Photochemistry of Pyrene with Water at Low Temperature: Study of

ARTICLE pubs.acs.org/JPCA

Photochemistry of Pyrene with Water at Low Temperature: Study of Atmospherical and Astrochemical Interest Zohra Guennoun, Christian Aupetit, and Jo€elle Mascetti* Institut des Sciences Moleculaires, Universite de Bordeaux, UMR 5255 CNRS 351, Cours de la Liberation, 33405 Talence cedex, France ABSTRACT: Photochemistry of a polyaromatic hydrocarbon, pyrene C16H10, with water has been investigated at cryogenic temperatures. Photoprocessing of this species, performed at λ > 235 nm, in argon matrices, adsorbed onto amorphous water surfaces, and trapped in solid water, led to the formation of ketonic isomers, C16H10O, and possibly quinones. These species have been identified for the first time by infrared spectroscopy with the support of isotopic substitution experiments and DFT calculations. These oxidized pyrene-like species, of atmospherical and astrochemical interest, most likely arise from a tautomeric rearrangement of their analogous hydroxylated molecules, these latter being formed by reaction of water with pyrene cations.

1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) and their substituted derivatives form a class of molecules which are atmospherically relevant because of their significant abundance, which is due to their efficient formation as byproducts of natural processes, such as biomass burning, or as byproducts of human activities, such as combustion of fossil fuels.1 In the atmosphere, PAHs with less than three rings are partitioned between the gas and solid phases due to their relatively higher vapor pressure,2 while PAHs with more than three rings are adsorbed onto particulate matter3,4 which is mainly composed of carbonaceous aerosols.5,6 The important portion of PAHs that are present on atmospheric particles shows the relevance to better understand their fate as, because of their extremely low solubility and hydrophobic nature, most PAHs are predominantly associated with particulate matter. Indeed, during their transport in the troposphere, they are exposed to atmospheric trace gases (NOx, SOx, OH, O3) and may undergo photochemical degradation forming byproducts such as nitro-PAHs, hydroxy-PAHs, or quinones.7-9 These compounds constitute a group of harmful mutagenic and carcinogenic micropollutants2,3,10-12 and are ubiquitous in the atmospheres of most regions of the world. Because of their toxic and potential carcinogenic nature, research has been undertaken to study their fate when exposed to sunlight. Subsequent reaction products need to be identified in order to assess their atmospheric impact and their potential effects on human health. In addition, PAHs are now generally acknowledged as an important source of the unidentified infrared bands (UIRs), which were detected for the first time in the early 1970s.13-15 These emission features are found to originate in a variety of regions in the interstellar medium.16 These bands can carry up to 40% of the infrared luminosity of a galaxy, and their intensity indicates that PAHs represent the most abundant interstellar polyatomic molecules known accounting for ca. 20% of the cosmic carbon.14,17,18 Over the last 25 years, many observational,13,19,20 experimental,21-26 and theoretical27-30 studies have been devoted to r 2011 American Chemical Society

PAHs as they are believed to play an important role in the physics and chemistry of the interstellar medium. These studies have shown that the mid-infrared interstellar emission features are due to a mixture of highly vibrationally excited polycyclic aromatic hydrocarbons, ionic PAHs, hydrogenated and dehydrogenated PAHs, nitrogen and silicon containing PAHs, PAH-metal ion adducts, and PAH clusters.31,32 Although not unambiguously identified, PAHs could be present in dense molecular clouds where they are likely to be frozen out into water-dominant ice mantles covering cold interstellar dust grains,33,34 thus contributing to the chemistry and physics taking place in ice which is governed by dynamic processes such as accretion/sublimation, barrier-less chemical reactions, and energetic processing by UV and cosmic ray particles.35,36 Photoprocessing of PAHs containing water-rich ices has been the subject of many in situ studies. Reactivity of small PAHs, such as naphthalene37 or anthracene,38 and larger ones (pyrene,39,40 4-methylpyrene,41 coronene,42,43 quaterrylene26,44), embedded in frozen water ices and subsequently exposed to energetic radiation or atom, proton, or electron bombardment, has been indeed investigated and has seen the formation of polyalcohols, ethers, ketones, and quinones. Recently, Bouwman and co-workers reported on the VUV photochemistry of pyrene, C16H10, diluted in water ice under astronomical conditions.39,40 By means of real-time optical spectroscopy, the formation of hydroxypyrene PyOH, cations such as Pyþ and PyOHþ, and possibly anionic species, such as Py- and PyO-, has been observed. In addition, Bouwman et al. have put in evidence that the photochemistry of this tetracyclic hydrocarbon trapped in water ice is dominated by ion-molecule interactions and processes at low temperatures (i.e., 235 nm. The aim of our study is twofold: • to provide infrared spectroscopic and complementary theoretical data on vibrational energy levels of the products arising from the photoreactivity of pyrene and water; • to identify the photoproducts and propose a photochemical pathway for their formation.

