Quantitative Infrared Intensity Studies of Vapor-Phase Glyoxal

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Quantitative Infrared Intensity Studies of Vapor-Phase Glyoxal, Methylglyoxal, and 2,3-Butanedione (Diacetyl) with Vibrational Assignments Luisa T. M. Profeta, Robert L. Sams, and Timothy J. Johnson* Pacific Northwest National Laboratory, Richland, Washington 99354, United States

Stephen D. Williams A. R. Smith Department of Chemistry, Appalachian State University, Boone, North Carolina 28618, United States ABSTRACT: Glyoxal, methylglyoxal, and 2,3-butanedione (diacetyl) are all known biomass burning effluents and suspected aerosol precursors. Pressurebroadened quantitative infrared spectra of glyoxal, methylglyoxal, and diacetyl vapors covering the 520
6500 cm
1 range are reported at 0.112 cm
1 resolution, each with a composite spectrum derived from a minimum of 10 different sample pressures for the compound, representing some of the first quantitative intensity data for these analytes. Many vibrational assignments for methylglyoxal are reported for the first time, as are some near-IR and far-IR bands of glyoxal and diacetyl. To complete the vibrational assignments, the far-infrared spectra (25
600 cm
1) of all three vapors are also reported, those of methylglyoxal for the first time. Density functional theory and ab initio MP2 theory are used to help assign vibrational modes. Potential bands for atmospheric monitoring are discussed.

1. INTRODUCTION Glyoxal, methylglyoxal, and 2,3-butanedione (commonly called diacetyl or biacetyl) are a series of water-soluble Rdicarbonyls with the structures shown in Figure 1. They are all intermediates during the photolytic degradation of volatile organic compounds (VOCs) in the atmosphere.1
4 VOCs are emitted from biogenic sources, as well as from various combustion processes, e.g., vehicle emissions, residential wood burning, and (prescribed) forest fires. For example, the production of glyoxal is indicative of the presence of several VOCs such as acetylene and glycolaldehyde.3,5 In terms of formation, all three R-dicarbonyls are also products of OH-initiated reactions with benzene, toluene, xylene, and trimethylbenzenes.6,7 All three compounds are precursors to other atmospheric species such as peroxyacetyl nitrate (PAN)1,6,8,9 and may also lead to formation of secondary organic aerosols (SOA). In terms of decomposition, glyoxal has four known photolytic pathways that occur under normal atmospheric conditions, listed in order of decreasing thermodynamic favorability:3

Tadi
c et al. found3 that reactions 1 and 2 occurred at all wavelengths of incident radiation used in their study (∼250
480 nm). Reaction 3 only occurs at wavelengths e 417 nm, and the photolysis reaction 4 occurs only when the incident radiation e 334 nm. The photolysis pathway (3) generates two HCO• radicals and is dominant at typical tropospheric temperatures and pressures,3 with the OH radical reaction being only a minor contributor to atmospheric glyoxal removal.10 As reported by Koch and Moortgat,1 methylglyoxal has three primary degradation pathways, namely, UV photolysis, UV/ visible excitation, or reaction with the OH radical:

CHOCHO þ hν f H2 þ 2CO

ð1Þ

CHOCHO þ hν f H2 CO þ CO

ð2Þ

where the products from either reaction 5 or 7 react further in the atmosphere1 to produce HO2 and CH3COO2 radicals along with CO.1 For reaction 6, an excited methylglyoxal molecule reacts with a methylglyoxal in the ground state to form CO and

CHOCHO þ hν f 2HCO•

ð3Þ

λ < 417 nm

CHOCHO þ hν f H þ CO þ HCO•

λ < 334 nm

r 2011 American Chemical Society

ð4Þ

CH3 COCHO þ hν f CH3 CO• þ HCO• λ < 380 nm ð5Þ  CH3 COCHO þ hν f CH3 COCHO 380 < λ < 440 nm ð6Þ CH3 COCHO þ OH• f CH3 COCO• þ H2 O

ð7Þ

Received: May 15, 2011 Revised: July 12, 2011 Published: July 14, 2011 9886

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Figure 1. Structures of glyoxal, diacetyl, and methylglyoxal, all in trans configuration.

HCHO along with acetyl radicals. As with glyoxal, the photolysis pathway (5) is the primary degradation route under tropospheric conditions. For diacetyl, photolysis is again the dominant atmospheric loss process resembling reaction 3 above, with a cleaving of the central C
C bond to form two acetyl radicals:6,10 CH3 COCOCH3 þ hν f 2CH3 CO•

λ < 470 nm

ð8Þ

All three of the dicarbonyl species experience strong diurnal variations with minimal overnight concentrations in the troposphere that may rise to low part-per-billion levels during the midday hours.10
13 However, their precursors and degradation products may have comparatively long atmospheric lifetimes— usually on the scale of several hours or more.14,15 In addition to the production of glyoxal, methylglyoxal, and diacetyl from other aromatic VOCs, they also are known (indirect) products arising from the ozonolysis of terpenes and hemiterpenes such as isoprene.16
21 Terpenes are also released during biomass burning (BB), and the BB process thus contributes to the total atmospheric burden of the three R-dicarbonyls.22
25 With the recent desire to better understand the effects of natural and prescribed fires, a better knowledge of the contribution from these R-dicarbonyls is clearly of great current interest. To date, biomass burning chemistry has been studied by multiple analytical methods including, e.g., gas chromatography (GC),26 high-performance liquid chromatography (HPLC),25,27 mass spectrometry (MS),28,29 proton transfer reaction mass spectrometry (PTR-MS),26 and differential optical absorption spectroscopy (DOAS),15 as well as certain hyphenated techniques.30,31 The mass spectrometric methods have been especially able to provide high sensitivity for certain species by detecting low concentrations of many smoke components.24
31 Such techniques, however, are not especially portable and in some cases may suffer from inlet effects. Open-path Fourier transform infrared spectroscopy (OPFTIR) has been used for biomass burning monitoring with good results for both identification and quantification of several smoke constituents.22,26,30,32
34 For example, Christian et al. reported in 2003 that the emission ratios (mmol/mol with respect to CO2) for African savannah burns were 66.4 for CO, 3.53 for CH4, 3.04 for NOx, and e0.09 for a variety of VOCs.22 OP-FTIR retains several advantages for BB studies including real-time measurements, minimal sample collection or manipulation, and simultaneous measurement of multiple chemical species. One important prerequisite for the OP-FTIR technique, however, is the availability of quantitative gas-phase reference data for analyzing the IR spectra. The reference data are used to process the field data, i.e., quantify

