Ground-State Intermolecular Proton Transfer of N2O4 and H2O: An

Letter pubs.acs.org/JPCL

Ground-State Intermolecular Proton Transfer of N2O4 and H2O: An Important Source of Atmospheric Hydroxyl Radical? Gefei Luo and Xuebo Chen* Department of Chemistry, Beijing Normal University, Xin-wai-da-jie No. 19, Beijing, 100875, People’s Republic of China S Supporting Information *

ABSTRACT: To evaluate the significance of the generation of atmospheric hydroxyl radical from reaction of N2O4 with H2O, CASPT2//CASSCF as well as CASPT2// CASSCF/Amber QM/MM approaches were employed to map the minimum-energy profiles of sequential reactions, NO2 dimerization and ground-state intermolecular proton transfer of trans-ONONO2 as well as the photolysis of HONO. A highly efficient groundstate intermolecular proton transfer of trans-ONONO2 is found to dominate the generation of hydroxyl radical under atmospheric conditions. Although proton transfer occurs with high efficiency, the precursor reaction of dimerization producing transONONO2 has to overcome a 17.1 kcal/mol barrier and cannot compete with the barrierless channel of symmetric O2N−NO2 formation from isolated NO2 monomers. Our computations reveal that the photolysis of HONO without a barrier definitely makes significant contributions to the concentration of the atmospheric hydroxyl radical, but its importance is influenced by the lack of trans-ONONO2 isomer in the atmospheric environment. SECTION: Environmental and Atmospheric Chemistry, Aerosol Processes, Geochemistry, and Astrochemistry

I

t is well-known that the hydroxyl radical (OH) is of importance as a “detergent” of the atmosphere by removing some greenhouse gases such as methane and so on.1−3 The classical OH source originates from ozone photolysis producing excited singlet-state O(1D) atoms, the majority of which decay to their ground state of O(3P) via collisional quenching while the minority of O(1D) (∼10%) gives rise to OH by the H abstraction reaction with water vapor.4−7 A new atmospheric source of OH was recently proposed by Sinha and co-workers,8 where OH radical was generated from electronically excited NO2 and H2O via reactions 1 and 2. NO2 + hν(λ > 420 nm) → NO*2

(1)

NO*2 + H 2O → OH + HONO

(2)

© 2012 American Chemical Society

(4)

1 NO** 2 → NO + O( D)

(5)

O(1D) + H 2O → 2OH

(6)

Although the barrier for H abstraction via two-photon absorption at ∼440 nm of reaction 6 is reduced to be ∼11.0 kcal/mol in our recent computations, the quantum yield of OH radical is lower than expectation because the second photon absorption is ∼1000-fold weaker than the first one (unpublished results). Therefore, the generation of OH radicals through the reaction of one- or two-photon excited NO2 with H2O is a minor channel in the atmospheric photochemistry.13,15 The new formation channel of the OH radical must be explored under atmospheric conditions, besides the classical OH source from ozone photolysis. It is well-known that the photolysis of the HONO through reaction 7 can give rise to OH.17,18

The formation mechanism and its role in atmospheric photochemistry underwent several investigations but are still controversial issues up to now.8−15 The OH radical was not observed by Blitz’s and Fitten’s groups. The barrier for the hydrogen abstraction reaction in the hot ground state via reactions 1 and 2 was calculated by our group to be more than 50.0 kcal/mol.12 Our computational results rule out the mechanism proposed by Sinha and are consistent with experimental observations conducted by Blitz’s and Fittschen’s groups.13,16 As an alternative mechanism, Crowley et al. suggested a facile sequential two-photon absorption, leading to O(1D) and thus to OH by the H abstraction reaction with water via reactions 3−6.9 NO2 + hν(499 > λ > 432 nm) → NO*2

NO*2 + hν(499 > λ > 432 nm) → NO** 2

HONO + hν(300 < λ < 405 nm) → OH + NO

(7)

However, the contribution of HONO to the total OH budget during the daytime was underestimated for a long time.19 The recent measurements show that up to 30% of the total OH is produced by HONO photolysis in a 24 h period under various Received: March 18, 2012 Accepted: April 11, 2012 Published: April 11, 2012

