The oxidized soot surface: Theoretical study of desorption mechanismsinvolving oxygenated functionalities and comparison with temperatureprogramed desorption experimentsGianluca Barco, Andrea Maranzana, Giovanni Ghigo, Mauro Causà, and Glauco Tonachini Citation: J. Chem. Phys. 125, 194706 (2006); doi: 10.1063/1.2360277 View online: http://dx.doi.org/10.1063/1.2360277 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v125/i19 Published by the American Institute of Physics. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

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The oxidized soot surface: Theoretical study of desorption mechanismsinvolving oxygenated functionalities and comparison with temperatureprogramed desorption experiments

Gianluca BarcoDipartimento di Chimica Generale ed Organica Applicata, Università di Torino, Corso MassimoD’Azeglio 48, 10125 Torino, Italy and Dipartimento di Scienze dell’Ambiente e della Vita DISAV,Università del Piemonte Orientale “Amedeo Avogadro,” Piazza Ambrosoli 5, 15100 Alessandria, Italy

Andrea Maranzana and Giovanni GhigoDipartimento di Chimica Generale ed Organica Applicata, Università di Torino, Corso MassimoD’Azeglio 48, 10125 Torino, Italy

Mauro Causàa�,b�

Dipartimento di Scienze dell’Ambiente e della Vita DISAV, Università del Piemonte Orientale “AmedeoAvogadro,” Piazza Ambrosoli 5, 15100 Alessandria, Italy

Glauco Tonachinia�,c�

Dipartimento di Chimica Generale ed Organica Applicata, Università di Torino, Corso MassimoD’Azeglio 48, 10125 Torino, Italy

�Received 30 June 2006; accepted 12 September 2006; published online 16 November 2006�

The desorption mechanism for oxygenated functionalities on soot is investigated by quantummechanical calculations on functionalized polycyclic aromatic hydrocarbon �PAH� models andcompared with recently published temperature programed desorption-mass spectrometry results.Substituents on PAHs of increasing size �up to 46 carbon atoms in the parent PAH� are chosen toreproduce the local features of an oxidized graphenic soot platelet. Initially, the study is carried outon unimolecular fragmentation �extrusion, in some cases� processes producing HO, CO, or CO2, inmodel ketones, carboxylic acids, lactones, anhydrides, in one aldehyde, one peroxyacid, onehydroperoxide, one secondary alcohol, and one phenol. Then, a bimolecular process is consideredfor one of the carboxylic acids. Furthermore, some cooperative effect which can take place byinvolving two vicinal carboxylic groups �derived from anhydride hydrolysis� is investigated forother four bifunctionalized models. The comparison between the computed fragmentation�desorption� barriers for the assessed mechanisms and the temperature at which maxima occur inTPD spectra �for HO, CO, or CO2 desorption� offers a suggestion for the assignment of thesemaxima to specific functional groups, i.e., a key to the description of the oxidized surface. Notably,the computations suggest that �1� the desorption mode from a portion of a graphenic plateletfunctionalized by a carboxylic or lactone groups is significantly dependent from the chemical andgeometric local environment. Consequently, we propose that �2� not all carboxylic groups go lost atthe relatively low temperatures generally stated, and �3� lactone groups can be identified asproducing not only CO2 but also CO. © 2006 American Institute of Physics.�DOI: 10.1063/1.2360277�

I. INTRODUCTION

Soot aerosol, of natural or anthropogenic origin, contrib-utes in a significant way to the total mass of atmosphericaerosol. It has an irregular agglomerate structure ofgraphenic layers, and both its structure and composition canvary depending on the source.1,2 These irregular sheets areclustered in globular particles, whose dimension varies be-tween 10 and 80 nm, approximately. In any case, a relativelylarge area of the particles is available to the interactions with

airborne inorganic and organic molecules. Since polycyclicaromatic hydrocarbons �PAHs� and compounds �PACs� aregenerated in the same combustion processes at low O2 con-centrations which bring about the relatively disordered grow-ing of the graphenic platelets, they share the same nature ofsoot and can be found in association with it.1,2 As PAHs andsoot form, they undergo oxidative attack by the small reac-tive species already present during the combustion process�HO, O2, NOx, etc.�. In fact, the functionalization of PAHsand that of soot could share essential mechanistic features.Oxidation can take place not only during the combustionphase but also at a later time, during the tropospheric trans-port of combustion-generated particulate. During transport,the relative amount of carcinogenic/mutagenic primary prod-ucts changes significantly, and other products form. The

a�Authors to whom correspondence should be addressed.b�Fax: ��39-0131-287416. Electronic mail: causa@al.unipmn.it. URL:

http://www.mfn.unipmn.it/�causac�Fax: ��39-011-2367648. Electronic mail: glauco.tonachini@unito.it.

URL: http://www.thecream.unito.it/

THE JOURNAL OF CHEMICAL PHYSICS 125, 194706 �2006�

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presence of polar hydrophilic groups alters the properties ofsoot much in the same way as for PACs. One importantaspect is that the compound or particle polarity increases, sodoes their hygroscopicity and water solubility, and, as aero-sol constituents, they can be more easily brought into contactwith, for instance, the lung tissues. Both PACs and soot par-ticles are consequently of general concern as regards humanhealth.1�d�

In recent times the interaction of soot with small inor-ganic oxidants, as NO2,3–5 HNO3,3 H2O,6 and O3,6,7,8�a� hasbeen investigated experimentally. In particular, in the presentpaper, reference will be made to desorption studies carriedout by the temperature programed desorption technique inconjunction with mass spectrometry �TPD-MS� and IRspectroscopy.9–12 This technique has proven useful to get abetter understanding of the nature of oxidized soot. However,TPD experiments produce in general broad desorption bandswhich render the interpretation of the spectra difficult. This isbecause, on one hand, different functional groups can desorbat similar temperatures, and, on the other hand, the chemical/geometric surroundings of one functional group can vary sig-nificantly. Consequently, the spectra have been found to bemore reliably interpreted only with the aid of curve fitting�deconvolution� procedures.9,10 Investigating by theoreticalmeans the desorption mechanisms of CO, CO2, HO, etc.,from different oxygenated functional groups bound to sootparticles could be instrumental for a safer interpretation ofthe valuable data collected experimentally, because the reac-tion barriers can be put into relation with the desorption tem-peratures. This would in turn help in clarifying the surfacecharacteristics of oxidized soot, which are related to its hy-groscopicity and capability of adsorbing and vehiculating po-lar molecules �such as some dangerous PACs�.

The lack of structural information on the atomic scaleabout soot and its possibly active surface sites �useful to setup a computational study and for subsequent comparativepurposes� poses some difficulties in defining adequate mod-els. Only a few experimental studies report on this issue tosome extent.8 Because of the origin and structural affinities,PAHs and PACs may appear as the natural candidates tomodel soot platelets. In fact, fragmentation �extrusion, insome cases� processes for PACs carrying some group as car-boxyl, carbonyl, hydroxyl, etc., can be thought of as corre-sponding to desorption of chemisorbed groups on a sootplatelet. The assessment of the barrier height for this processcan help in defining the temperature corresponding to onemaximum in the TPD spectrum, and thus to confirm the as-signment of that maximum to a particular group.