analytes of interest, as well as subtract out interferents and search for trace species. As we began to investigate the three R-carbonyls (all potential biomass burning effluents), we noted that although there were previously recorded IR studies, including vibrational and rotational assignments of certain bands, only a few of the works were quantitative.35
44 Cole and Osborne36,37 were the first to publish a high-resolution IR spectrum of glyoxal in 1970, using resolutions between 0.15 and 0.30 cm
1. While some of these results are partially quantitative, the data were obtained with a dispersive instrument and the focus was primarily on rotational assignments. More recently, Volkamer et al.44 have reported quantitative infrared data for four of the stronger glyoxal bands. Diacetyl has also been studied by infrared spectroscopy several times previously; however, similar to glyoxal much emphasis has been placed on rotational
vibrational assignments with few studies reporting quantitative results.41,45
47 For methylglyoxal, only four papers have reported infrared data: In the work published by Zhou et al.,4 a spectrum of methylglyoxal is used for quantitation of products of the OHinitiated degradation of acetylacetone. Raber and Moortgat48 used a portion of a quantitative methylglyoxal IR spectrum in their work quantifying methylglyoxal photolysis rates at different pressures and wavelengths. Kamei et al.49 reported six vibrational bands in their analysis of jet-cooled methylglyoxal for fluorescence and phosphorescence excitation studies. Similarly, Reid et al.50 also study jet-cooled methylglyoxal (in the singlet ground state) by using stimulated emission pumping (SEP) to assign some of the fundamental vibrational modes. However, their work focused only on low-frequency modes, with all spectra collected at reduced pressures. Thus, to our knowledge, there are few reported high-resolution (97% pure) was purchased from Sigma-Aldrich and used without further purification. Glyoxal and methylglyoxal (both 40% solutions in H2O) were also purchased from Sigma-Aldrich and purified using the methods of Brand62 and Dyllick-Brenzinger and Bauder,51 respectively, to eliminate CO2 and the water solvent using multiple freeze
thaw cycles at 77 K. Methylglyoxal was additionally desiccated using P2O5 during the freeze
thaw cycles; methylglyoxal was prepared fresh each time as it is known to decay with time.2,48 In all of our samples, trace impurities were found in the vapor-phase spectra. Glyoxal had trace amounts of CO and CO2, while diacetyl had trace amounts of CO2 and H2O vapor. Methylglyoxal had trace amounts of HCN, CO, and CH3COOH along with 2.0% HCHO, 0.7% CO2, and 0.46% HCl vapors. All impurity spectra were removed by quantitative spectral subtraction from each spectral burden to derive the final values. The 520
6500 cm
1 spectra were collected using a Bruker IFS 66v/S vacuum spectrometer with a thermostatted 19.96 cm gas cell. A description of the second aperture solution used to prevent ghosting as well as the “warm aperture” and nonlinearity effects has been reported previously.63 For methylglyoxal (vapor pressure 40 Torr at 298 K),64 measurements were made

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Figure 2. Quantitative IR spectra of vapor-phase glyoxal, methylglyoxal, and diacetyl in the fingerprint region of 550
1600 cm
1. The ordinate corresponds to a 1 m optical path length and a mixing ratio of 1 ppmv at 296 K. In this figure, methylglyoxal and glyoxal spectra are vertically offset for clarity. The Y-axis of the insets of glyoxal and diacetyl are not offset. Mode assignments are discussed in the text.

Figure 3. Quantitative IR spectra of vapor-phase glyoxal, methylglyoxal, and diacetyl in the carbonyl stretching region. The ordinate corresponds to a 1 m optical path length and a mixing ratio of 1 ppmv at 296 K. In this figure, methylglyoxal and glyoxal are vertically offset for clarity. Glyoxal and methylglyoxal are also vertically offset in the inset spectra of the difference and combination bands with the respective torsional frequencies. Mode assignments are discussed in text. Only for methylglyoxal are both the symmetric and asymmetric carbonyl stretches seen due to the lower symmetry compared to the parent molecules.

at 298 and 323 K, while glyoxal and diacetyl (vapor pressure 56 Torr at 298 K)64 had IR measurements made at 278, 298, and 323 K. [Our empirical results are consistent with glyoxal having a vapor pressure of ∼250
270 Torr at 25 °C. These are also consistent with the earlier vapor pressure measurements of Calvert and Layne (284 Torr at 26 °C),65 the Knovel solvents database values (255 Torr at 25 °C),66 and data from Dobeck et al.67 Other literature and MSDS values cite 25 °C values near ∼20 Torr, but we suspect this may be the vapor pressure of water above aqueous solutions of glyoxal.] The temperature was thermostatted and recorded from a thermocouple attached 9888

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Figure 4. Quantitative IR spectra of glyoxal, methylglyoxal, and diacetyl vapors in the C
H stretching region. The ordinate corresponds to a 1 m optical path length and a mixing ratio of 1 ppmv at 296 K. In this figure, methylglyoxal is vertically offset by 50  10
6 and glyoxal by 150  10
6 for clarity. Glyoxal has rovibrational bands which under low-pressure conditions would be fully resolved. Mode assignments are discussed in the text.

Figure 5. Quantitative vapor-phase IR spectra of glyoxal, diacetyl, and methylglyoxal in the C
H stretching and near-IR region. The ordinate corresponds to a 1 m optical path length and a mixing ratio of 1 ppmv at 296 K. The methylglyoxal and glyoxal spectra have been vertically offset for clarity.

to the gas cell with a stated accuracy of (0.02 K. For each burden at each temperature, 256 interferograms were averaged. Gain-ranged, single-sided interferograms were recorded, 2zerofilled and phase-corrected using Mertz’s method prior to the Fourier transform step. Decadic absorbance spectra were calculated as
log(I/Io) with a cell containing only nitrogen gas representing Io. The composite spectra seen in Figures 2
6 correspond to a 1 m optical path and a mixing ratio of 1 ppmv at 296 K. The far-infrared spectrum of methylglyoxal was obtained using a Bruker IFS 120 HR spectrometer equipped with a germanium on Mylar beamsplitter, polymethylpentene (TPX) windows (0.018” thick) and a helium-cooled silicon bolometer.68

The far-infrared spectra of glyoxal and diacetyl were obtained on a Bruker IFS 125 HR equipped with the same beamsplitter, windows, and detector. For methylglyoxal the far-IR samples were not back-filled with UHP N2 but were run neat; the spectral resolution was the same as for the mid-IR measurements. Glyoxal and diacetyl were prepared in the same fashion as the mid-IR counterparts; however, these spectra were also taken as neat samples. Five unique pressure burdens of methylglyoxal, two of diacetyl, and two of glyoxal were measured between 25 and 700 cm
1 in a 20.40 cm gold-coated gas cell, and the Norton
Beer apodization function was applied to the spectrum prior to the FFT step. The far-IR spectra have higher frequency calibration due to the frequency-stabilized HeNe 9889

