Two Photon Dissociation Dynamics of NO2 and ... - ACS Publications

Article pubs.acs.org/JPCA

Two Photon Dissociation Dynamics of NO2 and NO2 + H2O Nannan Wu 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 explore the dynamics of OH formation from two photon absorbed NO2 with H2O, a high-level multiconfigurational perturbation theory was used to map the potential energy profiles of NO2 dissociation to O (1D) + NO (X̃ 2Π), and subsequent hydrogen abstraction producing 2OH (X̃ 2Π) + NO (X̃ 2Π) in the highly excited SPP (Ẽ 2A′, 2ππ*) state. The ground state NO2 is promoted to populate in the SNP1 (Ã 2A″, 2nπ*) intermediate state by one photon absorption at ∼440 nm, one thousandth of which is further excited to SPP (Ẽ 2A′, 2ππ*) state and undergoes a medium-sized barrier (∼11.0 kcal/mol) to give rise to OH radicals. In comparison with the hydrogen abstraction reaction in highly vibrationally excited NO2 ground state, two photon absorption facilitates NO2 dissociation to O (1D) and O (1D) + H2O → 2OH (X̃ 2Π) but results in low quantum yield of NO2** since there is a weak absorption upon the second beam light at ∼440 nm. It can be concluded that the reaction of two photon absorbed NO2 with H2O makes negligible contributions to the formation of OH radicals. In contrast, single photon absorption at 420 nm) → NO2 *

(1)

NO2 * + H 2O → OH + HONO

(2)

his co-workers seem significant and allow one to rethink the ozone production and control strategies of pollutants.24 However, under conditions that better approximate the solar flux, Blitz and his co-workers did not reproduce the Sinha′s results but found an OH yield of λ > 432 nm) → NO2 **

(4)

1

NO2 ** → NO + O( D)

(5)

O(1D) + H 2O → 2OH

(6)

Similarly, no OH radicals were observed using an unfocused excitation laser beam at 565 nm in the recent experiment conducted by Fittschen and her co-workers, but OH formation was observed by focusing the beam.11 They proposed a complex mechanism including multiphoton absorption of NO2 to account for the observed discrepancy with Crowley et al.9 The barrier for hydrogen abstraction through reaction 2 in the

The rate constant for reaction 2 was calculated to be 1.7 × 10−13 cm3·molecule−1·s−1 with ∼0.001 yield.6 Although this reaction has low efficiency, it still contributes a major source of OH up to 50% of that produced from the traditional O (1D) + H2O reaction at high solar zenith angles under polluted conditions.6 The ozone concentration is accordingly estimated to an increase of up to 40% in polluted areas due to such additional OH generation.24 The results reported by Sinha and © 2012 American Chemical Society

