Visible Light-Driven Chemistry of Oxalic Acid in Solid Argon, Probed

Article pubs.acs.org/JPCA

Visible Light-Driven Chemistry of Oxalic Acid in Solid Argon, Probed by Raman Spectroscopy Adriana Olbert-Majkut,*,† Jussi Ahokas,‡ Mika Pettersson,‡ and Jan Lundell§ †

Faculty of Chemistry, Wrocław University, F. Joliot-Curie 14, 50-383 Wrocław, Poland Nanoscience Center, Department of Chemistry and §Department of Chemistry, P.O. Box 35, University of Jyväskylä, FI-40014 University of Jyväskylä, Finland

‡

S Supporting Information *

ABSTRACT: High-overtone induced chemistry of oxalic acid (OA) isolated in a low-temperature argon matrix was investigated using Raman spectroscopy. The Raman spectra of three conformers of OA are presented and discussed. Upon excitation of high overtone combination bands by 532 nm irradiation of the lowest energy cTc structure, the isomerization and unimolecular decomposition of OA were observed. Dissociation was induced presumably by absorption into the 5Ag + Bu vibrational state of the OH stretching mode of cTc. The photodecomposition leads to the formation of CO, CO2, and H2O products. The experimental observations were supported by computational studies and vibrational anharmonic calculations.

1. INTRODUCTION Unimolecular chemical reactions taking place within the ground electronic state, especially those induced by high-overtone and combination mode excitation, are interesting from the atmospheric chemistry point of view.1−3 The reactions are initiated by promoting a molecule to chemically relevant energies in highly excited vibrational states through thermal excitation or direct absorption of visible or near-infrared radiation. The advantage of the reactions initiated by direct absorption of a photon over thermally induced processes is that the former phenomenon can be treated without the need to explicitly consider the collisional activation processes. In general, it is required that the energy is deposited into the initially excited vibrational state and subsequently transferred by intramolecular vibrational redistribution (IVR) to other modes of the molecule including the reaction coordinate.4,5 Among the unimolecular reactions driven by high-overtone pumping, one can distinguish two main types of processes: unimolecular decomposition, where a sufficient amount of excitation energy is transferred into a weak bond in a molecule to result in its dissociation1,3,6−8 and isomerization. Here an energetically favorable vibrational mode acts as a reaction coordinate.9,10 Despite the fact that the cross sections for high overtone absorption are small, current advances in experimental techniques enable studies of such reactions. Molecules containing an OH-chromophore are the most convenient targets for these experimental studies, because the OH stretching vibrational transitions are characterized by higher frequencies and larger cross sections than C−H, S−H, and N− H vibrational transitions.11−13 In turn, the ν(OH) combination © 2013 American Chemical Society

modes have been observed with equally significant absorption intensities in the NIR as ν(OH) overtones.2 High-overtonedriven photodecomposition reactions have been successfully demonstrated for both organic8,14,15 and inorganic16−19 compounds, and a number of isomerization processes initiated by high-overtone pumping have also been reported.9,10,20,21 Conformational isomerization represents the particular case of isomerization processes characterized by an internal rotation around a single bond of the molecule. Here the reaction coordinate is essentially described by a torsional mode associated with a particular bond. Usually, due to the low barriers the conformers interconvert fast at ambient temperatures (Boltzmann equilibrium) and one has to cool the sample to trap high-energy species. This cooling can be achieved either by the supersonic jet method or by matrix isolation.22 There are many examples of isomerization processes induced by fundamental, combination, or first-overtone excitation leading to the formation of less stable conformers isolated in solid rare gases.23−27 Recently, we introduced a new method by which unimolecular reactions on the ground electronic state can be conveniently studied in the solid state. By combining high overtone excitation and probing by Raman spectroscopy with the same laser source, we were able to monitor the isomerization of formic acid (FA) in an argon matrix upon excitation of the fifth overtone of the νOH mode.28 Following this initial success, we now test the generality of the method by Received: November 29, 2012 Revised: January 16, 2013 Published: January 16, 2013 1492

dx.doi.org/10.1021/jp311749z | J. Phys. Chem. A 2013, 117, 1492−1502

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Figure 1. B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ (in parentheses) calculated energies of conformers of oxalic acid molecule. The cTc conformer represents the global minimum.

CO2 and HCOOH were the main products in the temperature range 120−180 °C.44 In turn, Kakumoto et al.45 found that thermal decomposition of OA vapor diluted in Ar at 577−1020 °C leads to the CO2, CO, and H2O products. This outcome was inconsistent with theoretical results wherein the barrier for the direct (one-step) decomposition to CO2 + HCOOH products is about 146 kJ/mol higher than the barriers leading directly to the CO2 + CO + H2O.38,45 Therefore, the authors considered two-step reactions with dihydroxycarbene (DHC) as an intermediate product. These mechanisms were recently confirmed by Chang et al. by high level calculations on the kinetics and mechanism of thermal decomposition of OA including the role of its isomerization in this process.46 The authors found two primary decomposition channels of OA leading to the formation of CO2 + DHC (with the subsequent DHC → FA isomerization) with the barriers of 138−150 kJ/ mol and CO2 + CO + H2O with barrier of 163 kJ/mol. At high temperatures the latter process becomes more competitive. Considering possible reactions taking place in the atmosphere, it is very important to determine the unimolecular reactions occurring upon the irradiation of the OA molecule with visible light and by excitation of the molecule to the high vibrational states. To the best of our knowledge there are no reports in the literature on the visible-light-driven isomerization or decomposition of the OA molecule. In this paper we present results on photoizomerization and unimolecular decomposition reactions observed for the OA molecule isolated in an argon matrix. Photons at 532 nm were used to induce the process and to probe it by Raman spectroscopy.28