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Figure 6. Quantitative vapor-phase far-infrared spectra of neat glyoxal, methylglyoxal, and diacetyl. The ordinate corresponds to a 1 m optical path length and a mixing ratio of 1 ppmv at 296 K. In this figure, methylglyoxal and glyoxal have been vertically offset for clarity. Assignments are discussed in the text. As discussed in text, the inset for diacetyl near 112 cm
1 was obtained from separate high-burden measurements, not digital amplification.

laser; frequency calibration is confirmed as the methylglyoxal band at 591.01 cm
1 agrees perfectly with the mid-IR measurements. Theoretical frequencies and intensities for trans-methylglyoxal were calculated in two ways: The first used DFT theory utilizing a 6-31G** basis set and the B3LYP functional69 to calculate the vibrational frequencies and infrared intensities via Spartan 2004.70 All DFT frequencies were scaled by a factor of 0.965 as B3LYP harmonic frequencies are known to be too high compared to the experimental fundamentals.71,72 For methylglyoxal and diacetyl, confirmation of vibrational assignments was provided by ab initio calculations73 in Gaussian 03 using frozen core MP2 theory74 with the aug-cc-pvdz basis set. Anharmonic frequencies75 were also calculated by Gaussian 03. Anharmonic frequencies and harmonic intensities were used to assist in assignments. In all cases the frequencies were computed for structures previously optimized with the same level of theory; no imaginary frequencies were found, indicating that these structures represent true energy minima. In all cases the optimum structures had the adjacent carbonyl groups trans to each other, and for methylglyoxal and diacetyl the methyl groups were eclipsed relative to the carbonyl oxygens. Optimizations that began with staggered methyl group(s) yielded staggered structures, but these had imaginary vibrational frequencies whose modes corresponded to methyl rotations. That the eclipsed structure was a minimum suggests a weak attractive interaction between the carbonyl O atom and the methyl H atom. The diffuse functions used in the basis set for the MP2 calculations were included to better represent this weak interaction. Many of our mode assignments are based on a combination of calculated frequencies and intensities combined with previous literature assignments and analogies between the different molecules.

3. RESULTS AND DISCUSSION The quantitative mid-infrared spectra of glyoxal, methylglyoxal, and diacetyl are shown in Figures 2
5 over different spectral ranges. In each, the y-axis of the 298 K data corresponds to an optical path of 1 m with an analyte mixing ratio of 1 ppmv at 296 K and 1 atm total pressure. The methylglyoxal and diacetyl spectra have been vertically offset for display purposes. For glyoxal, the displayed data are the weighted average of 12 burdens, while for methylglyoxal and diacetyl the spectra represent 10 pressure burdens. The (neat) far-infrared data are seen in Figure 6. 3.1. Glyoxal. As previously reported by Harris35 and Cole and Osborne37 glyoxal’s equilibrium configuration is trans and corresponds to the C2h point group (Figure 1). There are 12 normal modes of vibration consisting of five Ag, two Au, one Bg, and four Bu modes, of which the Au and Bu fundamentals are IR-active. Currie and Ramsay later confirmed76 that the trans configuration of glyoxal is approximately 1125 cm
1 lower in energy than that of the cis configuration, making it reasonable to base all the vibrational assignments on the trans-glyoxal configuration. Table 1 summarizes our vibrational assignments and includes assignments for previously unreported peaks in the glyoxal spectrum in the near-infrared along with the first quantitative integrated band intensities for several previously assigned bands. The assignments of both the IR37 and Raman77,78 frequencies of all the fundamental modes for glyoxal in both the solid and gaseous phases have been reported. Cole and Durig77 studied the singly- and doubly-deuterated isotopologues and assigned these bands as well, showing that for the singly deuterated species the symmetry is lowered and that the rule of mutual exclusion between the g and u modes is lifted. Because the present work concerns only C2H2O2 (natural isotope ratios), the exclusion rule applies loosely and only the Au and Bu modes are seen in the 9890

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Table 1. trans-Glyoxal Vibrational Assignments, Observed Vapor-Phase Frequencies and Band Integrals for Several Observed Far-Infrared and Mid-infrared Bandsa R

band obsd freq, cm


1

obsd band type

assignment

sym type (C2h)


2

mode description

intensity, cm

R atm


1

band

limits, cm
1

Fundamentals 801.21 127.63

C C

ν6 ν7

Au Au

OOP C
H wag C
C central torsion

2835.16

B

ν9

Bu

out-of-phase C
H stretch

432.4

1731.19

mainly A

ν10

Bu

out-of-phase CdO stretch

587.1

1661
1808

1313.30

B

ν11

Bu

in-plane C
H bend

30.0

1245
1387

341.52

B

ν12

Bu

C
CdO bend

8.3

727
871 2724
2940

Combination Bands 5558.22

B

ν1 + ν9

Bu

asym C
H + sym C
H

4571.14

B

ν2 + ν9

Bu

sym CdO + asym C
H

22.5

4456
4662

4386.68 3452.34

B A

ν2 + ν3 + ν11 ν2 + ν10

Bu Bu

sym C
H bend + asym C
H bend + sym CdO stretch in-phase CdO + out-of-phase CdO

28.9

3402
3512

3055.74

B

ν2 + ν11

Bu

in-phase CdO + asym C
H rock 4.1

2040
2133

2648.64

B

ν3 + ν11

Bu

sym C
H bend + asym C
H bend

2083.44

?

ν2 + ν12

Bu

in-phase CdO + C
CdO bend

1870.37

A/C

ν2 + ν7

Au

in-phase CdO + torsion

1616.36

mainly C

ν2
ν7

Au

in-phase CdO
torsion

a

Assignments follow primarily from refs 35, 36, 39, 77, and.78. Observed frequencies are cited as local maxima for A- and C-type bands; local minima, for B-type bands.

