4818
J. Phys. Chem. 1982, 86, 4618-4622
only aids the selection of stationary phases for gas chromatography but also appears to suggest a category of mixtures wherein entropy from orientation is of comparable importance to associated excess enthalpy. Further research in this direction should reveal new areas of experimental thermodynamics which can stimulate the development and refinement of appropriate solution theories. It has been suggested that strong selective interactions or solvent solubility parameters differing from those of solutes are desirable liquid-phase features for resolving similar isomer^.^,^ DBTCP is an example of a selective phase which gives a favorable activity coefficient ratio for cresol derivatives, while TCNP with a high cohesive energy density (solubility parameter) exemplifies a potential liquid-phase choice from solubility parameter considerations. Cohesive energy density differences alone, however, do not account for the activity coefficient ratios of TMSE derivatives in squalane and P P E (see Figure 8). Favorable activity coefficient ratios of the dual-character tolyl fluoroesters with T X P are due to entropic factors which can be as important as the energy factor (see Table V). The values of excess entropies of mixing are less favorable for the solution of meta isomers. Consideration of the relations between vapor pressures, activity coefficients, and activity coefficient ratios indicates
a rationale for selecting derivatives and stationary phases; derivative formation should tend to lower cohesive energy in order to increase solute vapor pressures. (The concept of a dual-character molecule is useful for selecting derivatizing agents.) The stationary phase should differ in nature from either the derivative or the parent hydroxyl compounds. Thus, a liquid phase with a high solubility parameter can be advantageous, e.g., the fluoroesterTCNP system. Alternatively, the selective interaction feature may be emphasized as with PFP derivatives with DBTCP or TXP solvents where orientation entropies for isomers are important (see Table V or Table VI, T;".~ values).
Acknowledgment. We thank Mr. Walter G. Taschek of the Army Research and Development Center for his help with the initial thermodynamic calculations. We also thank the University of Wisconsin and the National Science Foundation for support of this project. Supplementary Material Available: Tables I and 11, listing values for solutes with solvents of this study, and Table Id, listing vapor pressure data at 80 "C, with second virial coefficients, density data, and expansion coefficients (3 pages). Ordering information is available on any current masthead page.
Oxide Salt Reactlons in Matrix Isolatlon. Infrared Spectrum of the Ti,+C0,2- Triple Ion in Argon Matrices Shelle J. David and Bruce S. Ault' Department of Chemistry, Unlverslty of Clnclnnatl, Clnclnnatl, Ohlo 45221 (Recelved: February 22, 1982; I n Flnal Form: Ju& 19, 1982)
The salt/molecule reaction technique, which has been employed in the past to carry out halide ion transfer and anion formation in argon matrices, has been extended to oxide salt reactions. The codeposition of TlzO with samples of Ar/C02 and its isotopic counterparts gave rise to a series of product bands which have been assigned to the C032-anion in the T12+C032-triple ion. A lowering of the symmetry of the carbonate anion from D a h to probably (2%was observed, as a consequence of the T1+ cations. The antisymmetric C-0 stretching mode of the CO2-anion was split by 185 cm-', to 1506 and 1311 cm-', similar to the splitting observed for the COS2-anion in alkali carbonate melts. In addition, the spectra were in good agreement with those of the alkali-metalcarbonates in nitrogen matrices, formed through direct vaporization of the salt. Attempts to carry out analogous reactions with the alkali-metal oxides, MzO, were unsuccessful due to the decomposition during the vaporization process. The results here establish that oxide transfer to a suitable acceptor from T120 can take place, to isolate oxyanions of interest.
