The Conical Intersection Dominates the Generation of Tropospheric

Mar 17, 2010 - The conical intersections dominate the nonadiabatic relaxation processes after NO2 irradiated at ∼410 nm in the troposphere and furth...
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J. Phys. Chem. A 2010, 114, 4601–4608

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The Conical Intersection Dominates the Generation of Tropospheric Hydroxyl Radicals from NO2 and H2O Qiu Fang,†,‡ Juan Han,† Jieling Jiang,†,§ Xuebo Chen,*,† and Weihai Fang*,† Department of Chemistry, Beijing Normal UniVersity Xin-wai-da-jie No. 19, Beijing 100875, P. R. China and Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, Stockholm 10691, Sweden ReceiVed: December 2, 2009; ReVised Manuscript ReceiVed: March 1, 2010

In the present work, we report a quantitative understanding on how to generate hydroxyl radicals from NO2 and H2O in the troposphere upon photoexcitation at 410 nm by using multiconfigurational perturbation theory and density functional theory. The conical intersections dominate the nonadiabatic relaxation processes after NO2 irradiated at ∼410 nm in the troposphere and further control the generation of OH radical by means of hydrogen abstraction. In agreement with two-component fluorescence observed by laser techniques, there are two different photophysical relaxation channels along decreasing and increasing O-N-O angle of NO2. In ˜ 2B1 and A ˜ 2B2 (CI (2B2/2B1) first funnels NO2 out of the the former case, the conical intersection between B ˜ 2B1 and relaxes to the A ˜ 2B2 surface. Following the primary relaxation, the conical Franck-Condon region of B ˜ 2B2 and X ˜ 2A1 (CI(2B2/2A1)) drives NO2 to decay into highly vibrationally excited X ˜ 2A1 intersection between A -1 state that is more than 20 000 cm above zeroth-order |n1,n2,n3 ) 0〉 vibrational level. In the latter case, increasing the O-N-O angle leads NO2 to relax to a minimum of B˜2B1 with a linear O-N-O arrangement. ˜ 2A1 (CI(2B1/2A1)) and leads NO2 to relax into This minimum point is also funnel region between B˜2B1 and X 2 ˜ a highly vibrationally excited X A1 state. The high energetic level of vibrationally excited state has enough energy to overcome the barrier of hydrogen abstraction (40-50 kcal/mol) from water vapor, producing OH (2Π3/2) radicals. The collision between NO2 and H2O molecules not only is a precondition of hydrogen abstraction but induces the faster internal conversion (CIIC) via conical intersections. The faster internal conversion favors more energy transfer from electronically excited states into highly vibrationally excited ˜ 2A1 states. The collision (i.e., the heat motion of molecules) functions as the trigger and accelerator in the X generation of OH radicals from NO2 and H2O in the troposphere. Introduction Hydroxyl radicals are highly reactive, short-lived, and consequently difficult-to-detect molecules. Up to the year of 2008, the VIRTIS-Venus Express Technical Team first detected hydroxyl radical in the Venusian atmosphere, some 100 km above the surface, by using Venus Express’s visible and infrared thermal imaging spectrometer.1 However, the nightglow from the hydroxyl radical OH in the Earth’s atmosphere was discovered by Meinel using high-resolution spectra techniques much earlier than 1950.2 The significance of OH radicals is consequently shown in purging the atmosphere of pollutants harmful to the biosphere.3 The hydroxyl radical can function as the “detergent” of the troposphere because it controls the removal and, therefore, the concentrations of man-made gaseous pollutants as well as natural gases.4,5 The OH radical is mainly responsible for the oxidizing capacity of the troposphere and dominates the first and rate-determining step of chain of radical reactions in the atmosphere.4 The oxidation reaction with carbon monoxide mediates the eliminating of CO, producing carbon dioxide (CO2) in the atmosphere.6 This reaction takes place continuously, and ∼2380 million tons of carbon monoxide is annually turned over in the atmosphere.6 * To whom correspondence should be addressed. Phone: 86-1058809480. Fax: 86-10-58809480. E-mail: [email protected] (X. C.) or [email protected] (W. F.). † Beijing Normal University. ‡ Royal Institute of Technology. § Current address: The No. 4 Middle School, Miyun, Beijing 101500.