NO2 + hv(449 > λ > 432 nm) → NO2 *

Received: March 28, 2012 Revised: May 24, 2012 Published: May 30, 2012 6894

dx.doi.org/10.1021/jp3029825 | J. Phys. Chem. A 2012, 116, 6894−6900

The Journal of Physical Chemistry A

Article

ground state was calculated by our group15 to be 51.6 kcal/mol (554 nm) by using high level multiconfigurational perturbation theory. This high barrier is higher than the thermodynamic limit at ∼620 nm observed by Sinha and his co-workers6 and eliminates the possibility of the occurrence of reaction 2 at 565 nm. The one photon excitation (∼410 nm) promotes NO2 molecule to its low-lying excited B̃ 2B1 state, from which the conical intersections (CIs) of CI (B̃ 2B1/Ã 2B2), CI (Ã 2B2/X̃ 2A1), and CI (B̃ 2B1/X̃ 2A1) efficiently funnel NO2 decay to its ground state (X̃ 2A1) with abundant excess heat along decreasing and increasing O−N−O angle, respectively.15 The following two reasons are mainly responsible for high barrier hydrogen abstraction in the ground state: (i) the lager energy difference (30−40 kcal/mol) between reactant NO2−H2O (X̃ 2A1) and product HONO (X̃ ) + OH (2∏3/2) and (ii) energy consumption for the redistribution of singly occupied electron caused by O−H bond fission from the molecular plane to the vertical orientation. The high barrier of hydrogen abstraction in the ground state rules out the possibility of the generation of OH via the reaction 2 at long wavelength (>554 nm) by single photon absorption. However, the barrier for reaction in the third excited C̃ 2A″ state was calculated to be only ∼5.0 kcal/ mol at TD-DFT level,17 which supports the mechanism proposed by Sinha and his co-workers.6 Although the twophoton mechanism was proposed for the OH formation by Blitz and Crowley,7,9 Fittschen suggested a multiphoton process, and this added another level of complexity to the story.11 Two- or multiphoton process involves high-lying excited state of NO2, which is believed to be notoriously complicated and not well elucidated in the present stage.25 The complete active space self-consistent field (CASSCF) and multiconfiguration second-order perturbation theory (CASPT2) have been extensively used to investigate photodissociation mechanisms of carbonyl compounds and other complicated systems26−29 and were confirmed to be reliable methods for the treatment of mechanistic photochemistry. In the present work, excited electronic states of NO2 and the reaction mechanism with water photoinitiated by two photons will be explored using the combined CASSCF/CASPT2 approach. These computational results will be used to explore the dynamics of OH formation via the reaction of two photon absorbed NO2 with H2O.

Figure 1. Schematic minimum energy profile (MEP) of two-photon dissociation of NO2 along N1−O3 distance at CASSCF(9e/7o)/631G**//CASPT2 level of theory.

transition leading to failure attempt when O1−N3 distance was close to that in the Franck−Condon (FC) region in the presence of H2O. As an alternative solution, a reduced active space of 7e/6o was used by excluding the nonbonding orbital of O1 and σ, σ* orbitals of H4−O5 when the N1−O3 distance of NO2−H2O complex is ranged from 1.260 to 1.710 Å in the Ẽ 2A′ (2ππ*) state stationary point optimizations. To consider dynamical electron correlation effects, the refined single-point energy for all stationary points is calculated by the second-order perturbation method (CASPT2) with a 7 roots or 9 roots stateaveraged CASSCF wave function on the basis of the optimized structures. Since the elongation of O−N bond leads to the collapse of C2v symmetry, a reduced Cs rather than C2v symmetry is constrained during the structural optimization of NO 2 and NO 2 −H 2 O complex in this work. All the computations are performed by Gaussian0330 and Molcas31 program packages.

■

RESULTS AND DISCUSSION Two Photon Absorption of NO2 and NO2−H2O. Table 1 summarized the vertical excitation energies, oscillator strengths, and the character of singly occupied oribitals of different transitions of NO2 and NO2−H2O. The two lowest electronic states of NO2 are SNP1 (Ã 2A″, 2nπ*) and SNN (B̃ 2A′, 2nn) that are assigned to B̃ 2B1 and Ã 2B2 in the C2v point group, respectively.15 The S0 → SNP1 (Ã 2A″, 2nπ*) transition is originated from the redistribution of the singly occupied electron from the nonbonding orbital of N in the case of S0 (X̃ 2A′) to the virtual orbital of π* with two nodes. Whereas, the S0 → SNN (B̃ 2A′, 2nn) transition originates from the promotion of one electron from the nonbonding orbital of O to that of N leading to a lone pair around N and leaving a singly occupied electron around O. These findings have been discussed in detail in our previous publication.15 The vertical excitation energies for S0 → SNP1 (Ã 2A″, 2nπ*) and S0 → SNN (B̃ 2A′, 2nn) transitions are 65.2 (438 nm) and 74.7 (383 nm) kcal/mol at the CASSCF(9e/7o) level of theory with seven roots state averaged CASPT2 single point energy correction in this work. In our previous computations, these two values were determined to be 70.7 (404 nm) and 82.2 (348 nm) kcal/ mol, at CASSCF(9e/6o) optimized geometry followed by three roots state averaged CASPT2 corrections, and slightly higher