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The Journal of Physical Chemistry Letters

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conditions.19−21 The HONO in the atmosphere was first detected by Perner and Platt in 1979,22 which motivated different studies on the formation mechanism of HONO for 3 decades, but it is still under discussion.23 The heterogeneous disproportionation reaction of NO2 is believed to be the most favored contributor to HONO formation in the atmosphere 2NO2 + H 2O → HONO + HNO3

(8)

The NO2 dimer, N2O4, was found by Finlayson-Pitts and coworkers to play a predominant role in HONO formation and a is precursor of the ionization process of asymmetric ONONO2 through reactions 9−13.24−26 2NO2(g) ↔ N2O4(g)

(9)

N2O4(g) ↔ N2O4(surface)

(10)

N2O4(surface) → ONONO2(surface)

(11)

water

ONONO2(surface) ⎯⎯⎯⎯→ NO+NO−3(surface)

(12)

water

NO+NO−3(surface) ⎯⎯⎯⎯→ HONO(surface) + HNO3(surface)

(13) Figure 1. The numbering scheme of (a) the trans-ONONO2·(H2O)2 complex, (b) HONO, and (c) the QM/MM system.

The above reactions were modeled under experimental conditions of low temperature and dark, and the yield of HONO was good.27−34 In contrast, the photolysis of ONONO2 was proposed to be the primary step for the generation of HONO.33,34 The controversial mechanisms underwent several investigations by electronic structure calculations and classic trajectory simulations but were still under discussion.26,35−38 To clarify the controversy on the generation of HONO and the role of reaction of N2O4 with H2O under atmospheric conditions, accurate potential energy profiles were calculated in the high level of multiconfigurational perturbation theory in this work. To mimic the solvent environment around N2O4, a water solvent box positioned within 8.0 Å (279 water molecules) was built by the xleap module of Amber.39 A cutoff radius of 9.0 Å was used for the real space part of the electrostatic interactions and the van der Waals term. The Amber force field parameters were used in the present work.39 The entire system was subjected to a 1 ns long equilibration in the NVT ensemble at a normal ambient temperature (T = 298 K). The step size was set to 1.0 fs, and coordinates were saved every 100 fs. A cluster analysis of the sampled snapshots was done to choose the appropriate starting structure that evolves from hydrogen bonding between O3−H9 and O7−H12 (see Figure 1a for the numbering scheme). This starting structure was used for the following QM/MM calculations. As shown in Figure 1c, the QM part includes N2O4 and two water molecules, and the rest of the H2O molecules belong to the MM part. In the present work, the CASPT2//CASSCF/Amber (QM/ MM) protocol was employed to compute the minimum-energy profile (MEP) of the ground-state intermolecular proton transfer, where CASSCF and CASPT2 calculations were performed by the Gaussian40 and MOLCAS41 programs, respectively. The Amber force field (MM subsystem) was treated by the Tinker42 tool packages using standard potentials. The ab initio QM portion of the calculation was done at the CASSCF level with a 12e/9o active space using the 6-31G** basis set. To describe the bond fission of O4−N5 and O7−H9, the corresponding σ,σ* orbitals should be included in the active space. The rest of the 8e/5o originates from three π orbitals and one antibonding π* orbital of the NO3 moiety and