Hydroxyl radicals play an important role in eliminating some greenhouse gases such as methane and ozone.7-11 The first reaction with many volatile organic compounds (VOCs) is the removal of a hydrogen atom to form water and an alkyl radical.7

OH + CH4 f H2O + CH3•

(i)

Subsequently, a serious of chemical reactions leads to net HO2 production:

CH3• + O2 f CH3O2

(ii)

CH3O2 + NO f CH3O + NO2

(iii)

CH3O + O2 f CH2O + HO2

(iv)

The final product of radical HO2 arising from the above reaction chain undergoes further decomposition and eventually is recycled to OH.12 The radicals OH and HO2 are collectively known as HOx, which is the heart of the photochemistry of the troposphere.12-16 Although OH is initially consumed in the first reaction, subsequent reactions give rebirth to the OH radical. The oxidation by OH is autocatalytic, and its concentration is greatly enhanced by the presence of NO.7 It has long been accepted that the photolysis of ozone by ultraviolet light irradiation in the presence of water vapor is

10.1021/jp911455r  2010 American Chemical Society Published on Web 03/17/2010

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the primary generation mechanism of tropospheric OH radicals.17-19

O3 + hV (λ e 320 nm) f O(1D) + O2(1∆g)

(v)

O(1D) + M f O(3P) + M (M is N2 or O2)

(vi)

O(1D) + H2O f 2OH

(vii)

The majority of excited singlet state O(1D) atoms decay to their ground state of O(3P) via collisional quenching reaction (vi). The minority of O(1D) (∼10%) that form OH by H abstraction reaction (vii) is dependent on the concentration of H2O.17 Besides the photodissociation of ozone, the recent investigations conducted by Sinha and co-workers revealed that reaction of electronically excited nitrogen dioxide with water can be another important source of tropospheric hydroxyl radicals.20 NO2 exhibits a complex and irregular absorption spectrum with broad width ranging from 300.0 to 850.0 nm.21-25 Among these discrete, nearly continuous spectra, the two featured absorption bands lead to photodissociation of NO2. In the shorter than 420 nm wavelength absorption, the photolysis leads to the formation of O(3P) atoms in the atmosphere and is also an important source of tropospheric ozone:26,27

NO2 + hV (λ < 420 nm) f NO + O(3P)

(viii)

O(3P) + O2 + M f O3 + M

(ix)

In contrast, irradiation with longer than 420 nm wavelength light initiates the photochemistry of efficient OH formation in the presence of water vapor.20,28

NO2 + hV (λ > 420 nm) f NO*2

(x)

NO*2 + H2O f OH + HONO

(xi)

HONO could undergo further photodecomposition producing to additional OH radicals:20

HONO + hV (λ < 390 nm) f OH + NO

(xii)

Although numerous studies reported photochemistry of nitrogen dioxide above the dissociative limit,30 quantitative understanding of its photochemical mechanism below or near the photodissociation threshold has not been achieved up to now. On the other hand, the light of shorter than ∼300 nm is considerably shielded by ozone due to its filtering effect in the troposphere.29 This work therefore concentrates on excitation by the longer wavelength to elucidate how OH radical is generated in the troposphere from NO2 and H2O at the multiconfigurational perturbation theory of level. Computational Details The two reaction paths of the photophysical decay channel for nitrogen dioxide along increasing and decreasing O-N-O angle are mapped by multistep optimizations with fixed angle in the three lowest-lying electronic states (i.e., 2A1, 2B1, and

