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Gas phase infrared spectroscopic observation of the organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, and (CH3(CH2)8COOH)2
Chemical Physics Letters 402 (2005) 239–244 www.elsevier.com/locate/cplett
Gas phase infrared spectroscopic observation of the organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, and (CH3(CH2)8COOH)2 Teresa L. Eliason 1, Daniel K. Havey 1, Veronica Vaida
*
Department of Chemistry and Biochemistry and CIRES, University of Colorado, Campus Box 215, Boulder, CO 80309, USA Received 19 July 2004; in final form 3 November 2004
Abstract This work uses infrared absorption spectroscopy to study the organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, (CH3(CH2)8COOH)2, and their monomers. There is a scarce literature base about these acids and the dimers have not been previously studied in gas phase. Ab initio calculations of vibrational harmonic frequencies are used as a guide for spectroscopic investigations of C@O and O–H stretching regions. Correlation between experiment and theory as well as an interesting trend with acid chain length are discussed. Do and De are calculated for all dimers using the B3LYP/6-311++G[3df,3pd] method and are compared to literature values for short chain organic acid dimers. 2004 Published by Elsevier B.V.
1. Introduction Organic compounds have been shown to contribute significantly to the overall mass of aerosol particles [1–5]. Organic acids have been observed to preferentially partition to the surface of these aerosols [4–6], possibly with an inverted micelle structure [7]. Field measurements have shown that organic acids are substantial contributors to automobile exhaust [8], atmospheric aerosols [3], and a polluted atmosphere [2,9–11]. Formic and acetic acids were observed to be the most prevalent organic acids in the polluted troposphere but octanoic, nonanoic, and decanoic acids all contribute to the total organic mass. Total automobile emissions of octanoic and nonanoic acids in vapor phase and as particulate *
Corresponding author. Fax: +1 303 492 5894. E-mail addresses: [email protected] (T.L. Eliason), havey@ colorado.edu (D.K. Havey), [email protected] (V. Vaida). 1 These authors contributed equally to the development of this work. 0009-2614/$ - see front matter 2004 Published by Elsevier B.V. doi:10.1016/j.cplett.2004.12.021
matter were found to be 0.101 and 0.095 ppbv, respectively [8]. The gas phase lifetimes of octanoic, nonanoic, and decanoic acids are expected to be short in the atmosphere due to their low-vapor pressures. Thus, these acids are predicted to exist predominantly in condensed phase as small hydrogen bonded acid clusters or in aerosols. Characterizing the organic acid cyclic dimers [12–15,31] is of fundamental importance to understand the effect of hydrogen bonding in these systems. This work uses infrared absorption spectroscopy to obtain gas phase vibrational spectra of the organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, (CH3(CH2)8COOH)2, and their corresponding monomers [16]. These dimers have been previously unreported because experiments are hampered by the low-vapor pressures of the acid monomers. Theoretical calculations of vibrational harmonic frequencies as well as previous work on smaller organic acids [17] are used to investigate the C@O and O–H stretching regions. Correlation between experiment and theory as well as an
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interesting trend in the extent of dimerization with acid chain length are discussed.
2. Experimental Absorption spectra of (CH3(CH2)6COOH)2 (octanoic acid dimer), (CH3(CH2)7COOH)2 (nonanoic acid dimer) and (CH3(CH2)8COOH)2 (decanoic acid dimer) were measured in the IR/NIR from 1000 to 7500 cm 1. The sample chamber was a 100 cm pathlength optical flow cell with CaF2 windows, attached using Viton O-ring seals. Spectroscopic measurements were made using a Bruker IFS 66v/S Fourier transform spectrometer (FTS). A silicon carbide source, KBr beamsplitter, and HgCdTe detector were used. The FTS instrument was used in external mode with the light directed through a CaF2 window into the cell and then to a parabolic mirror focusing onto the detector. The experiments co-added 128 scans at 2 cm 1 resolution. The (CH3(CH2)6COOH)2 dimer was formed by bubbling filtered N2 at 4000 standard cubic centimeters per minute (sccm) through liquid octanoic acid (Aldrich, 99.5+%). The (CH3(CH2)7COOH)2 dimer was formed by flowing 4000 sccm filtered N2 through a bubbler containing nonanoic acid (Aldrich, 96%) immersed in an isothermal bath at 343 ± 5 K. Decanoic acid (Aldrich, 99%) is a solid at room temperature and the dimer was prepared by immersing a bubbler containing the solid in an isothermal bath at 363 ± 5 K. Filtered N2 at 4000 sccm was then bubbled through the melted sample. The cell was maintained at room temperature throughout all experiments. This work attempted to measure dimer features in the shorter chain heptanoic and hexanoic acids. However, their increased vapor pressures combined with a strong tendency to form large clusters and aerosols, under the conditions of these experiments, made it difficult to isolate dimer features from condensed features. Because water vapor absorbs in the spectral regions of interest it was imperative to ensure that spectral features were independent of H2O partial pressure. Spectra were recorded for each acid with varying partial pressures of water vapor from 0.0 to 0.3 Torr. There was no noticeable change in the bandshape or relative intensity of any of the spectral features studied. Thus, this work is confident that the spectral features discussed in this work depend solely on the partial pressures of the organic acids. It was necessary to ensure that all spectral features observed for the acid dimers were dependent on the partial pressure of the acid monomers in accordance with behavior predicted by the reaction equilibrium. Spectra were taken that changed the octanoic and nonanoic acid monomer partial pressures contained in the flow cell. This was accomplished by varying the
Fig. 1. Expanded spectrum (Tcell = 298 K) of the nonanoic acid system in the C@O stretching region with the bath maintained at (a) 298 K, (b) 343 K, and (c) 363 K.