investigating high overtone induced chemistry of OA in solid argon. OA is the smallest member in the group of dicarboxylic acids, which are ubiquitous in atmospheric aerosols.29 OA is considered to be a byproduct of fossil fuel combustion, biomass burning, and biogenic activity and has been shown in many studies to be the most abundant dicarboxylic acid in tropospheric aerosols.30 In crystalline form OA is described by an orthorhombic αphase and a metastable monoclinic β-phase.31−33 In both forms the individual molecules exhibit trans−trans−trans (tTt) conformation with both OCCO and HOCC dihedral angles equal to 180° (Figure 1).31,32 On the other hand, in the gas phase the most abundant form is the cTc conformer characterized by both H−OOC dihedral angles adopting the cis configuration with involvement in an intramolecular hydrogen bond and a OCCO dihedral angle equal to 180°.34 The cTc structure is also a predominant one in inert gas matrices at low temperatures.35−37 According to high-level theoretical studies, five conformers of OA have local minima.38−40 The lowest energy structure is represented by the cTc conformer, in agreement with experimental results obtained in the gas phase and in low temperature matrices. In the IR spectrum of OA isolated in rare gas solids the existence of a second low energy structure with cTt arrangement was also confirmed.36,37 The calculations predicted the tTt, tCt, and cCt species to be the third, fourth, and fifth low-energy conformers, respectively.41,40 Nevertheless, these conformers have not been observed in freshly deposited matrices.35−37 Irradiation with UV light of OA isolated in Ne, Ar, and Xe matrices was found to induce a cTc-to-cTt isomerization, and exposure to IR irradiation above 2000 cm−1 initiated the reverse process (cTt → cTc rotameric change).36 The third stable conformer of isolated OA (tTt) was identified by Maçôas et al. in argon matrices upon narrow band tunable irradiation in the 6800−6700 cm−1 region.37 The authors used selective IR pumping to promote conformational interconversion between cTc, cTt, and tTt rotamers.37 Unimolecular decomposition of OA by thermal excitation has attracted the attention of experimentalists for a long time.42−45 According to Lapidus et al. equimolar quantities of

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS The matrix samples were prepared by passing high purity argon (Aga, 99.9999%) through a U-tube made of steel that contained anhydrous OA (Aldrich, ≥99.00%) heated to 90 °C. The matrix-to-sample ratio could not be determined, but monomeric samples could be obtained by optimizing the temperature and the gas flow rate. The gaseous mixture was deposited onto a thin (150 μm) sapphire substrate kept at 14 K in a closed cycle cryostat (APD Cryogenics) equipped with quartz windows. The temperature 1493


1494

3424 1812 1787 1405 1295

1174

1167 826 820

639 636 667 543 460 405 259 122

3637 1847 1823 1429 1328

1218

1201 839 821

709 705 674 560 466 404 258 123

0 191 0 521 0 743 0 0

0 37 889

1433

1167 0 1642 156 0

0

Raman intensities

714 709 667 565 466 409 262 126

1211 835 830

1229

3641 1833 1801 1457 1336

3646

harmonic

676 669 659 544 458 406 258 121

1177 827 824

1189

3443 1802 1768 1416 1288

3450

anharmonic

MP2/AVTZ

1 136 0 585 0 558 0 0

0 31 1252

1214

1185 0 2033 82 0

0

Raman intensities

264e n.o.

459/458

657/656

622 460 405 264

669/664

1173

1321−1315 1267−1259*

1812/1810

3461/3459 3457/3453

Ar matrix (IR data)d

666

815

1278

1195

1826 1800c 1423 1330

3484

b

gas phase (IR and Raman)

405.5/399.0

545.0

676.0

n.o. 822.5 804.0* (FR)

1213/1201/1196 1138.0* (FR)

1793.0 1429.5

3452.0f

Ar matrix (Raman data, this work)

experimental

Ag Ag Bu Bg Ag Ag Au Bg Bu Ag Au Ag Bu Au

Ag Bu Ag Ag Bu

Bu

assignment

δCOH + νCO τOH (Au) + δopOCO (Au) νCO + δCOH δopOCO νCC 2δipCCO (Ag) τOH τOH δipOCO δipCCO δopOCO δipOCO δipCCO τCC

νOH νCO νCO νCO + δCOH δCOH + νCO

νOH

n.o. = not observed (see the text). bIR data from ref 35. cRaman data from ref 34. dIR data from ref 37. eIR data (Ne matrix) from ref 35. fBold wavenumbers are discussed in the text.

3427

3640

a

anharmonic

harmonic

B3LYP/AVTZ

Table 1. Observed and Calculated B3LYP/aug-cc-pVTZ Frequencies of the cTc Conformer of Oxalic Acida

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(cTc) exhibits C2h symmetry and is characterized by two intramolecular hydrogen bonds. A second structure (cTt form) is higher in energy by around 12 kJ/mol including the ZPVE correction, as compared with the cTc conformer. The cTt form is stabilized by one intramolecular hydrogen bond and shows Cs symmetry. Another centrosymmetric structure (tTt) is 19.65 kJ/mol (B3LYP) to 18.90 kJ/mol (MP2) higher in energy than the cTc conformer. This conformer does not have any intramolecular hydrogen bonds. As can be seen in Figure 1, the cis-arrangement of the carboxylic acid groups is not a favorable one, and the structures characterized by this arrangement are highest in energy (see conformers cCt and tCt in Figure 1). It is interesting to note that at a lower level of theory the conformer characterized by the cGc configuration appears to have an energy minimum with the energy about 60 kJ/mol higher than the cTc conformer.37−39,41 High-level calculations reveal that the cGc structure does not correspond to a true energy minimum. In agreement with earlier studies, our theoretical calculation with the aug-cc-pVTZ basis set indicates the cGc conformer to be a transition state. According to the calculated energies, in the gas phase at 353 K (the temperature of the sample of the U-tube) the relative abundance of particular conformers is cTc:cTt:tTt:tCt:cCt = 95.5:3.5:0.52:0.40:0.028. These numbers agree quite well with previously reported data,37 indicating that OA exists preferentially in the lowest energy structure cTc at 353 K. In the Raman spectrum of a freshly deposited OA/Ar matrix, recorded with 500 mW output laser power, the most intense bands are due to the cTc form (Table 1 and Figures 2−4). On the basis of the earlier gas phase Raman data34 and our theoretical calculations of harmonic and anharmonic frequencies, we were able to identify and assign eight out of nine Raman active modes of the centrosymmetric cTc conformer of