■

COMPUTATIONAL DETAILS The minimum energy profiles (MEPs) for two-photon dissociation of NO2 and NO2−H2O complex, i.e., O1−N3 (see Figures 1 and 2 for numbering) bond fission in the Ẽ 2A′ (2ππ*) state, were mapped by multistep optimizations with fixed O−N distance. An additional O3−H4 bond was also constrained for hydrogen abstraction in NO2−H2O complex in Ẽ 2A′ (2ππ*) state when O1−N3 distance is longer than 1.800 Å. The ab initio calculations were preliminarily done at the CASSCF level of theory with 6-31G** basis set. The principal 7 active electrons and 5 active oribitals (7e/5o) originate from two nonbonding orbitals of O and N and three π orbitals that have zero, one, and two node(s), respectively. To describe O1− N3 bond cleavage for NO2 and its water complex as well as hydrogen abstraction reaction for NO2−H2O complex, O1−N3 and H4−O5 σ and σ* orbitals are also added to the active space resulting in a total of 9 active electrons in 7 active orbitals (9e/ 7o) for NO2 and 11e/9o for NO2−H2O complex, respectively. The stationary point optimizations in the high-lying Ẽ 2A′ (2ππ*) state were often interrupted by a 1nπ* or 1σσ* 6895



Article

Figure 2. Schematic minimum energy profile of two-photon dissociation and hydrogen abstraction in NO2−H2O complex along N1−O3 (O3−H4) distance at CASSCF(11e/9o, 7e/6o)/6-31G**//CASPT2 level of theory.

> 420 nm). It should be noted that S0 → SNP1 (Ã 2A″, 2nπ*) and S0 → SNN (B̃ 2A′, 2nn) transitions have close vertical excitation energies and oscillator strengths (see Table 1). This inevitably leads to the strong coupling between these two states in FC region. Similarly, the mixed character between B̃ 2B1 and Ã 2B2 (Ã 2A″/B̃ 2A′) was experimentally assigned for the low lying electronic excitation of NO2.6,25,32 As illustrated in Table 1, the SPP (Ẽ 2A′, 2ππ*) state has 136.9 kcal/mol (209 nm) of energy with respect to the zero level of S0 (X̃ 2A′) and approximately doubles that of the S0 → SNP1 (Ã 2A″, 2nπ*) transition (65.2 kcal/mol), which is initially populated upon one photon excitation at λ > 420 nm. The electronic population analysis of SPP (Ẽ 2A′, 2ππ*) shows that three singly occupied electrons are found to distribute in the nonbonding orbital of N and two π orbitals with one and two node(s), respectively (see Table 1). As discussed above, one photon absorption of NO2 leads to a singly occupied electron being excited to the π orbital with two nodes from the nonbonding orbital of N in the S0 (X̃ 2A′) state leaving a vacuum environment around N. This allows the nonbonding orbital of N to accept one electron from π orbitals with one node initiated by the second beam of resonance light at a wavelength of ∼440 nm resulting in the SNP1 (Ã 2A″, 2nπ*) → SPP (Ẽ 2A′, 2 ππ*) transition. The two photon absorption of NO2 at ∼440 nm can be completed as follows: S0 (X̃ 2A′) + hν (∼440 nm) → SNP1 (Ã 2A″, 2nπ*) + hν (∼440 nm) → SPP (Ẽ 2A′, 2ππ*). Therefore, a sequential two-step one photon absorption process allows SNP1 (Ã 2A″, 2nπ*) to qualifiedly function as an intermediate state under ∼440 nm UV light irradiation. The existence of an intermediate state that must satisfy the criteria of electronic transition allowance and energy resonance is the crucial precondition for the occurrence of resonant two photon absorption. Besides the existence of an intermediate state, the transition dipole moment between the SNP1 (Ã 2A″, 2nπ*) intermediate and the SPP (Ẽ 2A′, 2ππ*) state is another

