Influence of Intramolecular Hydrogen Bonding on OH-Stretching

ARTICLE pubs.acs.org/JPCA

Influence of Intramolecular Hydrogen Bonding on OH-Stretching Overtone Intensities and Band Positions in Peroxyacetic Acid Montu K. Hazra,† Xiaobi Kuang,‡ and Amitabha Sinha*,† †

Department of Chemistry and Biochemistry, University of California, San Diego La Jolla, California 92093-0314, United States

bS Supporting Information ABSTRACT: Vapor phase absorption spectra and integrated band intensities of the OH stretching fundamental as well as first and second overtones (2νOH and 3νOH) in peroxyacetic acid (PAA) have been measured using a combination of FT-IR and photoacoustic spectroscopy. In addition, ab initio calculations have been carried out to examine the low energy stable conformers of the molecule. Spectral assignment of the primary features appearing in the region of the 2νOH and 3νOH overtone bands are made with the aid of isotopic substitution and anharmonic vibrational frequency calculations carried out at the MP2/aug-cc-pVDZ level. Apart from features associated with the zeroth-order OH stretch, the overtone spectra are dominated by features assigned to combination bands composed of the respective OH stretching overtone and vibrations involving the collective motion of several atoms in the molecule resulting from excitation of the internal hydrogen bonding coordinate. Integrated absorption cross section measurements reveal that internal hydrogen bonding, the strength of which is estimated to be ∼20 kJ/mol in PAA, does not result in a enhanced oscillator strength for the OH stretching fundamental of the molecule, as is often expected for hydrogen bonded systems, but does cause a precipitous drop in the oscillator strength of its 2νOH and 3νOH overtone bands, reducing them, respectively, by a factor of 165 and 7020 relative to the OH stretching fundamental.

1. INTRODUCTION Hydrogen bonds are a common feature in many areas of chemistry. In biological systems, these weak interactions influence the stability of protein conformers and assist in the binding of enzymes to substrates.1
4 In the earth’s atmosphere absorption of solar radiation by hydrogen-bonded complexes is thought to influence climate.5
11 Infrared spectroscopy has been a useful tool for probing hydrogen bonding (H-bonding) interactions in various molecules. An interesting trend observed from the infrared studies involves the intensity of the fundamental and overtone transitions associated with the hydrogen-bonded proton donor stretching coordinate. For example, measurements in liquid phase samples containing OH groups has shown that hydrogen bonding can increase the IR oscillator strength of the OH-stretching fundamentals while leading to a substantial decrease in the intensity of its first overtone compared to that typically found for OH groups of non-hydrogen-bonded (non-H-bonded) molecules.12
16 A similar trend in IR band intensities of H-bonded systems has also been noted in matrix isolation and helium droplet experiments.17
20 Gas-phase measurements of H-bonded systems apparently also support this trend, although the results are less conclusive.21 Several gas-phase studies have reported that overtone transitions of H-bonded hydroxyl groups become very difficult to detect with increasing H-bond strength and that the intensity enhancement due to H-bonding is limited to the OHstretching fundamental.21
33 Interestingly, however, even though r 2011 American Chemical Society

the number of gas-phase studies on H-bonded systems is quite extensive, published experimental values of absolute integrated absorption cross sections for both the fundamental and overtone transitions involving gas phase H-bonded molecules are rather limited. In the literature, intensity analysis of gas-phase H-bonded systems have been reported primarily in terms of changes in oscillator strength ratios with overtone order or theoretically predicted absolute oscillator strengths.21
37 Thus, gas-phase measurements that quantify the influence of H-bonding on absolute band intensities of overtone transitions provide a useful complement to the currently available database.21 Organic peroxyacids [R-C(O)OOH], which exhibit intramolecular hydrogen bonding,38
40 provide a convenient platform to systematically investigate the changes in absorption cross section with overtone order arising from hydrogen bonding. Unlike molecules with intermolecular hydrogen bonding, these peroxyacids can be generated in relatively large abundance, do not undergo predissociation upon vibrational excitation and their intramolecular CdO 3 3 3 H
O hydrogen bond strengths can be varied systematically by changing the identity of the R group Special Issue: A. R. Ravishankara Festschrift Received: July 12, 2011 Revised: September 14, 2011 Published: October 11, 2011 5784

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Figure 1. Optimized structures of the three stable conformers of peroxyacetic acid [CH3C(O)OOH  PAA] predicted at the MP2/ aug-cc-pVDZ level. Energies given are with respect to conformer I, with its intramolecular hydrogen bond indicated by the dotted line.

(RdH, CH3, or C6H5). Recently, we have reported on the overtone spectroscopy of the OH stretching vibration in peroxyformic acid (PFA).41 The strength of the hydrogen bond in PFA is ∼13.6 kJ/mol and our measurements revealed that the nominal intensity enhancement expected of its OH stretching fundamental due to hydrogen bonding was absent in this molecule and that the intensity drop in going from the fundamental to the first OH stretching overtone was comparable to that found in non-H-bonded molecules such as nitric acid. The molecule did however show significant drop in the intensity of its second OH stretching overtone. In the present work, we investigate the intensities of the low order OH stretching overtones of peroxyacetic acid (PAA), which like PFA also exhibits intramolecular H-bonding but with an hydrogen bond strength which is substantially (∼50%) stronger due to the presence of the electron donating methyl group. Peroxyacetic acid (PAA) is an important molecule in atmospheric chemistry due to its potential for generating hydroxyl radicals upon UV photolysis as well as its contribution to the formation of secondary aerosols.42
49 In the atmosphere, PAA is mainly produced through the reaction of HO2 with CH3C(O)OO radicals generated from the photochemical oxidation of volatile organic compounds.42 Furthermore, from an industrial perspective PAA is also an important oxidant for chemical processing, syntheses, bleaching, and disinfection.50 There exists limited spectroscopic information about PAA. The first infrared spectrum of PAA was reported by Giguere et al.38 and based on their study they concluded that intramolecular hydrogen bonding between the carbonyl oxygen and the acidic hydrogen atom results in a particularly stable five-membered ring conformer of PAA shown in Figure 1 (conformer I). This study also observed that the values for the OH stretch and OOH bending frequencies in PAA remain practically the same in the vapor phase as well as in liquid phase solution involving nonpolar solvents. Subsequently, Cugley et al.39,40 performed both microwave and matrix isolated infrared spectroscopy of PAA. The vibrational frequencies and normal-mode analysis resulting from this work were also consistent with the most stable geometry of PAA being the one involving intramolecular H-bonding, the same structure previously identified by Giguere et al. Recently, gas-phase UV absorption cross section measurements46 and UV laser photolysis have been carried out on PAA.43 The photochemical studies concluded that under tropospheric conditions, photolysis of PAA will be reasonably rapid and that its dissociation will ultimately lead to the production of the CH3 + CO2 + OH fragments. In this study, we report the first vapor phase overtone spectra of the OH stretching vibration in PAA. On the basis of the energy

