. Schulback and R. S . Drago, J . A m . Chem. Soc., 82, 4484 (1960).
0 1984 American Chemical Society
Barrier to Internal Rotation of DMA in SbC13.DMA sealed after the excess solvent was thoroughly pumped off. The characterization and thermochemistry of the adduct SbC13.DMA has been reported re~ent1y.l~ The 10-mm tube was inserted into a 12-mm tube by using Teflon spacers, and hexadeuteriodimethyl sulfoxide was placed in the annular region to provide a deuterium lock signal. The 13C spectra were obtained with a Varian XL-100 spectrometer operating at 25.2 MHz with proton broad-band idecoupling. The spectra of the methyl groups were obtained by usiing a sweep width of 1024 Hz (resolution 0.25 Hz/point) and with accumulation times of 2-15 min. After the spectra of the N-methyl region were expanded to 200 Hz, the spectra were digitized manually. Near the coalescence point, a smooth curve was drawn through the noise before the spectra were digitized. The rate constants were evaluated at 12 temperatures between 304 and 385 K by a total line-shape analysis using an iterative computer program previously des~ribed.'~,'' The analysis of a "two-site" exchange proce:ss is notoriously susceptible to systematic errors when the effective line widths in the absence of exchange and the chemical shift difference in the fast-exchange region cannot be accurately predicted.1s-20 For the complex SbC13.DMA no variation in the chemical shift difference between the N-methyl 13Cresonances was observed in the slow-exchange region, and the observed value of 64.3 f 0.1 Hz was assumed to be valid in the fast-exchange region. The line width of the acetyl carbon resonance provides a reliable estimate of the line width of the N-methyl resonances i n the absence of exchange. At ambient temperature all three methyl groups were observed to have the same spin-lattice relaxation times, and contributions to the line widths of these carbon resonances due to magnetic field inhomogeneity and incomplete proton decoupling should be the same. Line widths at half-height of the acetyl carbon resonance decreased from about 3 to 1.5 Hz over the temperature range (the viscosity decreased noticeably with temperature). The variation in the temperature determined before and after a spectrum was obtained, with use of a dummy sample containing a copper-constantan thermocouple, was at most 0.3 "C. The activation parameters were determined from a least-squares fit of In k vs. 1/ T and In ( k / T ) vs. 1/ T , and the eirrors limits were calculated by assuming only random errors.
Results and Discussion The 13C chemical shifts (G(Me4Si)) for the carbonyl, trans N-methyl, cis N-methyl, and acetyl carbons in the adduct, SbC13.DMA, were found to be 173.9, 40.1, 37.11,and 21.6 ppm, respectively, at 305 K. The chemical shifts were determined by assuming a chemical shift of 77.2 ppm for chloroform, present in the annular region of the coaxial tube, without correction for bulk diamagnetic susceptibilities. The relative positions of the N-methyl carbon resonances has been previously established.21 On formation of the adduct with SbC13, the I3C resonances of DMA are shifted to lower field by 4.6, 3.1, 3.7, and 1.0 ppm, respectively, relative to neat DMA. The larger effect of adduct formation on the cis N-methyl carbon resonance reduced the chemical shift difference between the two N-methyl carbons from 3.10 ppm in the unbound molecule to 2.55 ppnn in the adduct. The temperature difference of I3Ccomplex shifts was extremely small. The difference between the carbonyl carbon chemical shifts of DMA in the complex and in neat DMA decreased by only 0.16 ppm over a 100 K temperature range. In this temperature range no change in the average complex shift of the N-methyl resonances was observed, while a decrease of 0.39 ppm was observed for the difference between the acetyl carbon resonances, which is mainly due to a larger temperature dependence of the iinbound DMA. (16) L. W. Reeves and K. N. Shaw, Can. J . Chem., 48, 3641 (1970) (17) L. W. Reeves and K. W. Shaw, Can. J . Chem., 49, 3671 (1971). (18) A. Allerhand, H. S . Gutowsky, J. Jonas, and R. A, Meinzer, J . Am. Chem. SOC.,88, 3185 (1966). (19) T. Drakenburg, K. Dahlqvist, and S . ForsCn, Acta Chem. Scand., 24, 694 (1970). (20) R. R. Shoup, E. D. Becker, and M. L. McNeel, J. Phys. Chem., 76, 71 (1972). ( 2 1 ) W. McFarlane, J . Chem. SOC.D,418 (1970)
The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 1641 TABLE I: Barrier t o Internal Rotation of DMA in the Adduct SbC1;DMA
-
AS+,J E,, kJ
AG*,,,, kJ mol-'
molecule
mol-'
SbC1,.DMAa DMA,neatb
73.8 t 1.1 79.5 +- 0.4
72.6
?:
1.6
7 5 . 7 t 0.4
AH*:kJ mol-'
mol-' K-'
71.0 2 1.2 7 6 . 6 t 0.4
-5 ?: 4 32 4
a The errors quoted were calculated from the standard deviations of the linear least-squares fit assuming only random errors. Reference 22.