Table 1. Vertical Excitation Energies (E⊥, kcal/mol (nm)), Oscillator Strengths ( f), and the Character of Singly Occupied Oribitals of Different Transitions of NO2 and NO2−H2O at the CASSCF(9e/7o)/CASPT2 Level of Theory

than those in this work.15 As shown in Table 1, these two transitions together with S0 → SPN2 (F̃ 2A″, 2πn) (but with high vertical excitation energy (149 nm)) have 10- to 1000-folds larger oscillator strength over other transitions. This indicates that the nature of the initially populated electronic state varies with excitation wavelength, but one photon excitation at long wavelength (λ > 420 nm) promotes NO2 to the Franck− Condon (FC) region of the SNP1 (Ã 2A″, 2nπ*) state (∼410− 440 nm). Experimentally, it has been well established that the absorption spectrum of NO2 extends from about 250 to 650 nm and consists of substantial fine structures on a broad continuum with a maximum near 410 nm.25,32 The present and previous computations15 well reproduced the experimentally intense absorption upon photoexcitation at long wavelength (λ 6896



Article

Table 2. Electronic Configuration for Stationary Points Are Given along Minimum Energy Profile of NO2 in the SPP (Ẽ 2A′, 2 ππ*) Statea

The electronic configurations are described by the α (↑) and β (↓) spin state population in the 7 active orbitals. The electronic configurations of O (1D) splitting with different mJ (2, 1) caused by strong mutual interaction between O (1D) and NO (X̃ 2Π) are also described by the different spin state population of 7 distorted orbitals. a

predominate factor to influence the quantum yield of SPP (Ẽ 2A′, 2 ππ*). As shown in Table 1, the oscillator strength of the second photon absorption of SNP1 (Ã 2A″, 2nπ*) → SPP (Ẽ 2A′, 2 ππ*) is 4.48 × 10−6 and ∼1000-folds smaller than that of the S0 → SNP1 (Ã 2A″, 2nπ*) transition ( f = 2.04 × 10−3) by the first photon absorption. Therefore, the quantum yield of the two photon absorption of NO2 is reduced by a weak absorption of SNP1 (Ã 2A″, 2nπ*) → SPP (Ẽ 2A′, 2ππ*). The resonant two photon absorption is different from the direct S0 → SPP (Ẽ 2A′, 2 ππ*) transition photoexcited by using one beam light at 209 nm, where one electron of π orbital with one node is promoted to π* with two nodes (π → π* transition) leading to the configuration of three singly occupied electrons. Although direct S0 → SPP (Ẽ 2A′, 2ππ*) transition initiated by one photon absorption has a large oscillator strength (f = 3.47 × 10−4), it is not an important excitation in photochemistry of NO2 due to the requirement of high excitation energy (136.9 kcal/mol (209 nm)). The S0 → SNP2 (D̃ 2A″, 2nπ*) is another three singly occupied electron transition where one electron of lone pair of O is promoted to the π* orbital with two nodes, and its vertical excitation energy is 123.1 kcal/mol (232 nm). Like the S0 → SNN (B̃ 2A′, 2nn) excitation, both S0 → SPN1 (C̃ 2A″, 2πn) and S0 → SPN2 (F̃ 2A″, 2πn) transitions are originated from the promotion of one electron from π orbital with zero and one nodes to nonbonding orbitals of N, respectively, and eventually result in a lone pair around N that is different from the configuration of the single electron of X̃ 2A′ in the ground state. As shown in Table 1, the excitation energy of S0 → SPN2 (F̃ 2A″, 2 πn) state is energetically ∼100 kcal/mol higher than that of S0 → SPN1 (C̃ 2A″, 2πn), which indicates that promotion of an electron in the π orbital with zero nodes is much more difficult than that in a π orbital with one node. Since only three electronic states of S0 (X̃ 2A′), SNP1(Ã 2A″, 2nπ*), and SPP (Ẽ 2A′, 2 ππ*) are involved in the two photon absorption of NO2 and subsequent dissociation, other excited states will not be discussed in detail in this work. Overall, the present electronic structure calculations give accurate electronic transition energies and provide clear molecular orbital pictures for the complicated NO2 absorption spectrum. Two Photon Dissociation Dynamics of NO2. As shown in Figure 1, NO2 is instantaneously excited to the intermediate state of SNP1 (Ã 2A″, 2nπ*) photoinitiated by the first photon

