J . Phys. Chem. 1989, 93, 7252-7257
7252
as G, H+,G-,etc.) will orient solvent molecules to a greater extent in 1,2-propanediol-water media than in water. Consequently, this greater degree of orientation results in a more negative standard entropy of dissociation in the 1,2-propanediol-water solvent, which is produced by a decrease in the disorder of the system.
Acknowledgment. This project was supported by the National Natural Science Foundation of China. Registry No. H-Gly-OH, H20, 7732-18-5.
56-40-6;
HOCH2CH(OH)CH3,57-55-6;
Dissociation of Tritluoromethanesutfonic Acid in Aqueous Solutions by Raman Spectroscopy M. Sampoli, Dipartimento di Energetica, Universith di Firenze, via di S . Marta 3, 501 39 Firenze, Italy
N. C . Marziano,* and C. Tortato Dipartimento di Chimica, Universith di Venezia, Dorso Duro 21 37, 301 23 Venezia. Italy (Received: April 15, 1988; In Final Form: August 8, 1988)
The dissociation of aqueous trifluoromethanesulfonic acid (TFMSA) in the range 5-100 wt 7% has been studied by Raman spectroscopy and (Y values have been determined from integrated intensity of the band of the sulfonate group at 1035 cm-' relative to that of the C-S stretching at 770 cm-l taken as reference. Discrepancies are observed in comparison with previous IR estimates mainly at low acidities, and the possible reasons of the disagreement are discussed. The new results of TFMSA are compared with that of some other strong acids.
Introduction Trifluoromethanesulfonic acid (TFMSA) is one of the strongest of all known monoprotic acids as appears from the data available when the compound is studied as both solute and solvent in aqueous and nonaqueous ~ystems.l-'~ Many comparisons of its strength with that of other acids have been made in different solvents using conductivity data. In acetic acid, for instance, TFMSA has been found to be stronger than perchloric and fluorosulfuric acid: but in anhydrous sulfuric acid an inversion in the acid strength of fluorosulfuric and TFMSA has been ~bserved.',~ Values of Ho,el', HGF,lZ R013acidity functions (AF) calculated in the neat solvent and in aqueous solutions by Hammett's procedure14are also available. The A F method estimates the acidity of the medium from the equilibria of a homogeneous set of organic or potentiometric indicators (In) and the A F function is calculated by
where aHtis the hydrogen ion activity andfi,,fi,+ are the molar activity coefficients of un-ionized and ionized form of the indicator, respectively. In dilute solutions all the AFs are convergent to the pH scale, but in strongly acidic solutions, due to the specific interactions ( I ) Senning, A. Chem. Rev. 1965, 65, 385 and references cited therein. (2) Howells, R. D.; McCown, J. D. Chem. Rev. 1977,77,69 and references cited therein. (3) Fialkov. Y. Y.: Lieus. V. I. Dokl. Akad. NaukSSSR 1971. 197. 1353. (4j Fialkov; Y. Y.; L& V . I. Zh. Obshch. Khim. 1972, 42, 267; J. Gen. Chem. USSR 1972, 42, 256. (5) Balicheva, T. G.; Ligus, V. I.; Fialkov, Y. Y. Zh. Neorg. Khim. 1973, 18, 1735, Russ. J . Inorg. Chem. 1973, 18, 917. (6) Engelbrecht, A.; Rode, B. M. Monatsh Chem. 1972, 103, 1315. (7) Russell, D. G.; Senior, J. B. Can. J. Chem. 1974, 52, 2975. (8) Russell, D. G.; Senior, J. B. Can. J. Chem. 1980, 58, 22. (9) Grondin, J.; Sagnes, R.; Commeyras, A. Bull. SOC.Chim. Fr. 1976, -I .1 , 177 .. . . (10) Atkins, J.; Palling, D. J.; Poon, N. L.; Hall, C. D. J. Chem. SOC., Perkin Trans. 2 1982, 1107. ( I 1) Ridd, J. H., personal communication (preliminary results). (12) Cox, R. A.; Krull, U. J.; Thompson, M.; Yates, K. Anal. Chim. Acta 1979, 106, 5 1. (13) Carre, B.; Devynck, J. Anal. Chim. Acta 1984, 159, 149. (14) Hammett, L. P.; Deyrup, A. P. J. Am. Chem. Soc. 1932, 54, 2721.