nonbonding of O. For comparison, the MEP for ground-state intermolecular proton transfer in the gas phase was reoptimized at the CASSCF (12e/9o) level of theory with the same strategy of choice of active orbitals. The MEP for photolysis of HONO along the reaction coordinate of N1−O3 in the Ã 1A″(σπ*) state was mapped by multistep CASSCF (8e/6o) optimizations with a fixed N1−O3 distance (see Figure 1b for numbering). These orbitals are nonbonding of N1 and O2 and σ,σ* of N1− O3, as well as three π orbitals that have zero, one, and two node(s) respectively. To account for the dynamical electron correlation effects, the single-point energy was refined at the multiconfiguration second-order perturbation theory level (CASPT2) using a four-root state-averaged zeroth-order wave function. All of the computations were performed by Gaussian 0340 and MOLCAS41 program packages. Figure 2 shows the MEP of NO2 dimerization by N−N and O−N bonding along the reaction coordinates of the N2−N5 and N2−O4 distances, respectively. The supramolecular optimizations reveal that asymmetrically isolated NO2 monomers are ∼0.4 kcal/mol more stabilized than the symmetric one. As illustrated in Figure 2a, a downhill energy curve without barrier leads to a symmetric O2N−NO2 dimer, in which the N−N distance is shortened from 3.936 to 1.821 Å and the energy level is lowered by 10.8 kcal/mol. In our previous work,12 one singly occupied electron was found to distribute around the N nonbonding orbital in the ground-state configuration of NO2. This indicates that symmetric dimerization by the N−N connection is a radical combination reaction, which can explain why no barrier was found in the formation of the O2N−NO2 dimer. In contrast, a 17.1 kcal/mol barrier was determined in the MEP of asymmetric dimerization. As shown in Figure 2b, the energy level continuously increases with shortened N2−O4 distance and eventually reaches the maximum at ∼2.00 Å. Meanwhile, the angle of O4−N5−O6 is suddenly decreased from ∼133.0 to 111.2° at the maximum. Crossing the 17.1 kcal/mol barrier, the energy curve sharply falls down to the platform at the 1.50 Å N2−O4 distance, where the N2−O4 bond is almost formed with the broken 1148



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Figure 2. The MEPs of NO2 dimerization by (a) N−N and (b) O−N bonding along the reaction coordinates of the N2−N5 and N2−O4 distances, respectively, obtained at the CASSCF (12e/9o)/CASPT2 level of theory.

O4−N5 bond. The asymmetric ONONO2 dimer is finally formed by slight structural adjustment, and its energy level is 1.3 kcal/mol lower than that of isolated NO2 monomers. Apart from the direct formation of the asymmetric ONONO2 dimer, another source channel was proposed by Pimentel et al., namely, the isomerization from the symmetric O2N−NO2 dimer.37 The barrier was calculated to be 31.3 kcal/mol in the gas phase for the indirect channel,37 which cannot compete with the direct formation from the isolated NO2 monomers examined in this work. To examine the role of the water environment in the asymmetric dimerization of NO2, the MEP of NO2 dimerization in a water solvent box was mapped and illustrated in Supporting Information Figure S6. The barrier of asymmetric dimerization in the water box is decreased to be 12.6 kcal/mol, which is 4.5 kcal/mol lower than that in the gas phase. Unlike the almost isoenergetic reactant and product of NO2 asymmetric dimerization in the gas phase, the asymmetric ONONO2 dimer is 8.6 kcal/mol less stable than isolated NO2 monomers in the water box. Obviously, the generation of symmetric O2N−NO2 is the dominant reaction in the dimerization of NO2 due to no barrier existence; thus, asymmetric ONONO2 is a minor product. Figure 3 summarizes the schematic equilibrium geometries of bare ONONO2 and ONONO2·(H2O)2 complexes in the gas phase and water box environment, respectively. Figure 4 shows the MEPs of proton transfer for gas-phase and water-solvated ONONO2·(H2O)2 complexes in the ground state together with the schematic structure of critical points. Although six N2O4 isomers were found to be formed by O−O, O−N, and N−N bonding between two NO2 monomers,38 only O−N bonding isomers can undergo heterogeneous disproportionation producing NO+ and NO3− and will be investigated in this work. As shown in Figure 3, two NO2 monomers are dimerized by the covalent connection between N2 and O4, which results in cisand trans-ONONO2 isomers. The N2−O4 bond length is 1.340 Å and ∼0.15 Å longer than those of normal O1−N2 and O3−N2 double bonds in bare cis- and trans-ONONO2. Moreover, the O4−N5 bond is even weaker than those of N2−O4, O1−N2, and O3−N2 bonds, where the distances of O4−N5 are calculated to be 1.726 and 1.652 Å in bare cis- and

Figure 3. Schematic structures of ONONO2 isomers optimized by the QM (CASSCF (12e/9o)/6-31g** and CASSCF (12e/9o)/6-31g**)/ MM level of theory. (a) cis-ONONO2 and (b) cis-ONONO2·(H2O)2 complexes in the gas phase, (c) the cis-ONONO2·(H2O)2 complex in the water box, (d) trans-ONONO2 and (e) trans-ONONO2·(H2O)2 complexes in the gas phase, and (f) the trans-ONONO2·(H2O)2 complex in the water box.