The present study is preceded by two papers along thesame line. In the first one,13 we attempted to define a suitablemodel for soot and examined the features of the interactionof some small species �H, NO, NO2, and NO3� and the gas-solid interaction by which functionalization reactions cantake place. In the second paper14 we have addressed the ozo-nization mechanism for the internal positions of some PAHs,studied per se, but also intended as models of a soot platelet.In the present study, quantum mechanical calculations arecarried out on functionalized PAH-type systems: their gas-phase reactions can be seen as interesting by themselves, but

are again expected to be helpful in setting up a model for thesurface reactivity of functionalized soot, hence to allow ussome progress toward the description of the oxidized sootsurface. Within the vast literature dealing with carbonaceousparticulate, we like to call attention to some related theoret-ical work carried out by other groups, which used differentmodel systems and in some cases relied on different theoret-ical approaches �see Sec. II�.15–18

II. METHODS

In determining the dissociation energy profiles for thedifferent PAC models, the stable and transition structures�TSs� were determined by gradient procedures19 within thedensity functional theory �DFT�, and making use of theB3LYP functional.20 This functional is of widespread useand, even if prone to underestimate to some extent certainreaction barriers, has generally performed well as regardsgeometries and energetics.21�a� The split-valence shell polar-ized 6-31G�d , p� basis set22�a� was used in the DFT�B3LYP�optimizations. The nature of the critical points was checkedby vibrational analysis; inspection of the normal mode re-lated to the imaginary frequency was in most cases sufficientto confidently establish its connection with the initial andfinal energy minima. In some cases, an intrinsic reaction co-ordinate �IRC� calculation23 helped to confirm the connec-tion with the adjacent energy minimum. In addition, most ofthe reaction barriers were reassessed by using the more ex-tended 6-311G�2df , p� basis set22�b� in single-point energycalculations. Finally, the energetics relevant to the formationof H-bonded complexes �see Fig. 1� were refined by estimat-ing the basis set superposition error �BSSE� by the counter-poise method.21�b�

When searching at the UDFT�B3LYP� level of theory fora dissociation transition structure which may be endowedwith diradicaloid nature, the unrestricted self-consistent pro-cedure can converge, in fact, on a restricted wave functionwith zero spin densities. This not only gives an incorrectdescription of diradicaloid structures but can even affect thevery topology of the energy hypersurface. In these cases, ifthe wave function is put to the test to check its stability withrespect to orbital rotations,24 it can be found unstable. Whenthe orbitals are relaxed along the instability direction, a newstable unrestricted solution is found: this wave functionshows now nonzero spin densities. Yet, this handling of theUDFT monodeterminantal wave function gives rise to a sig-nificant spin contamination by the triplet, as evidenced by

the �S2� values, which may yield a value close to 1 �thenature of the wave function is intermediate between the sin-glet and triplet spin multiplicities�. Therefore, the energy val-ues so obtained need to be refined by some spin-projectionmethod, in order to get rid of the spin contaminants. This isaccomplished in an approximate way, by using the formulasuggested by Yamaguchi et al.25 which allows to eliminatethe largest contaminant of the singlet, i.e., the triplet.

All calculations were carried out by using the GAUSSIAN

03 system of programs.26

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Once the dissociation TS �or, in some cases, the disso-ciation limit� is defined, in correspondence with a startingreactant, the vibrational analysis is carried out on both. Thisallows the definition of the activation energy Ea=�H‡+RT.The desorption temperature for the maximum �Tmax� is asso-ciated with the activation energy Ea by the relation

Ea = RTmax�ln�Tmax�/�� − ln�Ea/RTmax�� �1�

�from the Polanyi-Wigner equation�27 in which the activationenergy itself is defined in an implicit way. The �H‡�T� and�S‡�T� values obtained from the calculations allow us toassess a theoretical Tmax for each fragmentation �desorption�process examined. This is done numerically, looking for theT value which satisfies the equation above. The frequency�preexponential� factor � is assessed as

��T� = ekT/h exp��S‡�T�/R� , �2�

while a value commonly chosen in the experiments is usedfor � �ca. 0.17 K s−1�. See the EPAPS supp-info.xls file forfurther details.

The MOLDEN program was exploited for the graphics.28

III. RESULTS

A. Choice of the molecular models

Some functionalized aromatic hydrocarbons �PACs�have been chosen as molecular models and compared. AllPACs share the same spin multiplicity, being closed shellsinglets. One obvious difficulty in deciding which modelscan be helpful for a better understanding of the desorptionspectra is the variety of structural and electronic situations�hence of energetic features� that can be present on oxidizedsoot. We have attempted to cover some different positions byputting the functional groups in connection with the so-called zigzag and armchair borders, though, of course, morecomplicated situations can be envisaged.

The monofunctionalized PACs from which loss of CO orCO2 was studied are fully represented in Chart A, see theEPAPS Chart A and Figs.doc file. Here �Chart 1� only asingle example for each functionality is displayed. The cho-sen functionalities are carbonyl �in the ketones K1, K2, andK3, and in the aldehyde A� and carboxyl �in the carboxylicacids CA1, CA2, and CA3�; then we considered the lactonesL1, L2, and L3, and the anhydrides AN1 and AN2. The en-docyclic ketones K1, K2, and K3 and the lactones L1 and L2share a structure that allows ring closure, upon CO or CO2

extrusion, giving a normal closed shell species. The anhy-drides can either lose CO, and undergo ring closure to pro-duce a lactone, or CO and CO2 together, and leave a diradi-caloid system in the case of AN1, an aryne in the case ofAN2. Some PAC models �A, CA1, etc.� derive from an an-thanthrenelike structure and can be seen as representative ofa portion of zigzag border. Others �CA2, CA3, and AN2�derive from benzo�ghi�perylene, or benzo�a�pyrene �L3�, andcan be seen as functionalized portions of an armchair border�as indicated by a dashed line�.

Then, some other PACs, shown in Chart 2, were consid-ered as possible HO sources. They are a hydroperoxide �HP�,a peroxyacid �PA�, a secondary alcohol �SA�, which bears

the hydroxyl bound to an sp3 carbon, and, finally, a phenol�P�. In fact, though we initially considered only unimolecularprocesses, we were subsequently induced to investigate forCA1 also a bimolecular reaction, possibly triggered by hy-droxyl �see below, Sec. III C�, which renders the assessmentof Tmax for the production of hydroxyl essential.

B. Unimolecular fragmentation processesin monofunctional molecules

The fragmentation energetics are reported in Table I,third and fourth columns, and will be presented now by mak-

CHART 1. Some of the molecular models adopted in this study for COand/or CO2 loss. Most of them �see Chart A in the EPAPS� show functionalgroups bound to what can be seen as a portion of zigzag border. A dashedline highlights a portion of an armchair border.