of 14 K avoided dimerization processes and gave good quality matrices, as is very important for Raman measurements. For obtaining an optically acceptable matrix the deposition rate was maintained below 0.15 mmol/min, and the total amount of the deposited gas was about 55 mmol. The thickness of optically clear matrices was ca. 50 μm. The sample temperature was maintained with a Lakeshore 330 temperature controller equipped with a silicon diode and a resistive heater. Raman measurements were performed using backscattering geometry as described in ref 47. The excitation laser was a solid-state diode-pumped laser (Verdi, Coherent) operating at 532.0 nm, and the power level was 350 mW to 3.5 W at the sample. The laser beam was focused on a sample with a 12 cm focal length achromatic lens. The Rayleigh scattering was attenuated with an edge filter (Semrock, laser line blocking: OD > 6). The Raman scattering from a sample was collected on a spectrograph’s slit with a 15 cm focal length achromatic lens. The slit width used for all measurements was 60 μm. The Raman signal was dispersed with a 50 cm focal length imaging spectrograph equipped with 600 and 1200 grooves/mm ruled gratings and a 2400 grooves/mm holographic grating. The Raman spectra were recorded with a CCD camera (Andor Technology, Newton) mounted on the spectrograph (Acton Research Corporation, SpectraPro 2500i). All calculations were carried out at the MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ levels using the Gaussian 09 package of computer codes.48 At the optimized geometries, the harmonic vibrational wavenumbers, IR intensities, and Raman activities obtained at the harmonic level were calculated to assist the analysis of the experimental spectra. The anharmonic wavenumbers were obtained by Barone’s method49 as implemented in Gaussian 09. However, this approach does not produce the anharmonic Raman activity values. The theoretical Raman intensities (IR) were calculated according to the formula:50 IiR =

(2π )4 h ·Si (ν0̅ − νi̅ )4 ·νi̅ −1· 2 45 8π cνi̅ Bi

(1)

where Si is the Raman scattering activity [in Å4/amu] of the normal mode Qi calculated by the DFT method, νi̅ is the calculated vibrational wavenumber [in cm−1] of the ith normal mode, Bi is a temperature factor which accounts for the intensity contribution of excited vibrational states and is represented by the Boltzmann distribution: ⎛ hν c ⎞ Bi = 1 − exp⎜ − i̅ ⎟ ⎝ kT ⎠

(2)

The Bi factor has been included for T = 14 K, which is in agreement with experimental conditions and additionally makes the value of this factor roughly equal to 1. The excitation frequency ν̅0 used in this work was equal to 18 797 cm−1 (532 nm).

Figure 2. 1850−1100 cm−1 spectral region of the Raman spectrum for oxalic acid (OA). Top frame: Raman spectra of OA/Ar matrix at 19 K: (A) freshly deposited matrix; (B)matrix after 3 h of irradiation with Raman laser (532 nm). Bottom frame: spectra of cTc (red), cTt (black), and tTt (blue) conformers of OA at the B3LYP/aug-cc-pVTZ level of theory. Calculated harmonic intensities for each species were scaled by different factors, to approximately simulate the corresponding observed intensities in the spectrum of irradiated matrix. The calculated wavenumbers are computed the anharmonic level. The spectra were simulated using Lorentzian functions centered at the calculated anharmonic frequencies and bandwidth-at-half-height equal to 1 cm−1.

3. RESULTS AND DISCUSSION 3.1. Conformers. In agreement with previous calculations,39,40 we found five structures with energy minima existing on the potential energy surface (PES) of the OA molecule. The energies calculated at the B3LYP/aug-cc-pVTZ and MP2/augcc-pVTZ levels and corresponding structures are shown in Figure 1. The optimized structures were the starting point for calculations of harmonic and anharmonic frequencies as well as Raman activities and IR intensities. The lowest energy structure 1495



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theoretical modeling and simulation of the spectra of the FA centrosymmetric dimer isolated in an Ar matrix.52 The Fermi triad observed in the gas phase was shown to be diminished upon matrix isolation due to detuning of relevant levels because of a significant shift of one of the modes in the matrix environment. We believe that an opposite effect can also be found. In the region under discussion of the IR spectra of an OA/Ar sample a similar splitting due to the FR was also reported.37 Analogously, the bands at 822.5 and 804.0 cm−1 (Figure 3) are ascribed to a FR doublet, resulting from interaction between the fundamental ν(CC)Ag vibration and δip(CCO) first overtone (Ag). As we have mentioned before, according to the theoretical predictions, weak bands due to the cTt and tTt conformers should appear in the spectra of a freshly deposited matrix. These peaks are expected to be mostly visible in the region of 850−700 cm−1 (Figure 3 and Tables 2 and 3) because some of the highest Raman intensities is predicted for the cTt and tTt bands in this spectral region. During Raman measurements and particularly upon increasing the Raman laser power up to 4 W, a significant decrease in the intensities of the bands of the cTc conformer and increase in the cTt and tTt bands was observed. By a comparison of the Raman spectra with the previously reported IR spectra36,37 and by observation of the band intensity changes upon Raman laser irradiation, we were able to assign unambiguously the Raman bands of both the cTt conformer and the centrosymmetric tTt structure. The abundance of cTt and tTt conformers increases upon irradiation due to the isomerization processes discussed in the next section. In Tables 2 and 3 we present the calculated and experimental data for the cTt and tTt conformers, respectively. The experimental wavenumbers observed in our Raman spectra are compared with IR data obtained for OA isolated in an Ar matrix. The theoretical frequencies are calculated with the harmonic and the anharmonic approaches. As can be seen in Table 2, the bands observed for the cTt rotamer in our Raman spectra are in agreement with previously reported IR analysis of cTt isolated in solid Ar.37 There are some features identified in the IR spectrum of this conformer that are not observed in the Raman spectra (e.g., νas(CO), δop,as(OCO), etc.). However, these bands are predicted to have very low Raman intensities. In addition, our experimental conditions allowed detecting and assigning the bands appearing below 500 cm−1, which were not reported previously. The feature at 420 cm−1 is assigned to the in-plane symmetric bending motion of the OCO moiety, whereas the 261 cm−1 band is assigned to the in-plane bending mode of the CCO group (Figure 4 and Table 2). According to the anharmonic theoretical predictions, two other peaks belonging to the δop,s(OCO) and τ(CC) modes should lie at ca. 433 and 77 cm−1, respectively. These features were not detected in our spectra probably because the Raman intensities, as predicted, are very small. In turn, there are two bands observed in our spectrum assigned to the νCO + δCOH and δasCOH + νasCO, modes that are shifted by ca. −10 and +15 cm−1 as are the results of Maçôas et al.37 These differences can be caused by the cTt conformer being formed according to a somewhat different mechanism of isomerization in our experiment compared with the previous studies.37 Eight out of nine Raman active modes of the tTt conformer were identified in the spectra (Table 3). The only feature not observed in our spectra belongs to the out-of-plane bending

