mechanism for oh-initiated oxidation of n-octane in the presence of o2 and no: a dft study

Chinese Journal of Chemistry, 2009, 27, 281—288 Full Paper

* E-mail: [email protected]; Fax: 0086-0531-88364435 Received July 24, 2008; revised and accepted December 19, 2008. Project supported by the National Natural Science Foundation of China (Nos. 20777047, 20737001 and 20873074) and the Shandong Provincial

Outstanding Youth Natural Sciene Foundation of China (No. JQ200804).

© 2009 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Mechanism for OH-Initiated Oxidation of n-Octane in the Presence of O2 and NO: A DFT Study

ZHAO, Yana(赵燕) WANG, Huib(王慧) SUN, Xiaomina(孙孝敏) ZHANG, Qingzhu*,a(张庆竹) WANG, Wenxinga(王文兴)

a Environment Research Institute, Shandong University, Jinan, Shandong 250100, China b Environment Science and Engineering Institute, Tsinghua University, Beijing 100080, China

Density functional theory was used to study the mechanism for the OH-initiated atmospheric oxidation of n-octane in the presence of O2 and NO. The geometries and frequencies of the reactants, transition states, interme-diates, and products were calculated at the B3LYP/6-31G(d) level and single-point energy calculations were carried out with a large basis set, 6-311＋G(3df, 2p). All of the possible product channels have been studied, but only the main ones are reported. The main products obtained are hydroxynitrates and hydroxycarbonyls, which have relative low vapor pressure and are inclined to form secondary organic aerosol. The theoretical results were compared with the available experimental observations.

Keywords n-octane, OH radical, reaction mechanism, atmospheric oxidation

Introduction

Atmospheric organic aerosol particles play a sig-nificant role in atmospheric chemistry and physics.1 Studies have shown that a large fraction of organic aerosol particles are formed as secondary organic aero-sol (SOA) resulting from the condensation of partially oxidized volatile organic compounds (VOC). SOA is the main contributor to ambient PM2.5 mass, accounting for up to 70% or more of the organic aerosol present in the urban plume.2 Now, it is realized that SOA has ma-jor impacts on visibility, cloud formation, direct and indirect radiative forcing, and human health.3-5

SOA is formed in the atmospheric oxidation of gas-phase organic compounds and subsequent gas- particle partitioning of lower-volatility reaction products. These organic gases are primarily VOC, including al-kanes, alkenes, and aromatic hydrocarbons. As reported, in urban air in cities such as Los Angeles, CA and Bos-ton, MA, the nonmethane VOCs are typically made up of the following: alkanes, ca. 40%—45%; alkenes, ca. 10%; aromatic hydrocarbons, ca. 20%; and oxygenates, ca. 10%—15%; plus unidentified VOC.2,6

Alkanes are important constituents of gasoline and vehicle exhaust7 and comprise ca. 50% of the non-methane organic compounds observed in ambient air in urban areas.8 Thus, it is necessary to understand the re-action mechanism of alkanes in the troposphere. At present, there have been a number of studies of the products formed from the OH radical-initiated reactions of alkanes.8-25 However, little attention has been paid to

the detailed mechanism for the SOA formed from al-kanes, which is mainly initiated by OH radicals in the troposphere.25 A large fraction of the products observed from the chamber, in which the OH-initiated reactions of n-alkanes (C8—C15 n-alkanes) under atmospheric conditions were simulated, have not been explained. To obtain more insight into the actual reaction pathways and explain the results of experimental observations, we performed a detailed study of the application of quan-tum calculations to the OH-initiated atmospheric reac-tion of n-octane in the presence of O2 and NO.

Computational methods

Using Gaussian 03 program,26 the density functional theory calculations have been carried out. The choice of basis sets and computational levels requires a compro-mise between accuracy and computational time. The geometries of the reactants, transition states, intermediates, and products have been optimized at the B3LYP level27 with the 6-31G(d) basis set.28,29 The vibrational frequencies have been calculated at the same level in order to determine the nature of the stationary points. Each transition state was verified to connect the desig-nated reactants and products by performing an intrinsic reaction coordinates (IRC) analysis.30 The single-point energies of the stationary points were calculated at the B3LYP/6-311＋G(3df,2p) level. The main possible re-action pathways involved in the OH-initiated reaction of n-octane (C8H18) were studied.

282 Chin. J. Chem., 2009, Vol. 27, No. 2 ZHAO et al.


Results and discussion

The optimized geometries for reactants, intermediates, transition states and products involved in the formation of the first and second generation products are listed in Figures 1 and 2, respectively. The spin mul-tiplicity (in parentheses) is given below each structure. In addition, the ground-state spin multiplicities are 2, 3, 2, 2 and 2 for OH, O2, NO, NO2 and HO2, respectively. Figure 3 shows the energy profiles for the reaction of n-octane with OH radical and the formation pathways of the first generation products in the presence of O2, NO and H2O. The profiles of the potential energy surface for the formation pathways of the second generation prod-ucts in the presence of O2 and NO are presented in Fig-ure 4.

