**5. Adsorption and reactivity of vocs on a mineral aerosol surface**

Clay particles are present in large quantities in mineral dust in atmospheric aerosols and their interaction with organic species may, in principle, influence atmospheric reactions and the prediction of environmental risks. Thus, it is interesting to study the kinetics and mechanisms of adsorption and reaction of atmospheric volatile organic compounds bound to a model clay surface in order to understand and describe, at the molecular level, its effect on the reaction.

In this section, mechanistic and kinetic data for OH-initiated oxidation reactions of VOC's adsorbed on mineral aerosols models are discussed.

The rigid tetrahedron SiO4 is the building block of all siliceous materials, from zeolites to quartz and amorphous silica. Clay minerals, or phyllosilicates, are formed by sheets of SiO4 tetrahedrons joined to a sheet of Al oxide octahedrons (Figure 1). The ideal surface of a phyllosilicate is characterized by the presence of a large number of siloxane Si-O-Si bridges, forming hexagonal rings. However, a natural clay surface presents many structural defects and fractures, and its chemical properties are largely due to the presence of active sites on the surface. These are mainly acid sites (Brönsted sites, associated to aluminol and silanol groups) and Lewis sites (such as in four-coordinated Al phyllosilicates) that have large specific surfaces and catalytic properties. Therefore, their presence in aerosols can be expected to play an important role in the heterogeneous chemistry of the troposphere.

Within the quantum mechanical methodology, small clusters of silicate groups are used to model silicate surfaces (Sauer et al., 1994). The basic premise behind the cluster models is that reactions and adsorption are local phenomena that are primarily affected by the nearby surface structure. The active site is described explicitly by the interactions between the local molecular orbitals of the adsorbate and the adsorbent. Clearly, this methodology has the disadvantage that the electronic system is represented only partially, due to the small size and the discrete nature of the cluster employed. However, it has been shown to be adequate to represent a mineral aerosol surface and to allow the use of high accuracy quantum methods to describe the energetics of adsorption, reaction paths, and intermediates formation.

Reactivity Trends in Radical-Molecule

*OH* Si(OH)4

with OH could be significant.

surface, as shown in Figure 9.

Tropospheric Reactions – A Quantum Chemistry and Computational Kinetics Approach 239

the frequency of the O-H stretching involved in the main interaction is 204 cm-1.

Fig. 8. Adsorption complex calculated infrared spectrum.

4062 cm-1 3858 cm-1 204 cm-1

adsorption complex calculated infrared spectrum is shown in Fig. 8. The observed shift in

In the main reaction channel, formaldehyde reacts with OH radicals by hydrogen abstraction, to form a water molecule and a bound formyl radical. We showed that the rate constant for the H-abstraction reaction is an order of magnitude smaller when formaldehyde is bound to Si(OH)4 than in the gas phase. Thus, on the basis of the calculated rate constants one can conclude that, when the OH-formaldehyde reaction occurs in the presence of dust, at atmospheric pressure, it is *slower*, by a factor of about ten, than the reaction in the gas phase. If one takes into account the fact that silicate aerosols are known to trap some of the OH radicals in the troposphere, the resulting decrease in the formaldehyde reaction rate

On a perfect silicate surface with no defects, there are no OH groups and adsorption is much weaker. This case can be modelled with a simple hexamer with no OH groups on the

Results obtained for the formaldehyde + OH reaction using larger cluster models (Iuga et al., 2008b) with OH groups on the surface yield results that are equivalent to those of the Si(OH)4 model and suggest that, at least for small molecules, the monomer model is

Adsorption of larger (C2-C5) aliphatic aldehydes on silicate clusters active sites, and their subsequent reaction with OH radicals was also studied (Iuga et al. 2010). Different adsorption complexes were found, and it is clear that the adsorption complex structure determines the subsequent path of its reaction with OH. The initial step in the OH reaction with acetaldehyde occurs according to the same mechanism as in the gas phase, i.e., the aldehydic hydrogen abstraction. Starting from propanal, another abstraction channel becomes increasingly important, involving mainly the abstraction of a hydrogen of the aliphatic chain. In the presence of a silica monomer model surface, the reaction rate of acetaldehyde with OH is found to be about one seventh its value in the gas phase. Because

sufficient to qualitatively predict the main effects of mineral aerosols.