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difference for inter conversion between the hydrogen bonded and non-hydrogen bonded conformers (conformers I and II; see Figure 1), the calculated H-bond strength of PAA is estimated to be 20.92 kJ/mol at the G2(MP2) level of theory.51 This is approximately 1.5 times larger than the hydrogen bond strength in peroxyformic acid (PFA). The integrated absorption cross sections for the 1νOH, 2νOH, and 3νOH bands of PAA have been measured relative to the known integrated absorption cross sections of n-propanol.52 Furthermore, the measured intensities are compared with theoretical predictions based on a onedimensional (1-D) local mode model using OH stretching potential energy curves calculated at the MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels. Finally, we also investigate the issue of the number of stable conformational minima in PAA by independently varying the dihedral angles around C
C, O
O, and C
O single bonds using relaxed scan calculations at the MP2/ aug-cc-pVDZ level. Geometry optimization and subsequent normal mode vibrational frequency calculations at the MP2, QCISD and B3LYP levels with aug-cc-pVDZ basis set show that PAA has three stable conformers as shown in Figure.1. However, as we discuss below, the room temperature spectra of PAA show evidence for transitions originating only from the lowest energy conformer, conformer I. Therefore, for conformer I, geometry optimizations at the MP2 and B3LYP levels were extended using the larger aug-cc-pVTZ basis set.

2. EXPERIMENTAL METHOD Peroxyacetic acid is known to be a potentially dangerous and explosive substance.39,53 It can be generated by the reaction of hydrogen peroxide with acetic acid. The process is described by the following reaction scheme: CH3 CðOÞOH þ H2 O2 h CH3 CðOÞOOH þ H2 O

ð1Þ

Both the forward and reverse reactions are fairly slow at room temperature, and a long time is necessary for equilibrium to be established.54 To achieve the concentration and purity required in our experiment, we synthesized the PAA and its deuterated analog PAA-d3 [CD3C(O)OOH] by following the procedure outlined (example 1) by Krimm.55 Briefly, 8.2 mL of 30% hydrogen peroxide (H2O2) is added dropwise to 12.5 mL of 99.5% sulfuric acid kept in a 250 mL round-bottom flask cooled to ice temperature. After this, 4.3 mL of 99.5% acetic acid (AA) or CD3C(O)OH (in the case of PAA-d3) was slowly added to the mixture. Sulfuric acid acts as a catalyst for the reaction by removing the product water molecules and driving the reaction further to the right side. The suspension was then stirred for approximately 48 h. After synthesis, H2O2 and AA were removed from the reaction mixture by vacuum distillation using a water aspirator through a 40 cm long vigreux column. Under our experimental condition, pure PAA consisted of the distilled fraction having a boiling point of ∼40
C at the column head. Finally, before use, the collected sample was further purified through several freeze
pump
thaw cycles using a liquid nitrogen bath in order to remove potential volatile impurities. The experimental methods for measuring FT-IR and photoacoustic spectra have been described in our recent work on peroxyformic acid.41 Briefly, the infrared spectra in the fundamental region (700
4000 cm
1) was recorded at a spectrometer resolution of 0.5 cm
1 using a commercial FT-IR spectrometer (Thermo Fisher, Nicolet 6700) equipped with a liquid-nitrogencooled Mercury-Cadmium-Telluride-A (MCT-A) detector and 5785

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In the above equation, S, I, P, and σ correspond, respectively, to the integrated signal, laser intensity, sample partial pressure, and integrated absorption cross section.

3. RESULTS AND DISCUSSION 3.1. Conformational Analysis, Optimized Geometry, and Vibrational Frequencies. Polyatomic molecules with flexible

Figure 2. Comparison of one-dimensional relaxed scan potential energy curves for variation about the three dihedral angles corresponding to rotation about the C
C, C
O, and O
O single bonds (Figure 1). The calculations have been performed at the MP2/aug-cc-pVDZ level and locations of three stable conformers (Figure 1) are indicated by arrows in the figure.

KBr beam splitter. The spectra were recorded at room temperature for various sample pressures ranging between 0.5 and 3 Torr placed in a 15 cm long glass FT-IR cell with either CaF2 or NaCl windows. The windows were attached to the cell body using TorrSeal epoxy. During the scans, the spectrometer was continuously purged with dry nitrogen to minimize IR absorption by atmospheric water vapor and carbon dioxide. For overtone spectra, tunable infrared radiation in the vicinity of the first and second OH-stretching overtones were generated by an optical parametric oscillator (OPO; Spectra Physics: MOPO730), pumped by the third harmonic of a seeded Nd:YAG laser (Spectra Physics: GCR-270). Few turning prisms directed the infrared light from the MOPO unit into a glass photoacoustic cell (2.5 cm diameter, 15 cm length) containing 6
11 Torr of PAA sample and the balance N2 buffer gas, thus, resulting in a total cell pressure of around 760 Torr. Typical MOPO infrared pulse energies measured near the entrance to the cell ranged between 1 and 2.5 mJ/pulse. The photoacoustic cell was equipped with a microphone (Knowles model BT-1759) and signal from the microphone was passed through a home-built preamp and filter circuit and then the resulting output signal sent to a gated integrator (LeCroy: 2249SG ADC). Subsequently, the signal was digitized and stored in a computer as a function of excitation laser wavelength. We verified the linear dependence of the signal on laser power and normalized the spectra for variation in laser power. Integrated absorption cross sections of the fundamental and overtone transitions of PAA were determined by comparing peak areas of the PAA bands with the corresponding OH stretching modes in n-propanol for which the absorption cross sections are known.52 We used the following equations to obtain the PAA integrated absorption cross sections from the relative signal measurements: SPAA α ILaser PPAA σ PAA

ð2Þ

σPAA ¼ ðSPAA =Sn-propanol Þ  ðPn-propanol =PPAA Þ  ðIn-propanol =IPAA Þ  σ n-propanol