H abstraction channels

As the n-octane molecule at ground state is of C2h symmetry, the 18 hydrogen atoms connected to different carbon atoms fall into four species. Therefore, there are four pathways for the H abstraction reactions. Calcula-tions show that the most favorable channel in energy is the loss of H bonded with C(4) or C(5), and a hydrogen- bonding intermediate, IM1, is formed firstly. It should be noted that the result is based on the energy parame-ters and there may be some limitations. In IM1 the length and the angle of the hydrogen bond are 1.710 Å and 170.2°, respectively. The energy of IM1 is 11.67 kJ/mol lower than the total energy of n-octane and OH radicals. After the formation of IM1, a hydrogen atom is abstracted from C(5)—H(1) bond of n-octane via a low potential barrier, 1.25 kJ/mol. The transition vector clearly shows the motion of H(1) between C(5) and O(1), with an imaginary frequency of 300i cm－1. This process is exothermic by 38.41 kJ/mol. Thus, H abstrac-tion from C(5)—H(1) bond of n-octane can occur easily and is expected to play an important role in the oxida-tion of n-octane in the troposphere. The product of H abstraction from C(5)—H(1) bond, labeled as IM2, is an open-shell species and will be further oxidized in the atmosphere.

Reaction of IM2 with O2/NO

In the troposphere, IM2 will further react with O2 to form an intermediate, IM3, via a barrierless association. The energy of IM3 is 313.05 kJ/mol lower than the total energy of IM2 and O2. The reaction enthalpy is －298.40 kJ/mol.

IM3 will react immediately with ubiquitous NO, forming an intermediate, IM4, which can further react via decomposition or isomerization. Two possible product channels from IM4 were found.

The first channel is the isomerization of IM4 to P1 via the transition state, TS2. P1 is an alkyl nitrate and was detected in the chamber.12 The structures of IM4,

TS2 and P1 are depicted in Figure 1. From these struc-tures, we can see the changes of the newly-formed N—O bond and the cleavage of O—O bond. The calcu-lated potential barrier and reaction enthalpy for this process are 66.11 and －115.39 kJ/mol, respectively.

The second one is the decomposition of IM4 in which NO2 is lost after covering a transition state, TS3. The potential barrier of this process is 34.73 kJ/mol, which is lower than that of the first isomerization reac-tion. The reaction enthalpy is －35.31 kJ/mol. So it can be concluded that the decomposition of IM4 to IM5 and NO2 is more favorable than the isomerization from IM4 to P1. As an alkoxy radical, IM5 is a key intermediate and may play an important role in the subsequent tro-pospheric degradation reactions of VOC.31 Under at-mospheric conditions, the major alkoxy radical removal processes involve reactions with O2, unimolecular de-composition, or isomerization. Therefore, there are four possible reaction pathways for the activated radical, IM5: addition with O2, dissociation to form butanal (P3), decomposition to pentanal (P4), and isomerization to IM7.

Firstly, IM5 can further react with the ubiquitous oxygen molecules in the troposphere to form two products: C4H9(CO)C3H7 (labeled as P2) and HO2. The pathway occurs through two steps. IM5 is attacked by O2 to form a five-membered ring intermediate, IM6. Then HO2 is removed to form P2 via the transition state TS4. The first step is a barrierless association, which is strongly exothermic by 156.98 kJ/mol. The second step has a low barrier, 23.47 kJ/mol and the reaction en-thalpy from IM6 to P2 and HO2 is －119.83 kJ/mol. Due to the low potential barrier of the H-atom abstrac-tion by O2, this removing process can occur easily.

Secondly, two similar decomposition channels were considered for IM5. The one decomposition channel of IM5 occurs via the cleavage of C(4)—C(5) bond, forming C4H9 (alkyl) and an aldehyde, P3. A transition state, TS5, was identified as being associated with the decomposition. The bond length of C(4)—C(5) bond is 2.164 Å, which is longer by 41.07% than the equilib-rium value of 1.534 Å in IM5. Calculations indicate that this decomposition has a high potential barrier of 58.32 kJ/mol and is endothermic by 36.86 kJ/mol. The other decomposition channel is similar to the former. This process has a transition state, denoted as TS6. The po-tential barrier and the reaction enthalpy are 58.24 and 22.43 kJ/mol, respectively. Because of the high poten-tial barriers, these two decomposition channels are of minor or no importance for n-octane, which is consis-tent with the experimental findings.8

The fourth possible reaction pathway of IM5 is the isomerization proceeding by a cyclic transition state. Because of the ring strain, the 1,4-H shift isomerization proceeding through a five-membered ring transition

n-Octane Chin. J. Chem., 2009 Vol. 27 No. 2 283




Figure 1 B3LYP/6-31G(d) optimized geometries for reactants, intermediates, transition states and products involved in the formation of the first generation products. Below each structure is given the total spin multiplicity (in parentheses). Distances are in Å and angles in (°).