temperature of the bath from 298 ± 5 to 343 ± 5 K. Based on Beers law, it can be shown that a linear increase in monomer partial pressure, while fixing other experimental parameters, leads to a predictable quadratic increase in integrated intensity of the dimer peak (Fig. 1). This assumes a typical 2:1 ratio of the C@O stretching absorption cross-sections of the dimer to monomer as calculated in this work and in previous studies [18].
3. Calculations Optimized geometries and vibrational harmonic frequencies for the monomers and dimers of octanoic, nonanoic, and decanoic acid were calculated using the B3LYP/6-31G[d,p] method within the GAUSSIAN 98 suite of programs [19]. All harmonic frequencies were left unscaled. The optimized B3LYP/6-31G[d,p] geometries were used to perform single point energy calculations on all species using the B3LYP/6-311++G[3df, 3pd] method. Previous work has shown that the B3LYP method is appropriate for describing hydrogen bonded organic acid hydrates as long as the basis set exceeds 6-31+G** [20]. The use of B3LYP to describe hydrogen bonded dimers is rationalized because of the extensive agreement between binding energies calculated using this method and those measured experimentally [21–25].
4. Results and discussion The vibrational spectrum of the CH3(CH2)6COOH/ (CH3(CH2)6COOH)2 system in the region 1000–7500 cm 1 is dominated by the C@O stretching vibrational
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dence for assigning spectroscopic contributions of the dimer at room temperature. The O–H stretching transition (3000–3600 cm 1) region also contained spectral features attributed to both the octanoic acid monomer and dimer. The O–H stretch of CH3(CH2)6COOH had a calculated harmonic frequency of 3747.8 cm 1 (Table 2) and the transition was observed at 3578.5 cm 1. This is in agreement with the calculations in this work as well as with transition frequencies empirically predicted by previous work [17]. A broad O–H stretching transition (Fig. 3) between 3050 and 3500 cm 1 was observed to low energy of the monomer O–H stretch and is attributed to the hydrogen bonded O–H stretch of the octanoic acid dimer. Harmonic frequency calculations indicate that 5–10 normal modes between 3000 and 3125 cm 1 contain a notable O–H stretching component. The broad bandshape of the feature is not surprising due to the effect of hydrogen bonding in the cyclic dimer structure (Fig. 4). The diffuse spectral feature observed between 3000 and 3500 cm 1 is attributed to small hydrogen bonded octanoic acid networks because the harmonic frequencies are red shifted from the monomer and due to the extremely broad bandshape. The most prevalent of these appears to be the (CH3(CH2)6COOH)2 dimer as evidenced by the correlation to the C@O stretching region.
mode [16]. For the octanoic acid monomer the C@O stretch was observed at 1781.5 cm 1 (Fig. 2). The harmonic frequency of this mode was calculated to occur at 1842.5 cm 1 (Table 1) and agrees well with experiment considering the anharmonicity of the C@O stretch [14]. A less intense transition was observed to low energy of the C@O stretch of octanoic acid at 1710.0 cm 1. This peak is attributed to the C@O stretching transition in the octanoic acid dimer (CH3(CH2)6COOH)2 which had a calculated harmonic frequency of 1780.7 cm 1. This 70 cm 1 frequency shift is in agreement with previous work on the acetic acid dimer which calculated a harmonic vibrational frequency of 1862.0 cm 1 (MP2/ 6-31G[d]) [18] for the C@O stretch of the monomer and 1825.1 cm 1 for the Bu symmetry C@O stretch of the dimer. The Bu symmetry C@O stretch of the acetic acid dimer has been observed previously at 1737.0 cm 1 [26]. Since the C@O transitions for the octanoic acid monomer and dimer are overlapping, each band was fit with a Lorentzian function to calculate the relative intensity of the dimer to the monomer C@O stretch (Table 1) which was found to be 0.4 ± 0.064. The presence of two distinct C@O stretches in the vibrational spectrum, the strong correlation between experimental frequencies and theoretical predictions, and predictable agreement in pressure dependent spectra, provide evi-
Fig. 2. Expanded spectrum (Tcell = 298 K) of the (a) octanoic (Tsample = 298 K), (b) nonanoic (Tsample = 343 K), and (c) decanoic (Tsample = 363 K) acid systems in the C@O stretching region generated by flowing 4000 sccm filtered N2 through the sample.