2

B2), respectively. The critical points (i.e., minima and conical intersections) involved in these photophysical processes are explicitly described by full system optimizations without any structural parameter constraint. For these computations, complete active space self-consistent field (CASSCF) method was employed using appropriate active spaces and the 6-31G** basis set. All the optimizations with fixed O-N-O angle for NO2 were performed in the restriction of C2V symmetry. The ground state (2A1) reaction path was calculated by using singly root CASSCF optimizations when the O-N-O angle is fixed to be 179.9, 170, 160, 150, 140, 130, 133.9 (minimum), 120, and 110°, respectively. We also optimized the following stationary ˜ 2B2 and B ˜ 2B1 by employing two roots points in the surfaces of A ˜ 2B2 (90°), A ˜ 2B2 (100°), state-averaged CASSCF calculations: A 2 2 2 2 ˜ B2 (102.9°), A ˜ B2 (110°), A ˜ B2 (120°), B˜ B1 (180.0°), B˜2B1 A 2 2 ˜ ˜ (170.0°), B B1 (160.0°), B B1 (150.0°), B˜2B1 (140.0°). To account for the chemical reaction of hydrogen abstraction from water vapor in the troposphere, we built a complex model with one NO2 and one H2O molecule by using supramolecular optimizations. The potential energy surfaces (PES) for photophysical processes of the NO2-H2O complex were characterized by critical points optimizations, whereas the reaction of hydrogen abstraction is described by a minimum energy path. Herein, we would like to give some more comments on the selection of active space. To describe the π electrons in NO2, three center π orbitals (Π3) should be included in the active space. Figure 1a presents these three orbitals along the x-axis direction (y-z is the plane of the molecule) with zero, one, and two node(s), respectively. The rest of the oribitals in the active space originate from no-bonding orbitals of two oxygen atoms and nitrogen atom.(see Figure 1a). We employed nine active electrons and six active orbitals (9e/6o) to calculate the reaction path for NO2. To account for the reaction of hydrogen abstraction, more orbitals and electrons from O4-H6 σ and σ* bond and nonbonding of O4 of water (4e/3o see Figure 1b) were added into the active space, resulting in a total of 13 active electrons in 9 active orbitals (13e/9o) for the PES mapping of NO2-H2O complex. The singly root optimizations were used to map PES in the ˜ 2A1), whereas two-roots state-averaged with ground state (X equal weights (0.5:0.5) were introduced for surface calculations of excited states (2B1 and 2B2). The conical intersections of CI(2A1/2B2), CI(2A1/2B1), and CI(2B2/2B1) for NO2 and its complex are rigorously determined by using a two-roots stateaveraged optimization procedure available in the Gaussian programs package.31 These two roots originate from the ground and first excited states for CI(2A1/2B2) and CI(2A1/2B1) with equal weights and from the first and second excited states for CI(2B2/ 2 B1) with weights of 0.00, 0.50, and 0.50, respectively. To consider dynamical correlation, the single-point energy is calculated with second-order perturbation method (CASPT2) on the basis of CASSCF optimized structures. The zeroth-order wave function used in the CASPT2 calculations was a threeroot state average CASSCF wave function. For comparison, the PES of hydrogen abstraction for NO2-H2O complex was reoptimized at the density functional theory (DFT) level by using the B3LYP functional and 6-31G** basis set. The nature of the minima and transition states was confirmed by using analytical or numerical second derivative calculations and the IRC method. In the present work, all calculations were performed by using Gaussian 0331 and Molcas32 program packages.

Tropospheric Hydroxyl Radicals from NO2 and H2O

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Figure 1. The numbering scheme and selected oribitals in the active space for NO2 (a) and NO2-H2O (b) complex.

Results and Discussion Critical Structures in the Reaction Paths of Photophysical Decay. Geometric structures of minima and conical intersections of NO2 are depicted in Figure 2, along with CASSCF(9e/6o)/ 6-31G** optimized bond parameters. The ground state of NO2 ˜ 2A1) adopts well-known C2v symmetry, where the N-O bond (X is 1.171 Å and the O-N-O angle is 133.9° at the CASSCF(9e/ 6o)/6-31G** level of theory. The calculated geometric param˜ 2A1 are quantitatively consistent with previous eters for X calculations and experimental observations.33-35 The population analysis using CASSCF(9e/6o) density reveals that two threecenter π orbitals (Π3) with zero and one node (Π3-0 and Π3-1) are occupied by two pairs of π electrons, respectively, whereas there is no electron in two node π orbitals (φ(Π3-2)). On the other hand, the singly occupied electron is observed to distribute in the no-bonding orbitial of NO2 (see φ(NB-N) in Figure 1), which can be represented as φSO ) 1.014φ2pz(N1) - 0.748φ2pz(O2) - 0.748φ2pz(O2) (y-z is the plane of molecule). These findings ˜ 2A1) indicate that π electronic occupation in ground state NO2 (X 4 3 adopts Π3 rather than Π3 configuration, and singly occupied electrons distribute in the nonbonding orbitals of the molecular plane, mainly localizing in the N atom region. The π electronic configuration of Π43 versus Π33 for NO2 gave rise to controversy in different published papers or even textbooks for a long time.

The electron spin resonance (ESR) spectroscopy provides undoubted evidence that the singly occupied electron distributes in nonbonding orbitals rather than in π orbitals.36 These also support our finding of the π electronic configuration of Π34 in the present work. Among Π34 configurations, two π electrons are nonbonding ones and two other π electrons contribute the formation of two O-N π bonds. Consequently, the total π bond order of NO2 in ground state is 1, which well interprets the nature of the double bond of O-N (1.171 Å). Since there is weaker repulsion between single electrons around the N atom and bonding electrons in comparison with lone pairs-bonding ˜ 2A1) electrons mutual interaction, the O-N-O angle of NO2 (X (133.9°) is much larger than the H-O-H angle in H2O (104.5°). This angle is also larger than the expected value (120°) from the viewpoints of hybrid orbital theory (in the case of sp2 hybrid). The structural arrangement supports the above assign˜ 2A1) in the present ment of electronic configuration for NO2 (X work. This indicates the present calculation provides reasonable description for the understanding of fundamental issue of NO2. ˜ 2B2-TS was found by means of The critical point of A CASSCF(9e/6o) optimizations with C2v symmetry constraint. The second derivative calculations at CASSCF(9e/6o) level of theory show that there is a negative frequency with 1004i cm-1 and following its displacement vector leads to a asymmetric

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Figure 2. Schematic structures of critical points for NO2 (angle in degree and bond length in Å) along CASSCF(9e/6o)/6-31G** optimizations.

