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Quantum Chemical Investigation on Photochemical Reactions of Nonanoic Acids at Air−Water Interface Pin Xiao, Qian Wang, Wei-Hai Fang, and Ganglong Cui* Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, China S Supporting Information *

ABSTRACT: Photoinduced chemical reactions of organic compounds at the marine boundary layer have recently attracted significant experimental attention because this kind of photoreactions has been proposed to have substantial impact on local new particle formation and their photoproducts could be a source of secondary organic aerosols. In this work, we have employed first-principles density functional theory method combined with cluster models to systematically explore photochemical reaction pathways of nonanoic acids (NAs) to form volatile saturated and unsaturated C9 and C8 aldehydes at air−water interfaces. On the basis of the results, we have found that the formation of C9 aldehydes is not initiated by intermolecular Norrish type II reaction between two NAs but by intramolecular T1 CO bond fission of NA generating acyl and hydroxyl radicals. Subsequently, saturated C9 aldehydes are formed through hydrogenation reaction of acyl radical by another intact NA. Following two dehydrogenation reactions, unsaturated C9 aldehydes are generated. In parallel, the pathway to C8 aldehydes is initiated by T1 CC bond fission of NA, which generates octyl and carboxyl radicals; then, an octanol is formed through recombination reaction of octyl with hydroxyl radical. In the following, two dehydrogenation reactions result into an enol intermediate from which saturated C8 aldehydes are produced via NA-assisted intermolecular hydrogen transfer. Finally, two dehydrogenation reactions generate unsaturated C8 aldehydes. In these reactions, water and NA molecules are found to play important roles. They significantly reduce relevant reaction barriers. Our work has also explored oxygenation reactions of NA with molecular oxygen and radical− radical dimerization reactions.

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INTRODUCTION Air−water interface in particular near the marine boundary layer provides a unique environment where heat, momentum, water, as well as a large number of chemical species can exchange. In addition, neutral molecules, radicals, and ions meet, interact, and react at air−water interfaces to generate new molecules and eventually cause new particle formation (NPF). These exchanges and releases have a considerable and complex influence on global climate system, and regional air quality, and local secondary organic aerosol (SOA) formation. Because of these importances, physical and chemical processes at air− water interfaces have been intensively studied in the past decade.1−13 In recent years, photochemical reactions of organic compounds at the marine boundary layer, that is, air−sea interfaces, attracted a lot of experimental attention.14−20 Very recently, Rossignol et al.21 have employed ultraviolet−visible absorption and mass spectroscopy techniques to explore photochemical reactions at air−water interface coated with a monolayer of nonanoic acids. Upon direct illumination with actinic radiation, saturated and unsaturated aldehydes are seen in gas phase, while highly oxygenated products appear in © 2017 American Chemical Society

aqueous phase. In order to explain experimental observations, they have proposed two possible photoinduced reaction mechanisms to rationalize observed C9 aldehydes as shown in the left panel of Figure 1. The first pathway is initiated by an intramolecular Norrish type I reaction, that is, the CO bond fission, followed by a series of hydrogenation and dehydrogenation reactions by NA, hydroxyl, and carboxyl radicals; the second pathway starts from an intermolecular Norrish type II reaction forming a diol radical, which is followed by several dehydrogenation and hydrogen transfer reactions. Moreover, they have proposed two feasible reaction pathways for generating highly oxygenated products (see the right panel of Figure 1). In recent years, George and his co-workers have explored photosensitized reactions of nonanoic acids at air− water interfaces under different experimental conditions,20,22,23 as well as reactions with methyl vinyl ketone leading to significant amounts of isoprene.24 Received: April 2, 2017 Revised: May 16, 2017 Published: May 17, 2017 4253

DOI: 10.1021/acs.jpca.7b03123 J. Phys. Chem. A 2017, 121, 4253−4262

Article

The Journal of Physical Chemistry A

Figure 1. Experimentally proposed photochemcial reaction pathways (R1 and R2) for the formation of C9 aldehydes (left) and highly oxygenated products (right). NA· and NAH stand for CH3(CH2)7COO· and CH3(CH2)7C·(OH)OH radicals, respectively.

excited-state structures and energies of polyatomic molecules.30−35 All CASSCF and CASPT2 calculations are carried out using MOLCAS8.0.36,37 Electronic Structure Calculations in the T1 and S0 States. Geometries are optimized and characterized by frequency analysis calculations to be minima or transition states (TSs) using density functional theory (DFT) method38 with globally hybride exchange-correlation functional B3LYP with D3 dispersion correction scheme of Grimme.39−43 The 631G* basis set is used to expand total electronic wave function.44 Single-point energies of all B3LYP-D3 optimized structures are further refined by B3LYP-D3 method with a larger 6-311++G** basis set.39−43,45,46 Vertical excitation energies at the Franck−Condon point are also calculated using TD-B3LYP-D3/6-311++G** method.47 All DFT and TD-DFT calculations are performed using Gaussian 09 program package.48