CHART 2. The molecular models for which the loss of a hydroxyl radicalwas studied.

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ing reference to Charts 1 and 2. It can first be seen that thedependence of the energy barriers from the basis set is mod-erate.

�1� The model endocyclic ketones K1 and K2 �upon spindecoupling and recoupling of the electrons pertainingto the two CO bonds� can lose a CO moiety, concur-rently closing a five-membered ring. A two-step extru-sion would entail an intermediate with diradical char-acter, having an sp2 radical lobe, facing an acyl–CvOgroup, which has also an unpaired electron in a p or-bital. We cannot find such a structure as a stable struc-ture. Instead, we find a nonplanar and highlyasynchronous—but concerted—TS �shown in theEPAPS Chart A and Figs.doc file�.29 These processesrequire overcoming energy barriers somewhat higherthan 120 kcal mol−1, while the reactions are endoergicby 40–44 kcal mol−1. The five-C ring closure can be

hampered by the stiffness of the surrounding condensedsix-membered rings, and this factor must be alreadyfully active in the lighter system, because the extensionof the model does not affect the energetics significantly.For this reason, the relatively time-consuming vibra-tional analysis was not carried out on the more de-manding system K2. The third model, K3, was chosento minimize the rigidity factor. The same process comesout indeed to be significantly easier: the barrier resultsonly 46 kcal mol−1 high, and the endoergicity9 kcal mol−1.

�2� Also the aldehyde A can undergo a concerted process,in which the loss of CO from the substituted carbon isaccompanied by a hydrogen transfer to the same car-bon. This requires surmounting a barrier of89–86 kcal mol−1, while the reaction is endoergic byonly 3 kcal mol−1.

�3� A similar process, with loss of CO2, can take place inthe carboxylic acids CAn: as the COOH group detachesfrom the ring carbon, its H can become bound to it. Thetransition structure for CA1 is shown in the EPAPSChart A and Figs.doc file to exemplify the traits of thiskind of concerted process. The concerted eliminationstep implies a barrier of 55–57 kcal mol−1 in the CA1and CA2 models, which raises to 70 for CA3. The re-action energy ranges from −14 to −3 kcal mol−1, andthe less exoergic is CA3. This is the only one present-ing the carboxyl group coplanar with the aromatic rings

TABLE I. Monofunctional models: Computed energies �when two �E‡ are reported for the fragmentationbarriers, these are relevant to two basis sets �see Sec. II� and the format: 6-31G�d , p� /6-311G�2df , p� is used.The �E values are only 6-31G�d , p�. Both �E‡ and �E are relative to the reactants �kcal mol−1�� activationenergies, entropy contributions, and predicted temperatures �unit in kelvins�. T increment rate used: �=0.17 K s−1 for the TPD maximum.

Substratea Loss of �E‡ �E Ea�Tmax�b �S‡�Tmax�

b,c Tmax

K1 CO 120.8/120.1 40.3 119.5/120.4 5.44/5.25 1579/1594K2 CO 121.6/¯ 43.7 ¯ ¯ ¯

K3 CO 46.4/45.2 9.4 45.4/44.3 1.46/1.49 663/647A CO 88.8/86.1 3.5 87.8/85.0 6.96/7.00 1155/1120

CA1 CO2 55.3/55.9 −10.3 52.3/52.9 0.31/0.30 770/779CA2 CO2 56.2/57.2 −13.8 53.3/54.4 −1.49/−1.50 803/818CA3 CO2 70.2/70.4 −2.5 67.1/67.3 2.85/2.85 943/946L1 CO2 80.3/80.5 −12.0 81.4/79.1 −2.13/−0.66 1209/1154L1 CO 70.0/67.5 1.7 69.1/66.5 1.08/1.12 992/956L2 CO2 80.3 −9.7 82.5 −2.00 1223L2 CO 67.5/65.1 0.6 66.6/64.1 0.79/0.83 962/927L3 CO2 99.5/93.6d 97.7/91.9 43.82/43.92 880/828L3 CO 105.2 46.1 104.4 5.00 1395

AN1 CO 86.2/84.4 27.4 85.4/83.5 −0.20/−0.17 1233/1207AN1 CO+CO2 120.0d 115.4 77.10 804AN2 CO+CO2 95.7d 92.0 79.32 635HP HO 4.7 3.3 3.2 0.61 55PA HO 37.9/37.2d 34.6/33.9 37.50/37.46 342/335SA HO 59.0/58.5d 56.0/55.5 34.29/34.29 561/556P HO 112.3d 109.9 34.34 1073

aSee Chart A, EPAPS.bEa and �S‡ are functions of the temperature at which a maximum is present in the TPD spectrum �Tmax�, anddetermine in turn Tmax �see text�.ccal mol−1 K−1.dNo backwards barrier ��E‡ and �E coincide�. A diradicaloid structure results for L3 and AN1.

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in the reactant molecule; in particular, given that forCA2 and CA3 the products are identical, the reactionenergy must reflect the stability of the initial modelPAC. Moreover, for CA2 and CA3, the reverse barriersare similar, and the small difference is ascribable to thesteric hindrance present for CA2.

�4� For the lactones, the extrusion process is concerted andsimilar to that which takes place in K. The lactones canlose in principle either CO or CO2, by concurrentlyclosing a six-or five-membered ring, respectively. Theformer process, which entails formation of an etherfunctionality within a cyclic structure, is more favor-able by 10–16 kcal mol−1 in the first two models, L1and L2 �Table I�. L2, the dibenzo homolog of L1, wasintroduced again to explore the consequences of astiffer structure, but this factor must be already effec-tive in the smaller system, because the extension of themodel does not affect the energetics significantly; as amatter of fact, the barrier for CO extrusion comes outto be even lower by 2.5 kcal mol−1. By contrast, forCO2 loss, the barrier was slightly higher, due to asomewhat hampered formation of the five-C ring. Thetransition structure for CO loss in L2 and CO2 loss inL1 are shown in the EPAPS Chart A and Figs.doc file.The preceding fragmentations take place from a func-tionalized zigzag border. The lactone L3 is, by contrast,a model for a functionalized armchair border. In L3 theconcerted “CO loss plus ring closure” process shows abarrier of 105 kcal mol−1: this reaction is significantlymore difficult than the preceding CO extrusions, by35–38 kcal mol−1, since a five-membered ether ringclosure is required. It is also endoergic by46 kcal mol−1, while the other two were almost isoer-gic. The alternative fragmentation, with CO2 loss,would leave a 1,4-diradical in which the hybrid lobescarrying unpaired electrons do not overlap significantly.This feature entails that the reaction energy is signifi-cantly higher.

�5� The anhydride AN1 can undergo CO loss with a barrierof 86 kcal mol−1. The product is a lactone and the re-action is endoergic by 27 kcal mol−1. By contrast, CO+CO2 loss in AN1 requires 120 kcal mol−1 and is en-doergic by the same amount, since no barrier for reas-sociation can be found. Also in this case a diradical isgenerated, in which the hybrid lobes, positioned 1,3and carrying unpaired electrons, are parallel. A similarfragmentation in AN2 is easier, since the potentialdiradical is actually a benzynelike structure, whosewave function results are stable in correspondence witha closed shell description.