2. EXPERIMENTAL AND THEORETICAL DETAILS The experimental system used consists of a stainless steel highvacuum chamber with a suspended CsBr window attached to the cold finger of a cryostat.45 The chamber is evacuated to P ≈ 10-7 mbar by an Edwards diffusion pump backed by an Edwards twostage rotary pump, and pressures are monitored using Pirani and Penning gauges (Edwards). The infrared transparent CsBr substrate is cooled down to 10 K by a Cryophysics Cryodine M22 closed cycle He refrigerator. The sample temperature is monitored with a Si diode thermometer affixed to the window, and spectra can be taken at selected temperatures between 10 and 300 K. The CsBr material can face either a deposition system, an irradiation source, or a Bruker 113 V FTIR spectrometer allowing the recording of transmittance spectra in the range 4000-600 cm-1. Each spectrum is the result of 250 coadded single-scan spectra measured at a resolution of 2 cm-1. In this work, three sets of experiments have been performed to study the photoreactivity of pyrene with water at 10 K: at first in argon cryogenic matrices, then with pyrene adsorbed onto amorphous water ice films, and finally with pyrene intimately mixed in solid water ices. Pyrene (99%) purchased by Aldrich and argon (99.9995%) supplied by Air Liquide were used without further purification. Water was filtered through a milli-Q purification system and then purified further by three freezepump-thaw cycles under vacuum to remove dissolved gases before vacuum transfer into a glass bulb. Pyrene powder, placed in the reservoir of a resistively heated oven, was sublimed and deposited together either with H2O/Ar (10/500) mixtures, prepared by standard manometric techniques in a gas manifold, or only with H2O to obtain water ice samples doped with pyrene. H2O/Ar mixtures and water vapors were passed through a separate inlet over pyrene at room temperature prior to deposition onto the cold CsBr window held at 10 K. For the adsorption experiments, H2O films have been deposited at 10 K to produce a porous solid surface prior to adsorption of pure pyrene. To help in the identification of the photoproducts, isotopic experiments have also been performed using deuterated water. In the experiments, samples have been typically deposited for about 4-6 h and subsequently irradiated with a high-pressure Hg arc lamp (λ > 235 nm) at 10 K. To study the photochemical processes and to identify the molecules formed upon photolysis, the depletion and the production of species were followed as a function of irradiation time. After irradiation, samples were warmed up and halted at regular intervals. Annealing experiments carried out in argon cryogenic matrices have been performed in 5 K steps, and spectra have always been recorded after recooling at 10 K. For the condensed-phase experiments, warm-up has been carried out in 20 K steps. To help in the identification of the photoproducts, theoretical calculations have been performed on a set of expected

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photoproducts. As density-functional theory (DFT) has been proved to be a powerful method for predicting the geometry and harmonic vibrations of organic substances,46 structures have been optimized using the B3LYP/6-311þþG(d,p) functional and basis set. All optimized geometries were confirmed to be those of energy minimum by harmonic frequency analysis. Isotopic frequency shifts have also been computed. All calculations have been performed with the Gaussian 09 package of programs.47