with the medium, each set of indicators generates its own acidity f~ncti0n.l~ Despite the serious shortcomings of the AF the AFs have been used for relative comparisons and the Ho functions of some aqueous acid systems vs the mole fraction of acid are reported in Figure 1. These plots suggest, in contrast with other comparison^,^^-^^ that in aqueous solutions TFMSA is of lesser strength than perchloric acid and stronger than the others reported in the same figure. Analogous trends can also be obtained from HOF AFs12~25~26 (HGF AFs are estimated from redox equilibria of electrochemical indicators, while Ho AFs from protonation equilibria of primary nitroanilines). TFMSA as solute in sulfuric acid and in oleum has been studied by NMRz7 but the ionization equilibrium appears to be rather complex. The I9F resonance shift increases gradually from dilute to 100% H2S04and this shift has been ascribed to the solvent effect; then a step rise is observed in oleum, indicating a deionization process. However, in the latter range, unstable solutions and decomposition process have also been r e c o g n i ~ e d . ~This ,~ situation does not allow any reliable evaluation of pK, from the acid-base equilibrium of TFMSA as indicator. Further progress in the knowledge of acidic properties of TFMSA can be achieved by studying its degree of dissociation CY in aqueous acid solutions, but CY values have been derived only ( I 5) Rochester, C. H. Acidity Functions; Academic Press: London, 1970. (16) Cox, A. R.; Yates, K. Can. J. Chem. 1983, 61, 2225. (17) Marziano, N. C.; Sampoli, M.; Gonizzi, M. J. Phys. Chem. 1986, 90, 4347. (1 8) Bagno, A.; Scorrano, G.; More OFerrall, R. A. Rev. Chem. Intermed. 1987, 7, 313. (19) Bascombe, K. N.; Bell, R. P. J. Chem. SOC.1959, 1096. (20) Johnson, C. D.; Katritzky, A. R.; Shapiro, S. A. J. Am. Chem. SOC. 1969, 91, 6654. (21) Yates, K.; Wai, H.J. Am. Chem. SOC.1964.86, 5408. Cox, R. A,; Yates, K. Can. J. Chem. 1981, 59, 2116. (22) Leuchs, M.; Zundel, G. J. Chem. SOC.,Faraday Trans. 2 1978, 74, 2256. (23) Leuchs, M.; Zundel, G. Can. J. Chem. 1980, 58, 311. (24) Zundel, G.; Fritsch, J. In The Chemical Physics of Soluation; Dogonadze, R. R.,Kalman, E., Kornyshev, A. A., Ulstrup, J., Eds.; Elsevier Science: Amsterdam, 1986; part B. (25) Janata, J.; Jansen, G. J. Chem. SOC.,Faraday Trans. 1 1972, 68, 1656. (26) Modro, T. A.; Yates, K.; Janata, J. J. Am. Chem. Soc. 1975,97, 1492. (27) Koeberg-Telder, A.; Cerfontain, H. J. Chem. Soc., Perkin Trans. 2 1975, 226.
0022-3654/89/2093-7252$01.50/00 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989 7253
Dissociation of TFMSA
H.