Apparently there is little, if any, dissociation of the adduct in the neat liquid phase. This is consistent with the observation that the adduct can be distilled at reduced pressures.l0 Table I presents the kinetic parameters for the hindered rotation of the amide bond in SbC13.DMA determined by total line-shape analyses of the I3C NMR spectra as well as those for neat DMAZ2 determined by 'H N M R for comparison. Several careful determinations of the barrier to internal rotation of DMA, neat and in solution, have shown that the solvent effects on the barrier are relatively The small decrease in the rotational barrier in this complex is surprising. It has been generally accepted that complex formation at the oxygen atom of the carbonyl group increases the barrier to internal rotation in amides by stabilizing the resonance from \
N,
t
=C
AU \
Determinations of the barrier to internal rotation in adducts of amides with various Lewis acids support this conclusion with some exceptions. Studies of the adducts of BF3,3SnC14,4TiC14,4and SbC1,4,5 with N,N-dimethylamides have shown that complex formation increases the barrier to internal rotation in the order of 10-15 kJ/mol. In the case of SiCl, and GeC14, the observed decrease in the barrier was attributed to coordination at the nitrogen atom.4 The interaction of small cations with carbonyl oxygen can also stabilize the charge-transfer resonance form. An increase in the coalescence temperature of N,N-dimethylformamide has been observed in the presence of the Li+ ion.23,24 In contrast, Ag' ions were observed to decrease the rotational barrier of DMA in aqueous solution, and it was suggested that small concentrations of amide molecules in other forms with a very low barrier could be responsible for the observed results.25 In the presence of rapid exchange between various species, the apparent rotomer exchange rate could be significantly lowered by the presence of small concentrations of amide complexes with a very low barrier. Studies of adducts of ureas with SbC15,5s6C O ( I I ) ,BF3,' ~ ~ and TiC148have shown that adduct formation also increases the rotational barrier in ureas. Acid catalysis of the hindered rotation in amides has been clearly establi~hed.*~-~~ Amides protonated at the nitrogen atom have no C-N double-bond character, and even a small fraction of the molecules in this form would reduce the barrier to rotation below detection by NMR. However, the fraction of N-protonated species, even in 5 N aqueous acid solution, must be extremely low since the barrier in N,N-dimethylformamide is only reduced by approximately 15 kJ/mol.' In preliminary studies of SbC13.DMA using less stringent precautions to avoid contamination with at(22) T. Drakenburg, K. Dahlqvist, and S . Forsen, J Phys. Chem., 76,2178 11972) ,--- I .
(23) W. Egan, T. E. Bull, and S . ForsCn, J . Chem. SOC.,Chem. Commun.,
____
I n99 (1 973) \ - - ' - I .
(24) B. M. Rode and R. Fussenegger, J . Chem. SOC.,Faraday Trans. 2, 71, 1958 (1975). (25) P. A. Temussi, T. Tancredi, and F. Quadrifoglio, J. Phys. Chem., 73, 4227 (1969). (26) D. R. Eaton and K. Zaw, Can.J . Chem., 49, 3315 (1971). (27) G. Fraenkel and C. Franconi, J . Am. Chem. SOC..82.4478 (1960). (28) C. A. Bunton, B. N. Figgis, and B. Nayck, Adu. Mol. Specrrosc., Proc. Inf. Meet. 4th, 1959, 3, 1209 (1962). (29) L. M. Jackman, T. E. Kavangh, and R. C. Haddon, Urg. Magn. Reson., 1, 109 (1969).
1642
J . Phys. Chem. 1984, 88, 1642-1648
mospheric moisture, the free energy of activation was not significantly different from the value presented here. Vibrational spectroscopic studies of the adduct, SbCl3-DMA, have clearly established that adduct formation occurs at the oxygen atom of the carbonyl group. A decrease in the CO stretch frequency and an increase in the CN stretch and O C N deformation frequencies have been observed on complex for ma ti or^.^^'^^' Although the presence of a small amount of adduct with coordination a t the nitrogen cannot be completely ruled out, there is no evidence which indicates the presence of this type of adduct. The Lewis acids, AlBr, and GaCl,, have been observed to form complexes with N,N-disubstituted amides in which two molecules of the acceptor are complexed to one molecule of the amide.30 Although the site of second acceptor could not be unambiguously determined, it was concluded that the center of coordination of the second acceptor molecule is not the nitrogen atom. In the molecular orbital description of the amide bond, the planar ground-state configuration is stabilized by mixing of the nitrogen lone-pair orbital with the carbonyl a bond, giving rise to a barrier to internal rotation.31 Adduct formation of Lewis acids with the oxygen of the carbonyl group should further stabilize the ground state. However, a decrease in the rotational barrier would occur if adduct formation had a greater stabilizing effect on the transition state. A rotation of 90° about the CN bond would place the nitrogen lone-pair and the oxygen lone-pair electrons in the same plane, and SbC1, could coordinate to both oxygen and nitrogen simultaneously. SbCI, does form five-coordination complexes with oxygen,10~1z-14*32 nitrogen,32and sulfur donor^.^^-^^ Also, the amide nitrogen would be a much stronger (30) L. A. Ganyuskin, I. P. Romm, E. N. Gur’yanova, and R.R. Shifrina, J . Gen. Chem. USSR (Engl. Transl.), 50, 1739 (1980). (31) J. F. Yan, F. A. Momany, R. Hoffmann, and H. A. Scheraga, J . Phys. Chem., 74, 420 (1970). (32) M. J. Gallagher, D. P. Graddon, and A. R. Sheikh, Thermochim. Acta, 27, 269 (1978). (33) G. Kiel and R. Engler, Chem. Ber., 107, 3444 (1974).