3. RESULTS 3.1. Adsorption of Pyrene on Amorphous Water Ice Film. Water vapor deposited at 10 K under the conditions described in the experimental section is found to be in its high-density amorphous form. This latter is believed to be the dominant form of water ice present in interstellar molecular clouds.48-50 Water forms microporous amorphous films and a great surface area by volume when it is slowly deposited on a cold surface. Amorphous porous water ice is characterized by the presence of threecoordinated H2O molecules with one OH group noninvolved in a hydrogen bond, referred to as dangling H (dH) sites of water ice surface. Traces a in Figure 1 show the mid-infrared spectrum of water vapor deposited at 10 K exhibiting a broad band centered at 3250 cm-1, assigned to the symmetric and antisymmetric OH stretching modes, a weaker one at 1650 cm-1, corresponding to the HOH bending vibration, and an absoprtion around 803 cm-1, attributed to the libration mode. In addition, the infrared spectrum of amorphous water ice displays a weak feature at 3695 cm-1 which is due to the free OH vibrational mode, namely, dangling H. These bonds of water ice surface are more reactive than the OH groups of isolated water molecules and consequently are more sensitive to interaction with molecules. The dH absorption is thus used as a probe to follow adsorption of molecules on water ice films. Over warm-up, amorphous water ice goes through several phase transitions with one of them occurring around 100 K, involving the crystallization and phase separation, and another one around 150 K with changes to a cubic crystalline form and then to a hexagonal one before sublimation at 180 K.49,51,52 Adsorption of pyrene onto a bare amorphous ice film at 10 K results in a decrease of the OH dangling bond feature at 3695 cm-1, as seen by the traces b in Figure 1, and in the formation of a weak feature located at 3583 cm-1 in the high-wavenumber wing of the bulk water ice broad band, as shown by the subtraction spectra (after and before adsorption, traces c) in Figure 1 which have been multiplied by a factor of 4 for a better visualization. The band at 3583 cm-1 results from a downshift of 112 cm-1 of the free OH band showing that pyrene accepts a proton from a water molecule on the ice surface. In addition, this shift, larger than the one observed of 72 cm-1 for coronene, C24H12,53 indicates that there is a strong interaction between pyrene and ice surface. However, no change in the position and shape of the PAH absorption bands is seen as a result from adsorption on amorphous water ice surface compared to those of pure solid pyrene. It is worthwhile to note that the downshift of 112 cm-1 of the dH bond feature is comparable to the one of about 110 cm-1 obtained for benzene.54 Lastly, as noticed for coronene,53 the dH absorption band does not totally disappear by adsorption of pyrene on water ice as its relatively large size prevents its penetrating into every ice pore. The lack of penetration of pyrene in amorphous ice could also be due to its weaker mobility at that temperature. 1845

dx.doi.org/10.1021/jp108713n |J. Phys. Chem. A 2011, 115, 1844–1852

The Journal of Physical Chemistry A

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Figure 1. Mid-infrared spectra resulting from an exposure of pyrene onto an amorphous water ice film maintained at 10 K between 3900 and 650 cm-1 (A) and 3850-2000 cm-1 (B): (a) bare amorphous ice, (b) after exposure of pyrene on water ice, and (c) subtraction spectra (b - a) which have been multiplied by 4. The bands which appear as positive peaks on the subtraction spectra are those of pyrene.

Adsorption of pyrene has also been performed onto an amorphous D2O surface resulting in the decrease of the dD feature at 2725 cm-1 55 and at the same time in the growing of a weak band at 2622 cm-1, shifted by 103 cm-1 to lower wavenumbers with respect to the free dD absorption. This is consistent with the result obtained by adsorption of pyrene onto amorphous H2O surface. Upon warm-up, onto amorphous H2O (D2O) films, the dH (dD) signal starts to decrease in intensity above 85 K and then disappears at 130 K due to the amorphous ice pores collapsing during their evolution toward a crystalline ice form. The intensity of pyrene absorption bands remains nearly unchanged between 80 and 150 K, which indicates a stable adsorption state, before an abrupt decrease at 150 K owing to the crystallization process of the water ice surface. 3.2. Photoreactivity of Pyrene and Water at λ > 235 nm. 3.2.1. Results. The electronic spectrum of pyrene, studied both theoretically56,57 and experimentally,58-62 shows four π-π* symmetry-allowed transitions above 235 nm, the range of the electromagnetic spectrum investigated in this work. Two of them lie at 373 and 275 nm and correspond to 1B3u r 1Ag band systems, and the other two, with a higher oscillator strength, fall at 340 and 243 nm and are assigned to 1B2u r 1Ag transitions. In addition to these four well-separated band systems, some forbidden and thus much weaker bands are also found in the respective band regions. Mixtures containing pyrene and H2O (D2O) either as solids or in argon cryogenic matrices have been prepared yielding infrared spectra that are consistent with those previously reported for the monomers (pyrene, H2O, D2O), respectively.55,63 Photoprocessing experiments have been carried at first on a frozen layer of a pyrene/ H2O/Ar (1/10/500) mixture at 10 K. Upon photolysis, a decrease of the bands of both pyrene and water is observed with a loss of about 8% of pyrene after 4 h of irradiation. At the same time, apart the formation of CO2, CO, and N2O formed as impurity over photoprocessing, the growth of weak absorptions is seen in the 18001150 cm-1 range at 1720, 1702, 1678, 1636, 1264, 1222, and 1167 cm-1 (Figure 2). Control experiments performed with pyrene/ H2O/Ar mixtures which were not photolyzed did not produce any detectable bands. These experiments indicate that these new appearing absorptions arise from the reaction between pyrene and water

