Low-frequency Raman spectra and molecular association in liquid

LOW-FREQUENCY RAMANSPECTRA IN FORMIC AND ACETICACIDS

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Low-Frequency Raman Spectra and Molecular Association in Liquid Formic and Acetic Acids'

by Peter Waldstein and L. A. Blatz University of California, Loa Alamoa Scientific Laboratory, Lo8 Akzmos, New Mexico 87644 (Received December 27, 1966)

The low-frequency Raman spectra of normal and perdeuterated liquid formic and acetic acids and their solutions in water and hydrocarbons are reported. All the lines seen can be attributed to out-of-plane vibrations of various hydrogen-bonded associated species of the acids. Only lines assignable to cyclic dimers were observed in the hydrocarbon solutions. Glacial acetic acid also consists primarily of cyclic dimers. I n anhydrous formic acid and in aqueous solutions of both acids, the predominant species is either an open dimer or a chain polymer.

Introduction

Experimental Section

Molecular association of formic and acetic acids in the liquid and in solution has been studied by a wide variety of physical rnethods."l7 Several infrared and Raman studies have been made,18-22but in most cases interpretation has been complicated by the difficulty in assigning particular bands to particular vibrations of particular associated species (monomer, cyclic dimer, open dimer, chain polymer) of the acid. Spectra below 303 cm-l are partially free from this difficulty. The monomers have no fundamentals below 500 cm-' except for the internal rotation of the methyl group in acetic acid, which is probably very weak and which, if observed, can easily be identified by its frequency shift on deuteration. Furthermore, some of the bands can be assigned to the cyclic dimer on the basis of the work of Rliyasawa and P i t ~ e r . The ~ ~ lowfrequency infrared spectra of these acids in the gas,2e in the pure liquid,n,z8and in CCI, solutionz8 have been reported and low-frequency Raman lines have been observed in the pure liquid^.^^^^^ No low-frequency Raman spectra of these acids in solution or isotopic effects on the Raman spectra of the pure liquids have been reported. Here we report the low-frequency Raman spectra of normal and perdeuterated formic acid in benzene, acetic acid in n-pentane, and both acids in the pure liquid and in aqueous solution.

The method of observing low-frequency Raman lines has been described e l s e ~ h e r e . ~ ~ No J l essential changes in method were made during this study.

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(1) Work performed under the auspices of the U. S. Atomic Energy Commission. (2) G. C. Pimentel and A. L. McClellan, "The Hydrogen Bond," W. H. Freeman and Co., San Francisco, Calif., 1960. (3) G. A. Allen and M. A. Caldin, Quart. Rev. (London), 7, 255 (1953). (4) N. I. Gulivets, A. E. Lutskii, and I. V. Radchenko, Zh. Strulct. Khim., 6 , 27 (1965). (5) H. Giesenfelder and H. Zimmermann, Ber. Bunsengea. Phyaik. Chem., 67, 480 (1963). (6) A. E.Lutskii and V. N. Solon'ko, Zh. Fiz. Khim., 39, 783 (1965). (7) A. E. Lutskii and S. A. Mikhailenko, Zh. Strukt. Khim., 4, 14 (1963). (8) D. Tabuchi, 2. Electrochem., 64, 141 (1960). (9) (a) D. L. Martin and F. J. C. Rossotti, Proc. Chem. SOC.,73 (1961); (b) D. L. Martin and F. J. C. Rossotti, ibid., 60 (1959); (c) E.E.Schrier, M. Pottle, and H. A. Scheraga, J . Am. Chem. SOC., 86, 3444 (1964). (10) M. Pancholy and S. P. Singal, Nuovo Cimento, 32, 847 (1964). (11) F. B. Stumpf and L. A. Crum, J. Acoust. SOC.Am., 39, 170 (1966). (12) (a) A. N.Campbell and J. M. T. M. Gieskes, Can. J . Chem., 43, 1004 (1965); (b) J. M.T. M. Gieskes, ibid., 43, 2448 (1965). (13) G. E. Maciel and D. D. Traficante, J . Am. Chem. SOC., 88, 220 (1966). (14) J. C. Davis and K. 9. Pitser, J. Phys. Chem., 64, 886 (1960). (15) (a) A. Parmigiani, A. Perotti, and V. Riganti, Gam. Chim. Ital., 91, 1148 (1961); (b) A. Perotti and M. Cola, ibid., 91, 1153 (1961).