Although experiments have given many useful information on photochemical reaction of nonanoic acids taking place at air−water interfaces, many atomistic mechanistic details remain elusive. For example, which reaction mechanism proposed by experiments for the formation of C9 aldehydes is more favorable energetically? Which elementary step is the ratelimiting one? How are saturated and unsaturated C8 aldehydes formed? What roles do water molecules play in these photochemical reactions? Obviously, this microscopic mechanistic information cannot be gained by experiments alone. In this regard, reliable electronic structure calculations play an irreplaceable role. Unfortunately, to date there is no theoretical work focusing on mechanistic issues of these photochemical reactions at air−water interfaces. In this work, we have employed density functional theory (DFT) method to systematically study photochemical reaction pathways of nonanoic acids that lead to saturated and unsaturated C8 and C9 aldehydes, as well as oxidation pathways to highly oxygenated products. This information certainly contributes important mechanistic insights for future experimental studies of similar photoreactions at air−water interfaces.

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RESULTS AND DISCUSSION Spectroscopic Properties. In order to understand initial population of excited singlet states of NA molecule, we have explored its excited-state properties at the Franck−Condon point. Table 1 collects TD-B3LYP-D3 and B3LYP-D3 computed vertical excitation energies of NA and its two water complexes to the lowest two excited states, that is, S1 and T1. Electronic structure analysis shows that these two singlet and triplet states are of nπ* character as found in many similar

COMPUTATIONAL METHODS

Electronic Structure Calculations in the S1 State. Minimum-energy structures, minimum-energy reaction paths, and linearly interpolated internal coordinate (LIIC) paths in the S1 state are optimized using state-averaged complete active space self-consistent field (CASSCF) method in which equal weights are used. In all CASSCF calculations, an active space of 10 electrons in 8 orbitals is used (see Figure S1). Single-point energies of all CASSCF optimized structures are refined using multistate complete active space second-order perturbation theory (MS-CASPT2) approach.25,26 In MS-CASPT2 computations, the same active space, that is, 10 electrons in 8 orbitals is used; the Cholesky decomposition technique with unbiased auxiliary basis set is used for accurate two-electron integral evaluation;27 the ionization potential-electron affinity (IPEA) shift is set to zero;28 the imaginary shift technique (0.2 au) is employed to avoid intruder-state issue;29 This combined MSCASPT2//CASSCF computational protocol has been recently demonstrated to be able to give an accurate description for

Table 1. TD-B3LYP-D3/6-31G* and B3LYP-D3/6-31G* Computed Vertical Excitation Energies to the S1 and T1 States at the Franck-Condon Point and Adiabatic Excitation Energies of S1 and T1 Minima Relative to the S0 Minimum (Units in kcal/mol) NA NA-1W NA-2W

S1(1nπ*)a

S1(1nπ*)b

T1(3nπ*)a

T1(3nπ*)b

135.0 137.9 139.1

109.9 111.0 112.2

121.4 125.4 126.7

94.8 96.9 96.5

a

TD-B3LYP-D3/6-31G* and B3LYP-D3/6-31G* computed vertical excitation energies. bS1 and T1 states at the Franck-Condon point and adiabatic excitation energies. 4254


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The Journal of Physical Chemistry A saturated carbonyl compounds.31,34,49−57 In addition, it can be found that water hydration has small influence on vertical and adiabatic excitation energies to these two singlet and triplet states. The vertical and adiabatic excitation energies of waterhydrated NA molecules are merely blue-shifted a little compared with those of isolated NA molecule. For example, vertical excitation energies to the S1 [T1] state are estimated to be 135.0 [121.4], 137.9 [125.4], and 139.1 [126.7] kcal/mol for NA, NA-1W, and NA-2W, respectively (see complexes in Figure 2). This can be understood very well taking into account

Figure 3. CASPT2//CASSCF/6-31G* computed linearly interpolated internal coordinate path connecting both S0 and S1 minima. See text for discussion.