In contrast with the just mentioned endoergic andpurely dissociative energy profiles for diradical forma-tion from L3 �upon CO2 loss� and AN1 �upon CO+CO2 loss�, some processes which are only moderatelyendoergic �loss of CO in K3, A, L1, and L2� or evenexoergic �loss of CO2 in CA1, CA2, CA3, L1, and L2�,and encompass formation of a new C–C bond, exhibitnonetheless significant barriers. This can be rational-ized by considering that these fragmentations are remi-

niscent of the simpler and well studied decompositionsof formaldehyde, to give H2 �in lieu of a new C–Cbond� and CO �K1, K2, K3, and A�, on one hand, andformic acid, to give H2 and CO2 �CA1, CA2, CA3, L1,and L2�, on the other hand, both of which present highfragmentation barriers.30

�6� The last four entries in Table I are the potential HO-producing molecules of Chart 2. For three of them nobackward barrier is found in correspondence with reas-sociation �in other words, for dissociation, �E‡ and �Ecoincide�. Loss of hydroxyl from HP is exceedinglyeasy and the reaction is almost isoergic. This can betraced back to the formation of a carbonyl group andconcomitant delocalization of a � electron �instead ofhaving an oxyl group and a closed shell � system�. HPitself can be seen as a rather unstable starting molecule.HO loss from PA, SA, and P requires 38, 59, and112 kcal mol−1, respectively.

It can be noted that the entropic contribution �sixth col-umn in Table I� is not large when �S‡ is related to a transi-tion structure, in which the separating moieties are stillbound together to some extent, but becomes significant �asexpected, since two independent moieties originate fromone� when the dissociative process simply leads to an energyplateau and does not present a rcassociation barrier �as in L3,PA, SA, and P�. �S‡ is still larger when the fragmentationproduces three molecules �as in AN1 or AN2 losing CO andCO2�. A significant positive �S‡ contribution tends to lowerthe peak temperature Tmax value.

In the case of carboxylic acids, the temperature predictedon the basis of the unimolecular process is fairly high andwould be in good agreement only with a high-T CO2 maxi-mum �such as the 900 K maximum, which is, however, usu-ally assigned to other functionalities�. Therefore, we lookedfor some alternatives.

First, the effect of size extension on the fragmentation

SCHEME 1. Size extensions for the carboxylic acid models.

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barrier for the concerted unimolecular fragmentation was ex-plored. Two new series of model molecules were built. Thefirst one was obtained by extending the size “monodimen-sionally” �Scheme 1, a�, from anthracene-9-carboxylic acidto pentacene-6-carboxylic acid, to get a substituted poly-acene structure with up to nine condensed six-memberedrings, having in the middle the one carrying the carboxylicgroup. The second one was built by condensing “anthracene”units on that side of the preceding acid which is opposite tothe COOH group, to get, for instance, from the anthracene-9-carboxylic acid the analogous anthanthrene-6-carboxylicacid �Scheme 1, b�, and so on, up to 15 condensed six-membered rings. The barriers related to the reoptimized frag-mentation transition structures drop upon size extension, butseem to tend asymptotically to the values of ca. 50 �a, ninecases� and ca. 55 �b, five cases� kcal mol−1, respectively �seethe EPAPS supp-info.xls file�. Though the trend is interest-ing, these limit values are still too high and cannot be appro-priately put in relation to the lower-temperature TPD maxi-mum.

Then, we considered that the concerted fragmentation ofCAn could be flanked by two-step processes producing CO2,CO, and HO. These are �1� Ar–COOH→Ar·+HOC·O, fol-lowed by C–O or H–O bond cleavage, to give HO+CO, orH+CO2; �2� Ar–COOH→HO+Ar–C·

vO, followed byC–C bond cleavage, to give Ar·+CO. A test was carried outon the benzoic acid, but the energetics imply Tmax

�1000 K. The result is useless in view of defining a mecha-nism corresponding to a low-T CO/HO maximum. How-ever, at the computationally assessed temperature, one findstwo parallel maxima for CO and HO in the experimentalspectra.9,31

In an attempt to find a process which could be associatedwith the rather low-temperature TPD maximum for CO2 andHO, expected for carboxylic acids, we proceeded along twofurther directions. We looked first for a possible bimolecular�and presumably easier� process. Then we took into accountthe possibility of having some cooperative effect in vicinalbifunctionalized PAHs, which could have been generated byanhydride hydrolysis.32

C. Bimolecular fragmentation processes

The presence of hydroperoxyl functionalities could pro-duce, in the early phases of the programed temperature rise,hydroxyl radicals by cleavage of the weak O–O bonds. Weestimate, in particular, that the peroxyacid PA or the second-ary alcohol SA could produce hydroxyl around Tmax=340and 560 K, respectively. It seems reasonable to surmise thatHO from the latter can trigger some process different fromthe unimolecular fragmentations examined up to now. Theattack of HO· onto anthanthrene-6-carboxylic acid, CA1, wasthen investigated.

The 6-311G�2df , p� energetics of the reaction pathwayare described in Fig. 1. The first step is the exoergic forma-tion from the reactants A of a hydrogen-bonded complex B��E=−11.6 kcal mol−1�. Its stability is overestimated be-cause of the BSSE, and the correction is estimated by the

counterpoise method21�b� to be 3.65 kcal mol−1, when thelarger basis set is used ��EBSSE=−8.0 kcal mol−1�.

Then, hydroxyl easily abstracts a hydrogen from theCOOH group, passing through a transition structure TS B-Cin which the H bond is fully maintained ��E‡

=1.8 kcal mol−1 only with respect to B�, to give waterand an Ar–COO radical in a further exoergic step��E=−32.7 kcal mol−1�. These moieties are held together bytwo H bonds in the �presumably vibrationally excited� sec-ond complex C. Again, the stability of this complex is over-estimated because of the BSSE, and this time the6-311G�2df , p� correction is estimated to be 6.3 kcal mol−1

��EBSSE=−26.3 kcal mol−1�. The initial exoergic capture ofHO by the acid to generate this complex can be seen asfacilitating the next step. This occurs in two respects. First,the complex itself is, as it forms, energetically excited. Moreprecisely, it will be vibrationally excited in a rather localizedway, at least in the beginning. This means that the vibrationalexcitation will involve modes dominated by coordinates rel-evant to the two hydroxyls which have interacted and areexpected to react in the following easy H-abstraction step.Indeed, B could be even unable to achieve thermalizationbefore giving way to C. The dissociation step �loss of CO2�from the thermalized complex would require overcoming anoverall barrier of ca. 27 kcal mol−1 �which can thus be seen

FIG. 1. Energy profile �6-311G�2df , p� data� for the reaction of HO· withthe anthanthrene-6-carboxylic acid. �A� Reactants. �B� First complex be-tween the reactants �arrow: BSSE correction�. TS B-C: H-abstraction TS.�C� Second complex between water and ArCOO· �arrow: BSSE correction�.�D� The two separate moieties. TS D-E: C–C bond cleavage TS. �E� Theresulting sp2-localized Ar· radical and carbon dioxide.