Table 1 Calculated frequency, observed frequency, and relative intensity for organic acid monomers (M) and dimers (D)
CH3(CH2)6COOH (CH3(CH2)6COOH)2 CH3(CH2)7COOH (CH3(CH2)7COOH)2 CH3(CH2)8COOH (CH3(CH2)8COOH)2
Calculated harmonic frequency (cm 1)
Calculated intensity (km/mol)
Observed frequency (±0.6 cm 1)
Relative intensity
1842.5 1780.7 1842.5 1781.9 1842.3 1782.1
317.5 741.4 320.0 745.7 320.9 746.4
1781.5 1710.0 1781.5 1709.1 Not observed 1710.7
0.4 ± 0.064 (D/M) 9 ± 1.4 (D/M) 0 (M/D)
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Table 2 Calculated, observed, and predicted [17] peak positions for the monomer O–H stretch
CH3(CH2)6COOH CH3(CH2)7COOH CH3(CH2)8COOH
Calculated harmonic frequency (cm 1)
Observed frequency (±0.6 cm 1)
Predicted peak position (cm 1) [17]
3747.8 3748.2 3747.5
3578.5 3578.2 Not observed
3578.7 3578.2 3577.7
Fig. 3. Expanded spectrum (Tcell = 298 K) of the (a) octanoic (Tsample = 298 K), (b) nonanoic (Tsample = 343 K), and (c) decanoic (Tsample = 363 K) acid systems in the O–H stretching region generated by flowing 4000 sccm filtered N2 through the sample.
Fig. 4. B3LYP/6-31G[d,p] optimized cyclic dimer structure for (CH3(CH2)6COOH)2.
The CH3(CH2)7COOH/(CH3(CH2)7COOH)2 and CH3(CH2)8COOH/(CH3(CH2)8COOH)2 systems exhibited similar but more prominent spectroscopic dimer features compared to octanoic acid. The CH3(CH2)7 COOH monomer had a C@O stretching transition calculated at 1842.5 cm 1 and was observed at 1781.5 cm 1 (Table 1). CH3(CH2)8COOH had a calculated harmonic C@O stretching frequency of 1842.3 cm 1 but was not observed in any experiment. An intense transition to low energy of the monomer C@O stretch was observed in both systems and are attributed to the organic acid dimers. The (CH3(CH2)7COOH)2 dimer had a calculated harmonic C@O stretch of 1781.9 cm 1 and agrees with the observed transition at 1709.1 cm 1. Similarly, the (CH3(CH2)8COOH)2 dimer had a calculated harmonic C@O stretch of 1782.1 cm 1 which matches up with the one observed C@O stretching transition at 1710.7 cm 1. The ratio of
nonanoic acid dimer to monomer relative intensities of the C@O stretch was found to be 9 ± 1.4 with the bath temperature maintained at 343 ± 5 K. This was obtained by integrating fitted mixed Lorentzian/Gaussian functions to the observed features. It was not possible to obtain this value for the decanoic acid system because the C@O stretch of the monomer was not within signal-to-noise. The O–H stretching regions for the nonanoic and decanoic acid systems were also studied. The harmonic O–H stretching frequency of CH3(CH2)7COOH was calculated as 3748.2 cm 1, while that for CH3(CH2)8COOH was found to be 3747.5 cm 1 (Table 2). The O–H stretch for the CH3(CH2)7COOH was observed at 3578.2 cm 1 and agrees with the empirically predicted peak position of previous work [17]. The O–H stretch for CH3 (CH2)8COOH was not observed. Both organic acid systems exhibited a broad condensed phase feature between 3050 and 3500 cm 1. These features are attributed to the nonanoic and decanoic acid dimers based on the arguments made for octanoic acid. The bandshape is predictably broad due to hydrogen bonding. Also, both compounds were calculated to have close to 10 normal modes containing O–H stretch in this region. An interesting trend was revealed by analysis of the C@O stretching region and O–H stretching region from CH3(CH2)6COOH/(CH3(CH2)6COOH)2 to
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Table 3 Calculated single point energies and binding energies for all studied organic acid systems (B3LYP/6-311++G[3df,3pd]) Calculated energy (Hartrees) CH3(CH2)6COOH (CH3(CH2)6COOH)2 CH3(CH2)7COOH (CH3(CH2)7COOH)2 CH3(CH2)8COOH (CH3(CH2)8COOH)2
465.142349 930.312972 504.469423 1008.962511 543.796526 1087.618126
Calculated zero-point correction (Hartrees) 0.233449 0.468503 0.261828 0.525397 0.290119 0.582393