Introduction The salt/molecule reaction technique has been used numerous times in conjunction with matrix isolation for the formation of unusual halide-containing anions in inert matrices.'" In this technique, an alkali halide salt molecule, often CsF, is vaporized and codeposited with a suitable halide acceptor diluted in argon. Halide anion transfer occurs, and the product anion is trapped in an ion pair with the alkali-metal cation. The extension of this technique to gas-phase reactions of oxide salts would be (1) Ault, B. S. J. Phys. Chem. 1979,83, 837. (2) Auk, B. S. Inorg. Chem. 1979, 18, 3339. (3) Ault, B. S.; Andrews, L. J. Chem. Phys. 1975, 63, 2466. (4) Ault, B. S. J . Phys. Chem. 1980, 84, 3448.
of the interest as well, and a range of unusual oxyanions might be formed in this manner. Vaporization of oxide salts is much more difficult than that of the halide-containing salts as a consequence of the much greater lattice energy. The group 2A oxides, such as CaO, require extremely high temperatures and then decompose upon vaporization.s Conflicting reports alternately suggest that the alkali oxide salts, M20, can or cannot be vaporized without d e c o m p ~ s i t i o n .However, ~~~ (5) Klabunde, K. "Chemistryof Free Atoms and Particles";Academic Press: New York, 1980. (6) Brewer, L.; Margrave, J. L. USAEC, National Science Foundation, UCRL-1864, 1952. (7) Klemm, W.; Scharf, N. J. 2.Anorg. Allg. Chem. 1960, 303, 263.
QQ22-3654l82I2086-4618$Q1.25lQ 0 1982 American Chemical Society
Oxide Salt Reactions in Matrix Isolation
it is clear that T120 does vaporize directly, without decomposition, and has been the subject of a matrix isolation s t ~ d y .Previous ~ ~ ~ salt/molecule studiedohave shown that Cs+ and T1+are the cations of choice in terms of minimum perturbation, so the TlzO is a clear choice for a test of the oxide salt reaction technique. An appropriate test system is needed to evaluate whether or not 02-transfer can occur to an acceptor to form a matrix-isolated triple ion. The carbonate anion C032- is an excellent choice, in that the spectrum of the crystalline anion is well-known.'l In addition, Raman spectra have been recorded for the anion in high-temperature ionic melts.12 Finally, while this present work was in progress an infrared matrix isolation study of the M2C03 system was reported, through the direct vaporization and deposition of alkali carbonate triple ions.13 The goal of the present work is to investigate the reactions of oxide salts, and this recent carbonate work provides excellent comparison spectra. With this intent, a study was undertaken to investigate the reaction of oxide salts with C02 in inert matrices. Experimental Section All of the experiments conducted in this investigation were carried out in a conventional matrix isolation apparatus which has been described previou~ly.'~T120(Cerac), Cs20 (Cerac), and Na20 (Alfa) were the oxide salts employed in these experiments and were outgassed under vacuum prior to the start of an experiment. Since the vaporization and/or decomposition temperatures for Na20 and CszO are not well established, outgassing was limited to relatively low temperatures. Dry ice was used as a source for C02 by collecting the vapor above a small piece of solid COz. The sample was purified by several freeze-thaw cycles at 77 K, before final vaporization from a -77 "C bath. 13C02(99% 13C, Merck), C1802(94% l80,Merck), and C1802(50% l80, Isotope Labeling Co.) were all purified by at least one freeze-thaw cycle under vacuum prior to sample preparation. Argon and nitrogen were used as the matrix gases in most experiments and were used without further purification. In addition, carbon dioxide (Matheson) was used as the matrix material in one experiment. Matrices were deposited at a flow rate of approximately 2 mmol/h for 20-24 h onto a CsI window held at 15 K. All infrared spectra were recorded on a Beckman IR-12 infrared spectrophotometer. Normal coordinate calculations were run on the University of Cincinnati computer, employing a program provided by the National Research Council of Canada which incorporates a general valence force field.