-1 4
-1a
-1
-I
-I
-* - 2
D.D
4 2 1
I
1
1
I
1
1
1
I
1
I
on
0.1
D. 2
D.a
D.4
D.S
0.6
0.7
04
0.8
1
Nac I O Figure 1. Plot of Ho acidity functions for aqueous acid solutions vs mole fraction of acid (Nad): (0) CH3S03H,ref 19; (A) H2S04, ref 2 0 ( 0 )CF3S03H, ref 1 1 ; (*) HCIO,, ref 21.
by IR spectroscopy22 where the high broadening of the characteristic bands and large diffuse absorption, together with the general difficulties encountered in the case of strong electrolytes, could complicate the extraction of a from experimental data. A new experimental investigation by a different analytical method is therefore desirable and the present paper reports the degrees of dissociation of TFMSA in aqueous acid solutions obtained by Raman spectroscopy. The ionization equilibrium is studied by following the concentration dependence of both the vibration of the ionizable sulfonic group a t ~ 1 0 3 cm-' 5 28*29 and 0 taken as reference. the C-S stretching mode at ~ 7 7 cm-' 30931
Experimental Section Reagents. The acid solutions were prepared by mixing doubly distilled water and TFMSA, and vacuum distilled twice from the commercially available product (Janssen 99%). The percentage composition selected by weighing was verified by automatic potentiometric titration against standard solution of sodium hydroxide. Titrations were performed as described previously.'' (28) Haszeldine, R. N.; Kidd, J. M. J . Chem. Soc. 1954, 4228. Haszeldine, R. N.; Kidd, J. M. J. Chem. Soc. 1955,2901. Gramstad, T.; Haszeldine, R. N. J . Chem. Soc. 1956, 173. Gramstad, T.; Haszeldine, R. N. J . Chem. SOC.1957, 4069. (29) Burger, H.; Burczyk, K.; Blashette, A. Monatsh. Chem. 1970, 101, 102.
(30) Gillespie, R. J.; Robinson, E. A. Can. J . Chem. 1962, 40, 644. (31) Clarke, J. H. R.; Woodward, L. A. Trans. Faraday SOC.1966, 62, 2226.
Since available density data4,11*32 are not quite in agreement at high acidity, TFMSA distilled twice and handled under an atmosphere of dry nitrogen has been used for determining the density of the pure product. The measurements have been made at 25 OC by weighing a known volume of TFMSA with a precision micrometric syringe (calibrated at the same temperature with distilled water). The density found for pure TFMSA, 1.6989 g/cm3, is very close to the values of ref 4 and 11. Raman Measurements. A fairly conventional apparatus for Raman spectroscopy has been used. An incident laser beam, polarized normal to the scattering plane with 1-W power at 488 nm, was moderately focused into the sample. The 90° scattered light is analyzed by a Coderg T800 triple monochromator (usually with the polarization analyzer along the direction of the incident electric field) and detected by a low-noise cooled photomultiplier tube. The outcoming signal is processed by a single photon counting digital system and recorded on a personal computer as well as on a strip chart recorder for visual inspection. An instrument resolution of about 5 cm-l was suitable for measuring the Raman bands of liquid solutions of interest. Integration times ranging from 2 to 20 s (depending on the signal) together with frequency steps of 1 cm-' are usually able to provide good recordings to be processed for estimating integrated intensities. The solutions were contained in an all-fused-silica cell with four plane rectangular windows (1 X 5 cm) of optical quality. The (32) Corkum, R.; Milne, J. J . Can. J . Chem. 1978, 56, 1832.
Sampoli et al.
7254 The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
200
1000
600
1400
v c" Figure 2. Raman spectra of aqueous solutions of TFMSA a t (a) 15.8 wt %, Io = 0.0; (b) 86.9 wt %, IO = 0.1; (c) 95.8 wt %, IO = 0.4.
silica cell was put inside a brass holder kept a t constant temperature within f0.5 OC. The spectra were usually measured at 25 OC but higher temperatures (up to 45 OC) have been used for high acid concentrations in order to avoid the precipitation of the stable monohydrate which is a solid at room temperature.2,28 Attention was paid to avoid temperature gradients and liquid turbulences. The cell was almost entirely surrounded by the thick brass holder and its thermal contact in the lower part below the small holes for incident and scattered light was enhanced by silicon oil. Further, the incident laser beam was moderately focused (focal length = 200 mm) so as to gather as much as possible scattered light without introducing significant fluctuations due to nonstationary liquid movements. Integrated Raman band intensities have been evaluated from the digital recordings of band shape with linear interpolation of base line. Although the investigated bands are isolated, relatively narrow and well-defined, as we can see from the reported spectra in Figure 2 and 3, the interpolation is the major source of uncertainties, amounting usually to less than 5%, but up to 20% in the worst case. As a matter of fact, in spite of the careful attention paid to use high-purity products, a large unstructured background scattering was present at high acid concentrations. This background being not very stable and dependent on various experimental arrangem e n t ~prevents ~~ reliable intensity estimates at very high acid (33) Careri, G.; Mazzacurati, V.;Sampoli, M.;Signorclli, G. Advances in Raman Spectroscopy;Heyden: London, 1972; Vol. I, p 624.