donor in the transition state, where conjugation with the carbonyl group is not possible. A coordination of SbC1, to both oxygen and nitrogen atoms could stabilize the transition state sufficiently to reduce the barrier to internal rotation. Another possible structure for the transition state which would produce the same effect is a cyclic structure in which the oxygen atom is coordinated to the antimony atom and the nitrogen coordinated to SbC1, through one of the chlorine atoms. In a recent calorimetric study of the reactions of antimony(II1) halides with Lewis bases, 3 mol of aliphatic amines was observed to react almost quantitatively with 1 mol of SbX3 in 1,2-dichloroethane solution.32 The evidence indicated that, in these adducts, the three molecules of base are not coordinated with the antimony atom but rather one to each halogen. This type of adduct formation was observed, however, only with the strongest proton bases and was not observed with heterocyclic bases, aromatic amines, oxygen donors, or phosphines. In the nonplanar amide molecule, the nitrogen may be sufficiently basic for this type of interaction to occur. The decrease in the barrier to internal rotation of D M A on adduct formation with SbC1, is apparently due to the greater stabilizing effect in the transition state. This effect may also be present in the other amide adducts in which a decrease in the rotational barrier was o b ~ e r v e d . ~T~h ~e *type of structures suggested for the transition state would not be expected for most amide complexes because of steric or electronic considerations.
Acknowledgment. W e wish to thank Prof. L. W. Reeves of the University of Waterloo, Waterloo, Ontario, Canada, for providing the computer program. The financial support of FAPESP is gratefully acknowledged. Registry No. SbC13-DMA,40325-80-2. (34) G. C. Pellacani, G. Peyronel, W. Malavasi, and L. Menabue, J. Inorg. Nucl. Chem., 39, 1855 (1977). (35) V. M. Schmidt, R. Bender, and C. Bruschka, Z . Anorg. Allg. Chem., 454, 160 (1979).
Mass-Action Model of Mixed Micellization R. F. Kamrath and E. I. Franses* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907 (Received: May 12, 1983)
We develop the single-micelle-sizemodel for dilute aqueous binary ionic surfactants with the same polar group and counterion. We determine the mixed micelle composition(s) by minimizing the total Gibbs free energy. We calculate first and second critical micellization concentrations (cmc’s), monomer, micelle, and counterion concentrations, and micelle mole fraction(s) x . The parameters are total surfactant concentration, mole ratio of surfactants, salt concentration, individual cmc ratio, degree of counterion binding 0,micelle aggregation number N , and the excess free energy function w ( x ) of mixing of the surfactant chains. For dilute solutions with no intermicellar interactions we develop general conditions of azeotropic mixed micellization and of demixing of partially miscible micelles. The results for = 0 apply to binary nonionic surfactants. We present numerical results for w ( x ) = wo, Le., the strictly regular solution model. For that model the necessary condition for demixing is wo > 2 and for azeotropy is lwol 2 (((1 + @ ) N - 1)/N) In (c2*/c1*). The mass-action model (MAM) should be preferred over the simpler pseudophase separation model (PSM) for N less than about 50.
Introduction Surfactants are used extensively to control the bulk and interfacial properties of solutions. Surfactants in solution can self-associate to form equilibrium, closed, colloidal aggregates called micelles. In aqueous solutions, micelles usually consist of 20 to several hundred molecules, which are associated so that the surfactant polar groups form a closed shell surrounding the hydrophobic surfactant hydrocarbon chains.’ The micelle interior region, being fluid-hydrocarbon-like, can solubilize hydrocarbons.* (1) Mukerjee, P. Adu. Colloid Interface Sci. 1967, I , 241.
Micelles indirectly affect interfacial properties. Their formation determines monomer activities and hence surfactant adsorption a t ,interfaces. The formation of mixed micelles, which contain more than one type of surfactant, is especially important since most commercially available surfactants are in fact mixtures. W e have developed general models of micellization of binary nonionic-nonionic and ionic-ionic surfactant mixtures in the pseudophase separation limit.3%4W e have considered the mixed (2) Roberts, R. T.; Chachaty, C. Chem. Phys. Letf. 1973, 22, 348. (3) Kamrath, R. F. M.S. Thesis, Purdue University, West Lafayette, IN, 1981.
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