SCHEME 1: The Change from Single Electron to Lone Pair around N Atom Leads to Swelled in-Plane P Orbital

stretching vibrational modes. The population analysis reveals the singly occupied electrons distributes in the orbital of φ(NBO(y), while the orbital of φ(NB-N) is doubly occupied (see Figure 1). This means that one electron is promoted from the O nonbonding orbital to the N nonbonding orbital, producing a lone pair around N and leaving singly electron in orbital of ˜ 2A1 f A ˜ 2B2 transition. Scheme 1 φ(NB-O(y) upon the X illustrates the deformational process of φ(NB-N) orbital initiated by this excitation, which provides important clues to understanding this process. The formation of a lone pair around the N atom increases mutual interaction between nonbonding electrons and bonding electrons, resulting in a smaller O-N-O ˜ 2A1. ˜ 2B2 in comparison with 133.9° in X angle of 102.9° in A Meanwhile, the N-O bond lengths are also slightly elongated by ∼0.071 Å due to increased interaction. ˜ 2B2 state, the ˜ 2A1 and A Unlike bending arrangement in X ˜ 2B1 adopts a linear geometry with D∞h symmetry. minimum of B The singly occupied electron is excited to the virtual orbital of ˜ 2 A1 f π* (see φ(Π3-2) in Figure 1) upon the transition of X 2 ˜ B B1. This change of electron occupation increases the number of π electrons from 4 to 5, resulting in the Π35 configuration. Meanwhile, the departure of a singly occupied electron leaves a vacuum environment around N atom that further leads to the significant decreasing of mutual interaction between single electron and bonding electrons. The freedom of this constraint results in remarkable increasing of the O-N-O angle and ultimately leads to a linear arrangement. On the other hand, the redistribution of electrons also favors linear geometry to arrange the redundant π electrons.

Figure 3. Schematic reaction paths for the photophysical processes of NO2 along CASSCF(9e/6o)//CASPT2 computations.

Photophysical Processes after Irradiation of Nitrogen Dioxide at 410 nm. Figure 3 illustrates the reaction paths for the photophysical processes of NO2 by CASSCF(9e/6o)// CASPT2 computations, and their relative energies are summarized in Table 1. The 70.7 kcal/mol (∼410 nm) excitation energies vertically promote the NO2 molecule to instantaneously populate on Franck-Condon point of the B˜2B1 surface, where the O-N-O angle is 133.9°. As mentioned above, the O-N-O ˜ 2B1 angle is enlarged due to the structural characteristic of the B minimum. The intrinsic driving force boosts the O-N-O angle gradually increased from 133.9° at the FC structure to ∼180° at the B˜2B1 minimum, located 43.8 kcal/mol above zero level ˜ 2A1. As shown in the Table 1, the energy difference between of X 2 B1 and 2A1 in this point is only 1.0 kcal/mol. This indicates that NO2 evolves to the conical intersection region once the O-N-O angle is increased to be ∼180°. We were not surprise by these findings, since we discovered that several other cases of radicals in their excited state with linear geometry would intersect with those in ground state.37 Actually, one singly occupied electron around the N atom changes its orientation ˜ 2B1) with respect to the plane ˜ 2A1) to vertical (B from parallel (X of molecule once NO2 is promoted to the excited state. The changes of the orientation of the singly occupied electrons force the O-N-O angle to gradually enlarge. When this angle is increased to be ∼180° (the linear arrangement), there is no

Tropospheric Hydroxyl Radicals from NO2 and H2O TABLE 1: Relative Energies of NO2 in Low-Lying States (A1, B1, B2) along Reaction Paths of Photophysical Processes at the CASSCF(9e/6o)//CASPT2 Computational Level angle O2-N1-O3 90° 102.9° 105° CI(2A1/2B2) 110° 120° 132.1° CI(2B2/2B1) 133.9° 140° 150° 160° 170° ∼180° CI(2A1/2B1)

states A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2 A1 B1 B2

relative energies (kcal/mol) 91.3 106.9 41.2 36.5 46.1 27.5 28.1 48.4 30.4 19.8 49.1 33.0 6.4 60.4 50.6 64.2 64.6 0.0 70.7 82.2 0.1 52.9 95.4 7.4 46.2 122.2 17.8 42.7 159.4 30.9 40.6 172.6 43.8 44.8 182.9