The Journal of Physical Chemistty, Vol. 86, No. 23, 1982
Ill
g5
nro
6~ , 5 1340
14
,
,
1300
"Rj40,
+
,
4619
ArlC02
,^_,
COO
e
,
30
790
,
,
E0
710
ENERGY ( c m ' l
Figure 1. Infrared spectrum of the reaction products of TI,O with samples of Ar/C02 with dilutions from 500/1 to 100/ 1, over selected spectra regions.
was employed, similar results were obtained, with a shift of several wavenumbers to lower energy. When NazO was heated and codeposited in a matrix experiment, a deep red color was observed, indicative of sodium atoms, which suggests that decomposition had 0~curred.l~In addition, several attempts were made to vaporize Cs20 at roughly 440 OC. No bands were observed which could be attributed to this species, and a slight indication of decomposition was detected. T120+ C02. TI20was vaporized at approximately 430 OC and codeposited with samples of Ar/CO2 in a number of experiments, with M/R ratios ranging from 100/1 to 500/1. When T120 was codeposited with a sample of Ar/COz = 500, three distinct product bands were observed in the final spectrum, which could not be attributed to a parent species. These were located at 720,1311, and 1506 cm-', with the latter band being the most intense of the three. In addition, each band was split slightly, by about 2 cm-', so that each appeared as a slight doublet. When a comparable level of T120was codeposited with a sample of Ar/C02 = 200, the same set; of three bands were detected, but with considerably increased intensity. In addition, a weak band was observed at 1038 cm-', and this band was also split into a slight doublet. T120was also codeposited with samples of Ar/C02 = 100 in several experiments, and all of the above bands were detected, with the same relative intensities. However, the bands were slightly broadened so that the doublet structure was not Results as apparent. In addition, a very weak but distinct band Before the reaction products of the oxide salts with COP was observed at 845 cm-'. Infrared spectra of the reaction were investigated, blank experiments were conducted on products of T120with C02 are shown in Figure 1. all of the oxide salts. When TlzOwas vaporized at roughly TlzO was also codeposited with pure COz in the absence 425 "C and codeposited into an argon matrix, the resulting of any matrix matrial and similar results were obtained. spectrum resembled very closely the literature s p e ~ t r u m . ~ Product bands were observed in the same regions, namely, v3 for monomeric TIZ0was observed at 644 cm-l, while around 700,850,1000,1300, and 1500 cm-' with roughly (TlzO)zwas observed at 513 cm-'. When a nitrogen matrix comparable intensities. However, the bands were distinctly broader and with multiplet structure in some cases, so that (8) Hinchcliffe, A. J.; Ogden, J. S. J. Chem. Soc. D 1969, 1053. accurate determination of band position and intensities (9) Brom, J. M.; Devore, T.; Franzen, H. F. J.Chem. Phys. 1971,54, was not readily accomplished. Finally, T120was codepo2142. sited with a sample of COP diluted in N2, with a total (IO) Hunt, R. L.; Auk, B. S. Spectrochim. Acta, Part A 1981,37,63. (11) Bhagavantum, S.; Venkatarayudu, T. Proc.--lndian Acad. Sci., dilution ratio of N2/C02= 250. Here, just as in the argon Sect. A 1939, 9, 224. matrix experiments, five product bands were observed, (12) Bates, J. B.; Brooker, M. J.; Quist, A. S.; Boyd, G. E. J. Phys. shifted just slightly from the argon matrix positions. The Chem. 1972, 76, 1565. (13)Ogden, J. S.; Williams, S. J. J. Chem. Soc., Dalton Trans. 1981, 456. (14) Auk, B. S. J. A m . Chem. Soc. 1978, 100, 2426.
(15) Andrews, L. J. Phys., Chem. 1969, 73, 3922.