CH3(CH2)8COOH/(CH3(CH2)8COOH)2. The ratio of spectroscopically observed monomer/dimer diminishes from octanoic to decanoic acid. This is evidenced by both the C@O stretching region and the O–H stretching region. For octanoic acid, assuming the C@O stretching cross-sections for the monomer is a factor of two less than the dimer, it appears that the monomer is favored to exist at room temperature (Fig. 2). However, the decanoic acid system seems to exist completely in its dimer form under these conditions within signal-to-noise. This is reinforced by the observation of a similar trend in the O–H stretching region (Fig. 3). Assuming that the monomers and dimers come to equilibrium in the flow cell, this work estimates an equilibrium constant Kp on the order of 5 · 105 for all species. The estimate used Gaussian intensities (Table 1), observed spectral intensities, and a path length of 100 cm. The value should be taken as an upper limit because the assumption that equilibrium between the monomer and dimer is achieved is arguable. However, the estimated values are consistent with equilibrium constants of approximately 103 obtained for shorter chain organic acids (acetic, propionic, butyric, and isobutyric acids) from their UV/VIS spectra [21]. Binding energies for each of the studied dimers were obtained from single point energy calculations using the B3LYP/6-311++G[3df,3pd] method within the GAUSSIAN 98 suite of programs (Table 3). These energies qualitatively make sense from a bond strength perspective with two strong hydrogen bonds of about 5–10 kcal/mol each. They also agree favorably with the calculated binding energy of (CH3CH2COOH)2 which has been observed and calculated to be between 13.8 and 17.0 kcal/mol [13,21–25,27–30]. The binding energies of (CH3CH2COOH)2 (13.7–15.6 kcal/mol) [13,21,24, 25,28], (CH3(CH2)2COOH)2 (14.4–15.7 kcal/mol) [21,25], and ((CH3)2CHCOOH)2 (14.1–15.2 kcal/mol) [21,25] also agree with the calculations in this work. The binding energy calculations are not expected to explain the absence of decanoic acid monomer from the observed spectra. There are many factors contributing to the extent of the dimerization of these acids and it is most likely that this effect is simply caused by the different partial pressures of the acids (due to their vapor pressures) in the flow cell.
De (kcal/mol)
Do (kcal/mol)
17.8
16.7
14.9
13.8
15.7
14.4
5. Conclusions The organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, and (CH3(CH2)8COOH)2 and their corresponding monomers have been observed in the mid-IR using infrared absorption spectroscopy. Calculated vibrational harmonic frequencies using density functional theory with the B3LYP functional were essential in assigning the dimer features. These organic acid dimers are interesting prototype systems for spectroscopically studying hydrogen bonding. The distinct red shift of the C@O stretch by about 70 cm 1 absorbs in a region very different from that of organic acid monomers. Additionally, the hydrogen bonded O–H stretching region provides a broad and intense absorption feature that is red shifted from that of the monomer. Knowledge of these frequency shifts compared to the spectroscopic literature base on smaller chain acid dimers will help to understand the extent of cyclic dimerization of carboxylic acids.
Acknowledgments K.J. Feierabend is thanked for useful discussions. T.E. thanks CIRES for funding. D.H. is thankful for a Sewall fellowship. V.V. acknowledges NSF for funding and the Radcliffe Institute for Advanced Study for a fellowship.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version at doi:10.1016/ j.cplett.2004.12.021.
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[4] H. Tervahattu, J. Juhanoja, K. Kupiainen, J. Geophys. Res.Atmos. 107 (2002) 4319. [5] H. Tervahattu, K. Hartonen, V.M. Kerminen, K. Kupiainen, P. Aarnio, T. Koskentalo, A.F. Tuck, V. Vaida, J. Geophys. Res.Atmos. 107 (2002) 4053. [6] D.J. Donaldson, D. Anderson, J. Phys. Chem. A 103 (1999) 871. [7] G.B. Ellison, A.F. Tuck, V. Vaida, J. Geophys. Res.-Atmos. 104 (1999) 11633. [8] K. Kawamura, S. Steinberg, I.R. Kaplan, Atmos. Environ. 34 (2000) 4175. [9] N. Kumar, U.C. Kulshrestha, P. Khare, A. Saxena, K.M. Kumari, S.S. Srivastava, Atmos. Environ. 30 (1996) 3545. [10] D. Grosjean, Atmos. Environ. Part A-Gen. 24 (1990) 2699. [11] H.A. Khwaja, Atmos. Environ. 29 (1995) 127. [12] C. Emmeluth, M.A. Suhm, D. Luckhaus, J. Chem. Phys. 118 (2003) 2242. [13] A. Winkler, P. Hess, J. Am. Chem. Soc. 116 (1994) 9233. [14] G.M. Florio, T.S. Zwier, E.M. Myshakin, K.D. Jordan, E.L. Sibert, J. Chem. Phys. 118 (2003) 1735. [15] M.A. Czarnecki, Chem. Phys. Lett. 368 (2003) 115. [16] NIST, NIST Chemistry Web Book, NIST Standard Reference Database 69 – March 2003 Release, 2003. [17] D.K. Havey, V. Vaida, J. Mol. Spectrosc. 228 (2004) 152.