remarkable difference in the orientation for the singly occupied electrons. These orientation changes finally lead the NO2 to relax to the region of degenerate energy. The conical intersection of CI (2A1/2B1) functions as an efficient nonadiabatic relay, leading ˜ 2A1) with abundant the NO2 to decay to the ground state (X excess heat (i.e., hot molecule). Besides the decay channel along increasing O-N-O angle, the decreasing angle could also drive the NO2 in Franck-Condon ˜ 2B1 state to relax to the ground state. As illustrated in Figure of B 3, two conical intersections (CI (2B2/2B1) and CI(2B2/2A1)) dominate this relaxation channel. Starting from Franck-Condon of B˜2B1, the NO2 molecule first decays to the CI (2B2/2B1) that ˜ 2A1. lies 64.2 kcal/mol higher in energy than the zero point of X As shown in Figure 1, this conical intersection is very close to the FC point of B˜2B1 in structure, in which the differences of O-N-O angle and N-O bond length are 1.8° and 0.06 Å, respectively. Funneling from CI (2B2/2B1), the nitrogen dioxide quickly relaxes along the 2B2 surface and ultimately reaches to CI(2B2/2A1) with a 105.0° O-N-O angle that is 30.4 kcal/mol ˜ 2B2 higher than that in the ground state. The local minimum of A 2 2 with 102.9° angle is very close to the CI( B2/ A1) in structure and lies at 27.5 kcal/mol (1.19 eV) above the same zero level ˜ 2B2 of energy. Although relaxation to the local minimum of A is one of two decay channels from CI(2B2/2A1), internal ˜ 2A1 has higher probability since the local conversion (IC) to X 2 ˜ minimum of A B2 appears to have stronger internal tension originating from the compressed O-N-O angle. In summary,

J. Phys. Chem. A, Vol. 114, No. 13, 2010 4605 the decay channel in the coordinate of decreasing O-N-O angle is dominated by two conical intersections (CI (2B2/2B1) and CI(2B2/2A1)) and finally returns to a hot molecule in the ground state. Experimentally, two-component fluorescence was observed and identified to be the fast process of stepwise vibrational quenching and the slow process of collision-induced change in the rotational quantum state in the initially excited state.38 The fast decay lifetime is about 50 µs and the slow one is around 200 µs following excitation with wavelengths between 600 and 530 nm.39 Sackett and Yardley observed a very fast decay on the order of 1 µs or shorter in the double-exponential components of the laser-induced fluorescence upon the 450-460 nm excitation.40 The lifetime of fluorescence depends on the wavelengths of excitation, and longer wavelength irradiation results in long-lived fluorescence.38-40 The observed doubleexponential components fluorescence could be well interpreted by present two-decay channels along the increasing or decreasing of O-N-O angle. The fast decay process is recently observed by Dai and his co-workers and explained41 that, following the excitation, the NO2 was promoted to the B˜2B1 state rovibronic level at 22 994.92 cm-1 (equal to 65.8 kcal/ mol; 70.7 kcal/mol in present calculations) and primarily decayed on the 1 µs time scale via electronic radiation (the conical intersection of CI (2B2/2B1) in the present work) and ˜ /A ˜ state (the conical intersection of CI(2B2/ relaxed to a mixed X 2 A1) in the present work) with a rate constant of 3.0 × 107 and ˜ finally returned to highly vibrationally excited levels in the X state (hot molecule) with a rate constant at least 1 order of magnitude slower. This mechanistic interpretation on the basis of real time probing undoubtedly agrees with the case of the decay channel along decreasing O-N-O angle on the basis of CASSCF/CASPT2 calculations. As shown in Figure 3, the pathway along the decreasing O-N-O angle is barrierless and downhill, which means this is fast relaxation channel. However, real time probing observed this time scale was 1 µs41 and much longer than the expected value. We noticed that the fast decay lifetime is about 50 µs and the slow one is around 200 µs following excitation with wavelengths between 600 and 530 nm.39 In comparison with longer wavelength excitation, the time scale has already reduced over 50-fold or more in the case of 410 nm UV light irradiation. Probably, photoinduced fluorescence of NO2 exhibits longer time scale in nature. More theoretic and experimental evidence should be obtained to address this issue. In comparison with the decay channel along decreasing angle, nitrogen dioxide undergoes larger O-N-O angle changes (∼46.1°) from the FC point (133.9°) to the minimum (i.e., CI ˜ 2B1 (∼180°)) along relaxation path of decreasing (2A1/2B1)) of B angle. These larger structural changes and the process of timeconsuming of redistribution of singly occupied electron from ˜ 2A1) to vertical (B˜2B1) are responsible for slower parallel (X process in two-component fluorescence. Dai et al. also proposes this single rovibronic state of B˜2B1 as one of two contributions to the LIF double-exponential decay and the other one from ˜ 2B2/X ˜ 2A1 that is mentioned above.41 The studies the mixed A by Paech et al. observed that a double exponential still dominated the fluorescence decay behavior but a very fast decay component began to appear following the excitation range near 480 nm.42 The radiationless transition would become an important decay channel with blue shift of excitation wavelength to the near-dissociative limit,43 which provides the energy source for following hydrogen abstraction in the troposphere.