4620
The Journal of Physical Chemistry, Vol. 86, No. 23, 1982
David and Ault
T120 C02
-Jd 1 T120 + C 16,1802
.'I 43-
1
53
7330 126319"*8?5 WAVENUMEERS (cm- )
795'542
'30
Figure 2. Infrared spectrum of the reaction products of TI,O with samples of Ar/'%O, at dilutions of 50011 and 100/1, over selected spectral regions.
overall yield was less in the nitrogen matrix experiment, but the product bands were much sharper and were not split into doublets. TlzO + 13C02. This pair of reactants was investigated in several experiments, with dilutions of 500/1 and 100/1, and an enrichment of 99% 13C. When TlzO was codeposited with a sample of Ar/l3CO2 = 500, three sets of doublets were observed, just as in the experiments employing the normal carbon isotope, but these were all shifted to lower energies. These three doublets were centered at 719,1278, and 1463 cm-' and showed the same relative intensities as in the 12C02experiments. When the concentration was increased to M/R = 100, these three bands grew considerably in intensity, and two weak bands were also observed at 820 and 1018 cm-l, analogous to the two weak bands in the 12C02experiments, as shown in Figure 2. TlzO + Cleo2. T120was codeposited with a sample of Ar/C1802 = 500, with 94% lSO enrichment, and three product bands were observed in the final spectrum. These bands, located at 700,1292, and 1485 cm-', had the same relative intensities as the major product bands in the '%02 and 13C02 experiments, as well as the same doublet structure. The lSO counterparts of the two weak product bands were not detected due to low overall intensity. T120+ C16J802 TlzO was also codeposited with a sample of Ar/C02 with a 50% enrichment of l80,with the l80 label statistically distributed, so that parent C1602, C160180,and Cleo2were all present, in a 1:2:1 relative abundance. When a M/R value of 50 was used, the three major product regions showed relatively distinct triplets, all with approximate 1:2:1intensity ratios. Unfortunately, the bandwidths of the product bands were nearly as large as the splittings so that some smearing occurred for the weaker bands. Nonetheless, band centers could be identified. These triplets were observed at 1485,1495, and 1506 cm-l for the upper region; at 1292,1301, and 1311 cm-' for the middle region; and at 698, 710, and 721 cm-I for the lower region. In addition, weak features were observed in the 840- and 1020-cm-' regions but intensities were sufficiently low that definite bandshapes and positions could not be determined. Figure 3 shows the infrared spectra of the reaction products of T120with samples of lSO isotopically labeled C02. Discussion Product Identification. When T120was codeposited with C 0 2 in either argon or nitrogen matrices, or with pure COz, a number of infrared absorptions were observed which could not be attributed to parent species. Even in the most dilute experiments three product bands were
I
830
1490
7345
1305
730
690
WAVENUMBERS (cm-')
Figure 3. Infrared spectra of the reaction products of TI20 with "0 isotopicalb labeled The top trace shows the reaction with c ' 6 ~ 2 , while the middle trace shows the spectrum of the product of the reaction with scrambled, 50% C'60''02. The lower trace shows the reaction product of TI,O with a sample of 94% enriched C''O,, all in argon matrices.
cop.
observed, and at high concentrations, M/R = 100, two additional weak bands were detected. These five bands, at 720,845,1038, 1311, and 1506 cm-', clearly indicate that a distinct reaction has occurred and a new product formed. Furthermore, these five bands maintained a constant intensity ratio with respect to one another, to the degree that intensities could be measured for the two weak bands. This result suggests that only a single product is formed and is responsible for all five absorptions. Finally, the presence of both carbon and oxygen in the product species is verified by the observation that all of the product bands showed shifts when carbon and oxygen isotopic labels were employed. The salt/molecule reaction technique, employing alkali halide salts, has led to the formation of a number of unusual anions in matrix-isolated contact ion pairs, through the transfer of a halide anion from the salt to a suitable halide acceptor. In view of these halide salt results, the reaction of TlZ0with C02to form the T12+C032-triple ion might be anticipated. The carbonate anion, CO:-, is well-known and provides a test as to whether such a reaction will occur. For crystalline C032-,the vibrational spectrum is well established, with two doubly degenerate modes and two singly degenerate modes (one infrared inactive) under D* sy"etry.16 These crystalline modes occur in spectral regions close to the product bands observed here, supporting the notion that the C032-anion has been formed through an oxide salt reaction. More than three modes were observed here, but this is not surprising in view of the fact that the alkali-metal cation in previous salt/molecule reactions has been shown to lower the symmetry of the product anion. In the present case, the T1+ cations are likely to lower the symmetry of the C032-anion to at least Czoand split the two doubly degenerate modes, leading to at least the number of observed bands. Similar behavior has been observed for the C032-anion in ionic (16) Nakamoto, K. *Infrared and Raman Spectra of Inorganic and Coordination Compounds",3rd ed.; Wiley-Interacience: New York, 1978.