concentrations, as will be discussed in the next section.
Results and Discussion The dissociation of TFMSA in aqueous solutions was determined from the relative integrated intensities of selected bands obtained by Raman spectroscopy. The spectral behavior of some TFMSA-H20 mixtures is reported in Figure 2. It can be seen that the band at =lo33 cm-I is a good candidate for a dissociation study. Indeed it is a strong band at low acid concentrations and its molar intensity decreases as the acidity of medium increases. This band has been assigned to the symmetric SO3stretching mode of the sulfonic g r o ~ p ~and ~ *its* behavior ~ with dilution is similar to that previously observed in the Raman spectra of CH3S03H-
H20 The band at =770 cm-l, already assigned to the C-S stretching mode30931is not related to the proton dissociation and has been taken as internal standard. The linear dependence of its integrated intensity against the acid molarity of the solution has been checked. The shapes of both SO3 and C-S bands are found to be practically independent of medium composition as we can see by comparing the spectra of Figure 3 at low and high acidities. The two bands are well separated and it is rather easy to evaluate their integrated intensity. However, the strong and unsteady unstructured background, already mentioned in the previous section, makes the evaluations quite difficult especially at high acid percentages. This background can strongly be reduced by the addition of a small quantity ( = l wt %) of nitric acid but such an addition can affect significantly the acid dissociation when the
-
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7256
The Journal of Physical Chemistry, Vol. 93, No. 20, 1989
Sampoli et al.
Figure 4. Degrees of dissociation vs mole fraction of acid (Nlcid) of (A) CH3S03H from Raman intensity of CH3SOc at 1050 cm-I, ref 17; (m) CF3S03H from Raman intensity of CF3SOf at 1035 cm-I, present work; ( 0 )CF3S03Hfrom IR absorption of nondisscciated acid at 930 cm-I, ref 22; (0)HCIO, by Raman intensity of Cl04- at 930 cm-I, ref 34; (+) HC1O4 by ‘H NMR, ref 36; (0)HCIO, by 3sC1NMR, ref 34; (*) H 2 S 0 4 from Raman intensity of HS0,- at 1040 cm-I, ref 37; (+) H2S04 from Raman intensity of nondissociated acid at 910 cm-’, ref 37. The x axis for the two last sets of experimental data has been shifted for clarity. The Raman data of present work are also reported shifted.