of these two photochemical reaction pathways and determined which one is more favorable in the view of energy. In the first reaction pathway, the formation of C9 aldehydes is proposed to start from the CO bond fission upon irradiation (see R1 in Figure 1). As discussed above, the excitation energy of 270 nm wavelength can populate the lowest excited S1 singlet state with very small probability due to small oscillator strength ( f = 0.002, CASPT2 level). In the S1 state, the direct CO bond fission that generates hydroxyl and carbonyl radicals is not efficient because the S1 CO bond fission needs to overcome a large barrier, 15.4 kcal/mol for isolated NA, and 14.5 kcal/mol for water-assisted NA-1W according to MSCASPT2//CASSCF computed S1 minimum-energy bondfission paths (see the top panels of Figure 4). Therefore, NA will first hop to the T1 state via the S1 → T1 intersystem crossing at the Franck−Condon region as already observed in many similar carbonyl compounds.34,49,52,54 In the T1 state, the CO bond fission of isolated NA from its T1 minimum 1* is still inefficient because the T1 energy of the TS(1−2) transition state, that is, 105.2 kcal/mol at B3LYPD3 level, is close to available total energy (exp. 106 kcal/mol; red lines in Figure 5). However, when a water molecule is bound with NA forming a complex 1a, the T1 energy of the TS (1a-2) transition state for the T1 CO bond fission is significantly reduced, which is 91.5 kcal/mol at B3LYP-D3 level (blue lines in Figure 5). Thus, with the assistance of one water molecule, the T1 CO bond fission becomes efficient and finally produces hydroxyl and carbonyl radicals, that is, OH and 2 radicals in Figure 5. Both water and NA molecules can react with radical species 2 to generate molecule−radical complexes 4′ and 4. However, water-assisted radical reaction, from 3′ via TS(3′-4′) to 4′, cannot compete with that assisted by NA itself, from 3 via TS(3−4) to 4. Their barriers at TS(3′-4′) and TS(3−4) are computed to be 14.9 and 12.8 kcal/mol at B3LYP-D3 level, respectively. In addition, the latter reaction is less endothermic thermodynamically. Subsequently, dehydrogenation reaction of molecule−radical complexes 4′ and 4 takes place immediately. Among them, the reaction associated with molecule−radical complex 4 is much easier than that associated with molecule− radical complex 4′ because the energy of the TS(4−5) transition state is about 10 kcal/mol lower than that of the

Figure 2. NA molecule and its two water-hydrated complexes NA-1W and NA-2W. Also shown are some selected hydrogen bond lengths (in Å) of the S0 equilibrium structures optimized by B3LYP-D3/6-31G* method. See Supporting Information for Cartesian coordinates.

that both singlet and triplet states originate from local n → π* electronic excitation of the carbonyl group. Hydrogen-bonding interaction due to water hydration to somewhat extent inhibits electronic transition of the lone-pair electron of the O atom of the carbonyl group. The S1 state cannot be directly populated at the Franck− Condon point by 270 nm wavelength, about 106 kcal/mol, because of large S0 → S1 vertical excitation energy, 135.0 kcal/ mol in Table 1. However, one can find that this excitation energy is close to the S0 → S1 adiabatic excitation energy, 109.9 kcal/mol at TD-B3LYP-D3 level in Table 1. In addition, at room temperature the NA molecule has sufficient thermal energy to fluctuate so as to deviate from its S0 minimum. Figure 3 shows linearly interpolated internal coordinate (LIIC) path connecting the Franck−Condon point of NA and its S1 minimum. It can be found that when NA molecule moves toward its S1 minimum-energy structure, which is mainly driven by the CO stretching mode and the improper torsional motion with respect to the central C atom of the COOH group, the S1−S0 energy gap will become small enough. As a result, this 270 nm wavelength can populate the S1 state of NA with a small probability. Furthermore, the S1 population is also enhanced by neat or concentrated solutions of NA, as suggested by recent experiments.21 Initiation and Formation of Unsaturated C9 Aldehydes. Rossignol et al. have experimentally proposed two photochemical reaction mechanisms for the formation of C9 aldehydes.21 Next, we have separately examined the feasibility 4255


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Figure 4. CASPT2//CASSCF/6-31G* computed minimum-energy reaction paths (kcal/mol) for the CO (top) and CC (bottom) bond fissions of (left) NA and (right) NA-1W in the S1 state. See text for discussion.