Figure 3. 850−500 cm−1 region of the same spectra as presented in Figure 2. S denotes the bands due to the sapphire window. cTc* is the band due the cTc overtone (see text).

Figure 4. 440−250 cm−1 region of the same spectra as presented in Figure 2. S denotes the bands due to the sapphire window.

OA. The comparison between the experimental and calculated frequencies shows good agreement. As can be seen, the Raman spectra presented here nicely complement the infrared data reported by Maçôas et al.37 Similarly, the matrix data coincide with the gas phase results,34 exhibiting only modest and typical frequency shifts due to the matrix environment.51 Nevertheless, there are two new observations that are discussed below. As can be seen in Figure 2 and Table 1, the peak at 1138 cm−1 marked as cTc* appears in addition to the group of fundamental bands at 1213/1201/1196 cm−1 (marked as cTc, split because of a matrix site effect). The cTc* band can plausibly be assigned to the τ(OH)Au + δop(OCO)Au mode. The enhanced intensity of this combination band could be a result of Fermi resonance (FR) with δCOH. In the Raman spectra of the gaseous sample34 only one band at 1195 cm−1 was observed and assigned to the δ(COH) + ν(CC) mode. The feature corresponding to the 1138 cm−1 peak in an Ar matrix could be not observed in the gas phase for two reasons. First, the bands observed in the spectra of the matrix-isolated species are much narrower as compared with the gas phase spectra,51 thereby allowing better observation of close lying bands a second reason could be related to the interaction of the guest molecule with the surroundings. Recently, Ito reported 1496



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Table 2. Observed and Calculated B3LYP/aug-cc-pVTZ Frequencies of cTt Conformer of Oxalic Acida B3LYP/AVTZ

MP2/AVTZ

harmonic

anharmonic

Raman intensities

3728 3660 1863 1787 1421 1331 1201 1163 839 800 682 656 654 544 439 420 265 87

3532 3448 1833 1757 1381 1280 1150 1126 824 781 604 650 659 548 433 414 264 77

1407 629 836 841 128 101 717 83 31 1387 95 115 198 407 83 690 133 29

harmonic

anharmonic

Raman intensities

Ar matrix (IR data)b

Ar matrix (Raman data, this work)

3740 3672 1837 1777 1450 1336 1210 1171 810 835 684 658 657 549 442 425 266 89

3558 3479 1802 1746 1412 1328 1160 1134 793 827 624 656 656 551 431 418 262 82

1178 654 809 1130 156 118 662 38 26 1554 74 146 182 534 54 535 106 14

3549 3484 1834 1762 1416 1319/1213 (FR) 1161 1137 799 796 650 633 621 558

3549.1 3481.0 1824.4 1761.5/1763.0 1415.0 n.o. 1179.0 n.o. n.o. 796 n.o. n.o. (S) 624 552 n.o. 420c 261 n.o.

assignment A′ A′ A′ A′ A′ A′ A′ A′ A″ A′ A″ A″ A′ A′ A″ A′ A′ A″

νOH′ νOH νCO + δCOH νCO′ + δCOH′ νsCO + νCC + δCOH δsCOH + νsCO δasCOH + νasCO νasCO δop,asOCO νCC τOH δip,asOCO τOH′ δipCCO δop,sOCO δip,sOCO δipCCO τCC

a

n.o. = not observed (see the text). (S) = the band overlapped with sapphire window band (Figure 2). bFrom ref 37. cBold wavenumbers are discussed in the text.

Table 3. Observed and Calculated B3LYP/aug-cc-pVTZ Frequencies for tTt Conformer of Oxalic Acida B3LYP/AVTZ harmonic

anharmonic

3739 3739 1827 1819 1402 1327 1200 1135 839 784 669 638 618 527 423 422 265 11

3533 3532 1801 1795 1371 1296 1137 1092 821 764 638 628 564 522 411 452 260

a

MP2/AVTZ Raman intensities

harmonic

0 2620 1634 0 354 0 351 0 37 2009 0 0 494 359 699 0 0 0

3747 3748 1809 1799 1432 1333 1203 1142 835 795 672 623 626 532 428 426 263 24

experimental

anharmonic

Raman intensities

Ar matrix (IR data)a

3565 3566 1774 1770 1390 1309 1152 1107 823 778 647 627 594 538 421 408 261 28

0 2306 1837 0 304 0 354 0 25 2109 0 0 347 501 610 0 0 0

3565

Ar matrix (Raman data, this work) 3568.0b 1797.5

1781 1378.2 1133.0 1112 n.o. 782.0 641 636 564 538 426

assignment Bu Ag Ag Bu Ag Bu Ag Bu Bg Ag Au Bu Bg Ag Ag Au Bu Au

νOH νOH νCO + δCOH νCO + δCOH δCOH + νCO δCOH + νCO νCO + δCOH νCO + δCOH δopOCO δipOCO τOH δipOCO τOH δipCCO νCC δopOCO δipCCO τCC

n.o. = not observed (see the text). aFrom ref 37. bBold wavenumbers are discussed in the text.