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Figure 4. Schematic structures of minima and conical intersections for NO2-H2O complex (angle in degree and bond length in Å) along CASSCF(13e/ 9o)/6-31G** optimizations.

Figure 5. Schematic mechanism of hydrogen abstraction for NO2* from H2O photoinitiated by 410 nm light in line with CASSCF(13e/ 9o)//CASPT2 computations.

Hydrogen Abstraction in the NO2* · · · H2O Complex. Figure 5 illustrates the PES of hydrogen abstraction for the NO2-H2O complex, and its critical structures are shown in Figure 4. The formation of NO2-H2O complex leads to the collapse of C2V symmetry of mono-NO2, and all critical structures exhibit C1 symmetry. However, we still label the critical points of the NO2-H2O complex in low-lying electronic states by using irreducible representation in C2V group for convenience of discussion. The supramolecular optimizations reveal that 2.410 Å is equilibrium distance when NO2 and H2O get closer. The strong O · · · H hydrogen bond cause asymmetry N-O bonds that have ˜ 2A1) slightly longer lengths (∼1.18 Å) in the ground state (X than those in mono-NO2 (∼1.17 Å). Similarly, the 69.5 kcal/ mol (∼ 410 nm) excitation energies vertically promote the NO2-H2O complex in the Franck-Condon point of the B˜2B1 surface. Starting from this branch point, two relaxation channels are well reproduced along increasing and decreasing O-N-O angle, respectively. The energy level of CI (2B2/2B1) (68.4 kcal/ mol) for NO2-H2O complex is very close to that of the ˜ 2B1 surface. This Franck-Condon point (69.5 kcal/mol) of the B 2 ˜ 2B2 surface. ˜ facilitates internal conversion (IC) from B B1 to A With the decreasing O-N-O from more than 130.0° to ∼109.0°, the NO2-H2O complex evolves into the region of

˜ 2A1, locating at 22.7 ˜ 2B2 and X energetic degeneracy between A ˜ 2A1. The CI(2B2/2A1) functions kcal/mol above zero level of X as an efficient nonadiabatic channel to drive NO2-H2O complex decay to the ground state with abundant excess heat. Like the decay channel of increasing O-N-O angle for mono-NO2, the CI(2A1/2B1) of NO2-H2O complex dominates nonadiabatic direct relaxation from B˜2B1 to the highly vibrationally excited levels of the ground state. As shown in Figure 5, a very sharp barrier connects the ˜ 2A1) and product of HONO (X ˜ ) + OH reactant of NO2-H2O (X 2 ˜ 2 A1 ) ( Π3/2). With respect to the zero level of NO2-H2O (X minimum, the barrier of hydrogen abstraction in the ground state is 51.6 kcal/mol at CASSCF(13e/9o)/6-31G**//CASPT2 level of theory. This value decreases to be 41.6 kcal/mol at DFT/ B3LYP/6-31G** level. It is well accepted that B3LYP calculation normally underestimates the barrier of chemical reaction. The barrier of 40.0-50.0 kcal/mol is really high for the thermal reaction in the ground state. Actually, this high barrier originates from relative larger energetic difference with 27.5 kcal/mol ˜ 2A1) and HONO (X ˜ ) + OH (2Π3/2). The between NO2-H2O (X change in enthalpy (∆H) from product to reactant is determined to be +33.7 kcal/mol at DFT/B3LYP/6-31G** level of theory, which agrees well with that in NASA (39.8 kcal/mol) data.27 On the other hand, the singly occupied electron redistribution caused by O-H bond fission is also an energy consumption step. Although O4-H6 distance is elongated to be 1.21-1.22 Å (see Figure 5), the energy level reaches the peak of energy contour. Interestingly, we observed that the situation of singly electron occupation was significantly changed at this turning point. The singly occupied electron is distributed in the nonbonding orbital of around N atom in the side of reactant and redistributed to the π orbital of OH moiety (vertical orientation with respect to molecular plane). Both the energetic ˜ 2A1) and product difference between reactant of NO2-H2O (X 2 ˜ of HONO (X) + OH ( Π3/2) and singly occupied electron redistribution caused by O-H bond fission are responsible for the high barrier (40-50 kcal/mol) of hydrogen abstraction in the ground state. This indicates that the zeroth-order |n1,n2,n3 ) 0〉 vibrational level can not functions as an efficient reaction precursor state to trigger the hydrogen abstraction. Herein, n1, n2 and n3 are numbers of quanta in the symmetric stretching, bending, and asymmetric stretching modes, respectively.