upon photoprocessing. As these absorptions are weak or lie close to each other, in particular those around 1700 cm-1, their kinetic evolution did not allow any correlation. However, the analysis shows that these features are due to the formation of final photoproducts as they grow until the end of the irradiation. In addition, these species are stable over warm-up as the intensity of their absorptions starts decreasing around 80 K and disappears at ca. 180 K. Photoprocessing of pyrene adsorbed onto amorphous water ice and of an icy pyrene/water (1/10) mixture led to the formation of the same bands as the ones obtained in argon matrices (Figure 2). However, although the amount of starting material is almost identical in the experiments performed, the absorptions are stronger in intensity than the ones produced in argon matrices. As seen in the case of coronene,53 the photoproduct bands formed upon irradiation are indeed 4 and 9 times stronger on amorphous water ice and in solid water ice, respectively. The loss of pyrene upon irradiation was about 13% when it is adsorbed onto amorphous water ice film, and ca. 18% when it is embedded in solid water ice. It is worthwhile to note that the formation of the same absorption bands upon irradiation indicates at first that the same photochemical processes take place between pyrene and water in argon matrices and in condensed phase, and second that these photoprocesses are more efficient in solid phase as evidenced by the higher amounts of photoproducts obtained. Crystallization of water ice, starting around 100 K, induces a slight decrease in the intensity of the photoproduct bands which vanish totally around 200 K. Isotopic experiments have also been performed using deuterated water. Photoprocessing of pyrene/D2O/Ar and pyrene/ D2O icy mixtures (Figure 3) and pyrene adsorbed onto amorphous D2O surface led to the decrease of the absorption bands of pyrene and D2O and to the formation of the same absorption bands cited earlier, as seen in Figure 3. As the bands produced by irradiation of pyrene and H2O are not affected by isotopic substitution, it can be suggested that the products issued from the reaction of pyrene and H2O (D2O) at λ > 235 nm are either oxygen-containing pyrene compounds or the presence of the H (D) atom in these products does not greatly influence the vibrational modes. These observations are consistent with results 1846



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Figure 2. Spectra of solid C16H10/H2O (1:10) at 10 K before and after irradiation at λ > 235 nm. In each panel, the bottom trace (a) depicts the spectral region before irradiation, the middle trace (b) shows the spectrum after 4 h of irradiation, and the top trace (c) corresponds to the subtraction spectrum (b - a). The right panel of the figure shows an enlargement of the spectral range between 1550 and 1150 cm-1, and the subtraction spectrum (c) has been multiplied by 5 for a better visualization. Pyrene absorptions are labeled Py and those due to the photoproducts C, F, and I (see text for the respective assignments). Of note, there are few amounts of deuterium (due to previous experiments) explaining the presence of HOD.

Figure 3. Spectra of solid C16H10/D2O (1:10) at 10 K before and after irradiation at λ > 235 nm. Trace a shows the spectral region before irradiation, trace b displays the spectrum after 4 h irradiation, and trace c is the subtraction spectrum (b - a). Pyrene absorptions are labeled Py and those due to the photoproducts with letters C, F, and I (see text for the respective assignments).

reported in literature showing that photoprocessing of polyaromatic species in the presence of H2O induces their oxidation.37,38,42,43,53 3.2.2. Nature of the Photoproducts. To help in the identification of the products formed upon irradiation of pyrene and water, we proceeded to the deposition at 10 K and irradiation at λ > 235 nm of a pyrene-like species: 1-hydroxypyrene (C16H10O, 98%, Aldrich), denoted hereafter A. Figure 4 shows the infrared spectrum of A (trace a) characterized by prominent absorption bands at 1606 and 849 cm-1 and weak ones at 1630, 1516, 1331, 1266/1258, 1124/ 1116, 898, and 792 cm-1. It is worthwhile to note also the presence of pyrene absorption bands (labeled Py in the figure), as the hydroxylated species breaks down in the gas phase prior to deposition. Irradiation of pure solid A for 4 h resulted in the decrease of its

Figure 4. Infrared spectrum of pure 1-hydroxypyrene at 10 K before (trace a) and after irradiation for 4 h at λ > 235 nm (trace b). Absorptions of pyrene, 1-hydroxypyrene, and the ketonic species are respectively labeled Py, A, and C.