vibration of free OH groups in the tTt structure. In turn, the difference between the frequencies of the νOH mode of free and hydrogen-bonded OH group in the cTt conformer is ca. 68 cm−1, which is almost 50% smaller than observed for cTc (see experimental values in Tables 1−3). It is interesting to consider the anharmonic data included in Tables 1−3. For the high-frequency region corresponding to the O−H stretching vibrations the MP2 calculations almost perfectly reproduce the experimental data. This region is less satisfactorily described by the B3LYP method. In turn, the DFT approach does very well for lower-frequency vibrations,

mode of the OCO moiety, which is computed to have very low Raman activity. The most intense tTt bands, on the other hand, are observed in the ν(OH) and δip(OCO) region. It is worth mentioning that the band due to the ν(CO) mode (1797.5 cm−1) partially overlaps with the band of the same mode in the most stable cTc conformer (1793 cm−1, Table 1), and it is visible as a shoulder of the cTc feature (Figure 2). It is interesting to notice evidence of the stabilizing intramolecular cooperative effect observed in the OA molecule. The wavenumber of the in-phase stretching mode of the OH group in the cTc conformer is observed to be ca. 116 cm−1 lower than the wavenumber of the symmetric stretching 1497



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values can differ from the experimental values according to the computational accuracy and experimental perturbation of OA structure by the embedding condensed phase. Another argument considers the broadening of the OH overtone’s transition line width. Recently, it has been reported that for the systems with an intramolecular hydrogen bond an anomalous peak width for νOH overtones is observed. The FWHM value increases with the hydrogen bond strength and with the increasing overtone steps.53,54 For pyruvic acid (PA), for which the intramolecular hydrogen bond shows a strength comparable with the cTc conformer of OA, the observed FWHM for the third overtone is equal to 414 cm−1.15 That makes the excitation of the 3ν(OH)Ag + 3ν(OH)Bu transition of OA by 532 nm irradiation still possible despite the estimated position of the center of the discussed feature as presented above. The tTt conformer formation can occur because of two possible isomerization processes. The first channel is provided by excitation of the above-mentioned combination transitions of the cTc and simultaneous rotation of both CO bonds, which was also observed by Maçôas et al. upon excitation of the ν(OH)Ag + ν(OH)Bu combination mode.37 Another possibility for tTt conformer formation is a high-overtone or a combination-mode induced isomerization of the cTt conformer at the CO bond in the CO−H moiety involved in an intramolecular hydrogen bond. The estimated wavenumber of the fifth overtone transition equals 18 588 cm−1 for the nonhydrogen bonded group and 17 805 cm−1 for the group involved in intramolecular hydrogen bonding. In the Supporting Information further information on how the overtones wavenumber were calculated is provided. As can be seen with these values, the 532 nm (18 797 cm−1) radiation falls very close to the 6 ← 0 resonance of the nonhydrogen bonded OH group and is ca. 1000 cm−1 away from the resonance with the hydrogen bonded OH moiety. Nevertheless, the excitation of the 5ν(OH)HB + ν(OH)free combination needs an energy equal to 18 898 cm−1, which is very close to the energy provided by the laser used. Additionally, taking into account that the broadening of the OH overtones transition line width occurs for intramolecularly hydrogen bonded systems, the excitation of the abovementioned combination transition is very probable. The above considerations indicate that in our system both cTt → cTc and cTt → tTt isomerizations can take place. Therefore, the tTt conformer can be formed from both cTc and cTt isomers. In Figure 5, the kinetic profile of the tTt isomer shows a fast increase for up to 20 min of irradiation time as is also observed for the cTt conformer. Nevertheless, similarly to the cTt system the possibility of the tTt internal isomerization to other observed conformers (cTc and cTt) under our experimental conditions should be considered here. By analogy to the centrosymmetric cTc conformer for the tTt structure, the transitions upon consideration are 5ν(OH)Ag + ν(OH)Bu, ν(OH)Ag + 5ν(OH)Bu, and 3ν(OH)Ag + 3ν(OH)Bu. With the anharmonicity values presented and discussed in the Supporting Information the transition closest to the laser line (18 797 cm−1) is the 3ν(OH)Ag + 3ν(OH)Bu combination. The calculated frequency for this vibration is equal to ca. 19 400 cm−1. This value still seems to be a bit away from the resonance. Nevertheless, we have to be aware that the calculations are performed for the gas phase molecule and that the interactions with a rigid matrix are not included here. Additionally, the broadening of the line of the overtone

especially for carbonyl group and the stretching motions of the CC or CO moieties. 3.2. Isomerization Processes. Upon 532 nm laser irradiation a significant decrease of the intensities of the bands due to the cTc conformer and an increase of the intensities of the cTt and tTt bands were observed (Figures 2−4 and kinetic profile in Figure 5), clearly indicating that

Figure 5. Irradiation kinetics for the OA/Ar system. Variations of the normalized Raman scattering intensity of cTc, cTt, tTt, and CO2 bands as a function of irradiation time T.

isomerization takes place. Previously, irradiation of a cTc sample with broad-band IR or UV light led to the light-induced isomerization of the lowest energy cTc form into the cTt conformer.36 In turn, narrow-band tunable irradiation in the near-infrared region allowed observation and spectroscopical characterization, in addition to the cTc and cTt conformers, of the third structure, the tTt rotamer.37 In Figure 5 the changes in the intensities of the bands belonging to the cTc, cTt, and tTt isomers are presented. For the cTc isomer the intensity changes of two bands have been followed. The band at 1793 cm−1 is assigned to the ν(CO) vibration, and the feature at 1201 cm−1 is due to the δ(COH) + ν(CO) mode. Clearly, the intensity of the latter band decreases much faster than the 1793 cm−1 feature. This behavior is explained by the band for the ν(CO) vibration overlapping with the band at 1797 cm−1, which belongs to the tTt conformer and increases in intensity upon irradiation. In the present study, the plausible origin for isomerization to the less stable cTt and tTt structures is the high overtone or combination band excitation. Because of the C2h symmetry of the cTc conformer the overtone transitions induced by a single photon to even states are forbidden. Therefore, the only plausible symmetry-allowed transitions are 5ν(OH)Ag + ν(OH)Bu, ν(OH)Ag + 5ν(OH)Bu, 3ν(OH)Ag + 3ν(OH)Bu. The estimated wavenumber of the 3ν(OH)Ag + 3ν(OH)Bu transition is equal to 18 366 cm−1, whereas for ν(OH)Ag + 5ν(OH)Bu it is 18 748 cm−1 and for 5ν(OH)Ag + ν(OH)Bu it is 18 720 cm−1. For the details on how the particular transitions wavenumber were estimated, see the Supporting Information. As can be seen, the transition energies obtained for 5 + 1 (or 1 + 5) combinations are very close to the 532 nm radiation (18 797 cm−1). Nevertheless, the possibility of excitation of 3ν(OH)Ag + 3ν(OH)Bu in our experiment cannot be ruled out, because of the following facts. First, the estimated values are theoretical ones based on gas phase calculations. Such 1498