reaction is rather easier because of small barrier in the T1 state, which is computed to be 4.1 kcal/mol at the TS(7−8) transition state. Nevertheless, the following hydrogen migration reactions are to certain extent inhibited as demonstrated by their relatively high barriers, which are all more than 16 kcal/ mol at B3LYP-D3 level (see Figure 7). Moreover, the energies at all these transition states are more than 99 kcal/mol at B3LYP-D3 level, even 129.0 kcal/mol at TS(8−9) for 1,2 hydrogen migration. With these reasons taken together, it is not difficult to find that the R2 path is unfavorable compared with the R1 path in the formation of unsaturated C9 aldehydes. Therefore, we have refrained us from further exploration of subsequent reactions of this R2 path. Initiation and Formation of Unsaturated C8 Aldehydes. In experiments of Rossignol and co-workers, C8 aldehydes have also been observed using mass spectroscopy techniques.21 But, corresponding photoinduced reaction pathways are not proposed. In this work, we have figured out an energetically favorable reaction path producing these saturated and unsaturated C8 aldehydes, as shown in Figures 8 and 9. The formation of C8 aldehydes are initiated by the CC bond dissociation. Such bond fission is not allowed in the S1 state because it is associated with a large barrier for NA (see the bottom-left panel of Figure 4). Although water hydration can reduce this barrier to some extent, this reaction still cannot

TS(4′-5) one (Figure 5). The resultant radical 5 can further evolve into final unsaturated C9 aldehyde P1 via the second dehydrogenation reaction. Different from the first dehydrogenation reaction, the reaction initiated by the OH radical is much easier than that by the 1 radical generated through intermolecular radical reaction between 2 radical and NA molecule. Figure 5 shows computed reaction path to form unsaturated C9 aldehyde from triplet NA molecule in which the most favorable one is in blue. In addition to forming α, β unsaturated aldehyde, we have explored reaction pathways to generate other kinds of unsaturated aldehydes from species 4, such as P1−X (X = 1−6). As shown in Figure 6, all these reactions are thermodynamically very exothermic and are also associated with small barriers. Among them, forming P1−6 is associated with the largest barrier, but only 7.0 kcal/mol at B3LYP-D3 level, which can be surmounted very easily concerning available total energy. Of course, among these different unsaturated aldehydes, P1 is the most stable one because of its conjugated CC and CO double bonds. Besides the above reaction path of forming unsaturated aldehydes, we have examined experimentally proposed another reaction path (see R2 in Figure 1). Different from the R1 path, the R2 path is initiated by intermolecular hydrogen extraction reaction between two NA molecules. This intermolecular 4256


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Figure 5. B3LYP-D3/6-311++G** computed reaction pathways to form saturated and unsaturated C9 aldehydes from triplet NA molecule initiated by the CO bond fission. See text for discussion.

Figure 6. B3LYP-D3/6-311++G** computed reaction pathways to form different unsaturated C9 aldehydes from saturated C9 aldehyde, that is, species 4. See text for discussion.

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Figure 7. B3LYP-D3/6-311++G** computed reaction pathways to form unsaturated C9 aldehydes from initial NA molecules upon irradiation. See text for discussion.

Figure 8. B3LYP-D3/6-311++G** computed photochemical reaction pathways to form saturated C8 aldehydes from triplet NA molecules initiated by the CC bond fission. See text for discussion.

reaction process needs to overcome a barrier of 59.9 kcal/mol and thus is forbidden in the view of energy. In the second one, the radical species 10 will recombine with OH radical to generate 11. Then, from neutral molecule 11, two dehydrogenation reactions happen sequentially. These two reactions are initiated by OH radicals and are thus very facile energetically. The generated molecule 13 will convert into saturated C8 aldehyde P2 through three reaction paths, among which the

happen. Therefore, the CC bond dissociation should occur in the T1 state as a consequence of the S1 → T1 intersystem crossing at the Franck−Condon region. The CC bondfission barrier in the T1 state is computed to be 6.7 kcal/mol, which can be overcome by photon energy of 270 nm wavelength. The generated radical species 10 has two reaction channels. In the first one, it reacts with the HOC(O) radical to generate species 11 with the release of carbon oxide. This 4258


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Figure 9. B3LYP-D3/6-311++G** computed reaction pathways to form different unsaturated C8 aldehydes from saturated C8 aldehyde P2. See text for discussion.

Figure 10. Photochemical pathways to generate saturated and unsaturated C8 and C9 aldehydes suggested based on present studies.