Values of the degree of dissociation of TFMSA in aqueous solutions are given in Table I and plotted on Figure 4 together with previous values estimated by IR spectroscopyz2and literature dissociation data of some other strong acids. As is evident, our Raman values are much lower than previous IR values by amounts outside of our experimental uncertainty. Environmental effects on IR and Raman bands can hardly be responsible for the large observed discrepancies. For instance, at mole fraction N = 0.4 the Raman estimate of a is about 0.7 against an IR estimate of 1 - CY very close to zero. It is noteworthy that IR dissociation data for TFMSA solutions are calculated from the absorption of undissociated acid, while Raman data, from the scattering of the anion and that can be taken into account in a different framework. It has been suggested that in other strong acid concentrated solutions the dissociation is preceded by the formation of ion pair; Le., the dissociation is a two-step process:3840 AH
+ H 2 0 s A-.H30+
s A-
+ H30+
Such a behavior has been hypothesized for aqueous sulfuric acid s o l ~ t i o n and s ~ ”dissociation” ~ ~ ~ ~ ~ ~ values calculated from the Raman intensities of characteristic bands of both HS04- anion and undissociated molecule have been reported on the right side (38) Irish, D. E.; Puzic, 0. J . Solution Chem. 1981, 10, 377. (39) Stewart, R. The Proton: Applications to Organic Chemistry; Aca.. demic Press: New York, 1985. (40) Brooker, M. H. In The Chemical Physics of Sohation; Dogonadze, R. R., Kalman, E., Kornyshev, A. A., Ulstrup, J., Eds.; Elsevier Science: Amsterdam, 1986; part B-. (41) Zarakhani, N. G . ; Librovich, N. B.; Vinnik, M. I. Zh. Fiz. Khim. 1971, 45, 1733. (42) Maiorov, V. D.; Librovich, N. B. Zh. Fir. Khim. 1973, 47, 2298. (43) Cox, R. A.; Haldna, U. L.; Idler, K. L.; Yates, K. Can. J. Chem. 1981, 59, 2591. (44) Malinowski, E. R.; Cox, R. A.; Haldna, U. L. Anal. Chem. 1984,56, 778.
of Figure 4. As we can see, the dissociation from HS04- is significantly lower than the corresponding from HZSO4. The difference is attributed to the presence of ion pairs which reasonably could contribute quite little to the anion and undissociated acid Therefore, the same reason could be the source of the discrepancy between IR and Raman values in the present case of TFMSA solutions. In Figure 4 the dissociation of TFMSA is also compared with that of p e r ~ h l o r i cand ~ ~ ,methanesulfonic ~~ acid.I7 For HClO, determined by different some of the available CY va1ues22~34~36~4548 techniques are taken into account. The present comparison is not in complete agreement with a similar plot reported by Leuchs and Zunde123*24 and suggests either a different acid strength scale, or that within the experimental spread of the data obtained by different techniques, it is quite hard for instance to assess that perchloric acid is of lesser strength than TFMSA.23,z4However, by measuring a characteristic band of a given species (anion) and by using the same technique (Raman), TFMSA ( N < 0.8) appears to be weaker than HC104, similar to HzS04, and obviously stronger than CH3S03H.The sequence is the same as that found in Figure 1 by employing HoAFs. For a suitable estimate of “acidity” of concentrated strong acid solutions and for describing protonation equilibria of organic and inorganic solutes, the “activity coefficient function” (Mc)49has been demonstrated to be a more powerful tool than AFs.’@I* New (45) 2067. (46) (47) (48) (49)
Hood, G . C . ; Redlich, 0.; Reilly, C.
A. J . Chem. Phys.
1954, 22,
Hood, G . C.; Reilly, C. A. J . Chem. Phys. 1960, 32, 127. Heinzinger, K.; Weston, R. E. J. Chem. Phys. 1965, 42, 272. Ratcliffe, C. I.; Irish, D. E. Can. J . Chem. 1984, 62, 1134. Marziano, N. C.; Cimino, G . M.; Passerini, R. J . Chem. SOC.,Perkin Trans. 2 1973, 1915. Marziano, N. C.; Traverso, P. G.; Tomasin, A,; Passerini, R. J . Chem. SOC.,Perkin Trans. 2 1977, 309. Marziano, N . C.; Tomasin, A.; Traverso, P. G. J . Chem. SOC.,Perkin Trans. 2 1981, 1070.
J . Phys. Chem. 1989,93,1251-1261 studies are in progress for determining the M , function for TFMSA aqueous solutions by using equilibria of different indicators. Preliminary studies using Raman spectroscopy of CF3S03H-HN03-H20 mixtures with H N o 3 as indicatorSoshow for ' a titration curve related instance that in the range 80-100 wt % (50) Sampoli, M.; Marziano, N. C., unpublished results.
7257
to protonation-dehydration equilibrium of nitric acid8 is obtained. H N 0 3 + CF3S03H
H 2 0 + NO2+ + CF3S03-
The curve is similar to that observed in H2SO4-HNO3-H20 mixtures35 and the half-ionization occurs at about the Same mole fraction of the solvent, as expected from the previous discussion. Registry No. TFMSA, 1493-13-6.