*OH* Complex

*OH*

On the natural clay surface, silanol groups are the most reactive sites for adsorption of organic molecules and for their reactivity. Orthosilicic acid Si(OH)4 has been validated as a good model for both isolated and geminal silanol hydroxyl groups and we have used it to mimic the OH reaction with formaldehyde and formic acid on a silicate surface (Iuga et al., 2008a). Vicinal silanol groups may be studied by means of the (HO)3Si-O-Si(OH)3 dimer. In this model, substitution of a Si cation by a tetrahedral Al is useful to represent a wide range of silicates with Lewis or Brönsted acid properties. The silicate hexagonal six-member ring characteristic of a clay surface may be studied by means of a cyclic hexamer model, with and without an OH surface group. In Figure 7 we show the silicate models employed.

The adsorption energy is defined as the difference between the total electronic energy of the surface-adsorbate complex and the sum of those of the isolated molecule and the model surface, including ZPE corrections:

Fig. 7. Silicate cluster models.

The most stable adsorption complex in the reaction of OH· radicals with formaldehyde adsorbed on an Si(OH)4 monomer is shown in Fig. 8. In this complex, the formaldehyde molecule is clearly oriented perpendicularly to the surface and it attaches to the model surface by two hydrogen bonds. Surface hydroxyls play the role of a weak hydrogen donor and the adsorbed molecule assumes the role of the base.

The vibrational properties of silanol groups can be easily studied by infrared spectroscopy. On highly dehydrated surfaces, a single, well-defined band due to the OH stretch is measured at about 3742 cm-1 on an otherwise featureless spectral region extending from 3730 to 2000 cm-1. It is then easy to study the perturbations of the OH stretching frequency upon adsorption of molecules from the gas phase. As an example, the formaldehyde

Fig. 8. Adsorption complex calculated infrared spectrum.

On the natural clay surface, silanol groups are the most reactive sites for adsorption of organic molecules and for their reactivity. Orthosilicic acid Si(OH)4 has been validated as a good model for both isolated and geminal silanol hydroxyl groups and we have used it to mimic the OH reaction with formaldehyde and formic acid on a silicate surface (Iuga et al., 2008a). Vicinal silanol groups may be studied by means of the (HO)3Si-O-Si(OH)3 dimer. In this model, substitution of a Si cation by a tetrahedral Al is useful to represent a wide range of silicates with Lewis or Brönsted acid properties. The silicate hexagonal six-member ring characteristic of a clay surface may be studied by means of a cyclic hexamer model, with and

The adsorption energy is defined as the difference between the total electronic energy of the surface-adsorbate complex and the sum of those of the isolated molecule and the model

*∆E* adsorption = *E* adsorption complex – (*E* molecule + *E* surface) + Δ(ZPE)

The most stable adsorption complex in the reaction of OH· radicals with formaldehyde adsorbed on an Si(OH)4 monomer is shown in Fig. 8. In this complex, the formaldehyde molecule is clearly oriented perpendicularly to the surface and it attaches to the model surface by two hydrogen bonds. Surface hydroxyls play the role of a weak hydrogen donor

The vibrational properties of silanol groups can be easily studied by infrared spectroscopy. On highly dehydrated surfaces, a single, well-defined band due to the OH stretch is measured at about 3742 cm-1 on an otherwise featureless spectral region extending from 3730 to 2000 cm-1. It is then easy to study the perturbations of the OH stretching frequency upon adsorption of molecules from the gas phase. As an example, the formaldehyde

without an OH surface group. In Figure 7 we show the silicate models employed.

surface, including ZPE corrections:

Fig. 7. Silicate cluster models.

and the adsorbed molecule assumes the role of the base.

adsorption complex calculated infrared spectrum is shown in Fig. 8. The observed shift in the frequency of the O-H stretching involved in the main interaction is 204 cm-1.