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transition has to be considered. Of course, this effect is not as significant as is observed for a hydrogen bonded moiety, but still it can take place. Therefore, it is plausible that upon 532 nm irradiation the rotamerization of tTt to cTt can take place. Moreover, the simultaneous rotation around both CO bonds leading to the cTc conformer is also probable. Taking into account the facts presented above, we can say that it is possible that all three OA conformers can interchange, and within 20 min of irradiation a photostationary state is established. As can be seen in Figure 5, after this period the slow decrease of all the conformers is observed. This outcome will be discussed in the next section. The role of the dark processes in the system was followed by keeping the matrix in darkness for 3 h after initial irradiation. The low power (500 mW) of the Raman laser was used for the measurements of the spectra after the dark phase process to minimize additional overtone pumping. The relative intensities of cTc, cTt, and tTt conformers did not change during that period, indicating that the conformational isomerization by intramolecular tunneling does not occur or is very slow. This behavior is different from (FA), acetic acid (AA), and propionic acid (PA), for which tunneling decay of the high energy conformer in a time scale of minutes was observed.55−57 This difference can be explained by comparison of the isomerization barrier parameters (height and width). According to our calculations, the barrier for cis → trans (in the case of FA and AA) and cTt → cTc (in the case of OA) transition differs significantly (Figure 6). The barrier for OA is ca. 5170 cm−1,

where θ (ε ) =

1 ℏ

x1

∫x2

2μ[V (x) − ε] dx

μ is the reduced mass for the torsional mode (from ab initio calculations) and the integral is the area under the isomerization barrier (integrated barrier height). x1 and x2 are the turning points where V(x) = ε. The tunneling rate constant at an energy ε is expressed by the equation ω k WBK = 0 P(ε) 2π where ω0 in our case is the torsional frequency of the less stable conformer. According to our calculations the tunneling rates (1/s) for AA and FA are approximately 2 orders of magnitude smaller than that obtained experimentally.59 Moreover, for AA we got the tunneling rate value equal to 1.37 × 10−4 s−1, whereas for FA 2.2 × 10−5 s−1, that gives the relative AA/FA values very close to the relative experimental results (AA, 2 × 10−2 s−1; FA, 2 × 10−3 s−1).59 These results indicate that the theoretical approach is reasonable even though we neglect interaction with the environment and internal mode coupling. In turn, the calculated tunneling rate obtained for cTt → cTc conversion for OA is ca. 19 orders of magnitude lower than in the FA case, a result which explains the experimentally observed stability of cTt. 3.3. Unimolecular Decomposition Reaction. Upon irradiation of the OA/Ar sample the bands due to the symmetric stretching vibration of CO2 molecule at 1381 and 1279 cm−1 (splitting due to the FR with the 2δCO2 mode) appear. The observed features are blue-shifted by 5 and 1.8 cm−1, respectively, as compared with the bands observed for the CO2 monomer isolated in solid argon. As seen in Figure 5, these bands show a steady increase during the irradiation. This process is accompanied by an exponential decrease of the cTc conformer bands. Additionally, after about 20 min of irradiation of the matrix, the cTt and tTt features also start decreasing. The growth of CO2 bands is accompanied by the appearance of new features in the region of CO and H2O vibrations at 2154 and 3628.0 cm−1, respectively. These peaks are by +13 and −14 cm−1, respectively, shifted compared with monomer bands. These observations are clear signs of unimolecular decomposition occurring in the system. Additionally, the products of the reactions are probably caught in the same matrix cage, where they are involved in three body interactions. Earlier experimental studies on gaseous OA indicated that thermal decomposition at 126−180 °C leads to the decarboxylation reaction, resulting in formic acid formation.44 The theoretical calculations show that the HCOOH + CO2 reaction channel is accessible starting from the cTt conformer and it has an energy barrier equal to 304.7 kJ/mol,46 an amount of energy that cannot be provided by the laser used (λ = 532 nm, ∼225 kJ/mol). As a result, we did not observe any traces of FA in our matrices. Additionally, in our previous work on FA isomerization we did not observe a decomposition process of FA.28 That eliminates the possibility of a sequential OA → FA → CO2 + CO + H2O reaction. According to the theoretical predictions, another possible decomposition channel is described by the concerted CC and CO bond fission of the cCt conformer followed by the formation of H2O + CO2 + CO products via a 163.8 kJ/mol energy barrier,46 which is lower than the energy provided by the

Figure 6. Intrinsic reaction paths of trans−cis conversion (for AA and FA) and cTc−cTt (for OA) conversion as obtained at the B3LYP/augcc-pVTZ levels of theory. The reference (zero) energy is set to the cis or cTt minimum.

whereas for FA and AA it is ca. 4530 and 4470 cm−1, respectively. The same trend is shown by the barrier widths. To account for the tunneling rates, we estimated theoretically the permeability P(ε) of the obtained barriers according to the standard WKB approximation, using the ab initio torsional potential (V) and the energy levels (ε) of the reactant conformers58 1 P(ε) = 1 + e 2θ(ε) 1499