absorption at ∼440 nm. Followed by above photon promotion, the second beam light of ∼440 nm takes the system to populate in the FC region of the SPP (Ẽ 2A′, 2ππ*) state. The wave packet of this excited state rapidly decays to a global minimum that is energetically 111.9 kcal/mol above the minimum of the S0 when the N1−O3 distance is elongated from ∼1.200 Å in the FC region of the SPP (Ẽ 2A′, 2ππ*) state to 1.510 Å. A downhill energy curve connects the FC and minimum of SPP (Ẽ 2A′, 2 ππ*) in which the O−N−O angle is significantly decreasing from ∼133.6° in FC to 106.0° in minimum. As illustrated in Figure 1, the energy level is gradually increasing when N1−O3 distance is longer than 1.510 Å. Finally, it reaches the maximum of the SPP (Ẽ 2A′, 2ππ*) state along the reaction coordinate of the N1−O3 distance at 1.810 Å. This maximum of SPP (Ẽ 2A′)Max energetically lies 11.6 kcal/mol above the minimum of SPP (Ẽ 2A′, 2ππ*). When this medium-sized barrier (∼11.0 kcal/ mol) is overcome, NO2 molecule gradually evolves into the phase of the product with a large N1−O3 distance along a downhill energy curve. As illustrated in Table 2, the product in SPP (Ẽ 2A′, 2ππ*) that is labeled by SPP (Ẽ 2A′)-Pro is of triradical configuration in which two singly occupied electrons distribute in the O3 moiety along the orientation in the molecular plane and in the vertical plane, respectively, and the other one localizes in the π orbital of the NO moiety. Compared with the characteristics of the singly occupied electrons at stationary points of SPP (Ẽ 2A′)FC, SPP (Ẽ 2A′)-Min, and SPP (Ẽ 2A′)-Max, it can be concluded that the N1−O3 bond fission leads to the homolytic cleavage of the π orbital with two nodes and redistribution of a singly occupied electron from nonbonding orbital of N to that of O and eventually results in triradical configuration in SPP(Ẽ 2A′)Pro. With increasing N−O distance, the collapse of triradical configuration leads to the O atom with 1D (mJ = 1) configuration in which two singly occupied electrons have different α (↑) or β (↓) spin state populations and distribute in the p orbital of O along the orientation in the molecular plane and in the vertical plane, respectively. The other singly occupied electron is held by the π orbital of NO leading to a X̃ 2Π configuration in the ground state. As shown in Table 2, another electronic state of the oxygen atom with O (1D) (mJ = 2) configuration was also determined by CASPT2 calculation. The two p electrons for O (1D) (mJ = 2) are found to occupy in one orbital along the vertical plane orientation, while those for 6897