trans-ONONO2, respectively. The electronic population analysis reveals that six π electrons distribute in the NO3 moiety, therefore resulting in a four-centers/six-electron (Π64) configuration, while the other two π electrons localize in the NO part with a Π22 configuration. As discussed in our previous work,12,14 NO2 adopts a Π43 configuration in the ground state, which leads to a six-center/eight-electron configuration in the NO2 dimer. In the processes of NO2 dimerization, the 1149



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Figure 4. The MEPs of ground-state proton transfer along the reaction coordinate of the O7−H9 distance for trans-ONONO2·(H2O)2 complexes in the gas phase and the polar environment of the water box obtained by the CASPT2//CASSCF (12e/9o)/6-31g** and QM (CASPT2//CASSCF (12e/9o)/6-31g**)/MM level of theory, respectively.

symmetric redistribution of π electrons generates symmetric the O2N−NO2 isomer while two- versus six-π-electron fission produces an asymmetric ONONO2 structure with weak a O4− N5 bond. The formation of the ONONO2·(H2O)2 complex further causes a much weaker O4−N5 bond that is ∼0.1 Å elongated in comparison with those of isolated cis- and trans-ONONO2 (see Figure 3). The generation of a hydrogen bond between O3 and H9 is responsible for these structural changes. As shown in Figure 3, the distances between O3 and H9 are 2.064 and 2.153 Å for cis- and trans-ONONO2·(H2O)2 complexes, respectively. In contrast, no obvious hydrogen bond was found between the NO moiety and the other water molecule in ONONO2·(H2O)2 complexes. This indicates that NO3 and NO moieties have different charge character. The Mulliken charge analysis shows that the NO3 part carries about 0.32−0.37 au negative charge, while the same magnitude positive charge distributes in the NO moiety. The formation of a hydrogen bond further induces much more electron transfer from the NO to NO3 part. The charge translocation calculation confirms that ∼0.1 electron migration takes place between two moieties upon the formation of cis and trans hydrogen bond complexes. In the polar environment of the water box, ∼0.5 electron transfers from the NO to NO3 part, which results in a −0.89 and −0.79 au negative charge center around the NO3 moiety of cis- and transONONO2·(H2O)2 complexes, respectively. Consequently, the dipole moments increase from 3.63−3.19 D in bare ONONO2 to 8.82−7.83 D in cis- and trans-ONONO2·(H2O)2 complexes in the water box. Meanwhile, O4−N5 bonds are weakened and are 2.226−2.092 Å long, which is 0.50−0.45 Å longer than those of bare cis- and trans-ONONO2. Overall, the formation of an intermolecular hydrogen bond and the polar environment of the water box significantly increased the abundance of negative charge around the NO3− moiety that can function as a potential acceptor for the following ground-state proton transfer. The weakened O4−N5 bond allows the reaction of heterogeneous disproportionation to occur easily in the case of water molecule participation.

The trans isomers are 1.4, 5.0, and 5.6 kcal/mol more stable than cis conformers of bare ONONO2 and ONONO2·(H2O)2 complexes in the gas phase and water box, respectively. The trans isomers, therefore, are believed to be dominant isomers in different environments and are chosen as the initial reactant for the following ground-state proton transfer. Figure 4 shows the MEPs of ground-state proton transfer along the reaction coordinate of the O7−H9 distance for trans-ONONO2·(H2O)2 complexes in the gas phase and polar environment of the water box together with schematic structures of critical points. The O4−N5 distance of the reactant trans-ONONO2·(H2O)2 complex the in gas phase is 1.735 Å. That is 0.357 Å shorter than that in the water box when the O7−H9 distance remains within a normal bond length (0.964 Å). The attraction from the negative charge center NO3− leads to the departure of the proton H9 toward O3. As illustrated in Figure 4, the bond fission of O7−H9 causes an uphill energy curve that finally reaches the maximum at a 1.210 Å O7−H9 distance in the ground-state proton transfer of the water-solvated transONONO2·(H2O)2 complex. Meanwhile, the O4−N5 is slightly elongated to 2.439 Å in the water-solvated maximum. The barrier is calculated to be 7.4 kcal/mol for the ground-state proton transfer of the trans-ONONO2·(H2O)2 complex in the water box. However, the barrier for the counterpart reaction in the gas phase is extremely increased to 20.3 kcal/mol when the polar environment of the water box disappears. This indicates that the polar environment is an important precondition for the efficient proton transfer of the trans-ONONO 2 ·(H 2 O) 2 complex because it facilitates charge transfer, resulting in a good negative center, which serves as an acceptor for proton transfer. As shown in Figure 4, the N5−O7 distance is shortened to 1.404 Å, resulting in the almost formation of HONO after trans-ONONO2·(H2O)2 overcomes a small barrier in the water box. The departure of proton H9 leaves a OH− anion that combines with the NO+ cation, producing HONO. In contrast to the case in the water box, the turning point for the formation of HONO appears in the longer O7− H9 distance at 1.250 Å in the gas phase. Once the HONO is 1150