Tropospheric Hydroxyl Radicals from NO2 and H2O Since there is abundant water vapor in the troposphere, collision between NO2 and H2O molecule frequently takes place and further initiates the occurrence of collision-induced internal conversion (CIIC). The fast pressure dependence of CIIC was observed by Dai’s group to occur with a rate constant of 3 × 107 via the collision between NO2 and Ar matrix.41 The proposed mechanism in the present work is that two fast ICs lead NO2 to ˜ 2A1 states that have a decay to highly vibrationally excited X 41 larger IR emission. High quantum yield of IR rather than fluorescence emission indicates that the abundant electronically excitation energy is transferred to abundant heat in highly ˜ 2A1 states. Experimental studies found vibrationally excited X ˜ 2A1 locates at that the energetic level of vibrationally excited X -1 more than 20 000 cm (57.2 kcal/mol) after the photocycle initiated by ∼410 nm light with respect to zeroth-order |n1,n2,n3 ) 0〉 vibrational level.41 It is enough to overcome the barrier (40-50 kcal/mol) of hydrogen abstraction in the ground state to generate OH (2Π3/2) radicals. The collision between NO2 and H2O accelerates the ICs to occur in shorter time scale and favors energy transfer from electronically excited states to highly ˜ 2A1 states. These important collisionvibrationally excited X induced ICs have been monitored by intramolecular dynamics of NO2 near the dissociative limit.41 On the other hand, the collision is the chemical reaction of hydrogen abstraction that is the initial step, which therefore plays a very important role in the generation of OH radical in troposphere from NO2 and H2O. Conclusions To get insights into how to generate hydroxyl radicals from NO2 and H2O in troposphere, the photophysical reaction path for mono-NO2 and NO2-H2O complex and PES of hydrogen abstraction for NO2-H2O complex in low-lying electronic states have been mapped by using multiconfigurational perturbation theory and DFT. In the present work, the configurations of NO2 ˜ 2A1 state are assigned to be 3-center-4 π electrons (Π34) in X and 3-center-5 π electrons (Π35) in B˜2B1 state. One electron is promoted from the O nonbonding orbital to the N nonbonding orbital, producing a lone pair around N and leaving a single ˜ 2 A1 electron in an orbital of nonbonding around O upon the X ˜ 2B2 transition. These findings make explicitly theoretic f A contributions to appease the controversy on electronic configuration in low-lying states for novel molecules of NO2. The conical intersections dominate the photophysical processes upon irradiation of NO2 by ∼410 nm light source in the troposphere and further control the generation of OH radicals by means of hydrogen abstraction. Like the two-component fluorescence observed, there are two different photophysical relaxation channels along decreasing and increasing O-N-O angle, respectively. In the former case, the conical intersection ˜ 2B2 (CI (2B2/2B1) first funnels NO2 out of between B˜2B1 and A ˜ 2B2 surface. Franck-Condon region of B˜2B1 and relaxes to A Following the primary relaxation, the conical intersection ˜ 2A1 (CI(2B2/2A1)) drives NO2 to decay into ˜ 2B2 and X between A ˜ 2A1 state that is more than 20 000 a highly vibrationally excited X -1 cm above zeroth-order |n1,n2,n3 ) 0〉 vibrational level. In the latter case, increasing the O-N-O angle relaxes NO2 to a ˜ 2B1 with linear arrangement. This minimum point minimum of B ˜ 2A1 (CI(2B1/2A1)) is also a funnel region between B˜2B1 and X ˜ 2 A1 and leads NO2 to relax to a highly vibrationally excited X state. The high energetic level of vibrationally excited state has enough energy to overcome the barrier of hydrogen abstraction (40-50 kcal/mol) from water vapor to produce OH (2Π3/2) radicals. The collision between NO2 and H2O molecules not