Article

O (1D) (mJ = 1) distribute in two orbitals along different orientations. As illustrated in Figure 1, the energy levels for O (1D) (mJ = 2) and O (1D) (mJ = 1) are exactly degenerate when the N1−O3 distance is longer than 1.760 Å. This reproduces the well-known Zeeman effect33 without the presence of a static magnetic field. However, energy splitting is gradually increasing when O and NO moieties get closer and reach a maximum value of 14.3 kcal/mol in the minimum of SPP (Ẽ 2A′, 2ππ*). This kind of energy splitting caused by strong mutual interaction between two fragments is different from the origin of the Zeeman effect.33 Adiabatically, the product O (1D) + NO (X̃ 2Π) energetically lies ∼113.0 kcal/mol above the minimum of S0 (X̃ 2A′). Overall, two photon absorption of NO2 with ∼136.0 kcal/mol excitation energy significantly results in O (1D) and NO (X̃ 2Π) with abundant potential energy (∼113.0 kcal/mol). This is in agreement with early experimental observations in which photodecomposition of NO2 to O (1D) + NO (X̃ 2Π) takes place in its second predissociation region of ∼250−214 nm (∼114−133 kcal/ mol).34 The high energy product O (1D) + NO (X̃ 2Π) provides abundant potential energy for the subsequent hydrogen abstraction reaction from H2O. Two Photon Dissociation Dynamics and Hydrogen Abstraction in NO2 −H2O Complex. Figure 2 illustrates the minimum energy profile of two photon dissociation and hydrogen abstraction in the NO2−H2O (NW) complex along the reaction coordinate of the N1−O3 (O3−H4) distance. Like the dissociation of isolated NO2, the same magnitude of barrier (10.9 kcal/mol) was found in the MEP of two photon dissociation of the NO2−H2O complex and labeled by Barrier (D). Moreover, the minimum of SPP (Ẽ 2A′)-NW-Min and maximum of Barrier (NW-D) were determined to locate at almost the same coordinate points with 1.610 and 1.800 Å for the N1−O3 distance, respectively. These indicate that the formation of the NO2−H2O complex exerts little influence on the generation of O (1D) in the SPP (Ẽ 2A′, 2ππ*) state since there is large intermolecular distance (∼2.600 Å) between NO2 and H2O. As shown in Figure 2, the generation of O (1D) is completed when the N1−O3 distance is elongated to be ∼2.400 Å. The product of the two photon dissociation of O (1D) + NO (X̃ 2Π) for the NO2−H2O complex has 120.5 kcal/ mol of energy with respect to the zero level of S0 (X̃ 2A′)-NW. This energy level is ∼7.0 kcal/mol higher than that of isolated NO2. The energy curve is slightly increasing when O (1D) starts to abstract a hydrogen atom from a water molecule. It reaches the local maximum labeled by Barrier (HA) when the O3−H4 distance is shortened to be ∼1.400 Å. The barrier of O (1D) hydrogen abstraction is calculated to be 3.3 kcal/mol at the CASSCF/CASPT2 level of theory in the present work. Experimentally, it is well established that O (1D) + H2O occurs with a gas kinetic rate constant of 2.2× 10−10 cm3·molecule−1·s−1 at 298 K and the measured activation energy is zero or slightly negative.32 In comparison with experimental value, the presently calculated 2−3 kcal/mol slightly overestimates this barrier but does so beyond the range of allowed error of excited state calculation.26−29 In many cases of the photochemistry reaction investigated by our group, the calculated barriers for the ultrafast processes in the time scale of picoseconds confirmed by laser techniques are usually calculated to be 1−5 kcal/mol. However, the measured activation energy is the change of free energy, while the calculated barrier in this work is the difference of potential energy. This is another reason that can account for the