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as an upper limit�. This value can be compared with the ca.55 kcal mol−1 required for the unimolecular process in CA1:a less demanding series of reaction steps can thus be openedby hydroxyl at significantly lower temperatures �of the orderof 500–600 K instead of 800–950 K�. What has just beendiscussed closely recalls the very comprehensive study byDe Smedt et al. on the gas-phase reaction of hydroxyl withacetic acid.33 In that study the role of the hydrogen bondedcomplexes was, in fact, stressed, in that they were shown toremain intact in the transition structures for H abstractionfrom the carboxyl group, thus assisting the process.

Furthermore, recalling that HO loss from PA requires38 kcal mol−1 and leaves just the Ar–COO· species shown asD in Fig. 1, it can be noticed that further CO2 loss from itrequires only 14 kcal mol−1. Thus, also the peroxyacid func-tionalities present on the soot platelet can be considered as apossible low-T CO2 source.

D. Unimolecular fragmentations in bifunctionalmolecules

Various authors have conjectured34 that anhydride andlactone groups present on the oxidized soot platelet can besubjected to hydrolysis in the presence of water. Of these, thebicarboxylic acids come of course from the former, and thehydroxy-carboxylic acids from the latter. Therefore, we con-sidered also fragmentation �desorption� taking place in thedicarboxylic acids shown in Chart 3, taken as models of ahydrolized anhydridic functionality of a soot platelet. Inthese molecules, the presence of two acidic groups close toeach other could give way to cooperative effects and to lowerbarriers for CO2 loss. DCA1 prefers to lose water rather thancarbon dioxide, hence is not a good model for the latterprocess. DCA2, by contrast, prefers to decarboxylate. Its dis-sociation could be said a “superconcerted” process, in whichthree chemical events are involved. Its description �see detailin Fig. 2; Fig. E, in the EPAPS Chart A and Figs.doc file,

reports the full structure� reflects, in fact, the asynchronicityof these events, as described by the transition vector �v� co-efficients �or equivalently inferred from an animation of thenormal mode related to the imaginary frequency; see theEPAPS xyz.txt file�.

The process encompasses �1� H transfer from the OH inthe leftmost carboxylic group, which will leave as CO2, tothe other hydroxyl �largest weight in v�; �2� this OH, in turn,gives away its H to the endocyclic carbon undergoing CO2

loss; finally �3� we see the loss of CO2 itself �with a smallerweight in v�. The process requires overcoming an overallbarrier of ca. 30 kcal mol−1 with respect to the initial reactantmolecule. This datum can put the cooperative process in re-lation to the low-temperature TPD maximum. An IRC calcu-lation �carried out on the similar TS of a smaller model, thenaphthalene 1,8-dicarboxylic acid� confirms the relation ofthis TS with the reagent, on one side, and with a dissociationplateau on the opposite side.

DCA3 and DCA4 �structurally similar to DCA2� loseCO2 with a barrier of ca. 30 kcal mol−1. The correspondingtemperature is in the range of 440–480 K.

IV. DISCUSSION

A. Comparison with TPD experiments

Making special reference to the more recent papers byMuckenhuber and Grothe9 and Figueiredo et al.,10 we firstremark that different commercial soots �Monarch 120,Printex U, NORIT ROX 0.8� can present some differences inthe positions of the intensity maxima, though several traitsare in general shared by the TPD spectra �see, for instance,Fig. 3 in Ref. 9�. The temperature programed desorption ex-periments produce in general broad desorption bands, whichrender the interpretation of the spectra difficult. Therefore,TPD is often used in conjunction with mass spectrometry�TPD-MS� and IR spectroscopies �Fourier transform infrared�FTIR� and diffuse reflectance infrared Fourier transformspectroscopy �DRIFTS��.9,10 These techniques allow us toconnect the different maxima to specific desorbed molecules,as HO, CO, CO2, etc., and to suggest assignments of indi-vidual peaks to functional groups �see, e.g., Fig. 4 in Ref.10�—not without some difficulties. Since different functionalgroups can desorb at similar temperatures, the spectra can bemore reliably interpreted with the aid of curve fitting proce-dures, by which they are deconvoluted �see, e.g., Figs. 5–7 inRef. 9 and Fig. 7 in Ref. 10�. Thermal treatment of the

CHART 3. The bifunctional molecular models considered in the study.DCA2, DCA3, and DCA4 carry functional groups bound to what can beseen as a portion of zigzag border. In DCA3 one carboxylic group �and bothin DCA1 and DCA4� can also be seen as lying on an armchair border�dashed lines point out parts of armchair borders�. By contrast, DCA2 pre-sents a sheer zigzag border situation.

FIG. 2. Detail of the transition structure for the cooperative mechanism,which encompasses two concerted H transfers and CO2 loss in DCA4. Bondlengths in angstroms.

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sample under inert atmosphere at different temperatures canalso be helpful in simplifying the analysis �see, for instance,Fig. 5 in Ref. 10�.

B. Temperatures of the TPD maxima

The energy barriers reported in Tables I and II, togetherwith the results of the vibrational analysis, allow the estimateof the activation energy and preexponential factor of anArrhenius-type equation: these quantities permit in turn toassess the temperature of the TPD maximum �see Sec. II�.Some insight into the mechanistic details of variousfragmentation/desorption processes can then be achieved bycomparing the Tmax values so obtained �Tables I and II� withthe experimentally assigned maxima. Figure 3 helps in sum-marizing the experimental results �upper part� and in com-paring with them the present computational data �lower part�.

The lowest-temperature HO loss coming from our calcu-lations is from HP, indeed very low and not reported in Fig.3. Next comes that from PA, which occurs at Tmax�340 K.This result can be compared to the HO spectrum fromPrintex U �Fig. 3 in Ref. 9�, which seems to hint, on theextreme left, to some maximum at T lower than 400 K.Then, some HO release still at low T �ca. 500 K� is experi-mentally observed and attributed to water.9 We can put ourdecomposition of SA �Tmax�560 K� aside this datum as apossible concurrent contribution. Another HO broad experi-mental maximum occurs at higher T, centered around 980 K,and is accompanied by a parallel CO2 maximum. Their attri-bution is deemed ambiguous by Muckenhuber and Grothe,who excluded the attribution to carboxylic acids �“too un-stable”�, to phenols �“should result in CO and OH maxi-mum”�, and to anhydrides �“would result in a combined CO2

and CO signal”�.9 If we assume, in an unconventional andperhaps rather eccentric way, that not all carboxylic groupsgo lost at the relatively low temperatures generally stated,and that particular geometric/chemical situations could ren-der some CO2 detachments more difficult, we can relate the980 K centered TPD band to some computed temperaturedata. We find, indeed, not far from this value, CO2 produc-tion by the unimolecular mechanism from CA3 at ca. 945 K,