Table 2. Comparisons of the Integrated Vapor-Phase IR Band Intensities for Glyoxal with Previously Published Literature Values mode (C2h)

obsd freq, cm
1

R Essen79 band , cm
2 atm
1

R Bremen44 band , cm
2 atm
1

R PNNL band , cm
2 atm
1

ν6

801.21

3.2

8.3

ν11

1313.30

10.9

30.0

ν10

1731.19

233.6

ν2 + ν12

2083.44

562.8

R

band limits, cm
1 727
871 1245
1387

587.1

1661
1808

4.1

2040
2133

ν9

2835.16

409.1

432.4

2724
2940

ν2 + ν10

3452.34

26.8

28.9

3402
3512

ν2 + ν9

4571.14

19.3

22.5

4456
4662

171.3

infrared. Our fundamentals assignments follow from these literature sources. Oelichmann et al. at Essen79 examined glyoxal using CNDO/2 methods to calculate the normal coordinates, vibrational frequencies, and IR intensities. While not shown in Table 2, their calculated frequencies are in good agreement with both calculated and experimental values reported here and elsewhere.35 They also were the first to determine experimental integrated IR band intensities, reporting intensities for four of the vibrational modes. With the Essen intensities converted to the more common unit of cm
2 atm
1, their experimental values are systematically a factor of ∼2.5 smaller than the values we report in Table 2. While Oelichmann et al. were the first to report glyoxal band intensities, we are confident in the Pacific Northwest National Laboratory (PNNL) intensity values due to the vetted procedures described above. We also compare our results to the more recent values reported by the Bremen group of Burrows et al.44 They also published integrated band intensities for four glyoxal bands but provide more experimental detail than the Essen study. Albeit for only four bands, the Bremen values agree well with the PNNL intensities, with the Bremen values averaging ∼2
3% lower than the PNNL values.

This may in part be ascribed to the difficulty in measuring an average optical path length for a single-pass White cell,44 but the Bremen values are still within our 3% experimental uncertainties. Feierabend et al.80 also briefly reported their findings of the integrated band strength for the out-of-phase carbonyl stretching band, which is approximately 6% smaller than the values reported here. In general, the PNNL, Bremen and Boulder values are in quite good agreement. While not in Table 2, Tadi
c et al.3 also reported the integrated band intensity (converted from base 10 to base e) for the glyoxal asymmetric carbonyl stretch as 2.37  10
17 cm molecule
1, which is also in agreement with our value. They reported the integrated band intensity as 1.66  10
17 cm molecule
1 for the CdO band, which is in agreement with the Bremen value, but about 4% smaller than the PNNL data. There is a possibility of trace amounts of formaldehyde that slightly distort their values. Upon examining Figures 2
4, glyoxal is the only one of the three R-dicarbonyls to display discernible rotational
vibrational structure at atmospheric pressure.42,81 It is a nearly symmetrical prolate top (where IA < IB ≈ IC) with distinct IR band shapes and resolved rovibrational lines; there is some band-shape mixing for a few of the bands due to changes in 9891

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Table 3. Diacetyl Vibrational Assignments and Observed Vapor-Phase Infrared and Far-Infrared Frequenciesa band obsd freq, cm
1

obsd band type

assignment

sym type (C2h)

mode description

R

R

band

intensity, cm
2 atm
1

limits, cm
1

c

Fundamentals C

ν11

1425.84

C

ν12

Au

912.45

A/C

ν13

340.23

A/C

112.2 47.73

70.4

2905
3053

in-phase OOP CH3 deformation

a

135.2

1392
1476

Au

in-phase OOP CH3 rock

b

870
994

ν14

Au

in-phase OOP C
C
C bend

?

ν15

Au

out-of-phase CH3 torsion

C

ν16

Au

central C
C torsion

3023.94 2945.37

A B?

ν22 ν23

Bu Bu

out-of-phase IP C
H stretch out-of-phase IP sym C
H stretch

1729.28

B

ν24

Bu

out-of-phase IP CdO stretch

1424.37

A

ν25

Bu

out-of-phase IP CH3 scissor

1359.77

A/B

ν26

Bu

out-of-phase CH3 umbrella

248.1

1297
1392

1115.84

A/B

ν27

Bu

out-of-phase IP C
CH3 stretch

313.6

1078
1154

949.69

A

ν28

Bu

out-of-phase IP CH3 rock

539.74

A

ν29

Bu

out-of-phase IP OdC
C bend

249.10

A/B?

ν30

Bu

out-of-phase IP C
C
C bend

4435

ν1 + ν12

Au

4381

ν5 + ν22

3439

2979.30

Au

in-phase OOP C
H stretch

102.3

see text c

70.4 70.4

2905
3053 2905
3053

753.7

1690
1769

c

a

135.2

b

1392
1476

102.3

870
994

109.5

515
589

sym C
H stretch + OOP CH3 deformation

d

4339
4496

Bu

IP C
H stretch + sym CH3 deformation

d

10.1

4339
4496

ν3 + ν24

Bu

in-phase CdO + out-of-phase CdO

23.2

3402
3497

1777.80

ν3 + ν16

Au

in-phase CdO + torsion

1683.81

ν3
ν16

Au

in-phase CdO
torsion

Combination Bands 10.1

a

Assignments follow primarily from refs 39 and.46. Observed frequencies are cited as local maxima for A- and C-type bands; as local minima, for B-type bands. Band integrals denote overlapping bands in some cases, and the composite integral is denoted by a shared superscript letter in the band intensity column.

dipole moment occurring at some angle between the planes of the principle axis system. Moreover, the vibrations of Bu symmetry can be aligned with either the x- or y-axes, allowing for hybrids of A- and B-type bands. If the sample were isotopically pure 16O, 12C, and 1H, the C2h point group dictates that no first overtones or combinations of two IR-active (Au, Bu) fundamentals would be seen in the infrared. Due to the long-wavelength cutoff of the MCT detector used for the mid-infrared studies, glyoxal peaks below ∼550 cm
1 are better seen in the far-IR data of Figure 6. However, of the six IRactive fundamentals, we observe four in the mid-IR: the ν6 outof-plane (OOP) C
H wag (Au, 801.21 cm
1), the ν9 out-ofphase C
H stretch (Bu, 2835.16 cm
1), the ν10 out-of-phase CdO stretches (Bu, 1731.19 cm
1), and the ν11 in-plane C
H bend (Bu, 1313.30 cm
1). The remaining far-IR active previously reported35,37 fundamentals are as follows: ν7 (Au), the torsion of glyoxal about the central C
C axis at 126.7 cm
1, and ν12, the C
CdO bend (Bu), at 338.5 cm
1. We report local maxima and minima as 127.63 and 341.52 cm
1, respectively, for these two far-IR bands. The 126.7 cm
1 observed ν7 mode, however, is in fact a hybrid including several hot bands as well as the ν7 fundamental, as discussed by Cole and Osborne.37 They also observed mid-IR fundamentals that all have relatively distinct band shapes, the ν6 OOP C
H wag has a clear C-type profile, and the asymmetric ν9 C
H stretch having a B-type contour. As with Cole and Osborne’s observations,37 several hot bands are present in the P branch of the ν6 OOP C
H wag and in the P and R branches of the ν9 asymmetric C
H stretch reported here. The in-plane C
H bend ν11 has predominately B-type shape while the out-of-phase CdO stretches