J. Phys. Chem. A, Vol. 114, No. 13, 2010 4607 only is a precondition of hydrogen abstraction but induces the faster internal conversion (CIIC) via conical intersection from photoinitiated Franck-Condon of B˜2B1. The faster internal conversion favors more energy transfer from electronically ˜ 2A1 states. The excited state to highly vibrationally excited X collision (i.e., the heat motion of molecules) plays the roles of trigger and accelerator in the generation of OH radical from NO2 and H2O in troposphere. We contribute an in-depth understanding on how hydroxyl radical is generated from NO2 and H2O in troposphere upon photoinitiated by 410 nm light source. Acknowledgment. This work was financially supported by FANEDD 200932 and NSFC20973025 to XBC and NSFC 20720102038 and Major State Basic Research Development Programs 2004CB719903 to W.H.F. Supporting Information Available: The absolute energies (au) and Cartesian coordinates for NO2 and NO2-H2O complex along reaction paths of photophysical processes and hydrogen abstraction at CASSCF(9e/6o) and CASSCF(9e/6o)//CASPT2 computational levels. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Piccioni, G.; Drossart, P.; Zasova, L.; Migliorini, A.; Ge´rard, J.C.; Mills, F. P.; Shakun, A.; Garcı´a Mun˜oz, A.; Ignatiev, N.; Grassi, D.; Cottini, V.; Taylor, F. W.; Erard, S. , the VIRTIS-Venus Express Technical Team. Astron. Astrophys. 2008, 483, L29–L33. (2) Bates, D. R.; Nicolet, M. J. Geophys. Res. 1950, 55, 301–327. (3) Kley, D. Sicence 1997, 276, 1043–1045. (4) Ehhalt, D. H. Science 1998, 279, 1002–1003. (5) Ehhalt, D. H.; Rohrer, F. J. Geophys. Res. 2000, 105, 3565–3571. (6) Ehhalt, D. H.; Dorn, H. P.; Poppe, D. Proc. R. Soc. Edinburgh, Ser. B 1991, 97, 17–34. (7) Wennberg, P. O.; Hanisco, T. F.; Jaegle, L.; Jacob, D. J.; Hintsa, E. J.; Lanzendorf, E. J.; Anderson, J. G.; Gao, R. S.; Keim, E. R.; Donnelly, S. G.; Del Negro, L. A.; Fahey, D. W.; McKeen, S. A.; Salawitch, R. J.; Webster, C. R.; May, R. D.; Herman, R. L.; Proffitt, M. H.; Margitan, J. J.; Atlas, E. L.; Schauffler, S. M.; Flocke, F.; McElroy, C. T.; Bui, T. P. Science 1998, 279, 49–53. (8) Ancellet, G. M.; Beckmann, M.; Papayannis, A. J. Geophys. Res. 1994, 99, 3451–3468. (9) Brunner, D.; Staehelin, J.; Jeker, D. Science 1998, 282, 1305–1309. (10) Lacis, A. A.; Wuebbles, D. J.; Logan, J. A. J. Geophys. Res. 1990, 95, 9971–9981. (11) Roelofs, G. J.; Lelieveld, J.; Dorland, R. A. J. Geophys. Res. 1997, 102, 23389–23401. (12) Kondratyev, K. Y.; Varotsos, C. A. EnViron. Sci. Pollut. Res. 2001, 8, 57-62. (13) Levy, H. Planet. Space Sci. 1972, 20, 919–931. (14) Crutzen, P. J. Pure Appl. Geophys. 1973, 106, 1385–1399. (15) Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 1981, 86, 7210–7254. (16) Esler, J. G.; Tan, D. G. H.; Haynes, P. H.; Evans, M. J.; Law, K. S.; Plantevin, P. H.; Pyle, J. A. J. Geophys. Res. 2001, 106, 4717–4731. (17) Monks, P. S. Chem. Soc. ReV. 2005, 34, 376–395. (18) Wine, P. H.; Ravishankara, A. R. Chem. Phys. Lett. 1981, 77, 103– 109. (19) Levy, H. Science 1971, 173, 141–143. (20) Li, S. P.; Matthews, J.; Sinha, A. Science 2008, 319, 1657–1660. (21) Douglas, A. E.; Huber, K. P. Can. J. Phys. 1965, 43, 74–81. (22) Walsh, A. D. J. Chem. Soc. 1953, 2266–2289. (23) Brand, J. C. D.; Hardwick, J. L.; Pirkle, R. J.; Seliskar, C. Can. J. Phys. 1973, 51, 2184. (24) Stevens, C. G.; Swagel, M. W.; Wallace, R.; Zare, R. N. Chem. Phys. Lett. 1973, 18, 465–469. (25) Tanaka, T.; Abe, K.; Curl, R. F. J. Mol. Spectrosc. 1974, 49, 310– 313. (26) Solarz, R.; Levy, D. H.; Abe, K.; Curl, R. F. J. Chem. Phys. 1974, 60, 1182–1974. (27) Sander, S. P. JPL Publ. 02-25; NASA Jet Propulsion Laboratory; Pasadena, CA, 2003. (28) Roehl, C. M.; Orlando, J. J.; Tyndall, G. S.; Shetter, R. E.; Va´zquez, G. J.; Cantrell, C. A.; Calvert, J. G. J. Phys. Chem. 1994, 98, 7837–7843.

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