somewhat on the left side �at lower T� of the experimentalmaximum �further computational results for carboxylic acidswill be discussed in more detail below�. On the other hand,HO loss from a carboxylic acid �leaving Ar–C·

vO� can beestimated to take place at a temperature somewhat higherthan 1000 K.31 Further, the decomposition of P could pro-duce HO, and we find Tmax�1070 K. Both these data couldbe put on the right side of the experimental 980 K centeredband. As mentioned, a phenol is expected to produce alsoCO. Now, for the Monarch specimen, a clear maximum inthe CO curve is missing, though some clear signal increasecan be observed. By contrast, the Printex specimen presentsa well-defined CO maximum. Turning back to our threemodel carboxylic acids, they are described as producing CO2

by a unimolecular process with the following estimates ofTmax: CA1 at ca. 770 K, CA2 at ca. 800 K, and CA3, asalready mentioned, at ca. 945 K. All these values are higherthan the T range attributed to carboxylic acids in Ref. 9, andalso fall beyond the range defined in attributions based onprevious experimental work �ca. 400–700 K, see the left-most part of Fig. 3�. The A4 and A5 samples of Figueiredoet al., which are the most intensely oxidized �5% O2 at698 K for 10 or 20 h, respectively�, show a broad TPD CO2

band between 750 and 1000–1070 K, approximately �seeFig. 6 in Ref. 10�. An even wider band is presented by CO,which starts in the same temperature region but stretches tomore than 1100 K. In fact, in Ref. 10, Fig. 4, the DRIFTSspectra recorded for the specimen A7 �coming from the oxi-dized A4 subsequently treated under N2 for 1 h at 1023 K�still exhibit the remnants of two bands �at 1200 and 1600 K,approximately� which are also attributable to carboxylic ac-ids �though not only to them�. So, one possibility is to ten-tatively relate our rather high-temperature unimolecular pro-cesses to the relatively high-T broad TPD band. Conversely,another possibility is thinking that the intramolecular processdepicted in Fig. B, EPAPS Chart A and Figs.doc file, is not arealistic representation of what comes to pass in the desorp-tion process. A better agreement with the common attributionfor carboxylic functionalities is offered by the alternative bi-molecular process �Fig. 1� which could be triggered by hy-

TABLE II. Bifunctional models: Computed energies �when two �E‡ are reported for the fragmentation barriers,the format: 6-31G�d , p� /6-311G�2df , p� is used, as in Table I. The �E values are only 6-31G�d , p�, Both �E‡

and �E are relative to the reactants �kcal mol−1�� activation energies, and temperatures ��unit in kelvins�. Tincrement rate �=0.17 K s−1, approximatively� for the TPD maximum.

Substratea Loss of �E‡ �E Ea�Tmax�b �S‡�Tmax�

b,c Tmax

DCA1 CO2 41.7 −24.9 ¯ ¯ ¯

DCA1 H2O 30.7/32.0 −6.1 27.7/29.0 −4.82/−4.87 455/474DCA2 CO2 28.9/31.1 −21.6 25.9/28.0 −3.13 416/449DCA2 H2O 36.9 −6.4 ¯ ¯ ¯

DCA3 CO2 30.0/31.8 −23.3 26.9/28.6 −5.73/−5.89 449/476DCA4 CO2

d 29.6/31.4 −24.1 26.5/28.2 −5.19/−5.35 438/466DCA4 CO2

e 41.4 −19.1 ¯ ¯ ¯

aSee Chart 3.bEa and �S‡ are functions of the temperature at which a maximum is present in the TPD spectrum �Tmax�, anddetermine Tmax �see text�.c�cal mol−1 K−1�.dLoss of the rightmost CO2 �see Chart 3�eLoss of the leftmost CO2 �see Chart 3�.

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droxyl radicals, possibly generated in rather easy O–O bondcleavages, such as that studied for SA. This easier pathwaycorresponds, in fact, to temperatures of 500–600 K. It can benoted that concurrent HO and CO2 TPD maxima can beobserved within this same temperature range. See, for ex-ample, the TPD spectrum for Printex U, Fig. 3 in Ref. 9 �butthis trait is not observable for Monarch�. An extended seriesof peaks is also observable in the CO2 TPD spectrum of Fig.3, Ref. 10, for sample A1, the original NORIT ROX carbon.Finally, also the cooperative mechanism �detail in Fig. 2; infull: Fig. E, EPAPS Chart A and Figs.doc file� studied forbicarboxylic model acids offers a temperature range closer tothat indicated in several papers, though it might be actuallydeemed a bit too on the low T side �ca. 420–480 K�. Bothmechanisms could, however, concur to the rather broad low-temperature maximum �see the two largest “acid lines” inFig. 3�.

Also lactone functionalities can be thought of as contrib-uting to the high-T bands of CO and CO2. Loss of CO froma lactone corresponds in our zigzag models L1 and L2 toTmax�930–990 K. On the other hand, loss of CO from thearmchair model L3 is predicted to take place at a very hightemperature, almost 1400 K. The results for CO2 loss con-trast this picture. We estimate the temperature for a signalmaximum within the range of 1150–1220 K for L1 and L2,

out of the experimental “lactone zone” defined in Ref. 9.�gray dashed box in Fig. 3�. Though the lactones are locatedin Ref. 10 at higher temperature, they are also considered asremoved above 1023 K. Moreover, the reported computedtemperatures are also higher than the corresponding range forCO loss. Therefore, CO loss appears favored over CO2 loss,in contrast with the common opinion found in the literature.By contrast, in the case of L3, we assess Tmax

�830–880 K for CO2 loss, which appears to be the favoredfragmentation mode. Moreover, this value matches nicelywith the experimental data, falling within the mentioned lac-tone zone. Our results for the models L1, L2, and L3 suggestthat the desorption mode from a lactone-functionalized por-tion of a graphenic platelet is significantly dependent fromthe chemical/geometric local environment. Thus, we can pro-pose that CO2 desorption cannot be the only outcome, andCO desorption should also be considered. If so, the CO ex-perimental maximum around 1000 K, present in both Mon-arch and Printex spectra �Fig. 3 in Ref. 9�, could build upwith a contribution from lactone functionalities.

CO loss from the anhydride functionality is assessed asnot easy: in the zigzag AN1 model Tmax�1230 K. By con-trast, CO+CO2 loss not only from AN1 but also from AN2�armchair� models comes out to be less demanding and cor-responds to lower temperatures: ca. 800 K the former, ca.