Vibrational Pressure Tuning Spectroscopy of the Polymethylene Chain. 1. Various n-Alkanes from C8H,8 to C38H71t W. W. Ley and H. G . Drickamer* Materials Research Laboratory and School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 (Received: February 27, 1989; In Final Form: May 9, 1989)
In this paper, we present the effect of pressure on the vibrational spectra of a series of 11 crystalline paraffin hydrocarbons from n-C8H18to n-C36H74plus n-CsDl8 and a few branched-chain compounds. The focus is on the increase in intensity of the CHI wagging and C-C stretching vibrations with pressure. The sum of the intensities increases in a manner that depends primarily on chain length. The distribution between the two vibrations depends on the crystal structure. The results imply that the primary event is an increase in the wagging vibration intensity due to increased intermolecular coupling. This intensity is redistributed to the C-C stretching vibration via some lattice mode (phonon-assistedcoupling or phonon-assisted resonance). This resonance is most efficient for the orthorhombic structure with 4 molecules/unit cell and least efficient for the triclinic structure with 1 moleculelunit cell. For n-C~oH62and n-C36H74, which have a monoclinic structure with 2 molecules/unit cell, the behavior is intermediate.
Introduction The properties and behavior of the n-paraffins are of great interest to physical and biological scientists alike since they serve as models and analogues for carbon-based molecules from polyethylene to phospholipids. The solid-state vibrational spectra of polymethylene chain molecules, both the n-alkanes and polyethylene, are well-known, and the theory of their normal modes of vibration has been worked out. N o attempt will be made here to describe this large body of work; instead, the reader is referred to the appropriate references.'-8 Further, and rather elegant, refinements to this body of work have been made by Strauss et al.,g-'l who describe the use of the CD2 probe and the use of high-temperature spectra to better understand these vibrations. There have been previous high-pressure investigations of the vibrational spectra of n-C16H3412 and other polymethylene chain molecules,"'6 but the effects observed and discussed in this paper were not as fully exploited due to the limitations in both the pressure cells and spectrometers available at the time. Experimental Procedure All spectra were recorded by a Nicolet Model I199 FTIR fitted with a 4X Perkin-Elmer beam condenser to focus the IR radiation on the sample. The samples were held in a diamond anvil cell with type-I1 diamonds and confined by an Inconel gasket;17 pressure was measured by the ruby fluorescence method.17 All samples were loaded neat and run for 300-1000 scans, depending on the strength, and hence S / N ratio, of the interferogram. Two samples, n-CZ3H4and n-C24H50,were run surrounded with mineral oil as a pressurizing fluid to verify the hydrostaticity of the neat hydrocarbon pressure runs. The n-alkane molecules CnHZh2( n = 8, 10, 12-16, 23, 24, 30, and 36) were studied as well as the deuterated molecule n-C8D18 and a 50 mol % mixture of n-C8H18 and n-C8DI8. The branched-chain hydrocarbons 2,2,4,4,6,8,8-heptamethylnonane, lThis work was supported in part by the Materials Science Division, Department of Energy, under Contract DE-AC02-76ER01198.
\
-500 1300 1100
Wavenum bers
Wavenum bers
Figure 1. IR spectra of n-CsHls. The CH2 wag is indicated by the heavy arrow; the C-C stretch is indicated by the thinner slanted arrow.
2,6,10,14-tetramethylpentadecane,and 4-methylheptane were studied also. (1) Painter, P. C.; Coleman, M. M.; Koenig, J. L. The Theory of Vibrational Spectroscopy and its Application to Polymeric Materials; Wiley-Interscience: New York, 1982. (2) Snyder, R. G. J . Mol. Spectrosc. 1960, 4 , 411-434. (3) Snyder, R. G. J . Mol. Spectrosc. 1961, 7, 116-144. (4) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85-1 16.
0 1989 American Chemical Society
