Measurement of the Fourth Oâ H Overtone Absorption Cross Section

ARTICLE pubs.acs.org/JPCA

Measurement of the Fourth O-H Overtone Absorption Cross Section in Acetic Acid Using Cavity Ring-Down Spectroscopy Israel Begashaw,†,‡ Marc N. Fiddler,† Solomon Bililign,*,† and Steven S. Brown§ †

Department of Physics and NOAA-ISET Center, North Carolina Agricultural and Technical State University, Greensboro, North Carolina 27411 § Earth System Research Laboratory, Chemical Science Division, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305 ABSTRACT: We report the absolute absorption cross sections of the fourth vibrational O-H (5νOH) overtone in acetic acid using cavity ring-down spectroscopy. For compounds that undergo photodissociation via overtone excitation, such intensity information is required to calculate atmospheric photolysis rates. The fourth vibrational overtone of acetic acid is insufficiently energetic to effect dissociation, but measurement of its cross section provides a model for other overtone transitions that can affect atmospheric photochemistry. Though gas-phase acetic acid exists in equilibrium with its dimer, this work shows that only the monomeric species contributes to the acetic acid overtone spectrum. The absorption of acetic acid monomer peaks at ∼615 nm and has a peak cross section of 1.84 10-24 cm2 3 molecule-1. Between 612 and 620 nm, the integrated cross section for the acetic acid monomer is (5.23 ( 0.73) 10-24 cm2 3 nm 3 molecule-1 or (1.38 ( 0.19) 10-22 cm2 3 molecule-1 3 cm-1. This is commensurate with the integrated cross section values for the fourth O-H overtone of other species. Theoretical calculations show that there is sufficient energy for hydrogen to transition between the two oxygen atoms, which results in an overtone-induced conformational change.

’ INTRODUCTION Presently, the overtone spectroscopy of atmospherically relevant compounds is a topic of substantial interest due to their recently discovered role in photochemistry. In 1997, Donaldson et al.1 put forward a mechanism for the photochemical production of OH radical without the photolysis of ozone. They proposed that some atmospheric species that contain an O-H bond absorb visible light to excite the vibrational overtone of the O-H stretch. This light deposits enough energy in the molecule to initiate dissociation to produce OH or HO2 (collectively, HOx) radical. This mechanism was referred to as direct overtone photodissociation (DOP). The HOx radicals are produced as a result of the high energy photons absorbed by the O-H vibrational overtones that lie near or above the dissociation limit for one of the bonds in the molecule. Since then, several species, including HNO3,2-5 HO2NO2,4,6,7 HONO,5,8 H2O2,3,4 and CH3OOH,9,10 have been investigated as potential sources for HOx radicals through direct overtone photodissociation. All of these molecules contain an O-H chromophore that is weakly bonded to other atoms. In polyatomic molecules that contain C-H, N-H, and O-H (X-H) bonds, these stretching overtone vibrations dominate the ground electronic state vibrational overtone spectra.11 The small mass of hydrogen in the X-H bond gives rise to X-H stretching frequencies that are much higher than other vibrational modes,11,12 which allows them to be considered as local oscillators that are uncoupled from the other vibrations. Similar photochemical processes in H2SO4 have been suggested to reconcile discrepancies between modeled and observed levels of r 2011 American Chemical Society

sulfur dioxide (SO2) in the stratosphere and mesosphere.13 If the O-H excitation were to lead to decomposition of H2SO4, then the O-H vibrational overtone excitation in sulfuric acid (H2SO4) was shown to have sufficient intensity that it could produce SO2 at levels commensurate with observations.14,15 In addition to the OH radical and SO2 production, the O-H stretch vibrational overtone excitation of molecules in the atmosphere has recently been shown to lead to yet another significant chemical processes, as summarized in a recent review article.16 These reactions are different from radical production by photolysis, since they have lower dissociation thresholds and result from intramolecular rearrangement reactions. The dehydration of H2SO4 and the decarboxylation of pyruvic acid (CH3(CdO)2OH) are discussed in detail showing the potentially significant implications overtone excitation has on atmospheric chemistry. In H2SO4, the overtone excitation energy is not sufficient to cause direct bond cleavage; rather the reaction takes place through the hydrogen atom hopping between equivalent oxygen atoms at a very short time scale.16 The decarboxylation of pyruvic and glyoxalic acids are also initiated through excitation of their vibrational O-H stretch to the fourth and fifth quanta, which proceeds through a five-membered transition state consisting of the OH group of the carboxylic acid and the carbonyl group in the R position.17-19 This low energy phenomenon is, however, limited to Received: September 13, 2010 Revised: December 9, 2010 Published: January 12, 2011 753

dx.doi.org/10.1021/jp1087338 | J. Phys. Chem. A 2011, 115, 753–761

The Journal of Physical Chemistry A

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Table 1. Standard Enthalpy and Free Energy Change, in kcal/mol and kJ/mol, for the Decomposition of Acetic Acid to the Listed Productsa ΔH0 (kcal 3 mol-1, kJ 3 mol-1)