FIG. 3. Comparison of the experimental and computed data. Upper lines: Assignments made on the basis of the experiments: �1� Ref. 34�a�; �2� Ref. 24�b�;�3� Ref. 12; �4� Ref. 34�c�; �5� Ref. 32�a�; �6� Ref. 34�d�; �7� Ref. 10; �8� Ref. 9. Lowest line: Computationally estimated temperatures for the TPD maxima�Tmax�; indication of the species which dissociates in parentheses. From left to right, the following is reported. Cooperative concerted mechanism in vicinalbicarboxylic acids: DCA4 �416 K�, DCA2 �438 K�, DCA1 �449 K�. PA �516 K� or SA �561 K� may trigger the OH-initiated bimolecular mechanism bywhich CO2 is released from carboxylic acids around those temperatures. Concerted unimolecular mechanism for carboxylic acids: CA1 �770, CO2�, CA2 �803,CO2�, on the left, and CA3 �943, CO2� very close to lactones L2 �962, CO� and L1 �992, CO�. Then P �1073, OH�, A �1155, CO�, AN �1233, CO�, lactoneL3 �1395, CO�, and K1 �1638, CO� follow. Mechanism in parentheses�.

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630 K the latter. Therefore CO loss is not to be expected. ForCO+CO2 loss a lower Tmax is assessed, notwithstanding thehigher E barrier �120 or 96 vs 86 kcal mol−1�, because of thehigh entropy gain. These relatively low-T values put ourmodels in correspondence with the lactone zone, making ref-erence to the attributions in Muckenhuber and Grothe’s pa-per, while the anhydrides are identified there as spanning the975–1040 K range, on the basis of concurrent CO and CO2

signals �Fig. 7 in Ref. 9�. However, the 800 and 630 K com-puted values of AN1 and AN2 are in fair accordance with theattributions of Refs. 34�d� and 10 �where the anhydrides aresaid to be removed above 873 K�. In fact, it can be noted inFig. 3 that a wide variety of temperatures is attributed to theanhydride functionality in the different studies �from650 to 1040 K�.

Finally, let us consider the carbonyl functionalities ofaldehydic and ketonic nature, for which we consider the lossof a CO molecule. The aldehyde A has Tmax

�1120–1155 K. This is within the broad CO TPD band�T�750–1200 K� of the oxidized sample A4 �NORIT�. Inthat paper, it is concluded that carbonyls are completely re-moved upon heating at 1373 K.10 Loss of CO from a ketoneresults very difficult for the K1 and K2 models, and theestimated temperature for the TPD maximum �K1: Tmax

�1640 K� is quite high, beyond the usual limit of the ex-periments. This is attributable to the rigidity of the con-densed six-membered rings which are contiguous to the clos-ing five-membered ring. By contrast, the K3 model, designedto limit this stiffness, exhibits a much lower barrier and,consequently, a lower Tmax value, in the 650–660 K range.This appears to be even low, if we compare it again with thebroad CO band of sample A4.10 The results obtained for thespecimens subjected to heat treatment �Fig. 5a in Ref. 10�provide further information. Actually, these spectra can sug-gest that above 873 K �A6 sample, maximum at ca. 1070 K�,and to some extent even above 1023 K �A7 sample, maxi-mum at ca. 1170 K�, extrusion of CO can still take place.Conversely, the diffuse reflectance FTIR spectrum of the A6specimen does not show anymore the typical CvO stretch-ing peak �Fig. 4 in Ref. 10�. A different information is pro-vided by the Monarch and Printex soots. A broad CO TPDband is found centered around T=1000 K in the TPD spec-trum of the Printex sample. By contrast, no maximum ispresent in the spectrum of the Monarch soot, though a prettyconstant rise can suggest the presence of a maximum at T�1200 K.9

V. CONCLUSIONS

In this study we have examined some possible fragmen-tation mechanisms in functionalized PAHs, chosen to modelthe structural features of oxidized soot platelets. The com-puted fragmentation �desorption� barrier heights allow acomparison with published TPD experiments. The computedbarriers for different mechanisms have been used to estimatethe temperature at which a maximum occurs in TPD spectrafor HO, CO, or CO2 desorption. Matching calculated tem-peratures with experimental spectral maxima can then sug-

gest an assignment of these maxima to specific functionalgroups and provide in turn a description of the oxidized sootsurface.

A. “Carboxylic acid” functional zone

Three different mechanisms have been explored. Boththe bimolecular process, triggered by some hydroxyl radical�whichever the source�, and the cooperative mechanism,studied for bicarboxylic model acids, offer a temperaturerange close to that indicated by the assignments done in re-cent experimental papers �T�400–700 K�. However, bothmechanisms rely on assumptions. The former, that HO isavailable to some extent, possibly from the decomposition ofperoxyacids or saturated alcohols. The latter depends on thehypothesis that some anhydride functionalities could havealready undergone hydrolysis, thus generating the “bicar-boxylic acid” functional zone. As a third suggestion, a uni-molecular mechanism, entailing H transfer from COOH tothe ring carbon abandoned by CO2, is characterized by aTmax�770–950 K, depending on the model. We can sup-pose, perhaps in a rather eccentric way, that not all decar-boxylations can take place at the relatively low temperaturesgenerally stated, because some geometric/chemical situationscan render CO2 detachment more difficult. By doing so, wecan tentatively relate the higher-temperature unimolecularprocesses to the broad CO2 TPD band in the 950 K zone�which corresponds to a similar band present in the HO TPDspectrum�. Again at high T, we find other bond cleavageprocesses that could generate hydroxyl.

B. “Lactone” functional zone

Desorption of CO is preferred over that of CO2 in two ofthe three lactone models investigated, with Tmax

�930–990 K. In the third one, the reverse is true, andTmax�830–880 K. The temperature range matches satisfac-torily the experimental assignments �ca. 700–1000 K� in allcases. This result indicates that the desorption mode from alactone-functionalized portion of a graphenic platelet can besignificantly dependent from the chemical/geometric localenvironment. Thus, we propose that CO2 desorption cannotbe the only outcome, and CO desorption should also be con-sidered.

C. “Anhydride” functional zone

In this case, CO+CO2 loss is preferred over less destruc-tive decompositions and corresponds in our models to Tmax

�800 K in one case, Tmax�630 K in the other. These rela-tively low-T values put our models in correspondence withthe lactone zone, making reference to Muckenhuber andGrothe’s paper,9 while the anhydrides are there identified asspanning the 975–1040 K range, on the basis of concurrentCO and CO2 signals. However, our Tmax values are in fairaccordance with the attributions of other experimentalstudies.10,34 Indeed, a wide range of temperatures is spannedby the attributions from different studies �from650 to 1040 K�.

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D. “Aldehyde” or “ketone” functional zone

To conclude, these carbonyl functionalities are predictedto lose a CO molecule at rather high temperatures. The alde-hyde has Tmax�1120–1155 K, while a first ketone has evenTmax�1640 K �this is attributable to the rigidity of themodel�. Another more flexible model ketone has insteadTmax�650–660 K. The feasibility of this extrusion is evi-dently quite dependent on the chemical environment of thefunctional group.