The Journal of Physical Chemistry, Vol. 86, No. 23, 1982
Oxide Salt Reactions in Matrix Isolation TABLE I: Comparison of Infrared Spectrum of the TI,CO, Triple Ion t o the Alkali-Metal Carbonates in Argon and Nitrogen Matricesa
T12C03b T12C03c K , C 0 3 d Rb,CO,Csd 146 7 1498 1472 1506 1317 1321 1311 1313 1016 971 1010 1038 845 816 870 871 720 718 697 691 a Band ositions in cm-,. Argon matrix. matrix. From ref 13.
B
CS,CO,~*~
1462 1319 1018 871 685 Nitrogen
melts, where ion pairing does occur. The observed spectrum, then, can be accounted for by the formation of the T12+C032-triple ion. While the current research was in progress, Ogden and co-workers13isolated several alkali carbonate triple ions M2C03by the direct vaporization of the carbonate salt. These workers observed a set of five product bands as well, with positions and intensity pattern in both argon and nitrogen matrices quite similar to the bands observed here. Table I shows a comparison of the nitrogen matrix results of Ogden to the argon and nitrogen matrix spectra obtained here. These results certainly add strong support to the present observations that the reaction of TlzO with C02 during condensation into an inert matrix does lead to formation of the CO2- anion in the T12+C0t-triple ion. Band Assignments. With the analogy to the infrared spectrum of crystallinell CO2-, the spectrum of the C o t anion in melts,12and the recent matrix isolation study of Ogden and co-workers,13band assignments are relatively straightforward. The crystalline spectrum contains an intense doubly degenerate stretching mode at 1415 cm-', which will be split by the presence of the thallium cations. The two bands at 1311 and 1506 cm-l are split nearly centrosymmetrically about this value, as is characteristic of cation-anion interactions. For comparison, Devlin found a splitting of 275 cm-' for the analogous mode of the nitrate anion NO3- in the Tl+N03-ion pair.17 Similar splittings were also observed for the carbonate anion in melts, and by Ogden for the carbonate anion in alkali-metal triple ions. Hence, the assignment of the bands at 1311and 1506 cm-' to the two split components of the antisymmetric C-0 stretch of the carbonate anion is made. The symmetric stretch of the DBhcarbonate anion has been observed at 1063 cm-I in Raman spectra and is infrared forbidden. Upon lowering of the anion symmetry by the thallium cations, this mode will be activated in the infrared, but should still be quite weak. Consequently,the band observed at 1038 cm-' is readily assigned to this mode, on the basis of both location and intensity. The symmetric out-of-plane bend has been observed for the unperturbed anion at 879 cm-l, and the weak band observed at 845 cm-' is best assigned to this mode. This vibration should show a large carbon isotope dependence, and did, with a shift to 820 cm-'. The final vibration is the doubly degenerate in-plane deformation mode, which has been observed at 680 cm-l for the D 3 h anion. This mode should split into two components upon distortion by the thallium cations, just as did the antisymmetric stretching mode. Moreover, this mode involves primarily the oxygen atoms and should have only a small 13Cshift. The band at 720 cm-' fits this description well with a 1-cm-' 13Cshift and is assigned as one of the two components of the doubly degenerate mode. For comparison, Devlin and co-workersls observed a 1-cm-l shift with 15N (17) Smith, D.; James, D. W.; Devlin, J. P. J. Chem. Phys. 1971, 54, 4437.