mode ν10 has mainly A-type shape; both of these bands have an observable amount of vibrational mixing. The remainder of the peaks in glyoxal’s mid-IR spectrum are only (u plus g) combination bands since first overtones are forbidden by the C2h symmetry. Harris35 reported two small shoulder peaks on each side of the 1731.19 out-of-phase CdO stretch as difference and combination bands of the 126.7 cm
1 far-infrared glyoxal torsional mode ν7 with ν2, the Raman-active35,77 in-phase CdO stretching mode located at 1745 cm
1. Our results seen in Figure 3 inset indicate that these peaks are indeed present in the gas-phase spectrum of glyoxal and are located at 1616.36 and 1870.37 cm
1 for ν2
ν7 and ν2 + ν7, respectively, though the band contours are quite complicated due to the contribution of several hot bands to the ν7 profile.37 A weak peak at 2083.44 cm
1 shows some B-type character to it and is assigned to a combination of the ν2 symmetric CdO stretch and the ν12 bend of the C
CdO skeletal group. Glyoxal also exhibits a ν3 + ν11 band corresponding to a combination of the symmetric and asymmetric C
H bends at 2648.64 cm
1, adjacent to the C
H region. As seen in Figure 5 top trace the Ag in-phase CdO stretching mode ν2 also combines with the asymmetric C
H rock (ν11) and the outof-phase CdO stretch (ν10) to result in combination bands at 3055 and 3452 cm
1, respectively.35 The weak A-type band shape of the out-of-phase CdO fundamental at 1731.19 cm
1 can be used to indicate when this vibration forms a combination band, helping to distinguish peaks assigned above 3000 cm
1. Several overtone and combination bands extend into the nearIR: The ν2 + ν9 combination band at 4571.14 cm
1 again 9892

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The Journal of Physical Chemistry A combines the in-phase CdO stretch (ν2), but this time with the asymmetric C
H stretch ν9; the relatively strong band has an integrated intensity of 22.6 cm
2 atm
1, and the B-type combination band takes on the band shape of the C
H stretch mode. 3.2. Diacetyl (2,3-Butanedione). Durig et al.39 presented a thorough analysis of the infrared and Raman spectra of transdiacetyl which, like glyoxal, has C2h symmetry, but with 30 fundamental modes (Figures 2
6 lower traces; Table 3). Fifteen of these are infrared-active, six transforming as Au symmetry and nine transforming as the Bu irreducible representation. In Table 3 we report observed frequencies and integrated band intensities along with assignments for previously unreported combination bands in the near-infrared, including vibrational assignments. Diacetyl can loosely be characterized as a symmetric prolate top. However, due in part to the free CH3 rotation, little rotational structure is observed in its pressure-broadened spectrum. The diacetyl spectra seen in the lower traces of Figures 2
6 display all 15 IR-active fundamentals. These include the ν11 in-phase C
H stretch (2979.30), the ν13 in-phase OOP CH3 rock (912.45), the ν22 out-of-phase IP C
H stretch (3023.94), the ν23 out-of-phase IP symmetric C
H stretch (2945.37), the ν24 out-of-phase IP CdO stretch (1729.28), the ν26 out-of-phase CH3 umbrella (1359.77), the very strong ν27 out-of-phase IP C
CH3 stretch (1115.84), the ν28 out-of-phase IP CH3 rock (949.69), and the ν29 out-of-phase IP OdC
OCH3 bend (539.74). One of the few ambiguous fundamental assignments concerns the two asymmetric CH3 deformations, ν12 and ν25. Durig et al.39 and G
omez-Zavaglia and Fausto46 note that the solid-phase spectroscopy of diacetyl reveals two distinct peaks for the two predicted CH3 deformations. In theory, the gas-phase band shapes should help distinguish82 between the two modes (Au vs Bu). Due to instrumental resolution, the vapor-phase IR spectrum exhibited only one peak in this region for Durig et al.39 The low-temperature argon-matrix experiments of G
omez-Zavaglia and Fausto46 yield three bands in this region, but the lack of rotational freedom obviously means there are no band profiles. Their amorphous 9 K spectrum exhibits broadening in this area of the spectrum similar to our gas-phase spectrum, which does not exhibit two distinct asymmetric CH3 deformation peaks. As seen in the Figure 2 inset, the spectrum displays an asymmetric band resembling a combination of a C-type peak with a B-type band, thus verifying the hypothesis by Durig et al. that there is an accidental degeneracy of the two modes. In the gas phase, the peak at 1425.84 cm
1 appears to be the center of the C-type band while a smaller peak appears at 1424.37 cm
1. The DFT results show the higher frequency Au mode asymmetric deformation as having the primary motion along the z-axis, while the lower frequency Bu deformation occurs along the y-axis at lower frequency. This lends credence to the experimental observations, and we thus concur with Durig’s suggestion that it is a coincidental degeneracy of an Au and Bu mode: ν12 corresponds to the 1425.84 cm
1 peak and ν25 to the band at 1424.37 cm
1. The other diacetyl gas-phase fundamentals that need clarification can be assigned using the improved far-IR spectrum of Figure 6, lower trace. In general, our results for diacetyl are in agreement with the far-IR findings of Durig et al.39 The present data, however, resolve the acetyl torsional mode revealing how this band of Au symmetry has a strong, clear C-type structure, making its assignment as ν16 straightforward. While near the spectrometer limit (using the 23 μm Mylar beamsplitter), this ν16 acetyl torsion has a clear C-type profile with a strong peak