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Figure 7. Supposed reaction channels observed in the studied system. aActivation energy values calculated at B3LYP/aug-cc-pVTZ level between optimized structures. bData taken from ref 46. The ZVE correction is included in all energy values.

to the formation of CO2 and W-DHC for cTc or S-DHC for cTt. Under our experimental conditions dihydroxycarbene should be formed in the same matrix cage as the CO2 molecule. We performed the B3LYP/aug-cc-pVTZ anharmonic frequency calculations of both S-DHC···CO 2 and W-DHC···CO 2 complexes. On the basis of the experimental spectroscopic data of S- and W-DHC61 and our theoretical vibrational data, we can exclude the presence of the bands due to neither DHC nor its complexes with CO2 in our spectra. This suggests that DHC decomposes in our system once it is formed. This process, in turn, results in the appearance of CO and H2O trapped in the matrix cage with CO2. As was shown in the previous section, under our experimental conditions the excitation of the 5ν(OH)Ag + ν(OH)Bu or ν(OH)Ag + 5ν(OH)Bu combination of the cTc conformer and the excitation of 5ν(OH)HB + ν(OH)free combination of the cTt conformer are very probable ones. In these transitions at least one OH moiety is involved in an intramolecular hydrogen bond, which, as we mentioned above, is needed for the decarboxylation process. Therefore, we think that in our system the first step of the two-step unimolecular decomposition reaction leading to the formation of CO2, CO and H2O molecules is induced by the excitation of the abovementioned transition of the cTc and cTt conformers. Because in the tTt conformer neither of the OH groups is involved in a hydrogen bond, the hydrogen chattering upon higher overtone excitation requires a conformational change and the decarboxylation reaction is less probable. Nevertheless, in the kinetic profile (Figure 5) a clear decrease in the intensity of the tTt conformer bands is observed after around 20 min of irradiation when the steady state between all the conformers is established. Two mechanisms can explain decrease of tTt: (i) Photoinduced isomerization to cTt and possibly further to cTc and subsequent photoinduced isomerization of cTt and/or cTc conformers. Because the kinetics in Figure 5 suggests that all three conformers are at photoequilibrium, photodissociation of any one conformer will manifest itself as an apparent photodecomposition of all the conformers. (ii) Direct onephoton induced dissociation of tTt. Despite the lack of a hydrogen bond in this form, the excitation provides enough

532 nm laser. However, this mechanism can also be excluded here. As a first step, the isomerization of cTt conformer to the cCt form is needed. This process involves the rotation of cTt structure around its CC bond. The calculated barrier for this process is relatively low and in principle reachable within our experimental conditions (ca. 25 kJ/mol).46 However, the rigidity and steric constraints of the matrix cage make this process improbable. The formation of CO2, H2O, and CO via a two-step reaction is a plausible mechanism. Both cTt and cTc conformers are predicted to undergo decarboxylation reaction, which is described as a concerted process that involves H-transfer and a CC bond fission (Figure 7). This mechanism leads to the formation of CO 2 and the dihydroxycarbene (DHC) molecule.39,46,60 The cTc decarboxylation process results in the formation of W-like form of DHC, whereas from the cTt conformer’s decarboxylation process S-like DHC is produced. Both reactions show an energy barrier of ca. 140 kJ/mol46 (Figure 7). Recently, the decarboxylation reaction of pyruvic acid (PA) induced by third and fourth overtone excitation has been studied experimentally and theoretically by Takahashi et al.15 These authors used direct dynamics calculations, where the potential was computed “on-the-fly” and all degrees of freedom were included. It was found that the first step of the decarboxylation reaction mechanism in PA is an H-atom chattering between two O-atoms involved in intramolecular hydrogen bond. The H-atom transfer process initiates energy flow into the CC stretching mode causing its bond length to gradually oscillate with a larger amplitude until cleavage occurs to produce CO2 and methylhydroxycarbene. The hydrogen chattering followed by CC bond fission does not occur in the trans conformer of PA, in which OH group is not involved in hydrogen bonding. Here the OH overtone excitation is slowly dissipated into bath modes of molecule.15 Because of the comparable intramolecular hydrogen bond strength, the PA example can be extended to the OA system characterized by two (cTc) or one (cTt) intramolecular hydrogen bonds. In these cases, the decarboxylation reaction following the H-atom chattering within hydrogen bridge leads 1500