4621
TABLE 11: Observed and Calculated Band Positions for the lI,CO, Triple Ionaib assignment V3b (B2) u 3 a (A,) u , (A,) v2
(B,) (AI)
"4b
12c16032I3CI6O3 2 obsd calcd obsd calcd
1506b 1311 1038 845 720
1505.4 1311.8 1030.1 845.7 723.1
l2CI60 1802-
1463 1278 1018 820 719
1460.8 1279.0 1025.9 819.2 721.3
12C160180
2-
2
assignment
obsd
(B,) vSa (A,) V I (A,) v 2 (B,) V& ( A I )
1495 1301
calcd
obsd
calcd
1496.5 1485 1486.3 1300.4 1292 1290.7 1015.6 1000.3 842.6 839.4 710 709.4 700 694.7 a Argon matrix frequencies with average of site splittings where necessary. Band positions in cm-'. V3b
substitution for the analogous vibrational modes of the NO, anion in M+N03-ion pairs. The second component should lie at lower energies, roughly at 640 cm-'. However, this spectral region is completely obscured, both by the bending mode of parent COP and by the antisymmetric stretch of parent T1,O. Consequently, there is no hope of seeing this vibration under the conditions of this study. It might also be noted that Ogden et al. were unable to locate this second component, presumably also due to C02 interference. In general, the above set of band assignments and positions is in quite good agreement with those of Ogden, given the usual slight variations with cation in ion pairs and triple ions. Previous studies of isolated ion pairs with alkali-metal cations have demonstrated that the dominant contribution to the splitting of degenerate anion vibrational modes is cation polarization. However, Devlin noted that the splitting of the NO< anionic modes by the T1+cation was greater than anticipated by this model, and found it necessary to invoke some covalent contribution to the anion distortion for the T1+cation. Similar results are observed here, with the antisymmetric stretch of the C032-anion being split by roughly 35 cm-' more with T1+ than with alkali-metal cations.13 It was noted above that in each of the argon matrix experiments, the product bands appeared,as slightly split doublets, with roughly a 2-cm-l splitting. This structure may be due to either distinct locations of the cations in the triple ion or to matrix site effects. However, nitrogen matrix experiments showed sharp, single features, suggestive of matrix splittings in the argon matrices. It has been observed in the past that argon matrices contain two distinct sites, while nitrogen has a single site.Ig Moreover, Ogden and co-workers found that the argon matrix experiments also yielded poorer quality spectra and somewhat broader lines. Hence, this substructure on each of the product bands is best explained as small matrix perturbations in the argon matrices. Normal Coordinate Calculations. To further support these assignments, normal coordinate calculations were undertaken, employing a general valence force field. Since Ogden et al. had carried out extensive calculations, the present calculations were carried out only to confirm the nature of the vibrations and band assignments. Force constants from this earlier work were transferred and then refined to fit the frequencies observed here. Spectral bands for all of the isotopic species were employed and refined (18) Devlin, J. P.,private communication. (19) Tevault, D.; Nakamoto, K. Znorg. Chem. 1975, 14, 2371.
J. Phys. Chem. 1982, 86, 4622-4626
4622
I
where T120 might be observed. However, the spectral region around 510 cm-', where dimeric TlzO absorbs, was free of absorption, indicating that very little dimer survived under these conditions. This likely implies that little if any monomer survived, and hence a low barrier to reaction. Information concerning reaction barrier might also come from comparison of yields in the M/R = 100 experiment relative to the pure CO, matrix experiment. Unfortunately, the bandwidths were excessive in the pure CO, experiments, and little can be concluded about the relative yields. The possibility that the reaction of T120with CO, under these conditions goes through an intermediate prior to triple-ion formation cannot be ruled out by these experiments. Certainly, no evidence for such an intermediate was obtained, although the experiments were conducted a number of times under widely varying conditions. Apparently, if such an intermediate species is present along the reaction pathway, very little barrier exists between this species and the observed triple-ion product.