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at 47.73 cm
1. This diacetyl torsional mode band profile in the lower trace also mimics the band profile of the analogous ν7 torsional mode of glyoxal as seen in the top trace. The Bu modes ν29 and ν30 (out-of-phase, in-plane bending modes of the OdC
C and C
C
C angles) are also easily assigned at 539.74 and 249.10 cm
1, respectively. The out-of-plane bend ν14 is an Au mode with a peak at 340.23 cm
1. For the remaining Au mode, however, we do not observe a weak peak near 200 cm
1 as reported by Durig et al.39 but rather assign the methyl torsion ν15 to an extremely weak band at 112.2 cm
1. While not obvious in our original spectra, Bell’s calculations83 had recommended the MP2/6-311+G(3df,2p) method and basis set to accurately predict the methyl torsional frequencies and IR intensities in acetyl molecules. Such a subsequent calculation suggested a band near 135
140 cm
1 that was ∼8700 weaker than the 47.7 cm
1 ν16 acetyl torsion; 140 cm
1 is unfortunately near a transmission node of the 23 μm beamsplitter. Therefore, using an alternate Ge/Mylar beamsplitter, nine additional very high pressure burdens at 9.6 m path were recorded that revealed a band centered at 112.2 cm
1 with a band integral 4970 weaker than the ν16 mode. A plot of the high-burden peak is seen above the lower trace of Figure 6. We thus assign this 112.2 cm
1 band as the ν15 methyl torsion. Such methyl torsions are expected to have weak IR intensities;83 the calculational method corroboration also adds confidence to our assignments for methylglyoxal. Diacetyl also displays several combination bands: Due to the C2h symmetry IR bands involving two modes are combinations of a Raman active (g) mode and an infrared active (u) mode. Similar to glyoxal, the in-phase and out-of-phase CdO stretching modes form a combination band ν3 + ν24 at 3439 cm
1 as seen in Figure 5. The strongest bands above 4000 cm
1 are the ν5 + ν22 combination band at 4381 cm
1 (a combination of the ν22 asymmetric C
H stretch and the ν5 symmetric CH3 deformation), along with the ν1 + ν12 band at 4435 cm
1. While the out-of-phase CdO stretches mode ν24 is the strongest band in the IR spectrum, there are other combination bands associated with the Raman active in-phase CdO stretch ν3. The analogy can be made from the glyoxal “parent” molecule: For glyoxal the symmetric CdO stretch combines with the backbone torsional mode to produce35 the sum and difference frequencies ν2 ( ν7 (1745 ( 126.7) at 1870 and 1616 cm
1, respectively. In a similar fashion for diacetyl, the 1731 cm
1 in-phase CdO stretch and ν16 torsional mode (47.73 cm
1, Figure 6) produce difference (ν3
ν16) and combination (ν3 + ν16) bands at 1683.81 and 1777.80 cm
1, respectively; these are all seen in the inset of Figure 3. 3.3. Methylglyoxal. Of the three dicarbonyls studied here, only methylglyoxal has not undergone a full vibrational analysis. Unlike glyoxal and diacetyl, methylglyoxal’s highest symmetry is Cs (assuming a rigid methyl group), which means all fundamentals are IR-active, as are all overtones and combination bands. There are 21 fundamental vibrations, 14 of which are of A0 and seven of which are of A00 symmetry. In contrast to glyoxal, methylglyoxal’s room-temperature IR spectrum displays some band contours but no distinct rotational
vibrational structure. In Table 4 we present the methylglyoxal vapor measured IR peak positions, band types, integrated band intensities, as well as the calculated frequencies that all lead to our vibrational assignments. The methylglyoxal fundamental modes were assigned using a combination of band profiles, two levels of theory, literature data, and comparing to the parent species of 9893

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The Journal of Physical Chemistry A

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Table 4. trans-Methylglyoxal Vibrational Assignments and Observed and Calculated IR Frequencies* Spartan calc

G03 anharmonic

obsd

obsd band

freq, cm
1

calc freq, cm
1

freq, cm
1

type

3065

3060

3026.27

band

sym type assignment

(Cs)

mode description

R

R

band

intensity, cm
2 atm
1 limits, cm
1

Fundamentals A

ν1

A0

IP CH3 C
H stretch

a

77.9

2847
3143

0

sym CH3 C
H stretch

a

77.9

2847
3143

240.3

2950

2950

2950.07

A

ν2

A

2850

2850

2828.01

B

ν3

A0

aldehyde C
H stretch

1757

1677

1733.27

A

ν4

A0

in-phase CdO stretches

b

1747

1675

1729.41

B?

ν5

A0

out-of-phase CdO stretches

b

0

654.6

2785
2873 1691
1773

654.6

1691
1773

methyl H
C
H bend (scissors)

c

58.7

1401
1476

A0 A0

sym CH3 deformation (umbrella) IP aldehyde C
H bend

139.5 d 94.3

1310
1401 1184
1296

A0

IP asym C
C stretch

d

1184
1296

1416

1407

1422.92

A

ν6

A

1353 1318

1352 1313

1367.38 1265.57

B B (shldr)

ν7 ν8

1206

1231

1228.81

B

ν9

0

94.3

978

991

1005.65

A

ν10

A

756

785

781.23

B

ν11

A0

IP sym C
C stretch

50.3

716
827

551

570

535.21 or

B

ν12

A0

IP Aldehyde O
C
C bend,

21.3

567
635

463

474

477.63

B

ν13

A0

IP O
C
C bend

e

0

IP CH3 rock

C
C
C bend, C
C
CH3 rock

591.22

89.5

415
507

240 3009

251 3007

257.76 2977.85

B C

ν14 ν15

A A00

IP C
C
C bend OOP CH3 C
H stretch

49.5 a 77.9

222
305 2847
3143

1425

1405

1425.22

C

ν16

A00

asym CH3 C
H bend

c

58.7

1401
1476

00

1037

1038

1052.04

C

ν17

A

876

878

887.12

C

ν18

A00

OOP aldehyde C
H bend, CH3 rock

450

452

480.04

C on B

ν19

A00

OOP C
CdO bend

e

89.5

415
507

ν20

A

00

C
CH3 torsion

f

83.0

57
151

ν21

A00

central C
C torsion

f

83.0

57
151

12.2

4518
4614

25.1

3388
3522

OOP C
C
C bend, OOP aldehyde C
H bend

134

118

82

94

121.09

C

103

Combination and Overtone Bands 4561.15

B

ν3 + ν4

A0

aldehyde C
H stretch + in-phase CdO stretches

4442.05

C?

ν6 + ν15

A00

CH3 scissors + CH3 C
H stretch

4383

C?

ν3 + ν16

A00

asym CH3 C
H bend + aldehyde

3452.85

A?

ν4 + ν5

A0

in-phase CdO + out-of-phase CdO

1996.55 1834

A? -

2ν10 ν4 + ν21

A0 A00

CH3 rock overtone in-phase CdO + C
C torsion

C
H stretch

201

1630

-

ν4
ν21

A00

in-phase CdO
C
C torsion

201

B?