Volume 71, Number 7 June 1967

PETER WALDSTEIN AND L. A. BLATZ

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The Steinheil (three-glass prism, f / l O optics) spectrograph was used with an entrance slit width of 0.300 mm and 15-mm height. The linear dispersion for the region 0-100 cm-l from the Hg 4358 A exciting line was 16.4 cm--l/mm. The exit slit had a radius of curvature of 100.0 mm. It was moved in the focal plane of the spectrograph a t rates of 19.68 and 9.84 cm-’/min wit,h time constants of 20 and 40 sec, respectively . In all cases the Raman line frequencies and relative areas were determined after the lines were reconstructed on flat, straight base lines. The lines were always separated from the backgrounds by the use of straight lines. In some cases subjectively estimated curved background Imes were drawn. This was found to make essentially no difference in the values obtained for the frequencies as long as the ratio of Raman line to background height was large. However, for such cases as water, where the background is larger than the Raman line, the nominal value of 190 f 5 cm-1 for the frequency of this line pertains only to the method of drawing background lines and for baffle settings (10.0 mm) such as were used for the acids. In all cases the lines were reconstructed by working from the high-frequency toward the low-frequency side assuming the lines to be :symmetric. The uncertainty in the absolute values of the frequencies is estimated to be f3 cm-l for the formic acid upper line (larger frequency) and f 5 cm-’ for the acetic acid upper line or the lower formic acid line. This uncertainty for a weak, broad line such as the acetic acid 47-cm-I line may be somewhat larger than f 5 cm-l. The reproducibility from one scan to another scan nearby in time was usually 1 cm-’ for the formic acid upper lint? and h 2 cm-l for the acetic acid upper line a t room temperature. At temperatures of 8 and 5 6 O , the reproducibility of these lines was somewhat poorer. When precise relative frequencies were needed as in comparing the normal acids and deuterated acids, the lines were scanned (a) with the slower scanning rate and longer time constant, (b) as near to each other in time as possible, (c) repeatedly, that is, five times each for the pure acids and solutions in n-pentane (two scans of each were used for the water solutions and somewhat poorer precision was obtained), and (d) in addition to determining the frequencies from line peaks, corresponding points of each spectrum were compared. In this way average frequencies of the formic acid upper line relative to the upper line from perdeuterioformic acid reliable to *l cm-l were obtained and =k2 cm-l for the acetic acid upper line relative to the upper line from perdeuterioacetic acid.

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The Journal of Phylysica2 Chemistry