ΔG0 (kcal 3 mol-1, kJ 3 mol-1)

CO2 þ CH4

-8.58, -35.9

-16.9, -70.7

70.5, 295

405

H2O þ OdCdCH2 CH3• þ HOCO•

34.3, 142.9 93, 391

23.3, 97.3 85, 357

70.5, 295 85, 357

405 335

products

a

ΔG‡ (kcal 3 mol -1, kJ 3 mol-1)

λmax (nm)

H• þ •CH2(CO)OH

95.2, 398.3

93.7, 392.2

93.7, 392.2

305

H• þ CH3(CO)O•

110, 462

106, 442

106, 442

270

CH3(CO)• þ •OH

109.9, 459.9

98.6, 412.7*

98.6, 412.7*

290

cis-AcOH

5.8, 24.4*

5.6, 23.4*

12.5, 52.5*

2280

CH/OH eclipsed AcOH (V3)

-0.1, -0.4*

1.3, 5.4*

1.3, 5.4*

22360

The activation energy (ΔG‡) and maximum wavelength to achieve that energy is also listed for each reaction. One free energy change value, for C-O bond dissociation, and two rotational energy values were calculated in this work, as denoted by an asterisk. Values for dehydration and decarboxylation are from the pyrolysis-shock tube experiments of Mackie and Doolan,34 while others are from a compilation.36

R-carbonyl carboxylic acids. Staikova et al. studied the overtoneinduced decarboxylation of malonic acid20 and found that this mechanism can compete with the other removal mechanisms of malonic acid, namely, wet deposition and gas-phase reaction with the OH radical. Similarly, overtone-induced decarboxylation of vinyl acetic acid has also been experimentally and theoretically investigated.21 The investigation suggests that the major contribution to decarboxylation will come from the third O-H overtone. The overtone-induced decarboxylation resulted in an approximate lower limit of 27 days for its atmospheric lifetime. Since there is no UV photochemical process available for vinyl acetic acid, the overtone-induced decarboxylation could compete with its other removal mechanisms, such as wet and dry deposition, that have a lifetime on the order of 10 days. The other possible removal mechanism is oxidation by reaction with the OH radical. The authors also compared the decarboxylation lifetime of formic acid with oxidation by the OH radical (26 days) to indicate the competitiveness of overtone-induced decarboxylation. These studies suggest that overtone-induced chemistry can serve as a potential sink for carboxylic acids with lifetimes comparable to oxidation. The absorption cross sections for these overtones, therefore, need to be accurately characterized. The loss of species “A” through a photochemical pathway is expressed as first-order decay.

and 4νOH vibrational overtones of acetic acid have also been studied both experimentally and theoretically.23,28,29 However, thus far, no measurement of the fourth O-H (5νOH) overtone has been reported, likely because of the very weak absorption of the high overtones. We have used cavity ring-down spectroscopy (CRDS), an ultra sensitive direct absorption technique that is well suited to measure such very weak absorptions.30,31 To our knowledge, this work represents the first experimental measurement of the fourth O-H (5νOH) overtone absorption cross sections for acetic acid. Several reactions of acetic acid have been previously examined,32-35 and the reaction thermochemical values are summarized in Table 1 and Figure 1. The rotational barriers of acetic acid, derived from theoretical calculations performed in this work, are also included in Figure 1. Homolytic bond dissociation energies and standard free energy changes were derived from the standard heats and free energies of formation36 and are expected to be barrierless processes. Dehydration of acetic acid yields water and ketene, while decarboxylation yields carbon dioxide and methane. Notably, decarboxylation is an exothermic process, owing to the stability of the products. However, the barrier to this process is large (see below). Although acetic acid is not photochemically active in the atmosphere at the excitation energy of its fourth overtone, the spectroscopy and absolute intensity of this transition are of fundamental interest since it adds to the understanding of overtone spectroscopy in atmospherically relevant compounds.

A þ hν f products The rate coefficient (J) is related to the concentration of species A, [A], as follows:11 d½A ¼ - J½A dt

ð1Þ

Z J ¼

½σðλÞ φðλÞ IðλÞ dλ

ð2Þ

The rate coefficient J is a function of the spectral actinic flux I(λ), the quantum yield for a particular reaction φ(λ), and absorption cross section of the molecule σ(λ).22 The spectral actinic flux is the flux of radiation from all directions incident on a volume of air. The quantum yield is the probability the molecule will follow a certain reaction channel after absorbing a photon. The absorption cross section, given in units of area per molecule, measures the probability of interaction between a molecule and a photon. The O-H overtones of several organic acids have been studied by Vaida and co-workers.16-18,21,23-27 The 1νOH, 2νOH, 3νOH,