ð3Þ

side groups can exist in multiple stable conformational minima associated with different orientations of the flexible groups. Transitions originating from these multiple conformational minima can, in turn, increase the spectral complexity of the room temperature spectra. The PAA molecule (Figure 1) has three flexible dihedral angles associated with rotation around C
C, C
O, and O
O single bonds. Figure 2 shows a comparison of the onedimensional (1-D) potential energy curves resulting from relaxed scans carried out at the MP2/aug-cc-pVDZ level, as a function of the three dihedral angles resulting from rotations about three single bonds mentioned above. The Gaussian 03 suite of programs is used to perform all the calculations presented here.56 Comparison of these potential energy curves shows that rotation around the C
O bond is much more hindered, having a barrier height of ∼59.5 kJ/mol, compared to rotation around the C
C and O
O bonds. The three stable conformers (conformers I
III), obtained from an analysis of the three individual relaxed scans, are shown in Figure 1 with their relative energies measured with respect to the most stable structure, the intramolecular H-bonded conformer I. Geometry optimization and subsequent frequency calculations of these configurations at the MP2/cc-aug-pVDZ, B3LYP/cc-aug-pVDZ, and QCISD/aug-cc-pVDZ levels confirm that the three configurations are true minima. Optimized geometries of these conformers in terms of Z-matrices and their normal mode vibrational frequencies are given in the Supporting Information. Further, a look at Figure 1 shows that with respect to the molecular plane formed by the heavy atoms, the orientation of methyl group in conformer III is different from that present in conformers I or II. We have found that for conformer III the particular orientation of methyl group, as given in Figure 1, is required to obtain a complete set of positive normal mode vibrational frequencies. Because the barrier for rotation around the C
C and O
O bonds are less hindered compared to the rotation around the C
O bond, we wondered if there were other stable minima associated with a slightly different orientation of the methyl and OH groups for configurations near those of conformers I and II. We initiated a much more systematic study to examine this by simultaneously scanning both the low barrier dihedral angles associated with rotation about the C
C and O
O bonds using relaxed scans. More specifically, for each discrete pair of values of the O2
C1
C6
H7 and C1
O3
O4
H5 dihedral angles (Figure 2) we optimized the geometry of PAA at the MP2/aug-cc-pVDZ level. This procedure was repeated as the values of these two dihedral angles were scanned simultaneously from
180.0
to +180.0
in steps of 15.0
; the resulting potential energy contour plot is shown in Figure 3. In the contour plot shown in Figure 3, the three stationary points (labeled as A
C), arising from the rotation about the above-mentioned two dihedral angles, correspond to the possible stable minima. Geometry optimization and subsequent frequency calculation on the configurations associated with these three stationary points at the MP2/aug-cc-pVDZ, QCISD/aug-ccpVDZ, and B3LYP/aug-cc-pVDZ levels show that the stationary points A and B are true minima, with geometries, respectively, 5786

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corresponding to the already detected conformers I and II, while the stationary point C is a saddle point. Therefore, our theoretical calculations predict a total of three stable conformers of PAA, as given in Figure 1. Our findings are consistent with the earlier less

Figure 3. Contour plot corresponding to potential energy surface scan as a function of dihedral angles about C
C and O
O single bonds. The calculation has been performed at the MP2/aug-cc-pVDZ level. The calculations find three stationary points, labeled A
C, in the contour plot. Stationary points A and B correspond to the stable minima associated with conformers I and II, respectively, while the stationary point C corresponds to a saddle point (see text).

exhaustive conformational search by Bach et al. who, in their notation, designated conformers I and II as, respectively, the syn and anti conformers.51 On the basis of the energy difference between the H-bonded syn conformer and the non-H-bonded anti conformer, we estimate the strength of the hydrogen bond in PAA to be ∼18.86, ∼19.04, and ∼20.46 kJ/mol, respectively, at the MP2, QCISD, and B3LYP levels using aug-cc-pVDZ basis set. This range of values is consistent with the previously reported value (20.82 kJ/mol) calculated at the G2(MP2) level of theory.51 Anharmonic frequency calculations for all three conformers have also been performed at the MP2/aug-cc-pVDZ level to aid in the spectral assignments. The MP2 level anharmonic frequencies of conformers II and III are given in the Supporting Information, while those of the most stable conformer are given in Table 1. It is important to note that although the calculated harmonic frequencies of conformer II are all positive with respect to MP2/aug-cc-pVDZ, QCISD/aug-cc-pVDZ, and B3LYP/aug-cc-pVDZ levels, we find that the anharmonic frequency calculation at the MP2/aug-cc-pVDZ level shows one negative fundamental frequency associated with the low-frequency OH-torsion vibration and further verification at B3LYP level shows that this result is method independent. To facilitate spectral assignment of PAA we have also recorded the spectrum of its deuterated analog, PAA-d3. Table 1 gives the anharmonic vibrational frequencies for the syn-conformer of the deuterated isotopolog as well, computed at the MP2/aug-cc-pVDZ level. As discussed below, the experimental spectra of PAA shows evidence for transitions arising only from the lowest energy synconformer (conformer I). As a result, we have placed greater

Table 1. Harmonic (Fundamental) and Anharmonic (Fundamental and First Overtone) Vibrational Frequencies (cm
1) of Conformer I in PAA and PAA-d3 [CD3C(O)OOH] Predicted at the MP2/aug-cc-pVDZ Levela PAA harmonic

a

PAA-d3 anharmonic

harmonic

anharmonic

normal modes

fundamental

fundamental

overtone

fundamental

fundamental

overtone

ν1

3472 (86.4)

3265

6294

3472 (86.4)

3264

6292

ν2

3212 (1.7)

3063

6054

2382 (1.0)

2299

4558

ν3

3190 (0.5)

3047

6031

2361 (0.3)

2282

4528

ν4 ν5

3092 (0.4) 1777 (190.7)

2972 1745

5908 3474

2222 (0.1) 1774 (192.5)

2169 1748

4319 3476

ν6

1479 (143.5)

1433

2852

1478 (135.9)

1433

2844

ν7

1472 (3.1)

1434

2861

1279 (200.1)

1237

2462

ν8

1466 (8.0)

1421

2831

1081 (19.9)

1055

2092

ν9

1387 (26.6)

1351

2688

1056 (2.8)

1034

2062

ν10

1258 (191.8)

1218

2425

1046 (15.4)

1025

2044

ν11

1052 (6.7)

1023

2047

926 (0.4)

906

1809

ν12 ν13

1022 (2.9) 960 (5.9)

999 935

1996 1863

905 (14.4) 903 (8.9)

884 888

1763 1776

ν14

861 (38.9)

841

1679

787 (18.2)

772

1542

ν15

651 (12.8)

638

1274

606 (11.4)

600

1192

ν16

623 (26.7)

603

1205

565 (38.9)

538

1069

ν17

470 (62.7)