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(2) Vaida, V.; Feierabend, K. J.; Rontu, N.; Takahashi, K. Int. J. Photoen., 2008, ID 138091 (3) Donaldson, D. J.; George, C.; Vaida, V. Environ. Sci. Technol. 2010, 44, 5321−5326. (4) Child, M. S. Acc. Chem. Res. 1985, 18, 45−50. (5) Henry, B. R.; Kjaergaard, H. G. Can. J. Chem. 2002, 80, 1635− 1642. (6) Miller, Y.; Chaban, G. M.; Finlayson-Pitts, B. J.; Gerber, R. B. J. Phys. Chem. A. 2006, 110, 5342−5354. (7) Donaldson, D. J.; Frost, G. J.; Rosenlof, K. H.; Tuck, A. F.; Vaida, V. Geophys. Res. Lett. 1997, 2651−2654. (8) Takahashi, K.; Plath, K. L.; Akson, J. L.; Nelson, G. C.; Skodje, R. T.; Vaida, V. J. Chem. Phys. 2010, 132, 094305. (9) Jasiński, J. M.; Frisoll, J. K.; Moore, C. B. J. Phys. Chem. 1983, 87, 3826−3829. (10) Snavely, D. L.; Grinevich, O.; Hassoon, S.; Snavely, G. J. J. Chem. Phys. 1996, 104, 5845−5851. (11) Kjaergaard, H. G.; Howard, D. L.; Schofield, D. P.; Robinson, T. W.; Ishiuchi, S.; Fujii, M. J. Phys. Chem., A. 2002, 106, 258−266. (12) Miller, B. J.; Mivsam, Y.; Sodergren, A. H.; Howard, D. L.; Dunn, M. E.; Vaida, V. J. Chem. Phys. A 2010, 114, 12692−12700. (13) Miller, B. J.; Howard, D. L.; Lane, J. R.; Kjaergaard, H. G.; Dunn, M .E.; Vaida, V. J. Chem. Phys. A 2009, 113, 7576−7583. (14) Renard, L. M.; Donaldson, D. J. J. Phys. Chem. A. 2002, 106, 8651−8657. (15) Takahashi, K.; Plath, K. L.; Skodje, R. T.; Vaida, V. J. Phys. Chem. A 2008, 112, 7321−7331. (16) Li, E. X. J.; Konnen, I. M.; Lester, M. I.; McCoy, A. B. J. Phys. Chem. A. 2006, 110, 5607−5613. (17) Vaida, V.; Kjaergaard, H. G.; Hintze, P. E.; Donaldson, D. J. Science 2003, 299, 1566−1568. (18) Reiche, F.; Abel, B.; Beck, R. D.; Rizzo, T. R. J. Chem. Phys. 2000, 112, 8885 (14 pages). (19) Scherer, N. F.; Zewail, A. H. J. Chem. Phys. 1987, 87, 97−114. (20) Jasinski, J. M.; Frisoli, J. K.; Moore, C. B. Faraday Discuss. Chem. Soc. 1983, 75, 289−300 and references therein. (21) Segall, J.; Zare, R. N. J. Chem. Phys. 1988, 89, 5704−5714. (22) Räsänen, M.; Kunttu, H.; Murto, J. Laser Chem. 1988, 9, 123− 146. (23) Sharma, A.; Reva, I.; Fausto, R. J. Am. Chem. Soc. 2009, 131, 8752−8753. (24) Maçôas, E. M. S.; Khriachtchev, L.; Fausto, R.; Räsänen, M. J. Phys. Chem. A 2004, 108, 3380−3389 and the references therein. (25) Maçôas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Fausto, R.; Räsänen, M. J. Phys. Chem. A 2005, 109, 3617−3625. (26) Pettersson, M.; Lundell, J.; Khriachtchev, L.; Räsänen, M. J. Am. Soc 1997, 119, 11715−11716. (27) Marushkievich, K.; Khriachtchev, L.; Räsänen, M. J. Chem. Phys. 2007, 126, 241102−04. (28) Olbert-Majkut, A.; Ahokas, J.; Lundell, J.; Pettersson, M. J. Chem. Phys. 2008, 129, 041101−03. (29) Sorooshian, A.; Varutbangkul, V.; Brechtel, F. J.; Ervens, B.; Feingold, G.; Bahreini, R.; Murphy, S. M.; Holloway, J. S.; Atlas, E. L.; Buzorius, G.; Jonsson, H.; Flagan, R. C.; Seinfeld1, J. H. J. Geoph. Res. 2006, 111 DOI: 10.1029/2005JD006880. (30) Yu, J. Z.; Huang, S. F.; Xu, J. H.; Hu, M. Environ. Sci. Technol. 2005, 39, 128−133. (31) Derissen, J. L.; Smit, P. H. Acta Cyst. B. 1974, 30, 2240−2242. (32) de Villepin, J.; Novak, A.; Bougeard, D. Chem. Phys. 1982, 73, 291−312. (33) Boczar, M.; Kurczab, R.; Wójcik, M. J. Vibr. Spectr. 2010, 52, 39−47. (34) Stace, B. C.; Oralratmanee, C. J. Mol. Struct. 1973, 18, 339−341. (35) Redington, R. L.; Redington, T. E. J. Mol. Struct. 1978, 48, 165− 176. (36) Nieminen, J.; Räsänen, M.; Murto, J. J. Phys. Chem. 1992, 96, 5303−5308. (37) Maçôas, E. M. S.; Fausto, R.; Pettersson, M.; Khriachtchev, L.; Räsänen, M. J. Phys. Chem. A 2000, 104, 6956−6961.

energy to drive isomerization and subsequent decomposition without the need for absorption of another photon.

4. CONCLUSIONS Raman spectra of oxalic acid isolated in an argon matrix have been recorded and analyzed for the first time. The experimental data were supported by anharmonic calculations performed at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels. Upon 532 nm irradiation, formation of the higher energy conformers (cTt and tTt) is observed. The isomerization process is promoted most probably by the excitation of the 5ν(OH)Ag + ν(OH)Bu and ν(OH)Ag + 5ν(OH)Bu combination bands of the cTc conformer. The cTc structure can convert to cTt or tTt conformer via rotation of one or both OH groups, respectively. The cTt → cTc isomerization process, via excitation of the fifth overtone on (OH)free moiety is evidenced to be probable. This process is accompanied by the cTt → tTt conversion induced by the 5ν(OH)HB + ν(OH)free combination band. According to the experimental observations, there is no tunneling process leading to the most stable structure (cTc) characterized by intramolecular hydrogen bonds. The performed calculations confirmed the experimental observations, showing that the tunneling process rate is 19 orders of magnitude lower than the one reported for formic acid system, making it very improbable. The unimolecular decomposition leading to the formation of CO2, CO, and H2O takes place in our experimental system upon irradiation with the Raman excitation laser (532 nm). The suggested first step is the simultaneous CC bond fission and hydrogen atom transfer between OH and CO groups in the cTc conformer. The decomposition of the cTc conformer is promoted probably by 5ν(OH)Ag + ν(OH)Bu and ν(OH)Ag + 5ν(OH)Bu transitions. The excitation of the 5ν(OH)HB + ν(OH)free combination band is considered as an origin of the decarboxylation process of the cTt conformer. The tTt conformer could decompose upon the 3ν(OH)Ag + 3ν(OH)Bu band excitation after internal rotation to cTt and a subsequent decarboxylation reaction in the same process.

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ASSOCIATED CONTENT

S Supporting Information *

Description of the cTc, cTt, and tTt isomerization processes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The allocation of computer time from the Wrocław Center for Networking and Supercomputing (Technical University, Wrocław, Poland) and the Center for Scientific Computing Ltd. (Espoo, Finland) are gratefully acknowledged.

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REFERENCES

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Visible Light-Driven Chemistry of Oxalic Acid in Solid Argon, Probed

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