’ EXPERIMENTAL SECTION A detailed description of CRDS can be found elsewhere.31,37 In simple terms, in CRDS, the rate of decay of light exiting an optical cavity is measured. In this work, the optical cavity is formed by two highly reflective mirrors aligned perpendicularly with a laser beam introduced directly through one of the end mirrors. The mirrors trap this beam, which results in an increased effective optical path length as the beam interacts with a sample placed between the two mirrors. The difference in decay constant or ring-down time (τ) of the light coming out of the cavity with (τ) and without (τ0) a sample furnishes the absorption due the sample, according to the following equation, where the extinction coefficient (R, cm-1) is related to the respective ring-down times, the ratio of the length in which the absorber is present to the total mirror separation length (RL), and the speed of light in air (cair). RL 1 1 ¼ R ¼ σn cair τ τ0 754

ð3Þ

dx.doi.org/10.1021/jp1087338 |J. Phys. Chem. A 2011, 115, 753–761


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Figure 1. The potential energy surface for reactions of acetic acid. Black lines refer to standard enthalpy changes (ΔH0) and blue refer to standard free energy changes (ΔG0); both are at STP. Energies corresponding to the vibrational excitation of the O-H bond (first through eleventh quanta) are superimposed using dashed lines.

Figure 2. Experimental setup of the system, including the lasers, optical configuration, ring-down cavity, UV cell, and the acquisition and control system. Objects are not to scale.

The extinction coefficient is, in turn, the product of the absorption cross section and the number density of the sample (n). If the cross section is known, then CRDS can determine the number density of very low concentration or very weakly absorbing species. If the number density of the species is accurately known, then CRDS can provide the absolute absorption cross sections, as is done in this work. The CRDS experimental setup (see Figure 2) used in this work is similar to that described by Brown et al.3 A frequency doubled Nd:YAG laser (Continuum Laser Inc.) pumped a widely tunable dye laser (Continuum Laser Inc.). The Nd:YAG laser had a pulse width of 4-6 ns, a line width of 1 cm-1, and a repetition rate of 20 Hz. The dye laser had a manufacturer specified line width of 0.08 cm-1 at 560 nm. Rhodamine 640 dye (Exciton) was used to

target the specified O-H overtone transition. The CRDS setup included an optical isolator and a telescope to optimize coupling of the laser beam to the lower order transverse electric modes of the cavity. The ring-down cavity was made out of two 30 cm long 1/2 inch outer diameter Teflon tubes placed inside two 1/2 inch inner diameter copper pipes for mechanical support. The two pieces were connected with a Swagelok Tee fitting and an inline pressure transducer (Omega, PX309) was connected to one leg of the Tee to monitor the pressure insides the cavity. The ring-down cavity was connected to the mirror mounts using Entegris Teflon fittings. The fittings also served as the inlet and exit ports of the ringdown cavity. The mirror mounts had flexible bellows that effectively created a buffer volume between the highly reflective mirrors and the sample in consideration when flowing through the ring-down 755



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Figure 3. Comparison of several water vapor spectra with simulated spectrum from HITRAN 2008 (red).40

cavity. The separation was 91.0 cm, and the sample gas occupied the central 76.8 cm. The cavity mirrors (Los Gatos Research, Inc.) had peak reflectivity of 99.995% at 620 nm. The light exiting out of the cavity was collected by a fiber collimator (Thorlabs, F220SMA-B) attached to a fiber optic cable (Thorlabs, M25L01) which threaded into a flange attached to photomultiplier tube housing. A photomultiplier tube (PMT) (Electron Tubes Ltd., 9558B), detected the small transmission of laser intensity out of the cavity. The signal from the PMT was digitized using a 16-bit dual channel waveform digitizer (Gage, Compuscope 1602). The digitizer card was plugged into a PCI slot of a personal computer. A 1 MΩ terminator was used to convert the current signal from the PMT to a voltage; the resulting resistor-capacitor time constant was short in comparison to the measured ring-down time constants, such that its contribution to the ring-down time could be neglected. Custom software written in LabVIEW (National Instruments, LabVIEW 8.5) was used to acquire ring-down data. The software also controlled the dye laser grating motor, in order to scan the region of interest. Typical ring down times were near 95 μs at 620 nm. Spectra were obtained by recording the response of the ring-down cavity without any absorber (only N2 flowing) across the spectral window of interest, followed by a scan across the same region with a flow of the absorber in N2 gas in the ring-down cavity. Blanks (i.e., N2 only scans) were performed by simply bypassing the acetic acid bubbler. Blanks were done on the same day and used the same flow rates as sample measurements to minimize any difference between the blank and signal scans due to mechanical or thermal drifts of the cavity that may have affected the ring-down time constant. Typical flow rates were 76-83 mL 3 min-1 for the purge flow that maintains mirror reflectivity (Figure 2), 31-38 mL 3 min-1 for the sample flow, and, when used, 135-145 mL 3 min-1 of dilution flow, which is mixed with the sample flow before the ring-down cell. Theoretical calculations were performed to assess the barrier toward translational movement of hydrogen between the two