405

741

460 (47.5)

404

761

ν18

432 (11.8)

424

847

412 (17.4)

401

801

ν19

321 (15.9)

312

624

299 (10.0)

293

585

ν20 ν21

227 (0.2) 61 (0.2)

213 44

423 83

215 (0.1) 44 (0.1)

205 39

408 80

The integrated band intensities (km/mol) are indicated in parentheses. 5787

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Table 2. Comparison of Predicted Geometrical Parameters of Conformer I of Peroxyacetic Acid [CH3C(O)OOH  PAA] Computed at Different Levels of Theory with Corresponding Experimental Values (Ref 39)a

MP2 bond lengths and angles

a

B3LYP

QCISD aug-cc-pVDZ

CCSD(T)

aug-cc-pVDZ

aug-cc-pVTZ

aug-cc-pVDZ

aug-cc-pVTZ

aug-cc-pVDZ

experimental values (ref 39)

C1
O2

1.223

1.213

1.215

1.208

1.219

1.223

1.22

C1
O3

1.365

1.353

1.356

1.352

1.362

1.366

1.33

C1
C6

1.503

1.494

1.501

1.498

1.509

1.511

1.49

C6
H7

1.097

1.085

1.095

1.086

1.099

1.101

1.09

C6
H8

1.099

1.087

1.097

1.089

1.101

1.103

1.09

C6
H9

1.099

1.087

1.097

1.089

1.101

1.103

1.09

O3
O4

1.455

1.439

1.437

1.438

1.446

1.465

1.47

O4
H5 CdO 3 3 3 HO O2
C1
O3

0.989 1.846

0.985 1.830

0.988 1.859

0.984 1.868

0.985 1.864

0.989 1.852

1.00 1.82

122.2

122.1

121.6

121.9

122.1

122.2

C1
O3
O4

109.9

110.2

111.1

111.2

110.4

109.9

O3
O4
H5

99.6

99.6

100.4

100.5

100.4

99.6

C6
C1
O2

127.3

127.3

127.2

127.0

126.9

127.0

C1
C6
H7

111.4

111.4

111.6

111.8

111.3

111.3

C1
C6
H8

108.4

108.4

108.6

108.6

108.4

108.4

C1
C6
H9 H8
C6
H7

108.4 110.1

108.4 110.1

108.6 110.0

108.6 109.9

108.4 110.1

108.4 110.1

H8
C6
H9

108.4

108.2

108.1

107.9

108.5

108.5

H9
C6
H7

110.1

110.1

110.0

109.9

110.1

110.2

C1
O3
O4
H5

0.0

0.0

0.0

0.0

0.0

0.0

Hydrogen bonding is indicated by the dotted line.

emphasis on the computational analysis for this conformer and optimized its geometry further at a higher level. In our earlier study of PFA, a comparison of the experimental and predicted values of the geometrical parameters calculated at the MP2 and CCSD(T) levels using the aug-cc-pVDZ basis set showed that the CCSD(T) level predictions were somewhat in better agreement with experiments.41 Therefore, in the case of PAA, the B3LYP and MP2 level geometries were further optimized at the CCSD(T) level using the aug-cc-pVDZ basis set. Further, because intramolecular hydrogen bond distances are expected to be sensitive to the size of the basis set, the B3LYP and MP2 predicted geometries of the syn-conformer were also optimized using the larger aug-cc-pVTZ basis set. The optimized geometry of conformer I at the different levels of theory and corresponding experimental values obtained from microwave measurements39 are summarized in Table 2. A look at Table 2 shows that all the levels of calculations considered are more-or-less equally reliable in predicting the bond lengths and angles of PAA, including the H-bond distance. Recently, the proton affinity of PAA has been investigated theoretically by Miller and Francisco,57 and in their work they also find that the predicted bond lengths of PAA at the MP2, B3LYP, and QCISD levels of theory in combination with different basis sets also show equally good agreement with experiment. 3.2. Oscillator Strength of OH Stretching Overtones. For calculating the integrated absorption cross sections of the OH

stretching overtones, we used a one-dimensional (1-D) local mode model58
63 employing an ab initio OH stretching potential energy curve and corresponding 1-D dipole moment function calculated at the MP2/aug-cc-pVDZ level. In a second set of calculations, we have also used a 1-D OH stretching potential calculated at the CCSD(T)/aug-cc-pVDZ level with corresponding dipole moment function obtained at the CCSD/aug-ccpVDZ level. The OH potential energy curves V(q) were generated by determining the change in molecular energy as the O
H bond distance in PAA is varied from its equilibrium value in steps of 0.05 Å, over the range from 0.59 to 2.09 Å. All other geometrical parameters of the molecule are kept fixed at their equilibrium values during this process and the Gaussian 03 keyword “NoSymmetry” has been used throughout these calculations. The vibrational overtone oscillator strength, f, for a transition from the ground state to an excited vibrational level v is given by the expression:58,63 f ¼ 4:702  10
7 ½cm D
2 ν0 f v jμ0 f v j2

ð4Þ

In the above equation, ν0 f v is the transition frequency in cm
1 and μ0fv = Æ0|μ(r
re)|væ is the transition dipole moment matrix element between the ground and excited vibrational level. The ab initio dipole moment function, μ(q = r
re), required in the above equation is obtained in a manner similar to V(q), that is, by stretching the OH bond from 0.59 to 2.09 Å in increments 5788

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Table 3. Measured and Calculated Integrated Absorption Cross Sections (cm/molecule) of the 1νOH, 2νOH and 3νOH Bands in PAA (Conformer I)a

transitions

1νOH (cm/molecule)

b


18

PAA (Peroxyacetic Acid) 2νOH (cm/molecule) 3νOH (cm/molecule)
20


22

1νOH/2νOH

1νOH/ 3νOH

3.51 (0.35)  10

2.12 (0.27)  10

5.00 (1.5)  10

165

7020

CCSD(T)

14.36  10
18

23.46  10
20

253.9  10
22

61

566

MP2

16.26  10
18

21.12  10
20

250.5  10
22

77

649

measured

PFA (Peroxyformic Acid) measured

6.05 (0.60)  10
18
18

1.89 (0.11)  10
19

c

7.05 (0.75)  10
21

32

860


19

21.83  10
21

52

530

19.43  10
21

61

663

CCSD(T)