2ν21

A0

torsional overtone

*Calculated frequencies are based on trans configuration. Observed vapor-phase frequencies are cited as local maxima for A- and C-type bands; local minima, for B-type bands. Band integrals denote overlapping bands in some cases, and the composite integral is denoted by a shared superscript letter in the band intensity column.

glyoxal and diacetyl when possible. Figures 2
6 again show the spectrum of methylglyoxal (middle traces) along with the spectra of glyoxal and diacetyl for comparison. Our assignments are shown in Table 4. In a recent paper on methylglyoxal’s equilibrium with methylglyoxal diol, Axson et al.84 simply list experimental and theoretical frequencies by numbering the bands, but do not assign modes in the conventional group-theoretical fashion. We also find their peak positions to differ from PNNL values in some cases, but this is likely due to their assigning P- or R-branch maxima for B-type band profiles rather than minima. Using stimulated emission pumping of the jet-cooled species, Reid et al.50 also assign 16 of the 21 fundamentals. Of the 16 fundamentals assigned, however, we find that we only agree with eight of their assignments. One advantage that

gas-phase IR absorption spectroscopy has is its ability to use band profiles82 to determine the symmetry of a normal mode; in our studies most of the A00 modes exhibit clear C-type band profiles. We also plan Raman studies to further corroborate these assignments. Also in the far-infrared both Reid et al.50 and Kamei et al.49 assign some frequencies, but the assignments are not complete. As seen in Figure 6, methylglyoxal in fact has two torsional modes in the far-IR: the torsion about the methyl-carbonyl C
C bond (ν20) and the torsion about the central C
C bond (ν21). As expected, Figure 6 shows that the ν21 torsional frequency of the τ(C
C(central)) at 103 cm
1 for methylglyoxal lies between the values for diacetyl (47.73 cm
1) and glyoxal (126.7 cm
1). This is consistent with an intermediate mass for the atoms twisting 9894

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The Journal of Physical Chemistry A about the central C
C bond. The 103 cm
1 value is a band center frequency, but the value is in good agreement with the 105 cm
1 value reported by Fateley et al.85 However, the complicated profile also makes clear that multiple hot bands are involved (as for glyoxal) requiring further analysis. The profile is further complicated by overlap with the ν20 C-type band at 121 cm
1. Interestingly though, Figure 3 also helps confirm the ν21 torsional assignment as it is an analogue to the glyoxal and diacetyl case: Similar to the parent molecules,35 the in-phase CdO stretching motion again generates sum and difference bands with this torsional mode about the C
C bond. In the case of methylglyoxal these are ν4 ( ν21 (1733 ( 103 cm
1) forming combination bands at 1834 (ν4 + ν21) and 1630 cm
1 (ν4
ν21) as seen in the inset of Figure 3. This also serves as implicit corroboration of our assignment of the 1733 A-type peak as being the ν4 in-phase CdO stretching motion. In the glyoxal and diacetyl cases the in-phase dual CdO stretching motion is IRforbidden: only the out-of-phase CdO motion is seen in the IR. For methylglyoxal, however, this out-of-phase carbonyl motion is the 1729 cm
1 ν5 B-type band which overlaps the weaker but allowed ν4 1733 cm
1 in-phase mode. All of our calculated values indicate the two carbonyl modes are nearly degenerate in methylglyoxal, but with the out-of-phase ν5 having the greater predicted intensity. We also note that the out-of-phase motion is at exactly the same 1729 cm
1 IR frequency as that for diacetyl, and very close to that of glyoxal: Apparently the presence of either one or two methyl groups on the end(s) of the molecule has negligible effect on this dual CdO stretching mode. While we do observe a miniscule shoulder blip at 1704 cm
1, we do not concur with Reid that it is assigned to ν5 due to the combination band discussed below. Nor do we concur with Axson et al.84 that one frequency is associated with the CdO stretch of the aldehyde carbonyl, the other to the ketone carbonyl—all calculations show both bands involve the simultaneous in-phase/out-ofphase motion of both carbonyl groups. In addition to the 103 cm
1 ν21 torsion about the aldehydeketone C
C bond in Figure 6 we also assign ν20, the methylglyoxal torsion about the methyl ketone C
C bond; it is a clear C-type band at 121.09 cm
1. The observed frequency is in excellent agreement with the 122.7 ( 4.9 cm
1 value predicted from the microwave work of Dyllick-Brenzinger and Bauder,51 while the fluorescence work of Gurnick et al.52 also involved this methyl torsion to recognize a deformation of methylglyoxal in the first S1 singlet excited state. Slightly higher in energy is the weak 2ν21 torsional overtone band centered at 201 cm
1. In Figure 6 the band at 257.76 cm
1 is ν14 of A0 symmetry, a pseudo-B-type band of the in-plane C
C
C bending mode. The strongest methylglyoxal fundamental band in the far-IR region is the ν13 in-plane bend of the ketone OdC bond relative to the C
C
C backbone at 477.63 cm
1. This A0 vibration exhibits a B-type band profile, but has the C-type profile of the weaker ν19 out-of-plane C
CdO bend (A00 symmetry) built atop of it with a peak at 480.04 cm
1. There are two broad bands of comparable intensity, one at 535.21 and the other at 591.22 cm
1, and we are unsure which of the two bands to assign to ν12, the in-plane C
CdO bending fundamental. It is likely the 535 cm
1 band as this frequency is somewhat more consistent with other features of the methylglyoxal spectrum and the antisymmetric C
CdO bu bend in diacetyl,39 but a frequency of 591 cm
1 is a better match to frequencies predicted86
88 using SQM where nine force constant scale

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factors were fitted to 28 diacetyl frequencies; these scale factors were used without modification to scale the methylglyoxal force constants, thus using the diacetyl assignments as a quantitative guide for assigning methylglyoxal. One fundamental band that Reid et al.50 do not clarify is the band they report at 1426.5 cm
1, which they assign as being two (different) asymmetric bends of the methyl group. As with the single-carbonyl analogue—acetaldehyde (an asymmetric top)89—a transition moment is not uniquely parallel to a principal axis—it is observed that the peak for methylglyoxal in this region is a hybrid band. The band possesses a complicated shape indicative of both A-type and C-type bands. By symmetry this combination is plausible for ν6 which is A0 and ν16 which is A00 corresponding to A-type and C-type bands, respectively. The DFT calculations (Spartan) place the ν6 asymmetric H
C
H bend (scissors) at 1416 cm
1 while the other asymmetric C
Hmethyl bend (ν16) occurs at 1424 cm
1. We assign ν6 to the peak at 1422.92 cm
1 and ν16 to the sharper C-type band at 1425.22 cm
1. The bands may better resolve at lower pressures. Two other bands that overlap are ν8 and ν9. The ν8 band appears as a ∼1267 cm
1 shoulder to the ν9 fundamental at 1228.81 cm
1. Both are B-type bands, making assignment of the pronounced C-type band at 1052.04 cm
1 as the ν17 the OOP C
C
C bend A00 fundamental straightforward. The remaining bands observed in the methylglyoxal midinfrared spectrum are previously unassigned combination or overtone bands. For example, Figure 5 presents another homologous series in the near-IR overtone region that makes assignment of the methylglyoxal bands quite straightforward: For all three species there is a strong band near ∼3450 cm
1 that is a combination band of one quantum of in-phase and one quantum of out-of-phase carbonyl stretches for each of the three molecules: ν2 + ν10 for glyoxal,35 ν3 + ν24 for diacetyl, and ν4 + ν5 in the case of methylglyoxal (at 3452.9 cm
1). We do not agree84 with Axson et al. who state that the features at 3458 and 3444 cm
1 represent independent carbonyl overtone bands from the aldehyde and ketonic portions of the molecule, respectively. Rather, we are convinced that these two features are the R- and P-branches of a single B-type rotational band contour for the combination band of this gas-phase molecule, whose spectrum shows many such rotational band contours. That is, both ν4 as well as the ν5 fundamental do not arise from independent CdO stretches, but rather are coupled CdO motions. The assignment is confirmed by theoretical calculations (the G03 anharmonic prediction for this combination band is 3340 cm
1), as well as the agreement from summing the two frequencies (e.g., ν4 + ν5: 1733 + 1729 = 3462 cm
1; observed, 3453 cm
1). Further credence is lent by the appearance and position of the analogous ν3 + ν24 combination band for diacetyl. In the C2h point group the first overtone of any au or bu isolatedmotion fundamental would correspond to ag symmetry and thus be IR-forbidden, but combination bands such as ν3 + ν24 and ν2 + ν10 are IR-allowed and are seen for diacetyl and glyoxal, respectively. Figure 5 also allows for assignment by analogy for the combination near 4565 cm
1. For glyoxal this is ν2 + ν9 at 4571.14 cm
1, the combination of the in-phase CdO stretchings with the aldehyde asymmetric C
H stretch. Similarly, for methylglyoxal we assign the B-type band at 4561.15 cm
1 as the asymmetric C
Haldehyde stretch with the in-phase carbonyl motion, ν3 + ν4. Such a band is obviously absent in the diacetyl spectrum due to the lack of any hydrogen bonded directly to a carbonyl carbon. This is also evidenced (Figure 4) by the 9895