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lying S 0 → S ΣP(Ã 1A″,1σπ*) excitation originated from promoting one electron of the N1−O3 σ orbital (see Figure 5 for numbering) to the virtual orbital of π* with two nodes. One electron of the π orbital with one node is excited to the virtual orbital of π* with two nodes and a N1−O3 σ* orbital, which eventually leads to S0 → SPP(B̃ 1A′,1ππ*) and S0 → S PΣ (C̃ 1A″, 1 πσ*) transitions, respectively. The oscillator strengths of S0 → SΣP(Ã 1A″,1σπ*) and S0 → SPP(B̃ 1A′,1ππ*) are 1.0−3.0 × 10−3, which are about 10-fold larger than that of the highest-lying excitation of S0 → SPΣ(C̃ 1A″,1πσ*) (f = 1.71 × 10−4). As shown in Table 1, the vertical excitation energy of S0 → SΣP(Ã 1A″,1σπ*) is 80.1 kcal/mol (360 nm) and much lower than those of S0 → SPP(B̃ 1A′,1ππ*) (138.0 kcal/mol, 207 nm) and S 0 → SPΣ(C̃ 1A″,1πσ*) (227.7 kcal/mol, 126 nm) transitions. Although S0 → SΣP(Ã 1A″,1σπ*) and S0 → SPP(B̃ 1A′,1ππ*) transitions have the same magnitude oscillator strength, the S0 → SPP(B̃ 1A′,1ππ*) excitation exhibits negligible contribution to atmospheric chemistry because the wavelength of light λ < 300 nm is considerably shielded in the higher atmosphere and does not reach the troposphere.9 Therefore, the lowest-lying S0 → SPP(Ã 1A″,1σπ*) transition is responsible for initial absorption of HONO under atmospheric conditions. The calculated absorption wavelength at 360 nm for S0 → SΣP(Ã 1A″,1σπ*) excitation well reproduces the wavelength condition of atmospheric chemistry (300 < λ < 405 nm) for the generation of hydroxyl radical through reaction 7.17,18 As illustrated in Figure 5, 360 nm photoexcitation promotes the HONO system instantaneously in the Franck−Condon (FC) region of SΣP(1σπ*). The intense intrinsic tension originating from the σ → π* transition causes rapid fission of the N1−O3 bond when the HONO molecule evolves from the FC region of the SΣP(1σπ*) state. A sharp downhill energy curve leads to O departure from the moiety of NO, resulting in a radical configuration in the ∼2.5 Å N1−O3 distance. During this relaxation, the energy level rapidly decreases from 80.1 kcal/mol in the FC point of SΣP(1σπ*) to 45.9 kcal/mol at the ∼2.5 Å N1−O3 distance. With a continuously increased N1− O3 distance, the photodecomposed HONO reaches the phase

formed, the remarkable structural changes are associated with shortened O3−H9 and eventually generate the final product of HNO3 along a downhill energy curve. The product of HONO + HNO3 + H2O is energetically 31.6 kcal/mol lower than that of reactant trans-ONONO2·(H2O)2 in the water box. Nevertheless, the energy of HONO + HNO3 + H2O relative to transONONO2·(H2O)2 in the gas phase is only −6.8 kcal/mol, which is a much smaller energy gap than that in the water box. The formation of additional hydrogen bonds should be responsible for the much lower energy for HONO + HNO3 + H2O in the water box. Table 1 summarizes the vertical excitation energies, oscillator strengths, and transition characteristics of HONO. The lowestTable 1. Vertical Excitation Energies (ΔE⊥, kcal/mol), Oscillator Strengths ( f), and Character of the Singly Occupied Orbital of Different Transitions of HONO at the CASPT2(8e/6o)//CASSCF Level of theory