11.58  10

2.23  10

MP2

12.89  10
18

2.11  10
19

Experimental uncertainties are indicated in parentheses. For example, the value of 2.12(0.27)  10
20 for the 2νOH transition signifies (2.12 ( 0.27)  10
20. For comparison, we have also provided the corresponding values for PFA. The calculations at both CCSD(T) and MP2 levels have been performed using the aug-cc-pVDZ basis set. b Integrated absorption cross section for 1νOH band was determined by integrating over the spectral region between 3250 and 3400 cm
1. The corresponding integration regions for the 2νOH and 3νOH bands are, respectively, 6200
6430 cm
1 and 8930
9160 cm
1. No corrections for state mixing or hot band contribution have been made. c From ref 41. a

of 0.05 Å. However, as Gaussian 03 does not calculate dipole moments at the CCSD(T) level of theory, we perform the dipole calculations at the CCSD/aug-cc-pVDZ level using the keyword DENSITY=CURRENT. This allows dipole moments to be calculated using the generalized density for the specified level of theory and provides dipole moments that are correct analytical derivatives of the energy.34 The values of the dipole moment over the grid points is then fit to a seventh order Taylor expansion about the equilibrium position: μðqÞ ¼

∑ ð∂nμ=∂qnÞjq ¼ 0qn=n!

ð5Þ

The vibrational wave functions, obtained by solving for the eigenfunctions of the 1-D OH stretching potential, along with each component of the ab initio dipole moment function, are then substituted into eq 4, and the resulting matrix element integral evaluated numerically, as outlined in our previous work.64 Although the oscillator strength is a dimensionless quantity it can be readily converted, by multiplying it by the factor 8.853  10
13, to an integrated absorption cross section with units of cm/molecule.63 The results of our intensity calculations are presented in Table 3. 3.3. Spectra of OH Stretching Fundamental and Overtones in PAA. Figure 4 shows the FT-IR spectrum of PAA over the region from 700 to 3500 cm
1, along with the assigned vibrational mode description of the spectral features. A theoretically predicted stick spectrum, generated by using the anharmonic frequencies and harmonic intensities calculated at the MP2/aug-cc-pVDZ level, is shown below the experimental spectrum. To facilitate comparison between the two spectra, the intensity of the experimental spectrum has been normalized with respect to the theoretical intensity of the C
O stretching vibration. From Figure 4, it is seen that the predicted and measured spectra agree well both with respect to relative intensities and observed frequency for the various transitions, thus confirming the purity and identity of the sample. The present assignments are consistent with the low-resolution FT-IR spectrum of Giguere and Almos.38 Because all the features appearing in the IR spectra can be readily assigned to monomeric PAA and the spectrum does not change when the PAA vapor pressure is increased from 0.4 to 5 Torr, we infer that contribution due to PAA dimer is negligible in the spectrum. This observation is consistent with prior studies that have concluded that PAA exist almost exclusively in their

Figure 4. Comparison of measured and theoretically predicted IR spectra of PAA over the region from 700 to 3500 cm
1. The theoretical stick spectrum has been generated by taking the harmonic intensities and anharmonic frequencies predicted at MP2/aug-cc-pVDZ level. Experimental spectrum has been normalized with respect to the predicted intensity of the C
O stretching transition.

monomeric form in the gas phase.65
67 A quick glance at Figure 4 shows that the C
H stretching fundamental of PAA has a very low absorption cross section given that its spectral feature, predicted to be at ∼3000 cm
1, is barely discernible. By contrast, the fundamental of the OH stretch is substantially stronger and its appearance at 3310 cm
1 is in good agreement with the computed anharmonic frequency of 3265 cm
1. Overtone spectra of PAA covering the first (2νOH) and second overtone (3νOH) of the OH stretch are presented in Figures 5 and 6. The sharp features appearing in both Figures 5a and 6a are assigned to water impurity transitions on the basis of the HITRAN database.68 The small amount of water vapor impurity in the overtone spectra arises from residual air and minor leaks in the N2 buffer gas line used to fill the photoacoustic cell. This impurity, however, does not affect the equilibrium vapor pressure of PAA, which is introduced into the photoacoustic cell and allowed to equilibrate prior to the introduction of the N2 buffer 5789

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Figure 5. (a) Photoacoustic spectrum of PAA in the vicinity of 2νOH overtone band. The sharp features in the spectrum are assigned to the water transitions on the basis of the HITRAN database (ref 68). (b) Resultant spectra in the vicinity of 2νOH band after subtracting out the water lines.

gas. The positions of these water transitions provide a convenient calibration for the spectral frequency. Subtracting out the prominent water peaks gives rise to the PAA overtone spectra shown in Figures 5b and 6b. We have also measured the overtone spectra for PAA-d3 in the region of the 2νOH band and a comparison with the 2νOH overtone spectra of PAA, along with the measured isotope shifts, is shown in Figure 7. The frequencies of the prominent absorption features of PAA and PAA-d3 appearing in Figures 5
7 are summarized in Table 4. Table 4 also gives tentative assignments of these peaks on the basis of anharmonic vibrational frequencies computed at the MP2/augcc-pVDZ level (Table1). For the region of the first OH stretching overtone, shown in Figure 5b, three prominent spectral features are seen. We assign the most intense absorption feature at 6315 cm
1 to the 2νOH band (note: 2νOH  2ν1). The MP2/ aug-cc-pVDZ level predicted anharmonic frequency for the 2νOH band is 6294 cm
1 and is in relatively good agreement with the measured value. We have ruled out the possibility of interference in this spectral region from H2O2 and CH3C(O)OH, which are used in the synthesis of PAA, because the 2νOH transition of these OH containing molecules occur substantially

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Figure 6. (a) Photoacoustic spectra of PAA in the vicinity of 3νOH overtone band. The sharp features in the spectrum are assigned to the water transitions on the basis of the HITRAN database (ref 68). (b) Resultant spectra in the vicinity of 3νOH band after subtracting out of the water lines. The feature at 9037 cm
1 is assigned to the 3νOH transition.

further to the blue at, respectively, 7040 and 6991 cm
1.69,52 For the region of the second overtone, shown in Figure 6a,b, the signal levels are very low and only two broad features having roughly comparable intensity appear prominently in the spectrum. Using an estimated value for the OH stretching anharmonicity of PAA, obtained from the observed position of its OH stretching fundamental and first overtone, we assign the first spectral feature at 9037 cm
1 in Figure 6b as the 3νOH transition. Further examination of the spectra in Figures 5b and 6b also reveal that a common aspect in both overtone regions is the appearance of a prominent second spectral feature roughly ∼302 cm
1 to the blue of the assigned 2νOH and 3νOH peaks. This situation is analogous to what we have seen in PFA.41 In the case of PFA, a second spectral feature ∼342 cm
1 to the blue of the respective 2νOH and 3νOH overtone bands appeared, and this secondary feature was assigned to a combination band of PFA involving the 2νOH/3νOH stretching overtone and either the low-frequency in-plane H-bond stretching vibration (ν12) or 5790