lK
Figure 4. This figure depicts the two most probable structures for the TI,CO, triple ion in argon matrices, both of C,, symmetry (after ref 13).
to a total of eight force constants. These calculations support the assignments above and the details are presented in Table 11. The overall fit, using a CZumodel, fell well within the experimental error in measurement of band positions. Several geometries may be assumed for the triple ion itself in either Czuor C, symmetry. The data obtained from the use of 50% labeled Cl8O2provides evidence that the two oxygen atoms originating with the COz remain equivalent in the product anion. If the anion remains planar, then two Czustructures might be envisioned, as shown in Figure 4. Ogden et al. considered this question a t length in their calculations, and ultimately favored structure 11, but could not rule out structure I on the basis of infrared spectral data alone. If the low-frequency Raman spectra of the carbonate melts is considered, then a slight preference for structure I might be found. However, this detailed structural investigation does not represent the thrust of the present work, and no further analysis was undertaken.
Conclusions The reaction of thallium(1) oxide, T120,with COz under matrix isolation conditions has given rise to a set of infrared absorptions which can be assigned to the carbonate anion, COZ-, in the T12+C0t-triple ion. Moreover, these assignments are supported by the direct vaporization of alkali-metal carbonate salts into inert matrices. This work demonstrates the potential for this technique for the study of highly reactive oxyanions which cannot be formed and stabilized at room temperature. With the extensive use of oxide salts in both heterogeneous and homogeneous catalysis, formation of these intermediate anions may provide further insight into the mechanism by which oxide salts and oxide surfaces serve to catalyze chemical reactions.
Mechanism of Formation The details of the formation process of the T12C03triple ion is certainly of interest as well. For example, the observation of isolated TlzO in the experiment employing a pure CO, matrix might provide some information about activation energies. Unfortunately, monomeric TlZ0absorbs very near v 2 of C02, and, when a pure C 0 2 matrix was employed, the bandwidth of v2 obliterated the region
Acknowledgment. We gratefully acknowledge support of this research by the National Science Foundation through grant CHE8100119. B.S.A. also acknowledges the Dreyfus Foundation for a Teacher-Scholar Grant.
Sound Attenuation in Simple Carboxylic Acids P. F. Keegan and S. L. Whittenburg' Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 (Received: h4arch 3, 1982; In Final Form: July 19, 1982)
The attenuation of the thermally induced sound wave in the simple carboxylic acids has been measured by using Brillouin light scattering. The maximum in the line width and the dispersion in the frequency shift of the Brillouin bands in formic, acetic, and propionic acid were fit to a three-variable linear-response theory. In the temperature range studied, 20-80 "C,the data support the assignment of the relaxation process giving rise to the attenuation as the breaking of the hydrogen-bonded network due to small-angle reorientations. Introduction Laser light scattering is a powerful probe for studying the motions of small molecules in the liquid state. Typically the depolarized component of the central or Rayleigh peak is analyzed. The width of the depolarized Rayleigh peak is related to the reorientation rate of the molecule.' (1) B. J. Berne and R. Pecora, 'Dynamic Light Scattering", WileyInterscience, New York, 1976.
0022-365418212086-4622$0 1.25/0
Fewer applications of the polarized spectrum have been made. Generally, the polarized spectrum consists of the Rayleigh peak plus a pair of symmetrically displaced peaks, the Brillouin doublet. In the polarized spectrum the central peak contains information on the thermal diffusivity. The shift and width of the Brillouin peaks contain physical information. In a nonrelaxing fluid the shift is equal to the longitudinal phonon frequency while the width is equal to the instrumental line width. If, however, a 0 1982 American Chemical Society