Figure 2 B3LYP/6-31G(d) optimized geometries for reactants, intermediates, transition states and products involved in the formation of the second generation products. Distances are in Å and angles in (°).



Figure 3 Profile of the potential energy surface for the reaction of n-octane with OH radical and the formation pathways of the first generation products in the presence of O2, NO and H2O.

Figure 4 Profile of the potential energy surface for the formation pathways of the second generation products in the presence of O2 and NO.

state were calculated to be much less important than the 1,5-H shift isomerization proceeding through a six- membered transition state. Thus, the six-membered ring transition state, TS7, was found, which has an imagi-nary frequency of 1581i cm－1. This process is a shift of H atom bonded with C(2) to the O atom bonded with C(5). The energy of TS7 is 14.27 or 14.35 kJ/mol lower than that of TS5 or TS6, respectively. Hence the isom-erization of IM5 to IM7 is more favorable than the de-compositions to the carbonyls (aldehydes in this study).

The isomerization of IM5 was confirmed experi-mentally by the products observed and their yields.8 Due to the higher potential barriers for the decomposi-tion and isomerization reactions than for the reaction with O2, the O2 reaction is dominant in the troposphere.

The O2 molecule attacks IM7 to form an energy-rich intermediate IM8. The profile of the potential energy surface was scanned by varying the newly formed C(2)—O(2) bond length. We found no energy exceed-ing the C(2)—O(2) bond dissociation threshold along the reaction coordinate, which shows that the reaction of IM7 with O2 proceeds via a barrierless association. The process is strongly exothermic by 305.31 kJ/mol. Sub-sequently, IM8 will further react with NO to form IM9 with the reaction enthalpy being －98.11 kJ/mol. Then, two possible reaction channels from IM9 would be

found. The product from the first channel (isomerization) is a hydroxyalkyl nitrate, P5. A transition state, TS8, was found, with a potential barrier of 64.98 kJ/mol. This process is exothermic by 118.87 kJ/mol. This reaction can occur in the troposphere and the reaction product was detected in the chamber.8

The second reaction channel from IM9 is the de-composition reaction, forming NO2 and a hydroxyl alkoxy radical, IM10. There is a transition state, TS9, in this decomposition process. This process has a potential barrier of 38.12 kJ/mol, which is lower than that of isomerization. This process is endothermic by 38.62 kJ/mol due to the formation of the open-shell radical. Then, IM10 will be isomerized to IM11 via a transition state, denoted as TS10. This process is the shift of the H atom bonded with C(5) to the O atom bonded with C(2). Furthermore, compared with the former isomerization from IM5 to IM7, this process has a lower potential bar-rier of 27.91 kJ/mol. IM11 is more stable by 23.89 kJ/mol than IM10. The reaction enthalpy is －26.40 kJ/mol. So this reaction channel is energetically favor-able, which is in good agreement with the experimental findings.8

The subsequent reaction of IM11 is the addition of O2 to form a five-membered ring intermediate, IM12, via a barrierless association. This process is strongly



exothermic by 323.26 kJ/mol. Calculations indicate that a hydrogen bonding intermediate, IM12, is formed be-fore the formation of hydroxycarbonyl (P6) and HO2. There is a typical hydrogen bond with the bond length of 1.906 Å. The energy of IM12 is 52.89 kJ/mol lower than the total energy of P6 and HO2. The process from IM12 to P6 via a transition state, denoted as TS11, which has a potential barrier of 56.69 kJ/mol. The hy-droxycarbonyl (P6) exists in the gas phase in equilib-rium with a cyclic hemiacetal isomer P7, which is in equilibrium with a substituted dihydrofuran, P8, formed by dehydration. In general, H2O molecule is ubiquitous in the atmosphere. Thus, P6 will react with one mole-cule of H2O and a six-membered ring transition state, TS12, was found in this process. In TS12 the O(1)—

H(1) and O(2)—H(2) bonds break while C(5)—O(1), O(3)—H(2) and O(2)—H(1) bonds form. The potential barrier of this process is 49.99 kJ/mol and the reaction enthalpy is －0.17 kJ/mol. Similar to the reaction from P6 to P7, P7 will react with one molecule of H2O and then dehydrate two molecules of H2O to form P8 via the transition state TS13. This process has a potential bar-rier of 150.08 kJ/mol and is strongly endothermic by 84.81 kJ/mol. Though it is difficult to overcome the po-tential barrier of 150.08 kJ/mol for the reaction, Mar-tin32 still confirmed the occurrence of analogous reac-tion of 4,5-dihydro-2-methylfuran to 5-hydroxy-2-pen-tanone in the environmental chamber. The above analy-sis shows that the participation of H2O molecule results in the formation of a six-membered ring, which reduces the barrier of OH transfer. The behavior of H2O indi-cates that it prefers an activator to a reactant.