Figure 5. The MEP of the photolysis of HONO at 360 nm along the N1−O3 distance at the CASPT2//CASSCF (8e/6o)/6-31g** level of theory. 1151



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the OH budget during the day was underestimated in the atmosphere.19−21

of the product. As shown in Figure 5, the energy level almost remains unchanged (44.0−45.0 kcal/mol) once the product is finally generated. The electron population analysis shows that two singly occupied electrons of the product distribute in the π* orbital of NO and the π orbital of the OH moiety. This indicates that the homolytic cleavage of the N1−O3 bond in the SΣP(1σπ*) state leads to the product of NO(X̃ 2Π) + OH(X̃ 2Π), where two fragments are populated in their electric ground state. As illustrated in Figure 5, no minimum and barrier were found in the energy curve of HONO photolysis upon irradiation of light at 360 nm, which represents a photodissociation by relaxation along the repulsive potential. The above downhill relaxation path indicates that the generation of OH radical is a fast and effective process from the source of HONO photolysis under atmospheric conditions. In this work, multiconfigurational perturbation theory was used to investigate the sequential reactions of dimerization of NO2 and ground-state intermolecular proton transfer of transONONO2 as well as the photolysis of HONO. A highly efficient intermolecular proton transfer of trans-ONONO2 in the ground state is found to dominate the generation of hydroxyl radical under atmospheric conditions. The formation of the hydrogen bond and polar environment were verified to be the precondition of occurrence of fast ground-state intermolecular proton transfer with a small barrier (7.4 kcal/ mol). This is supported by experimental findings where the disproportionation reaction 8 was observed to be catalyzed by anions at the surface of droplets,32,43 and the lifetime of ONONO2 was detected to be really short under atmospheric conditions.26 The vertical excitation of trans-ONONO2 was examined to evaluate the role of light in the formation of HONO. It was found that all charge-transfer transitions result in reverse direction charge translocation from the moiety of NO3− to NO+, which does not facilitate the fission of the N4− O5 bond (see Supporting Information Table S3). Moreover, diradical excitation makes negligible contribution to the charge redistribution of trans-ONONO2. Therefore, the light plays a minor role in the generation of HONO from N−O bond cleavage of trans-ONONO2. This is consistent with the experimentally proposed mechanism that heterogeneous disproportionation of trans-ONONO2 releasing HONO can take place on snow and ice surfaces in the dark without the participation of light.44,45 The water molecules were found to function as a polar environment to assist the ground-state proton transfer in this work . This is not in agreement with the mechanism proposed by Pimentel et al.,38 where the number of water molecules may play an important role in the reaction of N2O4 hydrolysis producing HONO and HNO3. Although ground-state intermolecular proton transfer occurs with high efficiency, the precursor reaction of dimerization producing trans-ONONO2 takes place with a medium-magnitude rate (17.1 kcal/mol barrier) and cannot compete with the barrierless channel of formation of symmetric O2N−NO2 from isolated NO2 monomers. In good agreement with experimental observations, 20 the photolysis of HONO producing OH radical at ∼360 nm was confirmed to be a fast barrierless process in the present work. The photolysis of HONO definitely makes significant contributions to the concentration of atmospheric hydroxyl radical, but its importance is influenced by the lack of the trans-ONONO2 isomer in the atmospheric environment. Other HONO sources have to undergo careful investigations as well to explain why

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ASSOCIATED CONTENT

S Supporting Information *

Figures, tables, and Cartesian coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by FANEDD 200932 and NSFC20973025 to X.B.C. and NSFC21033002 and Major State Basic Research Development Programs 2011CB808503 to W.H.F.

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REFERENCES

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