The half-widths of the lines refer to the values obtained by doubling the high-frequency half of the halfwidths. The line intensities are approximate values obtained by comparing the heights of the lines relative to the formic acid upper line (at 23’). The precision with which the relative areas of the acid Raman lines were determined in the water solution was less than f 5 % for the pura acid and about *3001, for the most dilute acid solutions. The formic acid was 97+ % superior grade acid. In one case a bottle of acid which had been opened about 9 months previously was distilled a t about 10-cm pressure from anhydrous copper sulfate. The background decreased fourfold a t 375 cm-’ following this distillation. Formic acid newly received from supply houses sometimes had only a slightly larger background than the distilled acid. The perdeuterio acids, both DCOOD (Nuclear Research Chemicals, Inc.) and CD3COOD (Merck Sharp and Dohme of Canada) had a minimum of 99% isotopic purity. The glacial acetic acid was reagent grade, minimum 99.7% acetic acid. Freshly opened bottles had backgrounds that were only slightly improved by filtration through 0.2-p pore size Gelman filters. Several bottles of acid which had been on the shelves for 2 years or more were found to give intolerably high backgrounds. The Eastman practical grade n-pentane was filtered through 0.1-p pore size Rlillipore filters. The conductivity water used was found to give only a negligibly different background after filtration through 0.01-p (16) E. Constant and A. Lebrun, J . Chim. Phys., 61, 163 (1964). (17) G. Werner, J . Prakt. Chem., 29, 26 (1965). (18) H. Dunken and P. Fink, Z.Chem., 2, 117 (1962). (19) (a) N. G. Zarakhani and M.I. Vinnik, Zh. Fiz. Khim., 37, 2550 (1963); (b) N. G. Zarakhani and M. I. Vinnik, ibid., 38, 632 (1964). (20) A. A. Glagoleva and A. A. Ferkhmin, Z h . Obshch. Khim., 28, 289 (1958). (21) L. J. Bellamy, R. F. Lake, and R. J. Pace, Spectrochim. Acta, 19, 443 (1963). (22) Y.Nagai and 0. Simamura, Bu22. Chem. SOC.Japan, 35, 132 (1962). (23) K. Nakamoto and S.Kishida, J . Chem. Phys., 41, 1554 (1964). (24) M.Haurie and A. Novak, J . Chim. Phys., 62, 137 (1965). (25) T. llliyazawa and K. S. Pitzer, J . Am. Chem. Soc., 81, 74 (1959). (26) G. L. Carlson, R. E. Witkowski, and W. G. Fately, Spectrochim. Acta, 22, 1117 (1966). (27) A. E. Stanevich, Opt. i Spektroskopiya, 16, 446 (1964). (28) V. Lorenzelli, Ann. Chim. (Rome), 53, 1018 (1963). (29) U. A. Zirnit and M. M. Sushchinskii, Opt. i. Spektroskopiya, 16, 903 (1964). (30) L. A. Blatz, Spectrochim. Acta, 21, 1973 (1965). (31) Low-frequency “liquid structure” lines of benzene, n-pentane, and a large number of other liquids are being reported elsewhere. L. A. Blatz, submitted for publication.

LOW-FREQUENCY RAMAN SPECTRAIN FORMIC AND ACETICACIDB

pore size Millipore filters. The benzene was reagent grade and was used as received.

Results All spectra reported below were obtained with unpolarized exciting light. Insertion of crossed or axial polarizers around the sample tube did not change the spectra of pure HCOOH and glacial CH3COOH except to reduce the relative intensity of the residual Rayleigh line (at the extreme left of the spectra shown here) and t o reduce the total intensity of the entire spectrum because of the attenuation of the Polaroid. The relative intensities of the Raman lines remained the same, as would be expected from depolarized Raman spectra. Acetic Acid. Spectra were obtained of 49, 34, and 20% (by volume) solutions of acetic acid in n-pentane and of glacial acetic acid and pure n - ~ e n t a n e , ~all' a t 23'. When the background and the appropriate fraction of the pentane spectrum are subtracted, the resultant spectra of all three solutions are essentially the same. The integrated intensity of the solution spectrum relative to the intensity of the glacial acetic acid spectrum is proportional to the volume fraction of acetic acid in the solution within the experimental error ( =t 5-1075 for the more concentrated solutions; *20% for the most dilute, where the pentane contriI