NA-assisted one has a relatively smaller barrier, 13.6 kcal/mol at B3LYP-D3/6-311++G** (the other two have 56.2 and 44.8 kcal/mol; see Figure 8). This small barrier can be gotten through very easily by the “hot” system because the energy at the TS(13a-P2) transition state is significantly lower than the

T1 energy of initial triplet NA species, that is, 1* by more than 100 kcal/mol. In this reaction step, NA molecule plays an important role in reducing the barrier. Of course, the generated saturated C8 aldehyde P2 can be further converted into unsaturated C8 aldehydes through two 4259


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and radical−radical coupling reactions. Our present results evidently demonstrate that the pathway initiated by the CO bond fission of NA in the T1 state, and not by the intermolecular hydrogen transfer between NAs, is energetically more favorable. Alternatively, we have computationally proposed a feasible photochemical pathway for the formation of unsaturated and saturated C8 aldehydes, which is initiated by the T1 CC bond fission. Moreover, we have found that water and NA molecules play important roles in photochemical reactions of NAs at some key reaction steps. They are directly involved in water- and NA-assisted hydrogen transfer reactions in such a way as to reduce significantly related reaction barriers. We have found that all oxygenation reactions of NA radicals with molecular oxygen and radical−radical coupling reactions can easily happen. Finally, our present work opens the way for exploring photochemical reactions at air−water interfaces and could contribute useful knowledge to understanding local new particle formation near the marine boundary layer.20,22,23

sequential dehydrogenation reactions. As discussed above, both hydroxyl and carboxyl radicals can be used to extract hydrogen atoms of saturated C9 aldehydes to form unsaturated C9 ones (see Figure 5). But, it is obvious that abstraction reaction by carboxyl radical is energetically more favorable than that by hydroxyl radical. Therefore, we have merely explored dehydrogenation reactions of the C8 aldehyde P2 by carboxyl radical. As shown in Figure 9, all reactions are energetically allowed because of small barriers. In addition to the formation of C9 and C8 aldehydes, oxygenation and radical coupling reactions have been observed in experiments (see the right panel of Figure 1).21 In order to get a comprehensive understanding of photochemical reactions of NAs at air−water interfaces, we have also explored all possible pathways of oxygenation and radical coupling reactions (see Supporting Information for details). In oxygenation reactions, hydroxyl radical first abstracts a hydrogen atom from different sites of NA molecule, resulting into eight 1_X (X = 1−8) radicals and one carboxylic radical (Figure S10). All these abstraction reactions are associated with small barriers and can proceed easily. Moreover, they are thermodynamically exothermic, more than 20 kcal/mol at B3LYP-D3 level. In the following, oxygen molecules can be readily added to these eight radicals in a barrierless way. Finally, we have calculated reaction energy changes of totally more than 35 radical−radical coupling reactions and found that all radical recombination reactions are extremely exothermic thermodynamically (more than 80 kcal/ mol at B3LYP-D3 level; Figure S11). Photochemical Mechanism. On the basis of our results, we have proposed two photochemical reaction pathways generating unsaturated and saturated C8 and C9 aldehydes, respectively. Upon irradiation, NA molecules are first excited to their S1 state, which is followed by an S1 → T1 intersystem crossing process. In the T1 state, Norrish type I CO and C C bond fissions become feasible. The left panel of Figure 10 shows the most favorable reaction path to produce unsaturated and saturated C9 aldehydes. The T1 CO bond fission generates hydroxyl and acyl radicals in the same time. This step is accelerated by water molecules near air−water interfaces. The newly generated acyl radical abstracts a hydrogen atom from nearby NA molecule forming a C9 aldehyde, that is, nonanal. From this saturated aldehyde, two sequential dehydrogenation reactions lead to the formation of unsaturated C9 aldehydes. On the other hand, if the CC bond is cleaved in the T1 state, as shown in the right panel of Figure 10, the generated octyl radical will recombine with hydroxyl radical to form an octanol. Through two consecutive dehydrogenation reactions initiated by hydroxyl radical, an enol is formed. From this species, an intramolecular hydrogen transfer will result into a saturated C8 aldehyde, that is, capryl aldehyde. Of course, this reaction step is also accelerated by water molecules, in particular by NA molecules, which make the reaction barrier lower by more than 11.4 and 42.6 kcal/mol at B3LYP-D3/6-311++G** level. Similarly, final unsaturated C8 aldehydes are generated by two steps of dehydrogenation reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b03123. Schematic structures, oxygenation and radical−radical coupling reaction profiles and discussion, and Cartesian coordinates of optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei-Hai Fang: 0000-0002-1668-465X Ganglong Cui: 0000-0002-9752-1659 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research & Development Program of China (2016YFC0202600) and the National Natural Science Foundation of China (21522302, 21520102005, and 21421003).

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CONCLUSIONS We have employed ab initio electronic structure calculations to explore photochemical reactions of nonanoic acids (NAs) at air−water interfaces using cluster models assisted by water and NA molecules.21 First, we have examined two experimentally proposed photochemical reaction pathways for the formation of unsaturated and saturated C9 aldehydes as well as oxygenation 4260


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