oxygen atoms in acetic acid through a four-membered transition state. It was also used to determine the standard free energy change associated with direct, homolytic bond cleavage to produce OH and acetyl radicals. In this case, the calculated standard enthalpy change (109.93 kcal 3 mol-1) was in very good agreement with previous determinations (109.91 kcal 3 mol-1).36 The ground and excited state of acetic acid were calculated using Gaussian 03 at the MP2/6-311þþG(d,p) level of theory.38 Additionally, the barriers for methyl and OH group rotation and the energy of the cis configuration of acetic acid were calculated at the same level of theory. Structural minima and transition states were confirmed using frequency calculations, which yielded no imaginary frequencies for the ground state and one for the transition state, which corresponded to hydrogen traversing between the two oxygen atoms. No energy scaling was used. Though earlier theoretical work on acetic acid rotational barriers used a higher level of theory (MP4/ cc-pVTZ), they did not distinguish between ΔH0 and ΔG0.39 Wavelength Calibration. The dye laser wavelength was calibrated by recording the spectra of water vapor near 629 nm, which was subsequently compared against HITRAN data.40 Multiple water vapor spectra were recorded at a wavelength interval of 0.005 nm and compared with a spectrum simulated using HITRAN line positions and intensities. The number density of water vapor molecules inside the cavity were determined by measuring the relative humidity (VWR Scientific) and a parametrization for the saturated vapor pressure of water,41 although, for spectral calibration, only the line positions, and not the absolute intensities, were required. The spectral matching allowed the calibration of the dye laser wavelength to (0.006 nm against HITRAN (see Figure 3). Sampling and Data Collection. All samples were introduced into the cavity by sweeping a small flow of N2 through the vapor in the head space above the liquid in a glass bubbler. The number density of sample molecules in the ring-down cavity was determined using UV absorption. A Zn lamp (BHK lamps) and a bandpass filter (Andover Corp.) provided monochromatic light at 756



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Figure 4. Response of the ring-down cavity for an empty cavity, diluted, and undiluted acetic acid vapor.

Figure 5. Mean cross sections for acetic acid monomer between 607 and 625 nm. The red trace is the mean cross section and the gray area is the uncertainty. An eight point box smoothing was performed on the mean cross section (black trace) and the uncertainty bunds (blue traces), which are listed numerically in Table 2.

214 nm. The light passed through an absorption cell that had a path length (l ) of 1 m and was detected by a solar blind phototube (Hamamatsu R765) detector. The UV absorption cross sections used were from Orlando et al.42 for both the monomer (σUV,M) and dimer (σUV,D) of acetic acid, which were 1.35 10-19 and 1.84 10-19 cm2 3 molecule-1, respectively. The UV absorption (A) is related to the number densities of monomeric (nUV,M) and dimeric (nUV,D) acetic acid in the UV cell, as shown in eq 4. A ¼ l σ UV, M nUV , M þ σ UV, D nUV, D

The dye laser wavelength was tuned to the absorption window near 615 nm. This window of absorption was chosen after carrying out a simple Birge-Sponer43 analysis from previously reported frequencies23 of the lower O-H overtones for acetic acid monomer. Spectra were recorded by stepping the dye laser from 607 - 625 at 0.05 nm intervals. Cross Section Calculations. Since acetic acid readily forms dimers at room temperature, the contribution of the monomer and dimer were separated in the following manner. Spectra were taken with and without a dilution flow, effectively changing the number density of the monomer and the dimer in the cavity. Using the well established equilibrium constant for dimerization,42 the temperature (T), and UV absorption, the number

ð4Þ

The vapor above glacial acetic acid (Acros Organics) with 99.8% purity was used to record the absorption of acetic acid. 757



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densities of monomeric (nRD,M) and dimeric (nRD,D) acetic acid inside the cavity were calculated. The equilibrium constant for dimerization, in terms of pressure (atm) of monomer (PM) and dimer (PD), has been previously determined, as shown in eq 5. Keq 0

PD ¼ 2 ¼ 7:1 10 - 9 e7705=T PM

Table 2. Average Cross Section and the Associated Uncertainty of Acetic Acid Monomer between 612 and 620 at 0.2 nm Intervals, After Eight Point Box Smoothing uncertainty

ð5Þ

Using the ideal gas law, this can be converted to number density: Keq

nUV , D ¼ 2 ¼ T 9:6746 10 - 31 e7705=T nUV, M

wavelength

cross section

(1σ,

(nm)

(cm2 3 molecule-1)

cm2 3 molecule-1)