In the main reaction channel, formaldehyde reacts with OH radicals by hydrogen abstraction, to form a water molecule and a bound formyl radical. We showed that the rate constant for the H-abstraction reaction is an order of magnitude smaller when formaldehyde is bound to Si(OH)4 than in the gas phase. Thus, on the basis of the calculated rate constants one can conclude that, when the OH-formaldehyde reaction occurs in the presence of dust, at atmospheric pressure, it is *slower*, by a factor of about ten, than the reaction in the gas phase. If one takes into account the fact that silicate aerosols are known to trap some of the OH radicals in the troposphere, the resulting decrease in the formaldehyde reaction rate with OH could be significant.

On a perfect silicate surface with no defects, there are no OH groups and adsorption is much weaker. This case can be modelled with a simple hexamer with no OH groups on the surface, as shown in Figure 9.

Results obtained for the formaldehyde + OH reaction using larger cluster models (Iuga et al., 2008b) with OH groups on the surface yield results that are equivalent to those of the Si(OH)4 model and suggest that, at least for small molecules, the monomer model is sufficient to qualitatively predict the main effects of mineral aerosols.

Adsorption of larger (C2-C5) aliphatic aldehydes on silicate clusters active sites, and their subsequent reaction with OH radicals was also studied (Iuga et al. 2010). Different adsorption complexes were found, and it is clear that the adsorption complex structure determines the subsequent path of its reaction with OH. The initial step in the OH reaction with acetaldehyde occurs according to the same mechanism as in the gas phase, i.e., the aldehydic hydrogen abstraction. Starting from propanal, another abstraction channel becomes increasingly important, involving mainly the abstraction of a hydrogen of the aliphatic chain. In the presence of a silica monomer model surface, the reaction rate of acetaldehyde with OH is found to be about one seventh its value in the gas phase. Because

Reactivity Trends in Radical-Molecule

for kinetic studies by the Truhlar group.

vibrational ground state.

**7. Acknowledgment** 

**8. References** 

88, 811.

Autónoma de México for computer time.

*valuesTheor Chem Acc* 2010, *126*, 75.

*Phys. Chem. A,* 2004, *108,* 2740.

Tropospheric Reactions – A Quantum Chemistry and Computational Kinetics Approach 241

models has been obtained. The relative site reactivity of the studied compounds towards OH radicals has been shown to be strongly influenced by intramolecular hydrogen-bond-like interactions that arise in the transition states. The usefulness of quantum chemical calculations to elucidate the detailed mechanisms of OH radical reactions with oxygenated VOCs has been proven. We show that the theoretical methodology employed provides accurate kinetic data that reproduce well the available experimental results and provide new data for a large number of tropospheric reactions. Although the calculations performed in the references cover a list of different quantum chemistry methods that have been used over the years, our latest work has been performed using the M05-2X density functional method developed specifically

The correct explanation of the anti-Arrhenius behavior common to many radical-molecule reactions has been elucidated. The importance of including reactant complexes in the modeling in order to obtain accurate values of the rate coefficients, has been shown. The best results are those obtained when it is assumed that such complexes are in their

The possibility of one-water-molecule catalysis in OH reactions with volatile organic

Since the main daytime tropospheric sink of oxygenated VOCs is their reactions with OH radicals, the mechanistic and kinetic information discussed in this work is essential in order to fully understand their tropospheric chemistry as well as their subsequent fate. Hopefully, the large amount of experimental and theoretical work that has been revisited here, which has been devoted to chemical reactions of environmental significance, could contribute in some extent to act in the right direction and prevent more damage to the atmosphere.