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bution to the spectrum is relatively large). Figure 1 shows the spectrum of a 34% solution pf CH3COOH in n-pentane. The line a t 124 It 5 cm-I can be identified with the B, fundamental of the cyclic dimer, for which Miyazawa and PitzerZ5predict a frequency of 128 cm-'. The isotopic shift confirms this assignment; in a pentane solution of CD3COOD, the line appears a t 115 f 5 cm-'. The shifts predicted for the A, fundamentals are about 3.5y0;for the 3, they are 8%. The line a t 47 f 7 cm-' is probably a "liquid structure" line. 31, 32 Figure 2 shows the spectrum of glacial acetic acid a t 23'. Spectra taken a t 8 and 56' were the same except for small changes in background. The spectrum is similar to that of the pentane solutions, but the two lines are not as well resolved and their maxima are somewhat closer together. This might lsuggest the presence of a weak third line around 95 cm-I perhaps owing t o an open dimer or to a polymer. If there is such a third line, its specific intensity must be about equal to the sum of the specific intensities of the 124and 47-cm-I lines, since the total intensity of t h e glacial acid spectrum is twice that of the spectrum of the 49% solution in n-pentane. The frequency of the upper maximum in glacial CH3COOH is 116 f 5 cm-l; in glacial CD3COODit is 106 f 5 cm-I, consisbent with

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Figure 1. A 34% (by volume) solution of CH&OOH in n-pentane, 23": ( a ) observed spectrum with linear background; (b) spectrum with linear background and residual line removed; (c) two-thirds of the n-pentane spectrum; (d) spectrum b minus spectrum 0.

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Figure 2. Glacial acetic acid, 23': (a) o b s e d spectrum with linear background; (b) spectrum with linear background removed.

(32) V. 8.Starunov, Opt. 4. Speldroskopiya, 18, 300 (1966).

Volume 71, Number 7 June 1 W

PETER WALDSTEIN AND L. A. BLATZ

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Figure 3. Aqueous solution, 32 mole % HzO, 68 mole % CH&OOH, 23": (a) observed spectrum with linear background; (b) spectrum with linear background removed.

the isotopic shift expected for the B, vibration of the cyclic dimer. Figure 3 shows the spectrum of an aqueous solution (32 mole yo HzO) of acetic acid a t 23'. Only one broad line can be distinguished; its maximum is located at 95 f 5 C M - ~ . The spectrum is the same, except for intensity, in all aqueous acetic acid solutions below 80 mole acid. The isotopic shift of this maximum is 9 f 2 cm-l (9.5 f 2%). A plot of specific intensity of the aqueous solution spectrum against concentration of acetic acid is shown in Figure 4. Formic Acid. The only solvent available for this work in which substantial amounts of formic acid could be dissolved without the formation of solutesolvent hydrogen bonds was benzene. Unfortunately, the benzene spectrum has a very strong, very broad "liquid structure" line31t32which completely obscures the formic acid spectrum below about 200 cm-I. One line could be observed in a saturated solution of HCOOH in benzene a t 23'; it was located a t 245 f 5 cm-'. In a saturated solution of DCOOD in benzene at 23', a line could be detected with poor precision at 210 f 10 cm-'. The isotopic shift (14 f 4%) and the frequency permit assignment of this line to the B, fundamental of the cyclic dimer, for which a frequency of 243 cm-1 and an isotopic shift of 19% are predicted. 25 The spectrum of anhydrous HCOOH a t 23' is shown The Journal of P h u h l Chemistry

Figure 4. Specific intensity of the aqueous solution spectrum of CH8COOH at 23" relative to the spectrum of glacial acetic acid. The solid curve is the curve to be expected for K = 100 if the 95-cm-1 line i s due to a dimer and has a specific intensity equal to the sum of the specific intensities of the 47- and 124-cm-1 lines.