612.0

3.38 10-26

6.73 10-26

-26

7.28 10-26

612.4 612.6

-26

4.37 10 7.13 10-26

7.48 10-26 5.80 10-26

612.8

1.28 10-25

4.11 10-26

2.21 10

-25

4.90 10-26

3.40 10

-25

5.82 10-26

4.96 10

-25

6.59 10-26

7.07 10

-25

8.15 10-26

9.70 10

-25

9.62 10-26

614.0 614.2

-24

1.27 10 1.57 10-24

1.19 10-25 1.46 10-25

614.4

1.74 10-24

1.45 10-25

1.80 10

-24

1.54 10-25

1.80 10

-24

1.71 10-25

1.74 10

-24

1.69 10-25

1.65 10

-24

1.75 10-25

1.51 10

-24

1.71 10-25

615.6 615.8

-24

1.35 10 1.18 10-24

1.44 10-25 1.25 10-25

616.0

1.05 10-24

1.17 10-25

9.58 10

-25

1.02 10-25

8.98 10

-25

9.40 10-26

8.46 10

-25

8.98 10-26

7.76 10

-25

8.37 10-26

6.86 10

-25

7.26 10-26

617.2 617.4

-25

5.93 10 4.93 10-25

7.07 10-26 7.52 10-26

617.6

3.98 10-25

6.83 10-26

2.93 10

-25

7.21 10-26

1.90 10

-25

7.70 10-26

1.32 10

-25

6.52 10-26

9.19 10

-26

5.10 10-26

6.00 10

-26

4.49 10-26

618.8 619.0

-26

2.84 10 1.69 10-26

5.57 10-26 6.44 10-26

619.2

1.99 10-26

5.47 10-26

-27

5.28 10-26

-2.69 10

-26

6.68 10-26

-2.31 10

-26

7.14 10-26

-27

5.07 10-26

612.2

ð6Þ

613.0

Substituting terms in the eq 4 with those in eq 6 allows us to relate absorptivity to the number density of the monomer in the UV cell. l

σUV , D Keq n2UV , M

þ l σUV , M nUV , M - A ¼ 0

613.2 613.4 613.6

ð7Þ

613.8

Since this is in the form of a quadratic equation, nUV,M can be determined as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi l 2 σUV , M 2 þ 4Al σ UV , D Keq σUV , M þ ð8Þ nUV, M ¼ 2l σUV , D Keq 2σ UV, D Keq

614.6 614.8 615.0 615.2

Dilution due to the purge flow was taken into account by assuming that half the purge flow (Fpurge) went to each mirror and the flow at detector side did not enter the sample portion of the cell (see Figure 2). Using the purge, sample (Fsamp), and dilution (Fdil) flows, the relationship between the number density of monomeric acetic acid in the UV cell (nUV,M) and the ring-down cell (nRD,M) can be determined: nRD, M

Fsamp þ Fdil þ Fpurge ¼ nUV , M Fsamp þ Fdil þ Fpurge =2

615.4

616.2 616.4 616.6

ð9Þ

616.8 617.0

Acetic acid dimer was assumed to not absorb in the visible wavelength range scanned for the overtone absorption, for reasons discussed below, so absorption would be only attributed to monomeric acetic acid. For each run, the extinction coefficient (R) was determined from the sample and baseline decay constant, using eq 3. However, in sample runs, the baseline ring-down times did not always return to the blank values. To adjust for this variation, a second order polynomial, which was the simplest function the produce a good fit to the blank, was fit to the R values from 608-611.5 and 620-625 nm. Values of this fit was subtracted from R and divided by the number density to yield the absorption cross section (eq 3). The cross section listed for a given wavelength is the average of all runs and the uncertainty is the standard deviation (1σ). This procedure yields a robust measurement for the peak and integrated cross section between 612 and 620 nm, but would underestimate the integrated cross section if there were any broad, diffuse bands that extend outside of this wavelength range. There is, however, no evidence for such a feature in our measured spectra.

617.8 618.0 618.2 618.4 618.6

619.4 619.6 619.8 620.0

2.67 10

4.61 10

1.46 10

a diluted flow of acetic acid through the cavity. Figure 4 shows a few of these measurements. The ring-down times followed the same pattern for the undiluted and diluted runs. During the undiluted run, however, the ring-down times did not fall to the background level outside the absorption feature. The reason for the shift in the baseline is not clear. It is possible that the undiluted run was more affected by aerosol produced in the acetic acid source that lead to a broad extinction at all measured wavelengths. However, there was a filter in line with the acetic

’ RESULTS AND DISCUSSIONS The response of the ring-down cavity with and without acetic acid vapor was recorded over a wavelength range between 607 and 625 nm. Ring-down time measurements were also taken with 758



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Figure 6. Two Gaussian peaks (blue lines) fit to the mean cross section values (red line).