[18] L. Turi, J.J. Dannenberg, J. Phys. Chem. 97 (1993) 12197. [19] M.J. Frisch et al., GAUSSIAN 98 (Revision A.6), Gaussian Inc., Pittsburg, PA, 1998. [20] P.R. Rablen, J.W. Lockman, W.L. Jorgensen, J. Phys. Chem. A 102 (1998) 3782. [21] P.E. Hintze, S. Aloisio, V. Vaida, Chem. Phys. Lett. 343 (2001) 159. [22] E.W. Johnson, L.K. Nash, J. Am. Chem. Soc. 72 (1950) 547. [23] A.W. Pross, F. Van Zeggeren, Spectrochim. Acta 16 (1960) 563. [24] D.M. Mathews, R.W. Sheets, J. Chem. Soc. A 15 (1969) 2203. [25] A.D.H. Clague, H.J. Bernstein, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 25 (1969) 593. [26] Y. Marechal, J. Chem. Phys. 87 (1987) 6344. [27] J. Chocholousova, J. Vacek, P. Hobza, J. Phys. Chem. A 107 (2003) 3086. [28] J.O. Halford, J. Chem. Phys. 10 (1942) 582. [29] D.J. Frurip, L.A. Curtiss, M. Blander, J. Am. Chem. Soc. 102 (1980) 2610. [30] D.D. Wagman, W.H. Evans, V.B. Parker, R.H. Schumm, I. Halow, S.M. Bailey, K.L. Churney, R.L. Nuttall, J. Phys. Chem. Ref. Data 11 (1982) 1. [31] J.J. Orlando, G.S. Tyndall, J. Photochem. Photobiol. A: Chem. (2003) 161.
Gas phase infrared spectroscopic observation of the organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, and (CH3(CH2)8COOH)2 Teresa L. Eliason 1, Daniel K. Havey 1, Veronica Vaida
*
Department of Chemistry and Biochemistry and CIRES, University of Colorado, Campus Box 215, Boulder, CO 80309, USA Received 19 July 2004; in final form 3 November 2004
Abstract This work uses infrared absorption spectroscopy to study the organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, (CH3(CH2)8COOH)2, and their monomers. There is a scarce literature base about these acids and the dimers have not been previously studied in gas phase. Ab initio calculations of vibrational harmonic frequencies are used as a guide for spectroscopic investigations of C@O and O–H stretching regions. Correlation between experiment and theory as well as an interesting trend with acid chain length are discussed. Do and De are calculated for all dimers using the B3LYP/6-311++G[3df,3pd] method and are compared to literature values for short chain organic acid dimers. 2004 Published by Elsevier B.V.
1. Introduction Organic compounds have been shown to contribute significantly to the overall mass of aerosol particles [1–5]. Organic acids have been observed to preferentially partition to the surface of these aerosols [4–6], possibly with an inverted micelle structure [7]. Field measurements have shown that organic acids are substantial contributors to automobile exhaust [8], atmospheric aerosols [3], and a polluted atmosphere [2,9–11]. Formic and acetic acids were observed to be the most prevalent organic acids in the polluted troposphere but octanoic, nonanoic, and decanoic acids all contribute to the total organic mass. Total automobile emissions of octanoic and nonanoic acids in vapor phase and as particulate *
Corresponding author. Fax: +1 303 492 5894. E-mail addresses: [email protected] (T.L. Eliason), havey@ colorado.edu (D.K. Havey), [email protected] (V. Vaida). 1 These authors contributed equally to the development of this work. 0009-2614/$ - see front matter 2004 Published by Elsevier B.V. doi:10.1016/j.cplett.2004.12.021
matter were found to be 0.101 and 0.095 ppbv, respectively [8]. The gas phase lifetimes of octanoic, nonanoic, and decanoic acids are expected to be short in the atmosphere due to their low-vapor pressures. Thus, these acids are predicted to exist predominantly in condensed phase as small hydrogen bonded acid clusters or in aerosols. Characterizing the organic acid cyclic dimers [12–15,31] is of fundamental importance to understand the effect of hydrogen bonding in these systems. This work uses infrared absorption spectroscopy to obtain gas phase vibrational spectra of the organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, (CH3(CH2)8COOH)2, and their corresponding monomers [16]. These dimers have been previously unreported because experiments are hampered by the low-vapor pressures of the acid monomers. Theoretical calculations of vibrational harmonic frequencies as well as previous work on smaller organic acids [17] are used to investigate the C@O and O–H stretching regions. Correlation between experiment and theory as well as an
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interesting trend in the extent of dimerization with acid chain length are discussed.