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Figure 5. Anhydrous formic acid, 23" : (a) observed spectrum with linear background; (b) spectrum with linear background removed.

in Figure 5.m Two lines can be distinguished. The lower line, a t 71 f 5 cm-l, is probably a "liquid structure" line. The upper line is about 4.5 times as intense as the upper line of the glacial CHaCOOH spec-

LOW-FREQUENCY RAMAN SPECTRA IN FORMIC AND ACETICACIDS

trum and about 1.3 times as broad ( i e . , about 100 cm-1). Its frequency is temperature dependent. The frequency is 209 f 3 cm-I at 23', 215 f 5 em-' a t 8', and 205 f 5 cm-' a t 56'. The frequency of the upper line of the anhydrous DCOOD spectrum a t 23' is 176 3 cm-'. The 16% isotopic shift is about what is expected for an out-of-plane bend of an associated species of formic acid. However, the frequency is much lower than the 245 cm-' observed for the B, vibration of the cyclic dimer in benzene solution. The spectra of aqueous solutions of HCOOH a t 23' look about the same as the spectrum of the pure acid. The location of the upper maximum is concentration dependent, dropping from 209 em-' in pure HCOOH to 196 f 5 cm-1 in a 19 mole % aqueous solution. The isoljopic shift in these solutions is the same as that in the pure acid, 16%. Figure 6 shows the spectrum of a 19 mole % aqueous solution of HCOOH. Water has a broad, weak line around 190 cm-1,32 which can contribute an appreciable fraction of the observed intensity in the 180-200-~m-~region of the spectra of the more dilute formic acid solutions. This fraction has been subtracted from the intensities shown in Figure 7, a plot of the specific intensity of the upper formic acid line as a function of the acid concentration in water.

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Discussion Acetic Acid. The spectrum of glacial acetic acid

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Figure 6. Aqueous solution, 19 mole % HCOOH, 81 mole % HtO, 23": (a) observed spectrum with linear background; (b) spectrum with linear background removed. 1.2,

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can be interpreted as the superposition of a weak line around 95 cm-' on the spectrum of acetic acid in pen8 0.4 tane solution. The dimerization equilibrium constants for acetic acid in alkanes34predict almost com0.2 plete dimerization of the acid a t the concentrations used in this work. Since no lines attributable to any species except the cyclic dimer have been found in the 20 40 60 80 OO pentane solutions, it is assumed here that all the dimers MOLE e/' HCOOH at these concentrations in pentane are cyclic. The Figure 7. Specific intensity of the upper line in the spectrum 95-cm-' component can have no more than 15% of the of aqueous solutions of HCOOH a t 23", relative to the upper total intensity of the spectrum without destroying the line of anhydrous HCOOH. The solid curves represent the intensity us. concentration relationships expected for various resolution between the 124- and 47-cm-I lines. This values of K if the line is due to a dimer a t all concentrations puts a lower limit of 85% to the fraction of acid present and if its specific intensity is independent of hydration effects. as cyclic dimers in glacial acetic acid. X-Ray diffraction results4 and low-frequency infrared s p e ~ t r aare ~~,~~ consistent with this conclusion. is constant within the experimental error for solutions As water is added to the acid, the 95-cm-' component between 7.3 and 50 mole % acid, so that not much of the spectrum becomes more prominent and in soluinformation can be obtained on equilibria involving the tions with less than 50 mole % acid it probably accounts (33) G. E. Walrafen, J . Chem. Phys., 44, 1546 (1966). Walrafen for most of the observed intensity. The isotopic shift reports this line a t 175 cm-1. On our instrument, a t the baffle identifies this component as an out-of-plane bend settings used in this work and with the background drawn as shown in Figures 5 and 6, the maximum is located a t 190 f 5 cm -1. (or possibly more than one out-of-plane bend) but gives (34) (a) H. A. Pohl, M. E. Hobbs, and P. M. Gross, {bid.. 9, 408 no information as to whether the species responsible (1941); (b) M. Davies, P. Jones, D. Patnaik, and E. A. Moelwynis an open dimer or a polymer. The specific intensity Hughes, J . Chem. Soc., 1249 (1951). Volume 71,Number 7 June 1067

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species responsible for the spectrum. If the species is indeed a dimer, then the dimerization equilibrium constant, K = (dimer)/(monomer)2 in mole fraction units, must be greater than about 100 in the 10-20 mole % concentration region. In much more dilute solutions (