Atmospheric reaction of OH-P8 adduct, IM13 and IM19

First, the reaction pathway of OH addition to the C—C double bond was analyzed. Because the two car-bon atoms in the carbon-carbon double bond are not equivalent, two adduct isomers, IM13 and IM19, are formed. Thus, two reaction pathways were found for the addition of OH to the carbon-carbon double bond. Cal-culations show that the additions are barrierless associa-tion. The geometrical parameters of the two OH-P8 ad-ducts are shown in Figure 2. The evaluation of the vi-brational frequencies confirmed that IM13 and IM19 represented two minima on the potential energy surface. The addition of OH to the carbon-carbon double bond of P8 is a strongly exothermic process. The energies of IM13 and IM19 are 149.16 and 145.81 kJ/mol lower than the total energy of the original reactants (P8 and OH). The high reaction energies are retained as the in-ternal energy of the adducts. The energy-rich adducts, IM13 and IM19, can react via unimolecular decomposi-tion or with atmospheric O2/NO in atmosphere.

Reaction pathways of IM13

Similar to the reaction of IM3 with O2/NO, IM13

will first react with O2 to form IM14, which then further reacts with NO to form an intermediate, IM15. The process of IM13 with O2 is strongly exothermic by 300.66 kJ/mol. The energy of IM15 is 89.04 kJ/mol lower than the total energy of IM14 and NO. Thus, the intermediate, IM15, is an energy-rich adduct, which will further decompose to IM16 or isomerize to P9. So, two possible pathways would be open for IM15. The first channel is the isomerization from IM15 to a hydroxyni-trooxy, P9. A transition state, TS14, corresponding to the isomerization of IM15 was located. This product was detected in the chamber.3 This process has a poten-tial barrier of 64.81 kJ/mol and is exothermic by 106.73 kJ/mol. The second reaction channel involves decompo-sition, isomerization, association and elimination. First, IM15 will decompose to form IM16 and NO2 via a tran-sition state, denoted as TS15, which has a potential bar-rier of 45.35 kJ/mol. This process is endothermic by 21.00 kJ/mol. Then IM16 will be isomerized to IM17 via a transition state (denoted TS16). This process is a ring-open isomerization with the cleavage of C(4)—C(5) bond, which has a very low potential barrier of 0.38 kJ/mol. The reaction enthalpy is －74.73 kJ/mol. Sub-sequently, IM17 further reacts with O2 to form IM18. This process is strongly exothermic by 285.18 kJ/mol. The process from IM18 to P12 is similar to that from IM12 to P6. The product is a carbonyl ester. This proc-ess is endothermic by 51.92 kJ/mol and the potential barrier of the transition state (denoted as TS17) is 55.56 kJ/mol.

Reaction pathways of IM19

IM19, an activated radical, will also further react with the ubiquitous O2/NO to form hydoxynitrooxy (P10) and carbonyl ester (P12). The only difference from the reaction pathways of IM13 is the formation of tetrahydrofuran (P11). This process of IM22 with O2 to P11 is similar to the reaction of IM5 with O2 to from P2.

The reaction pathway leading to P12 is more favor-able than the other two pathways. Moreover, the P12 formation from IM19 is more favored than that from IM13. So the experimentally observed P12 is mainly from IM19 with O2/NO.

Conclusion

In this paper, a theoretical study on the OH-initiated atmospheric oxidation of n-octane in the presence of O2 and NO was performed. The comprehensive mechanism study has led to the following valuable conclusions.

(1) The isomerization of IM5 (an open-shell alkoxy radical) to an open-shell intermediate is more favorable than the decomposition to the carbonyls (aldehydes).

(2) As discussed above, most second-generation products are formed via the addition of P8 with OH radicals. Therefore, the isomerization from P6 to P7 and the dehydration from P7 to P8 were expected to play



important roles, especially for the formation of SOA. In addition, H2O molecule acts as an activator of OH transfer/or dehydration.

(3) The main first generation products of the OH- initiated atmospheric oxidation of n-octane are alkyl nitrates (P1) and hydroxycarbonyls (P6), and the second generation products are primarily hydroxycarbonyl tet-rahydrofuran (P11) and carbonyl ester (P12).

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(E0807242 Cheng, F.; Dong, H.)

mechanism for oh-initiated oxidation of n-octane in the presence of o2 and no: a dft study

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