2. Experimental Absorption spectra of (CH3(CH2)6COOH)2 (octanoic acid dimer), (CH3(CH2)7COOH)2 (nonanoic acid dimer) and (CH3(CH2)8COOH)2 (decanoic acid dimer) were measured in the IR/NIR from 1000 to 7500 cm 1. The sample chamber was a 100 cm pathlength optical flow cell with CaF2 windows, attached using Viton O-ring seals. Spectroscopic measurements were made using a Bruker IFS 66v/S Fourier transform spectrometer (FTS). A silicon carbide source, KBr beamsplitter, and HgCdTe detector were used. The FTS instrument was used in external mode with the light directed through a CaF2 window into the cell and then to a parabolic mirror focusing onto the detector. The experiments co-added 128 scans at 2 cm 1 resolution. The (CH3(CH2)6COOH)2 dimer was formed by bubbling filtered N2 at 4000 standard cubic centimeters per minute (sccm) through liquid octanoic acid (Aldrich, 99.5+%). The (CH3(CH2)7COOH)2 dimer was formed by flowing 4000 sccm filtered N2 through a bubbler containing nonanoic acid (Aldrich, 96%) immersed in an isothermal bath at 343 ± 5 K. Decanoic acid (Aldrich, 99%) is a solid at room temperature and the dimer was prepared by immersing a bubbler containing the solid in an isothermal bath at 363 ± 5 K. Filtered N2 at 4000 sccm was then bubbled through the melted sample. The cell was maintained at room temperature throughout all experiments. This work attempted to measure dimer features in the shorter chain heptanoic and hexanoic acids. However, their increased vapor pressures combined with a strong tendency to form large clusters and aerosols, under the conditions of these experiments, made it difficult to isolate dimer features from condensed features. Because water vapor absorbs in the spectral regions of interest it was imperative to ensure that spectral features were independent of H2O partial pressure. Spectra were recorded for each acid with varying partial pressures of water vapor from 0.0 to 0.3 Torr. There was no noticeable change in the bandshape or relative intensity of any of the spectral features studied. Thus, this work is confident that the spectral features discussed in this work depend solely on the partial pressures of the organic acids. It was necessary to ensure that all spectral features observed for the acid dimers were dependent on the partial pressure of the acid monomers in accordance with behavior predicted by the reaction equilibrium. Spectra were taken that changed the octanoic and nonanoic acid monomer partial pressures contained in the flow cell. This was accomplished by varying the
Fig. 1. Expanded spectrum (Tcell = 298 K) of the nonanoic acid system in the C@O stretching region with the bath maintained at (a) 298 K, (b) 343 K, and (c) 363 K.
temperature of the bath from 298 ± 5 to 343 ± 5 K. Based on Beers law, it can be shown that a linear increase in monomer partial pressure, while fixing other experimental parameters, leads to a predictable quadratic increase in integrated intensity of the dimer peak (Fig. 1). This assumes a typical 2:1 ratio of the C@O stretching absorption cross-sections of the dimer to monomer as calculated in this work and in previous studies [18].
3. Calculations Optimized geometries and vibrational harmonic frequencies for the monomers and dimers of octanoic, nonanoic, and decanoic acid were calculated using the B3LYP/6-31G[d,p] method within the GAUSSIAN 98 suite of programs [19]. All harmonic frequencies were left unscaled. The optimized B3LYP/6-31G[d,p] geometries were used to perform single point energy calculations on all species using the B3LYP/6-311++G[3df, 3pd] method. Previous work has shown that the B3LYP method is appropriate for describing hydrogen bonded organic acid hydrates as long as the basis set exceeds 6-31+G** [20]. The use of B3LYP to describe hydrogen bonded dimers is rationalized because of the extensive agreement between binding energies calculated using this method and those measured experimentally [21–25].
4. Results and discussion The vibrational spectrum of the CH3(CH2)6COOH/ (CH3(CH2)6COOH)2 system in the region 1000–7500 cm 1 is dominated by the C@O stretching vibrational
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dence for assigning spectroscopic contributions of the dimer at room temperature. The O–H stretching transition (3000–3600 cm 1) region also contained spectral features attributed to both the octanoic acid monomer and dimer. The O–H stretch of CH3(CH2)6COOH had a calculated harmonic frequency of 3747.8 cm 1 (Table 2) and the transition was observed at 3578.5 cm 1. This is in agreement with the calculations in this work as well as with transition frequencies empirically predicted by previous work [17]. A broad O–H stretching transition (Fig. 3) between 3050 and 3500 cm 1 was observed to low energy of the monomer O–H stretch and is attributed to the hydrogen bonded O–H stretch of the octanoic acid dimer. Harmonic frequency calculations indicate that 5–10 normal modes between 3000 and 3125 cm 1 contain a notable O–H stretching component. The broad bandshape of the feature is not surprising due to the effect of hydrogen bonding in the cyclic dimer structure (Fig. 4). The diffuse spectral feature observed between 3000 and 3500 cm 1 is attributed to small hydrogen bonded octanoic acid networks because the harmonic frequencies are red shifted from the monomer and due to the extremely broad bandshape. The most prevalent of these appears to be the (CH3(CH2)6COOH)2 dimer as evidenced by the correlation to the C@O stretching region.