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low-frequency out-of-plane COOH torsion (ν11).41 However, unlike in the case of PFA, where theory predicts only one H-bonded vibration, ab initio frequency calculations for PAA predict two H-bonded stretching vibrations corresponding to an in-phase (ν19) and out-of-phase (ν18) motion of the methyl group. Atomic displacements vectors associated with these two H-bonded vibrations are shown in Figure 8. The calculated frequencies of these two H-bonded vibrations at the MP2/aug-cc-pVDZ level are respectively 312 and 424 cm
1. Interestingly in the region of the first overtone, shown in Figure.5b, there are two absorption features centered at 6617 and 6748 cm
1 which are respectively 302 and 433 cm
1 to the blue of the assigned 2νOH transition at 6315 cm
1. Therefore we assign the feature at 6617 cm
1 to a combination band involving the OH stretching overtone vibration (2νOH) plus the lowest frequency H-bond vibration (ν19) and assign the second feature at 6748 cm
1 to a combination band involving the 2νOH stretching overtone plus the second H-bond stretching vibration (ν18). We note that the calculated frequency for the out-of-plane COOH torsional vibration (ν17) in PAA, also shown in Figure 8, is 405 cm
1 and is therefore of similar value to that of ν18. However, as we discuss below, we prefer the assignment of the peak at 6748 cm
1 as the 2νOH + ν18 combination band over 2νOH + ν17 on the basis of isotope shift data.

Figure 7. Comparison of photoacoustic spectra of PAA [CH3C(O)OOH] and PAA-d3 [CD3C(O)OOH] in the region of the second OH stretching overtone. The measured isotopic shifts (Δ) in cm
1 units are indicated above the main features in the PAA-d3 spectrum. These shifts can be compared with the predicted values given in Table 5 to provide a consistency check of the spectral assignments.

Figure 7 compares the spectrum of PAA-d3, where all three hydrogen atoms on the methyl group are replaced by deuterium atoms, with that of PAA in the region of the first OH stretching overtone. The broad nature of the peaks makes the experimental isotope shifts difficult to quantify. However, using water absorption features as a reference and the relatively large observed redshift of the second and third absorption features in the region of the first OH stretching overtone of PAA-d3 compared to PAA helps firm up the measured shifts. The predicted frequency shifts due to isotopic substitution for some selected bands expected in the vicinity of the first OH stretching overtone are presented in Table 5. The experimentally measured isotope shifts for the 2νOH stretch and the two features at 6617 and 6748 cm
1, shown in Figure 7, are, respectively, 5, 17, and 28 cm
1, while the corresponding MP2 predicted shifts for 2νOH, 2νOH + ν19, and 2νOH + ν18 are, respectively, 2, 21, and 25 cm
1. Thus, the observed and predicted shifts are in good agreement. The predicted frequency shift for the COOH torsional vibration ν17 due to isotopic substitution on the methyl group is only 3 to
1 cm
1. Thus, this result rules against the feature at 6748 cm
1

Figure 8. Nuclear displacements for the normal modes of PAA corresponding to the low-frequency vibrations ν19, ν18, and ν17. Some of these modes are potentially associated with the combination bands appearing in the experimental overtone spectra (see text).

Table 4. Tentative Assignments of the Prominent Peaks of PAA and PAA-d3 Appearing in the Photoacoustic Spectra Shown in Figures 5
7a PAA observed

a

assignments

PAA-d3 predicted

observed

assignments

predicted

6315

2νOH

6294

6310

2νOH

6292

6617

2νOH + ν19

6294 + 312 = 6606

6600

2νOH + ν19

6292 + 293 = 6585

6748 9037

2νOH + ν18 3νOH

6294 + 424 = 6718

6720

2νOH + ν18

6292 + 401 = 6693

Assignments are made with the aid of anharmonic frequencies computed at the MP2/aug-cc-pVDZ level (Table 1). 5791

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Table 5. Computed Isotope Shifts of Certain Vibrational Bands of PAA in the Vicinity of the First OH Stretching Overtone Arising from Deuterium Substitution on the Methyl Group of PAAa predicted shift (cm
1) transitions

MP2/aug-cc-pVDZ

B3LYP/aug-cc-pVDZ

2ν1  2νOH

2

1

2ν1 + ν19

21

20

2ν1 + ν18

25

25

2ν1 + ν17

3


1

ν1 + 2ν5


1


2

a

The isotope shifts have been computed at both MP2/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ levels.

being the 2νOH + ν17 combination band. Similar isotope shift considerations also rule out this feature from being a combination band involving two-quanta of CdO stretch and one quanta of OH stretch (1νOH + 2ν5), which is predicted to occur nearby at 6739 cm
1 but is expected to have an isotope shift of only
1 to
2 cm
1. Finally, using the above information obtained from the analysis of the 2νOH region, we assign the second feature that occurs ∼302 cm
1 to the blue of the 3νOH peak at 9037 cm
1 to the 3νOH + ν19 vibration. After obtaining the frequencies of the 1νOH, 2νOH, and 3νOH stretching band centers, the harmonic frequency (ωe) and anharmonicity constant (ωeχe) for the OH stretching coordinate are obtained from a Birge
Sponer plot, presented in Figure 9. From the slope and intercepts of this plot, we obtain a ωe value of 3606.6 cm
1 and a ωeχe value of 148.8 cm
1. The linearity of the Birge
Sponer plot provides further support for our assignment of the features that correspond to the 2νOH and 3νOH transitions in PAA. In the case of peroxyformic acid, the measured values of ωe and ωeχe for the OH stretching coordinate are, respectively, 3612.8 and 135.5 cm
1. For non-H-bonded molecules,52,70 typical values for the OH stretching anharmonicities range between 75 and 90 cm
1. The lower value of ωe and higher value of ωeχe in the case of PAA compared to peroxyformic acid (PFA) is consistent with the stronger H-bond strength of PAA relative to PFA. Theoretically, the predicted values for the OH stretching anharmonicity at the MP2/aug-cc-pVDZ and B3LYP/ aug-cc-pVDZ levels are, respectively, 118.2 and 125.9 cm
1. Apparently, with respect to the measured value of ωeχe, it seems that the B3LYP level of calculation does a better job compared to MP2. However, when we consider the observed frequencies for the OH stretching fundamental and first overtone transitions, the B3LYP level predicted anharmonic frequencies underestimate the measured values, respectively, by 81 and 108 cm
1. By contrast, for the same transitions, MP2 level calculations underestimate these frequencies by 45 and 21 cm
1 respectively. Finally, we point out that we have not detected any evidence for transitions associated with conformers II or III. Due to the absence of hydrogen bonding in these conformers, their OH stretching frequency and anharmonicity are expected to result in their OH stretching bands occurring substantially to the blue of the corresponding peaks of conformer I. A search of these spectral regions, however, did not provide any evidence for transitions originating from these higher energy conformers. This result is also consistent with the calculated energies of these conformers, which suggests that, at room temperature, the Boltzmann population of conformers