�See end of Ref. 29.�

ACKNOWLEDGMENTS

We thank Dr. Hinrich Grothe �Materials Chemistry Insti-tute, Technical University, Wien� for a very fruitful discus-sion and for disclosing his TPD-MS data to us before publi-cation. We would also like to thank Professor Luigi Costa�University of Torino� for helpful discussions. Financial sup-port by the Italian MIUR is gratefully acknowledged �PRIN-COFIN 2004, “Studio Integrato sul Territorio Nazionale perla Caratterizzazione ed il Controllo di Inquinanti Atmosferici�SITECOS�”�. A generous grant provided by the RegionePiemonte is also acknowledged �DD n. 1 �18.1.2006�; DD. n.64 �2.12.2005�; Bando Ricerca Scientifica-Settore SviluppoSostenibile�. This work was conducted in the frame of ECFP6 NoE ACCENT �Atmospheric Composition Change, theEuropean NeTwork of Excellence�. Supplementary materialis available. This material includes the geometries of all op-timized structures �one txt file�, the corresponding energeticsin one Excel �xls� file, and a more complete version of Chart1 together with some MOLDEN structures embedded in aWord 2000 �doc� file.35

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21 �a� F. Jensen, Introduction to Computational Chemistry �Wiley, NewYork, 1999�, Chap. 6; �b� ibid, Sec. 5.8.

22 �a� 6-31G: W. J. Hehre, R. Ditchfield, and J. A. Pople, J. Chem. Phys. 56,2257 �1972�; 6 -31G�d� P. C. Hariharan and J. A. Pople, Theor. Chim.Acta 28, 213 �1973�; �b� M. J. Frisch, J. A. Pople, and J. S. Binkley, J.Chem. Phys. 80, 3265 �1984�; Since, upon extending the basis set, thechanges in computed activation energies and predicted temperatures forthe TPD maxima were modest, and considering that the computed valueshad to be compared to broad experimental maxima, we did not extend thecalculations to some more demanding or less promising systems.

23 C. Gonzalez and H. B. Schlegel, J. Chem. Phys. 90, 2154 �1989�; J.Phys. Chem. 94, 5523 �1990�, and references therein.

24 R. Seeger and J. A. Pople, J. Chem. Phys. 66, 3045 �1977�; R. Bauern-schmitt and R. Ahlrichs, ibid. 104, 9047 �1996�; H. B. Schlegel and J. J.McDouall, in Computational Advances in Organic Chemistry, edited byC. Ogretir and I. G. Csizmadia �Kluwer Academic, The Netherlands,1991�, p. 167.

25 S. Yamanaka, T. Kawakami, K. Nagao, and K. Yamaguchi, Chem. Phys.Lett. 231, 25 �1994�; K. Yamaguchi, F. Jensen, A. Dorigo, and K. N.Houk, ibid. 149, 537 �1988�.

26 M. J. Frisch, G. W. Trucks, H. B. Schlegel et al., GAUSSIAN 03, RevisionB.02, Gaussian, Inc., Pittsburgh, PA, 2003.

27 K. W. Kolakinski, Surface Science: Foundations of Catalysis and Nano-science �Wiley, New York, 2002�, p. 173 ff. The treatment based on thePolanyi-Wigner equation is sufficient for our purpose �assigning broadmaxima to individual functionalities through some mechanism of HO,CO, or CO2 detachment�. However, more elaborate treatments have ap-peared in the literature: J. A. W. Elliot and C. A. Ward, J. Chem. Phys.106, 5677 �1997�.

28MOLDEN, G. Schaftenaar and J. H. Noordik, J. Comput.-Aided Mol. Des.14, 123 �2000� �http://www.cmbi.ru.nl/molden/molden.html�.

29 The CO moiety is noticeably out of the plane which is defined approxi-mately by the “PAH carbons.” In the TS, the two cleaving �O�C–C�bonds �C� belonging to the polycyclic system� are elongated by differentamounts, ca. +10% and +6% of their initial values �from 1.488 to 1.639and 1.538 Å, respectively�. The C�–C� distance �1.665 Å� is shortenedby 34% with respect to its initial value �2.532 Å�. Given that in theproduct, upon five-ring closure, this distance drops to 1.505 Å, its short-ening in the TS corresponds to 84% of the total. Correspondingly, theC–O distance undergoes in the TS a more moderate shortening, corre-sponding to 38% of the total. This is the description of a highly asyn-chronous concerted process. Figure A in the .doc file provided in theEPAPS illustrates it. See also Refs. 15�a�, 15�b�, and 16, and the com-

194706-11 Oxidized soot surface J. Chem. Phys. 125, 194706 �2006�

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ments in Ref. 17, p. 3443. Our TS search was performed by the treatmentoutlined in Sec. II. See also, in the EPAPS, the supp-info.xls file. Somesearches for a two-step mechanism were also carried out, but the possibleintermediate collapses back to K1. �Please see diagram in regard to thisreference above the Acknowledgments.�

30 J. D. Goddard, Y. Yamaguchi, and H. F. Schaefer III, J. Chem. Phys. 96,1158 �1992�; J. S. Francisco, ibid. 96, 1167 �1992�; B. S. Jursic, J. Mol.Struct.: THEOCHEM 418, 11 �1997�; X. Li, J. M. Millam, and H. B.Schlegel, J. Chem. Phys. 113, 10062 �2000�.

31 Ph-COOH→Ph·+HOCO· ��E=107.2 kcal mol−1�; HOCO·→HO+CO��E=36.0 kcal mol−1�; HOCO·→H+CO2 ��E=8.8 kcal mol−1�;Ph–COOH→HO+Ph–C·–O ��E=112.0 kcal mol−1�; Ph–C·–O→Ph·

+CO ��E=31.3 kcal mol−1�. The highest barriers correspond approxi-mately to Tmax=1040 K.

32 �a� Q.-L. Zhuang, T. Kyotani, and A. Tomita, Carbon 32, 539 �1994�;�b� M. Starsinic, R. L. Taylor, and P. L. Walker, Jr., ibid. 21, 69

�1994�.33 F. De Smedt, X. V. Bui, T. L. Nguyen, J. Peeters, and L. Vereecken, J.

Phys. Chem. A 109, 2401 �2005�.34 �a� J. Driel, in Activated Carbon: A Fascinating Material, edited by A.

Capelle and F. de Vooys �Norit, Amersfoort, 1983�, pp. 40–57; �b� B.Marchon, J. Carrazza, H. Heinemann, and G. A. Somorjai, Carbon 26,507 �1988�; �c� Q.-L. Zhuang, T. Kyotani, and A. Tomita, Energy Fuels8, 714 �1994�; �d� U. Zielke, K. J. Huttinger, and W. P. Hoffmann,Carbon 34, 983 �1996�.

35 See EPAPS Document No. E-JCPSA6-125-010640 for the energies of allcritical points � Tmax computation �Excel file: supp-info.xls�; Chart Aand 8 structures �Word 2000 file: Figs.doc�; cartesian coordinates of allcritical points: xyz.txt. This document can be reached via a direct link inthe online article’s HTML reference section or via the EPAPS homepage�http://www.aip.org/pubservs/epaps.html�.

194706-12 Barco et al. J. Chem. Phys. 125, 194706 �2006�

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