mode [16]. For the octanoic acid monomer the C@O stretch was observed at 1781.5 cm 1 (Fig. 2). The harmonic frequency of this mode was calculated to occur at 1842.5 cm 1 (Table 1) and agrees well with experiment considering the anharmonicity of the C@O stretch [14]. A less intense transition was observed to low energy of the C@O stretch of octanoic acid at 1710.0 cm 1. This peak is attributed to the C@O stretching transition in the octanoic acid dimer (CH3(CH2)6COOH)2 which had a calculated harmonic frequency of 1780.7 cm 1. This 70 cm 1 frequency shift is in agreement with previous work on the acetic acid dimer which calculated a harmonic vibrational frequency of 1862.0 cm 1 (MP2/ 6-31G[d]) [18] for the C@O stretch of the monomer and 1825.1 cm 1 for the Bu symmetry C@O stretch of the dimer. The Bu symmetry C@O stretch of the acetic acid dimer has been observed previously at 1737.0 cm 1 [26]. Since the C@O transitions for the octanoic acid monomer and dimer are overlapping, each band was fit with a Lorentzian function to calculate the relative intensity of the dimer to the monomer C@O stretch (Table 1) which was found to be 0.4 ± 0.064. The presence of two distinct C@O stretches in the vibrational spectrum, the strong correlation between experimental frequencies and theoretical predictions, and predictable agreement in pressure dependent spectra, provide evi-
Fig. 2. Expanded spectrum (Tcell = 298 K) of the (a) octanoic (Tsample = 298 K), (b) nonanoic (Tsample = 343 K), and (c) decanoic (Tsample = 363 K) acid systems in the C@O stretching region generated by flowing 4000 sccm filtered N2 through the sample.
Table 1 Calculated frequency, observed frequency, and relative intensity for organic acid monomers (M) and dimers (D)
CH3(CH2)6COOH (CH3(CH2)6COOH)2 CH3(CH2)7COOH (CH3(CH2)7COOH)2 CH3(CH2)8COOH (CH3(CH2)8COOH)2
Calculated harmonic frequency (cm 1)
Calculated intensity (km/mol)
Observed frequency (±0.6 cm 1)
Relative intensity
1842.5 1780.7 1842.5 1781.9 1842.3 1782.1
317.5 741.4 320.0 745.7 320.9 746.4
1781.5 1710.0 1781.5 1709.1 Not observed 1710.7
0.4 ± 0.064 (D/M) 9 ± 1.4 (D/M) 0 (M/D)
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Table 2 Calculated, observed, and predicted [17] peak positions for the monomer O–H stretch
CH3(CH2)6COOH CH3(CH2)7COOH CH3(CH2)8COOH
Calculated harmonic frequency (cm 1)
Observed frequency (±0.6 cm 1)
Predicted peak position (cm 1) [17]
3747.8 3748.2 3747.5
3578.5 3578.2 Not observed
3578.7 3578.2 3577.7
Fig. 3. Expanded spectrum (Tcell = 298 K) of the (a) octanoic (Tsample = 298 K), (b) nonanoic (Tsample = 343 K), and (c) decanoic (Tsample = 363 K) acid systems in the O–H stretching region generated by flowing 4000 sccm filtered N2 through the sample.
Fig. 4. B3LYP/6-31G[d,p] optimized cyclic dimer structure for (CH3(CH2)6COOH)2.