infrared absorptions and in the formation of a strong band at 1702 cm-1, characteristic of a CdO stretching vibrational mode, and much weaker ones located at 1222 and 1167 cm-1 (trace b). These bands are similar in shape, position, and intensity to the ones obtained by photoprocessing of pyrene and water (Figure 2, Figure 3). As seen in Figure 5, irradiation of 1-hydroxypyrene could lead to the production of two ketonic forms C16H9O (B) and C16H10O (C) which could be responsible for the features obtained. To discriminate between both candidates, DFT calculations have been performed to compute their respective infrared spectra and isotopic shifts. Table 1 and Table 2 list their harmonic frequencies and intensities. Based on these data, it could be seen in the CdO stretching region above 1600 cm-1 that B is characterized by two absorptions at 1632 and 1620 cm-1 (Table 1) while C shows features at 1723 and 1629 cm-1 (Table 2). The bands of molecule 1847



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Figure 5. Aromatic ketonic species C16H9O (B, E, H)/C16H10O (C, F, I) possibly formed upon irradiation at λ > 235 nm of 1-hydroxypyrene (A), 2-hydroxypyrene (D), and 6-hydroxypyrene (G), respectively.

B are too low in frequency compared to the one at 1702 cm-1 formed by irradiation of pyrene and water. However, the calculated one at 1723 cm-1 for the ketonic species C seems to be in good agreement with the experimental one. In addition, the computed spectrum of C displays a band lying at 1235 cm-1 which can consistently be assigned to the corresponding weak feature at 1222 cm-1 produced upon photolysis of pyrene/water icy mixtures. Finally, the band obtained at 1167 cm-1 could also be assigned to the ketonic species C as it correlates with the calculated one at 1180 cm-1, although the experimental and theoretical intensities do not match (Table 2). Discrepancies in the intensities may be due to the fact that calculations are performed for the free molecule and thus do not take into account matrix effects or interactions with solid water. All these observations suggest thus the formation of the ketonic photoproduct C16H10O (C) upon photoprocessing of

pyrene and water at λ > 235 nm. This is further confirmed by isotopic substitution as the calculated bands are scarcely shifted in agreement with experiment. Lastly, it is interesting to note that none of the bands of the hydroxylated pyrene were seen experimentally, indicating that this species is most probably an intermediate in the photochemical process. Similarly, as the structure of pyrene allows for the possibility of two other pyrenol species (Figure 5), formation of 2-hydroxypyrene D and 6-hydroxypyrene G have also been considered as intermediate products by photoprocessing of pyrene and water. Irradiation of these latter molecules could give respectively two ketonic compounds referred to as E/F and H/I, as shown in Figure 5. The calculation of their harmonic frequencies as well as their isotopic shifts are reported in Table 3, Table 4, Table 5, and Table 6 respectively. Comparison of the harmonic frequencies of species E and F shows that while the calculated infrared spectrum of E displays strong bands 1848



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Table 1. Position and Intensity of the Vibrational Bands of the Ketonic Aromatic Species C16H9O (B) Computed at the B3LYP/6-311þþg(d,p) Level of Theorya harmonic

Table 3. Position and Intensity of the Vibrational Bands of the Ketonic Aromatic Species C16H9O (E) Computed Using the B3LYP/6-311þþg(d,p) Functional and Basis Seta

harmonic

harmonic

harmonic

frequencies of B

scaled

frequencies of B

scaled

frequencies of

scaled

frequencies of

scaled

(intensity)b

frequenciesc

(intensity)b

frequenciesc

E (intensity)b

frequenciesc

E (intensity)b

frequenciesc

728 (14.4)

719

1573 (46.2)

1554

625 (5.9)

617

1437 (7.4)

1420

747 (5.7)

737

1598 (43.6)

1579

701 (25.5)

693

1457 (6.1)

1440

854 (100.0) 1117 (9.4)

844 1103

1602 (8.2) 1620 (32.3)

1583 1600

749 (18.9) 777 (6.4)

740 768

1463 (74.0) 1470 (10.1)

1445 1452

1185 (11.8)

1170

1632 (98.2)

1612

841 (66.8)

831

1562 (39.0)

1543

1205 (6.0)

1190

3156 (7.0)

3118

885 (100.0)

874

1652 (70.7)

1632

1262 (9.8)

1247

3162 (5.4)