pyruvic acid (∼38 kcal 3 mol-1)18 and malonic acid (∼20 kcal 3 mol-1).20 A 70.5 kcal 3 mol-1 barrier34 is thermochemically equivalent to an excitation at 405 nm, approximately the eighth O-H overtone (417 nm). Since the excitation energy is well below that of the barrier, it is unlikely that the unimolecular decomposition rate coefficient with respect to, for example, tunneling, could compete with rapid collisional quenching at atmospheric pressure. There are, however, conformational changes that can take place due to overtone absorption. The barriers to both methyl and OH rotation can be overcome and the higher energy cis configuration of acetic acid can be accessed.46 In a process distinct from simple bond rotation, the acidic hydrogen can move translationally to the carbonyl oxygen to assume a saddle point structure, where the methyl hydrogen is eclipsed with the OH group. The barrier for this process has been calculated in this work, based on transition state and potential energy surface calculations, and was found to be 32 kcal 3 mol-1 for both ΔH‡ and ΔG‡. This is low enough to be overcome by the fourth and even the third overtone (Figure 1). As previously mentioned, the laser was tuned near 615 nm, where the fourth O-H overtone absorption for the monomer is expected from Birge-Sponer analysis.43 The O-H fundamental frequency for the acetic acid dimer is not well characterized as it overlaps with the C-H stretching and the O-H stretch is coupled to the vibrational motion of the carbonyl group of the adjacent acetic acid. However, this combination band is observed at a much lower frequency than the monomer.28 This is likely owing to the broadness of the fundamental from 3000 to 3300 cm-1 (3030-3333 nm) compared to the relatively sharp fundamental stretch of acetic acid monomer at 3585 cm-1 (2790 nm). Consequently, the overtone O-H in the dimer will be significantly shifted from where the monomer is observed. Atmospheric levels of acetic acid are typically too small for the dimer to play an important role, although acetic acid may form complexes with other atmospheric species, such as water vapor. The spectroscopy of these complexes has not been investigated here. The overtone absorption feature itself is asymmetric and can be regarded as the combination of two absorption features. Figure 6

acid flow in order to eliminate aerosol transmission to the ringdown cell; furthermore, the signal was not characteristic of aerosol, which tend to be statistically noisier than gas-phase absorbers.44 Differences in pressure could give rise to a change in the Rayleigh scattering background, though measured pressure changes were insufficient to explain the background shift in these experiments. Prior work by Headrick28 suggests the potential for a resonance between the 5vOH and 6vCH overtones in this region. However, such a resonance is not evident from any of the structured absorptions in our measurements and it is unlikely to be diffuse enough to provide a constant background absorption throughout the measured spectral region. Several scans were taken at 0.05 nm interval between 607 and 625 nm to record the fourth O-H overtone of acetic acid. The absorption cross section was calculated by combining the ringdown data with concentration values derived from UV absorption measurements, which yielded the cross section spectrum in Figure 5 and smoothed values in Figure 5 and Table 2. The absorption for the acetic acid monomer has a peak value of (1.84 ( 0.17) 10-24 cm2 3 molecule-1 at 614.75 nm and has a baseline width of ∼6 nm. The integrated cross section for the acetic acid monomer over this broad width stretching from 612 to 620 nm was determined using the trapezoidal method and was found to be (5.23 ( 0.73) 10-24 cm2 3 nm 3 molecule-1 or (1.38 ( 0.19) 10-22 cm2 3 molecule-1 3 cm-1. Since there are no previously reported results for the fourth O-H overtone cross sections of acetic acid, a direct comparison was not possible. However, the cross sections reported in this work are similar to other fourth O-H overtone integrated cross sections reported in the same units for HNO3 ((2.57 ( 0.24) 10-22),3 for H2SO4 ((2.38 ( 0.57) 10-22),14 and for CH3OOH ((2.1 ( 1.3) 10-22).45 Although the integrated cross section reported here for acetic acid is similar to that of the species listed above, excitation of the O-H stretch to the fourth overtone is less likely to initiate unimolecular reactions in the acetic acid system. Acetic acid has a higher energy barrier for both dehydration (70-80 kcal 3 mol-1) and decarboxylation (70-75 kcal 3 mol-1),32-35 compared to 759


The Journal of Physical Chemistry A shows the fit of two Gaussian functions to the mean cross section values. The Gaussian functions are centered at 614.66 and 616.55 nm and their sum reproduces the observed absorption feature quite well. Their integrated cross sections are 3.51 10-24 and 1.69 10-24 cm2 3 nm 3 molecule-1 for the peaks at 614.66 and 615.55 nm, respectively. Their sum is 5.21 10-24 cm2 3 nm 3 molecule-1, which is close to the experimental mean of (5.23 ( 0.73) 10-24 cm2 3 nm 3 molecule-1, and their ratio is 2.08:1. The origin of the asymmetric band structure is presently unknown and an exact identification is difficult. Throughout the νOH series, there is another peak consistently present and shifted slightly to the blue.28 Although the separation slightly increases from 1νOH through 3νOH, the two features likely coalesce due to peak broadening at 4νOH. Pressure broadening of the peak, however, is negligible under present operating conditions due to the relatively small number densities in the cavity. Since the νOH is somewhat more intense than the unknown peak, the component centered at 614.66 nm has been tentatively assigned to the 5νOH vibrational overtone. A hot band of the CdO stretching vibration has been investigated as a potential candidate for the unknown peak, but this has been discounted.28 Seeing as it consistently follows the νOH series, it is likely that they are related.