This work is a result of the FONCICYT Mexico-EU 'RMAYS' network, Project Nº 94666. We gratefully acknowledge the Laboratorio de Visualización y Cómputo Paralelo at Universidad Autónoma Metropolitana-Iztapalapa and the Dirección General de Cómputo y de Tecnologías de Información y Comunicación (DGCTIC) at Universidad Nacional

Alvarez-Idaboy J. R. & Galano A. *Counterpoise corrected interaction energies are not* 

Alvarez-Idaboy, J. R., Cruz-Torres, A., Galano, A. & Ruiz-Santoyo M. E*. Structure-Reactivity* 

Alvarez-Idaboy, J. R., Diaz-Acosta, I. & Vivier-Bunge, A. *Energetics of the mechanism of the* 

Alvarez-Idaboy, J. R.; Mora-Diez, N.; & Vivier-Bunge, A. *A Quantum Chemical and Classical* 

*Substituted Ethenes* J. Am. Chem. Soc. 2000, *122,* 3715.

*systematically better than uncorrected ones: comparison with CCSD(T)CBS extrapolated* 

*Relationship in Ketones + OH Reactions: A Quantum Mechanical and TST Approach J.* 

*OH-propene reaction at low pressures in an inert atmosphere*, J. Comput. Chem. 1998,

*Transition State Theory Explanation of Negative Activation Energies in OH Addition To* 

compounds is discussed, and it is shown that it does not occur.

*EADS* = 3.05 kcal/mol

Fig. 9. Formaldehyde adsorbed complex.

of the abundance of this contaminant, our results may have implications in tropospheric chemistry. For larger aldehydes, the rate constant is also consistently smaller than in the gas phase. Results are summarized in Table 2.


NIST data base (http://kinetics.nist.gov/kinetics/index.jsp).

Table 2. Calculated total rate constants (in cm3 molecule-1 s-1) and branching ratios at 298 K, for OH H-abstraction reactions of C2-C5 aldehydes on the Si(OH)4 model. Experimental values correspond to the gas phase reaction have been taken from the NIST data base.

Adsorption of other contaminants, such as carboxylic acids and several polyaromatic heterocycles have also been studied on silicate model clusters. In general, these compounds use their most reactive groups to add on the silanol groups, and consequently their OH reactivity is smaller than in the gas phase.

### **6. Conclusions**

In this article, we have reviewed our work on the mechanisms and kinetics of selected VOCs towards OH free radicals.

Quantum chemistry and computational kinetics methods have been used to model mechanisms and kinetics of the reactions of OH radicals with several groups of organic compounds (alkanes, alkenes, dienes, aromatics, aldehydes, carboxylic acids, alcohols, ethers, etc.) under tropospheric conditions. We have calculated reaction profiles, rate constants and branching ratios for numerous volatile organic compounds with OH and other radicals. Insight into reactivity trends, both in the gas phase and in the presence of mineral aerosol models has been obtained. The relative site reactivity of the studied compounds towards OH radicals has been shown to be strongly influenced by intramolecular hydrogen-bond-like interactions that arise in the transition states. The usefulness of quantum chemical calculations to elucidate the detailed mechanisms of OH radical reactions with oxygenated VOCs has been proven. We show that the theoretical methodology employed provides accurate kinetic data that reproduce well the available experimental results and provide new data for a large number of tropospheric reactions. Although the calculations performed in the references cover a list of different quantum chemistry methods that have been used over the years, our latest work has been performed using the M05-2X density functional method developed specifically for kinetic studies by the Truhlar group.

The correct explanation of the anti-Arrhenius behavior common to many radical-molecule reactions has been elucidated. The importance of including reactant complexes in the modeling in order to obtain accurate values of the rate coefficients, has been shown. The best results are those obtained when it is assumed that such complexes are in their vibrational ground state.

The possibility of one-water-molecule catalysis in OH reactions with volatile organic compounds is discussed, and it is shown that it does not occur.

Since the main daytime tropospheric sink of oxygenated VOCs is their reactions with OH radicals, the mechanistic and kinetic information discussed in this work is essential in order to fully understand their tropospheric chemistry as well as their subsequent fate. Hopefully, the large amount of experimental and theoretical work that has been revisited here, which has been devoted to chemical reactions of environmental significance, could contribute in some extent to act in the right direction and prevent more damage to the atmosphere.