3124

1156 (12.8)

1142

3173 (13.3)

3135

1295 (10.2)

1279

3173 (9.2)

3135

1199 (13.8)

1185

3181 (31.0)

3143

1423 (7.9)

1406

3180 (20.1)

3141

1252 (5.6)

1237

3185 (22.1)

3147

1469 (7.7)

1451

3189 (19.9)

3151

1308 (9.3)

1292

3188 (30.8)

3150

1508 (20.6) 1531 (10.3)

1490 1512

3193 (7.3)

3155

1332 (60.9)

1316

a

a

Frequencies are given in wavenumbers, and intensities, written between parentheses, are expressed as a percentage of the strongest band. b For the sake of clarity, only the vibrational bands with intensity equal to or greater than 5 are tabulated. c Frequencies were scaled with a factor of 0.988 (see text).

Table 2. Position and Intensity of the Vibrational Bands of the Ketonic Aromatic Species C16H10O (C) as Well as Isotopic Shifts Calculated at the B3LYP/6-311þþg(d,p) Level of Theorya


Table 4. Position and Intensity of the Vibrational Bands of the Ketonic Aromatic Species C16H10O (F) as Well as Isotopic Shifts Calculated at the B3LYP/6-311þþg(d,p) Level of Theorya

harmonic harmonic scaled frequencies of frequencies of Δν frequenciesc of C C16H10O (intensity)b C C16H9DO (intensity)b (νH - νD) C16H10O 762 (5.0) 845 (9.9) 854 (11.1) 1034 (5.4) 1180 (5.3) 1235 (18.9) 1244 (5.2) 1281 (8.5) 1336 (6.7) 1358 (7.5) 1374 (5.3) 1406 (7.7) 1587 (6.9) 1629 (11.3) 1680 (5.2) 1723 (100.0) 3178 (7.6) 3187 (6.3)

756 (3.6) 827 (4.2) 851 (17.6) 1039 (3.1) 1180 (5.2) 1236 (15.1) 1240 (5.8) 1276 (6.5) 1312 (5.9) 1357 (6.0) 1373 (6.8) 1250 (2.3) 1586 (7.1) 1629 (11.4) 1679 (5.4) 1723 (100.0) 3178 (7.6) 3187 (6.3)

6 18 3 -5 0 -1 4 5 24 1 1 156 1 0 1 0 0 0

753 835 844 1022 1166 1220 1229 1266 1320 1342 1358 1389 1568 1610 1660 1702 3140 3149

a

Frequencies and isotopic shifts (Δν) are given in wavenumbers, and intensities, written between parentheses, are expressed as a percentage of the strongest band. b For the sake of clarity, only the vibrational bands with intensity equal to or greater than 5 are tabulated. c Frequencies were scaled with a factor of 0.988 (see text).

in the spectral range 1800-600 cm-1, that of F is characterized by few intense bands, which is in agreement with what is observed experimentally. A more detailed analysis of the data shows that the strongest bands of F—scaled with 0.988 in view of the ratio obtained for the 1702 (experimental)/1723 cm-1 (theoretical) band of the ketonic species C (Figure 5)—are calculated to fall at 1677, 1644, and

harmonic

harmonic

scaled

frequencies of

frequencies of

frequenciesc

F C16H10O

F C16H9DO

(intensity)b

(intensity)b

of

844 (10.1) 1268 (27.7)

843 (9.0) 1266 (21.9)

1 2

834 1253

1568 (18.0)

1567 (18.1)

1

1549

1614 (8.5)

1614 (8.2)

0

1595

1666 (29.1)

1666 (29.5)

0

1646

1699 (100.0)

1699 (100.0)

0

1679

Δν (νH - νD) C16H10O

a

Frequencies and isotopic shifts (Δν) are given in wavenumbers, and intensities, written between parentheses, are expressed as a percentage of the strongest band. b For the sake of clarity, only the vibrational bands with intensity equal to or greater than 5 are given. c Frequencies were scaled with a factor of 0.988 (see text).