The CH3(CH2)7COOH/(CH3(CH2)7COOH)2 and CH3(CH2)8COOH/(CH3(CH2)8COOH)2 systems exhibited similar but more prominent spectroscopic dimer features compared to octanoic acid. The CH3(CH2)7 COOH monomer had a C@O stretching transition calculated at 1842.5 cm 1 and was observed at 1781.5 cm 1 (Table 1). CH3(CH2)8COOH had a calculated harmonic C@O stretching frequency of 1842.3 cm 1 but was not observed in any experiment. An intense transition to low energy of the monomer C@O stretch was observed in both systems and are attributed to the organic acid dimers. The (CH3(CH2)7COOH)2 dimer had a calculated harmonic C@O stretch of 1781.9 cm 1 and agrees with the observed transition at 1709.1 cm 1. Similarly, the (CH3(CH2)8COOH)2 dimer had a calculated harmonic C@O stretch of 1782.1 cm 1 which matches up with the one observed C@O stretching transition at 1710.7 cm 1. The ratio of
nonanoic acid dimer to monomer relative intensities of the C@O stretch was found to be 9 ± 1.4 with the bath temperature maintained at 343 ± 5 K. This was obtained by integrating fitted mixed Lorentzian/Gaussian functions to the observed features. It was not possible to obtain this value for the decanoic acid system because the C@O stretch of the monomer was not within signal-to-noise. The O–H stretching regions for the nonanoic and decanoic acid systems were also studied. The harmonic O–H stretching frequency of CH3(CH2)7COOH was calculated as 3748.2 cm 1, while that for CH3(CH2)8COOH was found to be 3747.5 cm 1 (Table 2). The O–H stretch for the CH3(CH2)7COOH was observed at 3578.2 cm 1 and agrees with the empirically predicted peak position of previous work [17]. The O–H stretch for CH3 (CH2)8COOH was not observed. Both organic acid systems exhibited a broad condensed phase feature between 3050 and 3500 cm 1. These features are attributed to the nonanoic and decanoic acid dimers based on the arguments made for octanoic acid. The bandshape is predictably broad due to hydrogen bonding. Also, both compounds were calculated to have close to 10 normal modes containing O–H stretch in this region. An interesting trend was revealed by analysis of the C@O stretching region and O–H stretching region from CH3(CH2)6COOH/(CH3(CH2)6COOH)2 to
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Table 3 Calculated single point energies and binding energies for all studied organic acid systems (B3LYP/6-311++G[3df,3pd]) Calculated energy (Hartrees) CH3(CH2)6COOH (CH3(CH2)6COOH)2 CH3(CH2)7COOH (CH3(CH2)7COOH)2 CH3(CH2)8COOH (CH3(CH2)8COOH)2
465.142349 930.312972 504.469423 1008.962511 543.796526 1087.618126
Calculated zero-point correction (Hartrees) 0.233449 0.468503 0.261828 0.525397 0.290119 0.582393
CH3(CH2)8COOH/(CH3(CH2)8COOH)2. The ratio of spectroscopically observed monomer/dimer diminishes from octanoic to decanoic acid. This is evidenced by both the C@O stretching region and the O–H stretching region. For octanoic acid, assuming the C@O stretching cross-sections for the monomer is a factor of two less than the dimer, it appears that the monomer is favored to exist at room temperature (Fig. 2). However, the decanoic acid system seems to exist completely in its dimer form under these conditions within signal-to-noise. This is reinforced by the observation of a similar trend in the O–H stretching region (Fig. 3). Assuming that the monomers and dimers come to equilibrium in the flow cell, this work estimates an equilibrium constant Kp on the order of 5 · 105 for all species. The estimate used Gaussian intensities (Table 1), observed spectral intensities, and a path length of 100 cm. The value should be taken as an upper limit because the assumption that equilibrium between the monomer and dimer is achieved is arguable. However, the estimated values are consistent with equilibrium constants of approximately 103 obtained for shorter chain organic acids (acetic, propionic, butyric, and isobutyric acids) from their UV/VIS spectra [21]. Binding energies for each of the studied dimers were obtained from single point energy calculations using the B3LYP/6-311++G[3df,3pd] method within the GAUSSIAN 98 suite of programs (Table 3). These energies qualitatively make sense from a bond strength perspective with two strong hydrogen bonds of about 5–10 kcal/mol each. They also agree favorably with the calculated binding energy of (CH3CH2COOH)2 which has been observed and calculated to be between 13.8 and 17.0 kcal/mol [13,21–25,27–30]. The binding energies of (CH3CH2COOH)2 (13.7–15.6 kcal/mol) [13,21,24, 25,28], (CH3(CH2)2COOH)2 (14.4–15.7 kcal/mol) [21,25], and ((CH3)2CHCOOH)2 (14.1–15.2 kcal/mol) [21,25] also agree with the calculations in this work. The binding energy calculations are not expected to explain the absence of decanoic acid monomer from the observed spectra. There are many factors contributing to the extent of the dimerization of these acids and it is most likely that this effect is simply caused by the different partial pressures of the acids (due to their vapor pressures) in the flow cell.
De (kcal/mol)
Do (kcal/mol)
17.8
16.7
14.9
13.8
15.7
14.4
5. Conclusions The organic acid dimers (CH3(CH2)6COOH)2, (CH3(CH2)7COOH)2, and (CH3(CH2)8COOH)2 and their corresponding monomers have been observed in the mid-IR using infrared absorption spectroscopy. Calculated vibrational harmonic frequencies using density functional theory with the B3LYP functional were essential in assigning the dimer features. These organic acid dimers are interesting prototype systems for spectroscopically studying hydrogen bonding. The distinct red shift of the C@O stretch by about 70 cm 1 absorbs in a region very different from that of organic acid monomers. Additionally, the hydrogen bonded O–H stretching region provides a broad and intense absorption feature that is red shifted from that of the monomer. Knowledge of these frequency shifts compared to the spectroscopic literature base on smaller chain acid dimers will help to understand the extent of cyclic dimerization of carboxylic acids.
Acknowledgments K.J. Feierabend is thanked for useful discussions. T.E. thanks CIRES for funding. D.H. is thankful for a Sewall fellowship. V.V. acknowledges NSF for funding and the Radcliffe Institute for Advanced Study for a fellowship.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version at doi:10.1016/ j.cplett.2004.12.021.
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