Figure 9. Birge
Sponer plot for the OH stretching vibration of PAA. The linear experimental plot, shown in blue, corresponds to a harmonic frequency of 3606.6 cm
1 and an anharmonicity of 148.8 cm
1. Theoretically predicted Birge
Sponer plots, represented by the dashed red and black lines, correspond, respectively, to the MP2/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ level predictions. For the theoretical curves, the position of the 1νOH and 2νOH states are just the anharmonic frequencies obtained from the calculations while the position of the 3νOH band is obtained using the predicted anharmonicities and the values of ωe based on the predicted 1νOH and 2νOH anharmonic frequencies.

II and III is expected to be less than 0.15% of the population of conformer I. 3.4. Integrated OH Stretch Band Intensities and Influence of Hydrogen Bonding. The measured values of integrated band intensities for the 1νOH, 2νOH, and 3νOH bands of PAA along with the values calculated using the 1-D local mode model are given in Table 3. The specific spectral regions used to compute the peak areas of the bands are indicated below Table 3. For comparison, values of the absorption cross sections for analogous transitions in PFA, which corresponds to a molecule with a relatively weaker internal H-bond, are also given. A comparison of the calculated absorption cross sections given in Table 3 for the 1νOH, 2νOH, and 3νOH bands in both molecules at the CCSD(T) and MP2 levels shows that the computed values are internally consistent and do not vary appreciably between the two ab initio methods. Calculations at both the CCSD(T) and MP2 levels predict a slightly higher absorption cross section for the OH stretching fundamental of PAA compared to that of PFA and for both the fundamental and the overtone transitions, the 1-D model predicts absorption cross sections that are substantially larger than the actual measured value for both molecules. The deviations are significantly larger for the stronger H-bonded PAA molecule and the magnitude of the deviation increase as one goes to successive higher OH stretching overtones. For the 1-D calculations using the CCSD(T) method, for example, the computed absorption cross sections for the 1νOH, 2νOH, and 3νOH bands of PAA are, respectively, a factor of ∼4, 11, and 50 times larger than the measured values. In case of PFA, the agreement is better and the computed absolute intensities for the 1νOH, 2νOH, and 3νOH bands are, respectively, only ∼1.6, 1.2, and 3.1 times larger compared to measurement. If instead of absolute intensities one looks at the ratio of integrated absorption cross sections, the agreement between measurement and the results of the 1-D model are slightly better. The calculations predict, on average, a 5792

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Table 6. Comparison of Integrated Absorption Cross Sections for the OH-Stretching Fundamental and Intensity Ratios between the OH-Stretching Fundamental and Successive Overtones for Several Hydrogen-Bonded and NonHydrogen-Bonded Molecules measured integrated cross sections of 1νOH

1νOH/

1νOH/

(cm/molecule)

2νOH

3νOH

peroxyacetic acid

3.51  10
18

165

7020

peroxyformic acid

6.05  10
18

32

860

pyruvic acida

7.80  10
18

18b

molecules

trifluoroethanol dimeric complexc

30

trifluoroethanol dimeric complexd

400

glycolic acide nitric acidf acetic acidf

20 9.46  10
18 8.73  10
18

propanoic acidg

29 15

330 280

14

333

methanolf

3.28  10
18

12

140

ethanolf

2.73  10
18

7.5

120

a

Integrated absorption cross section of intramolecular H-bonded pyruvic acid conformer (ref 22). b Reference 83. c Weak hydrogen-bond (ref 21). d Strong hydrogen-bond (ref 21). e Reference 32 and where both the alcoholic-OH and acidic-OH groups contribute equally (within 5%). f Reference 52. g Reference 70.

factor of ∼70 drop in the integrated absorption cross section when going from the fundamental to the first OH stretching overtone of PAA and a factor of ∼56 drop in PFA (see Table 3). The actual measured drops in absorption cross section in going from the fundamental to the first OH stretching overtone are, respectively, a factor of 165 and 32 for PAA and PFA. Thus, when considering integrated absorption cross section ratios, experiment, and calculations agree roughly within a factor of 2. It is important to mention here that we have checked the integrity of our 1-D model calculations by reproducing the reported oscillator strengths of non-hydrogen-bonded molecules such as nitric acid at the MP2/6-311+G** level (see Supporting Information) as well as for acetic acid at the MP2/aug-cc-pVDZ level.41 It has been mentioned previously that several studies of hydrogen-bonded systems have indicated that the presence of H-bonding is reflected in an intensity enhancement of the H-bonded OH stretching fundamental and excessive weakness in the H-bonded OH stretching first overtone relative to that of a non H-bonded OH stretch.21,23,28,33 To examine this aspect and facilitate a comparison between PAA and other molecules, we present in Table 6 the ratios of measured integrated absorption cross sections between the OH stretching fundamental and its successive overtones for several non-hydrogen-bonded and H-bonded molecules. From Table 6 we see that for PAA the integrated absorption cross section of its OH stretching fundamental is not significantly enhanced by hydrogen bonding and its magnitude is comparable with those of non-H-bonded alcohols like methanol and ethanol. This behavior is also seen in PFA and pyruvic acid, which are the other two molecules in Table 6 that exhibit intramolecular hydrogen bonding. Furthermore, comparing the OH-stretching fundamental absorption cross sections of PAA, PFA, and pyruvic acid reveals that PAA has the lowest value followed by PFA and then pyruvic acid. This trend in oscillator strength is opposite the trend