1253 cm-1, which appear to be in good agreement with the ones observed experimentally at 1678, 1636, and 1264 cm-1. Here again, results obtained by isotopic substitution confirmed the present assignment as the calculated bands for the deuterated species of F are slightly shifted with respect to the hydrogenated one. Among the bands produced upon photoprocessing, only the band lying at 1720 cm-1 has not been assigned yet. Analysis of the data reported in Table 6 shows that the infrared spectrum of I is characterized by a prominent absorption expected around 1720 cm-1 and another one three times weaker around 1265 cm-1. As shown in Table 6, the calculated band at 1720 cm-1 is not affected by isotopic substitution while the one at 1265 cm-1 is predicted to be shifted by 5 cm-1 to higher wavenumbers. Comparison with experiment shows that the band produced upon photolysis of pyrene/H2O(D2O) icy mixtures at 1720 cm-1 is in good agreement 1849



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Table 5. Position and Intensity of the Vibrational Bands of the Ketonic Aromatic Species C16H9O (H) Computed Using the B3LYP/6-311þþg(d,p) Functional and Basis Seta harmonic

harmonic

frequencies of

scaled

frequencies of

scaled

H (intensity)b

frequenciesc

H (intensity)b

frequenciesc

676 (7.7)

668

1466 (7.2)

1448

718 (27.6)

710

1518 (6.2)

1500

776 (6.3) 849 (78.5)

767 839

1561 (100.0) 1594 (20.4)

1542 1575

1166 (5.6)

1152

1607 (30.6)

1588

1213 (5.0)

1198

1633 (10.0)

1613

1260 (5.5)

1245

1648 (9.7)

1628

1309 (44.0)

1293

3180 (15.8)

3142

1339 (7.0)

1323

3182 (16.9)

3144

1354 (6.8)

1338

3184 (6.5)

3146

1399 (11.2)

1382

3191 (14.4)

3153

Figure 6. Chemical structures of 1,6- and 1,8-pyrenequinones.

Table 7. Position and Intensity of the Vibrational Bands of the 1,6-Pyrenequinone (C16H8O2) Calculated Using B3LYP/ 6-311þþg(d,p)a harmonic frequencies (intensity)b

a


Table 6. Position and Intensity of the Vibrational Bands of the Ketonic Aromatic Species C16H10O (I) as Well as Isotopic Shifts Calculated at the B3LYP/6-311þþg(d,p) Level of Theorya harmonic

harmonic

frequencies of

frequencies of

scaled

I C16H10O (intensity)b

I C16H9DO (intensity)b

frequenciesc of C16H10O

730 (13.5)

727 (9.4)

Δν (νH - νD) 3

721

850 (26.6)

849 (26.1)

1

840

1280 (32.7)

1285 (25.8)

-5

1265

1369 (5.7)

1365 (3.1)

4

1353

1618 (14.1)

1618 (14.4)

0

1599

1741 (100.0)

1740 (100.0)

1

1720

852 (7.8)

842

879 (8.4)

868

1297 (12.7)

1281

1370 (12.5)

1354

1593 (9.1)

1574

1647 (8.6) 1695 (100.0)

1627 1675

a


Table 8. Position and Intensity of the Vibrational Bands of the 1,8-Pyrenequinone (C16H8O2) Calculated Using B3LYP/ 6-311þþg(d,p)a harmonic frequencies (intensity)b 818 (7.8)

a

Frequencies and isotopic shifts (Δν) are given in wavenumbers, and intensities, written between parentheses, are expressed as a percentage of the strongest band. b For the sake of clarity, only the vibrational bands with intensity equal to or greater than 5 are tabulated. c Frequencies were scaled with a factor of 0.988 (see text).

with the one calculated for the species I. However, the absorption growing at 1264 cm-1 by irradiation of pyrene/H2O(D2O) cannot be assigned to the computed one at 1265 cm-1 of I as no shift due to deuteration has been observed for this absorption. Although the assignment of the ketonic compound I relies on a single band, its formation seems to be possible and would be consistent with the production of C and F species. A study reported by Sigman et al. on the photochemistry of pyrene in aqueous solution at 350 nm has evidenced the formation of 1,6- and 1,8-pyrenequinones (C16H8O2) as stable products.64 These latter, having the chemical structures displayed in Figure 6, were suggested to be produced by oxidation of 1-hydroxypyrene. Geometry optimization and calculation of the harmonic frequencies have been performed for both quinones. The results are summarized in Table 7 and Table 8. From these data, it is seen that the strongest absorptions of 1,6- and

scaled frequenciesc

scaled frequenciesc 808

876 (25.8)

865

1206 (9.0)

1192

1284 (22.2)

1269

1287 (11.3) 1601 (35.1)

1272 1582

1702 (100.0)

1681

1703 (87.9)