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’ ACKNOWLEDGMENT This research is supported by that National Science Foundation Award No. 0803016 and National Oceanic and Atmospheric Administration-Educational Partnership Program Award No. NA06OAR4810187. We thank Geoff Tyndall and John Orlando of NCAR for their valuable discussions. ’ REFERENCES (1) Donaldson, D. J.; Frost, G. J.; Rosenlof, K. H.; Tuck, A. F.; Vaida, V. Geophys. Res. Lett. 1997, 24, 2651. (2) Donaldson, D. J.; Orlando, J. J.; Amann, S.; Tyndall, G. S.; Proos, R. J.; Henry, B. R.; Vaida, V. J. Phys. Chem. A 1998, 102, 5171. (3) Brown, S. S.; Wilson, R. W.; Ravishankara, A. R. J. Phys. Chem. A 2000, 104, 4976. (4) Zhang, H.; Roehl, C. M.; Sander, S. P.; Wennberg, P. O. J. Geophys. Res. 2000, 105, 14593. (5) Miller, Y.; Chaban, G. M.; Finlayson-Pitts, B. J.; Gerber, R. B. J. Phys. Chem. A 2006, 110, 5342. (6) Fono, L.; Donaldson, D. J.; Proos, R. J.; Henry, B. R. Chem. Phys. Lett. 1999, 311, 131. (7) Stark, H.; Brown, S. S.; Burkholder, J. B.; Aldener, M.; Riffault, V.; Gierczak, T.; Ravishankara, A. R. J. Phys. Chem. A 2008, 112, 9296. (8) Reiche, F.; Abel, B.; Beck, R. D.; Rizzo, T. R. J. Chem. Phys. 2002, 116, 10267. (9) Matthews, J.; Sinha, A.; Francisco, J. S. Proc. Natl. Acad. Sci. 2005, 102, 7449. (10) Matthews, J.; Fry, J. L.; Roehl, C. M.; Wennberg, P. O.; Sinha, A. J. Chem. Phys. 2008, 128, 184306. (11) Donaldson, D. J.; Tuck, A. F.; Vaida, V. Chem. Rev. 2003, 103, 4717. (12) Donaldson, D. J.; George, C.; Vaida, V. Environ. Sci. Technol. 2010, 44, 5321. (13) Vaida, V.; Kjaergaard, H. C.; Hintze, P. E.; Donaldson, D. J. Science 2003, 299, 1566. (14) Feierabend, K. J.; Havey, D. K.; Brown, S. S.; Vaida, V. Chem. Phys. Lett. 2006, 420, 438. (15) Mills, M. J.; Toon, O. B.; Vaida, V.; Hintze, P. E.; Kjaergaard, H. G.; Schofield, D. P.; Robinson, T. W. J. Geophys. Res. 2005, 110, D08201. (16) Vaida, V. J. Phys. Chem. A 2009, 113, 5. (17) Takahashi, K.; Plath, K. L.; Skodje, R. T.; Vaida, V. J. Phys. Chem. A 2008, 112, 7321. (18) Plath, K. L.; Takahashi, K.; Skodje, R. T.; Vaida, V. J. Phys. Chem. A 2009, 113, 7294. (19) Takahashi, K.; Plath, K. L.; Axson, J. L.; Nelson, G. C.; Skodje, R. T.; Vaida, V. J. Chem. Phys. 2010, 132, 094305/1. (20) Staikova, M.; Oh, M.; Donaldson, D. J. J. Phys. Chem. A 2005, 109, 597. (21) Dunn, M. E.; Shields, G. C.; Takahashi, K.; Skodje, R. T.; Vaida, V. J. Phys. Chem. A 2008, 112, 10226. (22) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Chemistry of the Upper and Lower Atmosphere; Academic Press: New York, 1999. (23) Havey, D. K.; Feierabend, K. J.; Black, J. C.; Vaida, V. J. Mol. Spectrosc. 2005, 229, 151. (24) Havey, D. K.; Feierabend, K. J.; Takahashi, K.; Skodje, R. T.; Vaida, V. J. Phys. Chem. A 2006, 110, 6439. (25) Lane, J. R.; Kjaergaard, H. G.; Plath, K. L.; Vaida, V. J. Phys. Chem. A 2007, 111, 5434. (26) Rontu, N.; Vaida, V. J. Phys. Chem. B 2008, 112, 276. (27) Vaida, V.; Feierabend, K. J.; Rontu, N.; Takahashi, K. Int. J. Photoenergy 2008, 138091. (28) Headrick, J. E. Structure, Stability, Spectroscopy and Atmospheric Significance of Select Hydrogen-Bonded Complexes; PhD Thesis, University of Colorado at Boulder, 2002. (29) Lange, K. R.; Wells, N. P.; Plegge, K. S.; Phillips, J. A. J. Phys. Chem. A 2001, 105, 3481.