in hydrogen bond strength in these molecules, which increases progressively from pyruvic acid to PFA and, finally, PAA. With regard to the large intensity drop in going from the fundamental to the first OH stretching overtone that is expected for hydrogen-bonded systems, from Table 6 we see that the intensity of the 2νOH transition of PAA drops by a factor of ∼165 relative to its fundamental. In case of non-H-bonded molecules like acetic and nitric acid, the magnitude of this drop is a factor of 15 and 29, respectively.52 The intensity drop for the first overtone of PFA and pyruvic acid, which exhibit successively weaker internal hydrogen bonding compared PAA, are, respectively, a factor of ∼32 and ∼18. Thus, the magnitude of the drop in the first overtone intensity for these three molecules correlates with the strength of their internal hydrogen bond. Finally, Table 6 also compares the results for PAA with that of the trifluoroethanol dimeric complex, which is a system exhibiting intermolecular hydrogen bonding.21,28 There are two different OH groups within the trifluoroethanol dimeric complex. In the region of the fundamental, the intensity of the H-bonded proton donor-OH group with strong H-bonding (hydrogen atom of proton donor-OH group bound strongly to the oxygen atom of the binding partner) is a factor of 4 larger compared to the intensity of the H-bonded proton acceptor OH group with weak H-bonding28 (hydrogen atom of the proton acceptor-OH group bound weekly to the fluorine atom of the binding partner). Furthermore, the intensities of the OH stretching first overtones are, respectively, about a factor of 400 and 30 times smaller than the corresponding OH stretching fundamentals in the above-mentioned strong and weak H-bonds.21 Although the trifluoroethanol dimeric complex has two hydrogen bonds while PAA has just one, the overall hydrogen bond strengths in the two molecules are comparable. On the basis of ab initio calculation, the hydrogen bond strength of the dimeric complex is estimated to be 24 kJ/mol,71 while that of PAA is around 20 kJ/mol. From Table 6, we see that even though the two systems have comparable H-bond strengths, PAA does not exhibit the large increase in its OH stretching fundamental band strength nor does it exhibit as a large drop in the intensity of its first overtone, as found in the trifluoroethanol dimeric complex. Finally, in going to the second OH stretching overtone (3νOH), the integrated band intensity of PAA drops by a factor of 42 relative to the first overtone and a factor of 7020 when compared to the fundamental. By contrast, in PFA the second overtone (3νOH) is a factor of 27 smaller than its first overtone (2νOH) and a factor of 860 times smaller than its OH stretching fundamental. Thus, unlike non-H-bonded molecules, the intensity of the higher OH-stretching overtone transitions in hydrogen-bonded molecules fall off considerably more rapidly. In the case of both PAA and PFA, the largest drop in successive OH stretching overtone transition strength, over the overtone orders investigated, appears to be in going from the fundamental to the first overtone. Hence, taken together, the available data for both intramolecular and intermolecular H-bonded systems presented in Table 6 suggest that an enhancement of OH-stretching fundamental intensity due to H-bonding is not always the case, although the magnitude of the enhanced intensity drop in going to successive overtones does appear to correlate with increased hydrogen bond strength.

4. SUMMARY AND CONCLUSION Vapor phase absorption spectra and integrated absorption cross sections of the OH stretching fundamental as well as first 5793

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The Journal of Physical Chemistry A and second overtones (2νOH and 3νOH) in peroxyacetic acid (PAA) have been measured using a combination of FT-IR and photoacoustic spectroscopy. In addition, ab initio calculations have been carried out to examine the low energy conformers of the molecule. We find that there are three stable conformers of PAA, with the most stable being the intramolecular hydrogenbonded conformer (conformer I) also designated as the syn conformer. On the basis of the energy difference between conformers I and II, produced by rotation around the O
O bond, the strength of the hydrogen bond in PAA is estimated to be, respectively, ∼18.86, ∼19.04, and ∼20.46 kJ/mol at the MP2/ aug-cc-pVDZ, QCISD/aug-cc-pVDZ, and B3LYP/aug-cc-pVDZ levels. Spectral assignment of primary features appearing in the region of the 2νOH and 3νOH bands have been made with the aid of isotopic substitution and anharmonic vibrational frequency calculations carried out at the MP2/aug-cc-pVDZ and B3LYP/aug-cc-pVDZ levels. The experimental spectra only show features belonging to the lowest energy conformer. Apart from spectral features associated with the pure zeroth-order OH stretch (2νOH and 3νOH), spectra in the OH stretching overtone region are dominated by features we assign to as combination bands composed of the respective OH stretching overtone and vibrations involving the collective motion of several atoms arising from excitation of the internal hydrogen bonding coordinate (ν18 and ν19). Our integrated absorption cross section measurements, using n-propanol band intensities as a reference, reveal that internal hydrogen bonding does not result in enhanced oscillator strength for the OH stretching fundamental of peroxyacetic acid (nor in PFA or pyruvic acid). This is counter to what is often expected for hydrogen-bonded molecules. Our measurements do however show that hydrogen bonding results in a precipitous drop in the oscillator strength of the 2νOH and 3νOH bands in PAA, reducing them, respectively, by a factor of 165 and 7020 relative to the OH stretching fundamental. This is a much faster drop than what is observed for non-hydrogenbonded molecules. One-dimensional local mode model calculations at the MP2/aug-cc-pVDZ and CCSD(T)/aug-cc-pVDZ levels predict an intensity increase for the OH stretching fundamental and a large drop in the first overtone transition of PAA. However, with respect to absolute values of the intensities, the agreement between experiment and the 1-D model is uniformly poor for the fundamental as well as the overtone transitions of PAA. It is worth noting that in these room temperature experiments the measured areas of the vibrational bands used for the integrated cross section estimates may have small contributions from hot bands for which we have not accounted. However, hot band corrections are not the reason for the observed difference between the 1-D local mode model and the measurements, as removing any potential hot band contribution from an OH stretching band will only lower the experimental integrated absorption cross section, making the difference between the calculation and measurements even larger. The discrepancy, especially for the higher overtones, could have a contribution from vibrational state mixing that tends to distribute the zeroth-order OH stretch oscillator strength over several modes.72
77 However, we find that including additional areas from neighboring peaks in the integrated absorption cross section analysis to take into account potential fractionation of bright state character does not improve the agreement substantially, especially in the case of the 3νOH band. Future work utilizing jet cooling and higher dimensional calculations will be required to address these issues more fully. In this regard, both PAA and PFA are ideal candidates because the

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simultaneous presence of both a peroxide and OH chromophors makes these molecules amenable to action spectroscopy using vibrationally mediated photodissiociation, as has been demonstrated for several other peroxide molecules.78
82

’ ASSOCIATED CONTENT

bS

Supporting Information. Optimized geometries of three conformers of PAA at MP2/aug-cc-pVDZ level of calculations, harmonic and anharmonic vibrational frequencies of three conformers at different levels of calculations, as mentioned in the text, and oscillator strengths for the OH fundamental and overtone transitions of nitric acid at MP2/6-311+G** level of calculation. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses ‡

Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, CA 90095
1565.

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