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The Journal of Physical Chemistry A presence of a strong aldehyde C
H stretch fundamental near 2830 cm
1 for glyoxal and methylglyoxal only. All 21 fundamental modes of methylglyoxal were observed in this study and have been assigned. Those modes which have not been reported previously include the following: the ν1 inplane C
Hmethyl stretch (3026.27), the ν2 symmetric C
Hmethyl stretch (2950.07), the ν3 asymmetric C
Hald stretch (2828.01), and the ν15 out-of-plane C
Hmethyl stretch (2977.85). We have also reported and assigned the ν13 and ν17, as well as the ν8 to ν10, fundamentals, along with several combination and difference bands for the first time. Few IR band intensities for methylglyoxal have been reported in the open literature: In their work Axson et al. used theoretical values to determine methylglyoxal concentrations.84 However, we were able to compare our band intensities (Table 4) to unpublished results from the Max Planck Institute in Mainz, Germany.90 For the mid-IR spectra, the Mainz group recorded quantitative data over the same range at 1 cm
1 resolution, which was helpful in searching for trace contaminant peaks in the PNNL data, namely, HCHO. Comparing integrated band intensities with the MPI data, we found that the values agreed to about the ∼10% level, reasonably good agreement. The only other reported literature values include a peak intensity of 1.07  10
19 cm2 molecule
1 at 2835 cm
1 (base 10) from Zhou et al.4 This compares to the present value of 1.35  10
19, a ∼20% difference. This is reasonable agreement for a species that is somewhat difficult to prepare. Clearly further experimental comparisons would be useful. The results of our calculations for methylglyoxal clearly show the global minimum as the trans-conformer with respect to the two carbonyls (Figure 1). The energy difference between the eclipsed and anticonformer is a significant difference (∼7 kcal/ mol), making it likely no other conformer exists at room temperature to complicate the spectrum. The scaled DFT and MP2 anharmonic theoretical predictions are in reasonable agreement with the experimental data. All of the quantum chemistry results indicate some vibrational mode mixing, which confirms the lack of completely pure fundamental vibrations in both the mid- and far-infrared. 3.4. Ambient Monitoring Considerations. Recent reports have shown that the present R-dicarbonyl compounds are likely precursors to secondary aerosol (SOA) formation,23,91
93 as well as the fact that they are anticipated to be generated at significant levels by biomass burning. We thus consider the possibilities of using IR spectroscopy for either extractive or open-path monitoring of such compounds. While each has its own pathways, all three are expected or known to have short atmospheric lifetimes, τ ∼ 2
4 h, limited chiefly by photolysis. Maximal tropospheric mixing ratios are only 1
2 ppbv, though much higher concentrations are likely found in BB plumes. Detection with any method is thus challenging. Some success to date has been had for glyoxal: Volkamer and colleagues15,94 have used the structured vibronic bands in open path DOAS (differential optical absorption spectroscopy) for successful detection, primarily in polluted atmospheres with concentrations between 1.8 ppbv and their 150 pptv detection limit. Keutsch and co-workers95 have also recently demonstrated sensitive detection of glyoxal using a laser-induced fluorescence method with 18 pptv/min detection limits and ambient concentration measurements ranging from 20 to 250 pptv. Along with NO2, Washenfelder et al.96 et al. have demonstrated similar glyoxal detection limits (29 pptv) using a broadband cavity enhanced method.

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Broadband IR methods cannot generally achieve such sensitivities, but infrared laser methods using, e.g., quantum cascade lasers,97 difference frequency generation,98 or cavity ringdown99 techniques can now often achieve detection of optical densities of ∼1  10
7 or even smaller over path lengths of 100
200 m or more. While the present data are measured at atmospheric pressure,100 our experience indicates that for all three molecules most are subbands whose lines resolve at low pressure. One resolved line at 2816.23 cm
1 suggested44 for glyoxal monitoring lies in the ν9 band. The pressure-broadened line has a crosssection of ∼3.2  10
4 (ppm m)
1, meaning that for a 1 m path an optical system would need a sensitivity of 3.2  10
4 to detect 1 ppm, or for, e.g. a 100 m optical path 3.2  10
5 OD to detect 1 ppbv, or 3.2  10
7 OD to detect 10 ppt. As modern QCL systems now have broader tuning ranges, another possibility is to detect the entire Q-branch of the very strong ν10 carbonyl band at 1731.2 cm
1. With a strong differential cross-section of ∼6.1  10
4 (ppm m)
1, a 1 m laser system would need to detect 6.1  10
7 OD for 1 ppbv sensitivity. But both of these lines are in relatively congested spectral regions, the 2816 cm
1 region for the C
H modes of many organics, as well as N2O and H2O lines, the 1730 cm
1 region cluttered by the rovibrational lines of the water bending mode. An alternative could be the weaker but potentially useful ν6 Q-branch at 801.2 cm
1; the Q-branch has a differential cross-section of 2.0  10
4 (ppm m)
1 in the longwave infrared (LWIR) window and a full width at half-maximum (FWHM) of