’ CONCLUSIONS We have measured the fourth O-H overtone absorption cross section for acetic acid monomer. The integrated cross sections is (5.23 ( 0.73) 10-24 cm2 3 nm 3 molecule-1 or (1.38 ( 0.19) 10-22 cm2 3 molecule-1 3 cm-1, which is commensurate with the fourth O-H vibrational overtone of other species. The peak cross section is (1.84 ( 0.17) 10-24 cm2 3 molecule-1 at 614.75 nm. The acetic acid dimer has negligible absorption within the spectral window selected. Rotational excitation is achievable and the translational movement of hydrogen between the oxygen atoms of acetic acid is likely occurring, since the energy provided by absorption of the fourth overtone is sufficient to overcome the energy barrier. However, excitation of this overtone in acetic acid is unlikely to initiate unimolecular reactions, as the energy barrier is higher than that supplied by the excitation photon. Unimolecular dissociation by UV absorption is not a significant loss process for acetic acid, since it possesses a high-energy electronic excited state47 that cannot be reached by UV radiation from the Sun. Therefore, reactions with OH and other radicals and wet and dry deposition are the only likely atmospheric removal processes for acetic acid.22 The wavelength of the absorption feature is consistent with Birge-Sponer analysis of lower overtones and seems to consist of two overlapping absorption features at 614.66 and 615.55 nm. The former peak has been tentatively assigned to the 5νOH vibrational overtone, while the latter has an unknown source, though it seems to be related to the νOH series. This work will be expanded to other compounds whose overtone-induced photolysis is more likely in the atmosphere. It would aid greatly in the study of the O-H vibrational overtones of peracetic acid (CH3(CdO)OOH), which cannot be isolated from water and acetic acid monomer and dimer.42

’ AUTHOR INFORMATION Corresponding Author

*Phone: (336) 285-2328. E-mail: [email protected]. Present Addresses ‡

LI-COR Biosciences, Lincoln, Nebraska 68504, United States. 760



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(30) Fiddler, M. N.; Begashaw, I.; Mickens, M. A.; Collingwood, M. S.; Assefa, Z.; Bililign, S. Sensors 2009, 9, 10447. (31) Brown, S. S. Chem. Rev. 2003, 103, 5219. (32) Duan, X. F.; Page, M. J. Am. Chem. Soc. 1995, 117, 5114. (33) Fang, W. H.; Liu, R. Z.; Zheng, X. M.; Phillips, D. L. J. Org. Chem. 2002, 67, 8407. (34) Mackie, J. C.; Doolan, K. R. Int. J. Chem. Kinet. 1984, 16, 525. (35) Moreira, I. d. P. R. Theochem.;J. Mol. Struct. 1999, 466, 119. (36) Sander, S. P.; Finlayson-Pitts, B. J.; Friedl, R. R.; Golden, D. M.; Huie, R. E.; Keller-Rudek, H.; Kolb, C. E.; Kurylo, M. J.; Molina, M. J.; Moortgat, G. K.; Orkin, V. L.; Ravishankara, A. R.; Wine, P. H. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation number 15, JPL Publication 06-2; Jet Propulsion Laboratory: Padadena, CA, 2006. (37) Berden, G.; Peeters, R.; Meijer, G. Int. Rev. Phys. Chem. 2000, 19, 565. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M. L., X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.03; Gaussian, Inc.: Wallingford CT, 2004. (39) Senent, M. L. Mol. Phys. 2001, 99, 1311. (40) Rothman, L. S.; Gordon, I. E.; Barbe, A.; Benner, D. C.; Bernath, P. F.; Birk, M.; Boudon, V.; Brown, L. R.; Campargue, A.; Champion, J. P.; Chance, K.; Coudert, L. H.; Dana, V.; Devi, V. M.; Fally, S.; Flaud, J. M.; Gamache, R. R.; Goldman, A.; Jacquemart, D.; Kleiner, I.; Lacome, N.; Lafferty, W. J.; Mandin, J. Y.; Massie, S. T.; Mikhailenko, S. N.; Miller, C. E.; Moazzen-Ahmadi, N.; Naumenko, O. V.; Nikitin, A. V.; Orphal, J.; Perevalov, V. I.; Perrin, A.; Predoi-Cross, A.; Rinsland, C. P.; Rotger, M.; Simeckova, M.; Smith, M. A. H.; Sung, K.; Tashkun, S. A.; Tennyson, J.; Toth, R. A.; Vandaele, A. C.; Vander Auwera, J. J. Quant. Spectrosc. Radiat. Transfer 2009, 110, 533. (41) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; John Wiley & Sons: Hoboken, NJ, 2006. (42) Orlando, J. J.; Tyndall, G. S. J. Photochem. Photobiol. A 2003, 157, 161. (43) Birge, R. T.; Sponer, H. Phys. Rev. 1926, 28, 259. (44) Pettersson, A.; Lovejoy, E. R.; Brock, C. A.; Brown, S. S.; Ravishankara, A. R. J. Aerosol Sci. 2004, 35, 995. (45) Homitsky, S. C.; Dragulin, S. M.; Haynes, L. M.; Hsieh, S. J. Phys. Chem. A 2004, 108, 9492. (46) Macoas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Fausto, R.; Raesaenen, M. J. Am. Chem. Soc. 2003, 125, 16188. (47) Hintze, P. E.; Aloisio, S.; Vaida, V. Chem. Phys. Lett. 2001, 343, 159.

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Measurement of the Fourth Oâ H Overtone Absorption Cross Section

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