1682

a


1,8-pyrenequinones are calculated to lie around 1675 and 1682/1681 cm-1, respectively. This appears to be in good agreement with the band formed upon irradiation at 1678 cm-1. Further analysis of the theoretical data shows that the infrared spectra of these two quinones exhibit absorptions expected at 1354/1281 and 1272/1269 cm-1. The calculated bands at 1272/1269 cm-1 for 1,8-pyrenequinone appear to be in agreement with the one at 1264 cm-1 obtained by irradiation of pyrene and H2O (D2O). However, the ones computed in this range for 1,6-pyrenequinone are not seen although they could be overlapped by absorptions of N2O at 1301 and 1285 cm-1. Although there are 1850



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Figure 7. Possible reactional mechanism for the formation of the products issued from photoprocessing of pyrene and water at λ > 235 nm and 10 K.

some disagreements between experiment and theory, the possibility of a quinone species formation cannot be excluded.

4. DISCUSSION From the data reported in the previous section, it is seen that a rich chemistry results from photoprocessing of pyrene and water at 10 K and λ > 235 nm, as the tetracyclic aromatic species undergoes oxidation and reduction yielding the formation of ketonic species (C, F, and I), and possibly quinones, identified by infrared spectroscopy. In addition, this study points out that photochemistry of pyrene is enhanced when it is embedded in solid water ices as seen from the larger amounts of photoproducts obtained. This is similar to what was observed in a previous work reporting on the photochemistry of coronene and water studied at 10 K and λ > 235 nm.53 As water does not photodissociate at these wavelengths, it has been shown that the formation of oxygen-substituted coronene species arise from coronene cations, generated through UV processing, which could subsequently react with water. This is based on extensive works carried out by Gudipati and Allamandola26,44 and Woon and Park65 showing that PAHs are easily ionized in water ices as their ionization potential is lowered by up to ≈2 eV, allowing thus the formation of PAH cations, even at λ > 235 nm. Pyrene has an ionization energy of about 7.43 eV,66 similar to the one of coronene (7.29 eV67). Embedded in solid water ice, its ionization potential could thus be lowered down to ≈5.4 eV making formation of pyrene cations possible under our experimental conditions. Once generated, cations are trapped by water as shown in the proposed mechanism in Figure 7, inducing the production of [Py-H2O]þ cations which dissociate yielding hydroxylated pyrene species. For the sake of clarity, the mechanism is described only in the case of the 1-hydroxypyrene species. The monoalcohol pyrene-like molecules (A, D, and G), reactive intermediates, could then undergo a tautomeric rearrangement resulting in the formation of the ketonic molecules identified in this work. This is consistent with data reported by Bernstein and co-workers who showed by mass spectrometry that pyrene embedded in amorphous water ices and upon photolysis produces mono-oxidized species.25 Lastly, as said in the previous section, quinones could form from further oxidation of 1-hydroxypyrene. Additional work, including theoretical approaches, will be necessary to carefully describe step-by-step mechanisms involved in the photoreaction of pyrene and water. Similar photoprocesses have been found to take place also by irradiation of pyrene in aqueous solution at room temperature and λ = 350 nm.64 In such an environment, the first step has been shown to be the formation of pyrene cations which react with water to form

1-hydroxypyrene. This latter, also acting as an intermediate, is further oxidized yielding the production of 1,6- and 1,8-pyrenequinones. However, none of the ketonic species, identified in the present study, have been detected in aqueous solution. Although only 1-hydroxypyrene has been detected in the atmosphere to date,68 formation of ketonic species may occur in the atmosphere where most important gas-phase reactions are photochemical reactions of PAHs with atmospheric water and oxygen. Such processes are also likely to occur in the interstellar medium, in particular in dense molecular clouds where the temperature is low ( 235 nm, show that pyrene, as other PAHs, can be oxidized in the presence of water leading to the formation of ketonic species, identified for the first time with the support of infrared spectroscopy, isotopic experiments, and DFT calculations. Their production most likely occurs through the generation of their analogous hydroxylated molecules after ionization of pyrene. These data may help in the detection of these relevant interstellar and atmospheric substituted PAHs. ’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]; tel. þ33 (0)5 40 00 63 60; fax þ33 (0)5 40 00 84 02.

’ ACKNOWLEDGMENT Dr. Z. Guennoun acknowledges the PRES of University of Bordeaux for financial support. The theoretical part of this work was conducted with the assistance of “Pôle de Modelisation de l‘Institut des Sciences Moleculaires”. 1851



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Photochemistry of Pyrene with Water at Low Temperature: Study of

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