difference between experimental and calculated values. Therefore, the calculated barrier for the O (1D) + H2O reaction does not lead to the principal discrepancy with that of experimental measure. In other words, the O (1D) + H2O reaction can take place with high efficiency in the photochemistry of NO2 and makes important contributions to the formation of OH radical. Crossing a small barrier, the energy level of the NO2−H2O complex is gradually going down, while the O3 and H4 distance gets closer, and the O3−H4 bond is partially formed. As shown in Figure 2, the energy of system is decreasing from 123.8 kcal/ mol in Barrier (HA) to 116.5 kcal/mol in the stationary point with 1.150 Å O3−H4 distance. The energy level sharply falls down from 116.5 to 90.3 kcal/mol when the O3−H4 distance is slightly shortened from 1.150 to 1.100 Å. Meanwhile, the H4−O5 distance is rapidly increased from 1.115 to 2.132 Å. This indicates that the formation of the O3−H4 bond leads to rapid departure of the O5−H6 radical since there is strong molecular repulsion between the two OH radicals. The sharp decrease in the energy level is accounted for by the released energy from the pairing between one of the singly occupied electrons of O (1D) and that of H4. Since electronic paring takes place in the molecular plane, the singly occupied electron along the vertical plane orientation is retained in the O3−H4 radical resulting in a product in its X̃ 2Π state. Meanwhile, another OH (X̃ 2Π) is also formed from H2O in its ground state with the departure of H atom. The product 2OH (X̃ 2Π) + NO (X̃ 2Π) is finally generated by the adjustment of distance (2.200−3.000 Å) among the radical fragments and lies 83.2 kcal/mol above the zero level of S0 (X̃ 2A′)-NW.

■

CONCLUSIONS The complete active space self-consistent field (CASSCF) and multiconfigurational perturbation theory (CASPT2) were used to quantitatively understand two photon dissociation dynamics of NO2 and subsequent hydrogen abstraction of O (1D) with H2O. The ground state NO2 is promoted to populate in the SNP1 (Ã 2A″, 2nπ*) intermediate state by one photon absorption at ∼440 nm and continuously excited to the high-lying SPP (Ẽ 2A′, 2ππ*) state by the second beam of resonant light of ∼440 nm, i.e., S0 (X̃ 2A′) + hν (∼440 nm) → SNP1 (Ã 2A″, 2nπ*) + hν (∼440 nm) → SPP (Ẽ 2A′, 2ππ*). This two photon absorption significantly leads to the occurrence of the N−O bond fission in the SPP (Ẽ 2A′, 2ππ*) state that overcomes a medium-sized barrier (∼11.0 kcal/mol) and eventually results in O (1D) with ∼113.0 kcal/mol of excess energy. The high energy O (1D) allows the hydrogen abstraction from H2O to take place with high efficiency producing the OH (X̃ 2Π) radical, where a small barrier (3.3 kcal/mol) is encountered. There are two possible processes for NO2* upon one photon excitation. The NO2* is continuously excited to the high-lying SPP (Ẽ 2A′, 2ππ*) state by the second beam or decays to the ground state (X̃ 2A1) with abundant excess heat. As discussed in our previous publication,15 the existence of conical intersections of CI (B̃ 2B1/Ã 2B2), CI (Ã 2B2/X̃ 2A1), and CI (B̃ 2B1/X̃ 2A1) of NO2 allows highly efficient internal conversion to its ground state from SNP1 (Ã 2A″, 2nπ*) (B̃ 2B1) state. However, the absorption of SNP1 (Ã 2A″, 2nπ*) → SPP (Ẽ 2A′, 2ππ*) is extremely weak (∼1000 lower than that of one photon excitation) under the second beam light irradiation. As a result, promotion to the SPP (Ẽ 2A′, 2ππ*) state by the second photon absorption cannot compete with IC to the ground state via the conical intersections. In the experimental conditions, the situation for the two photon process becomes even worse. 6898



Article

excited sates Ẽ 2A″, 2ππ* obtained by the CASSCF(9e,7o)/631G*// CASPT2 level of theory. This material is available free of charge via the Internet at http://pubs.acs.org.

The collision between NO2* and air (N2/O2) induces the occurrence of collision-induced internal conversion (CIIC)36 and accelerates the process of IC to the ground state (X̃ 2A1) and extremely reduces the occurrence possibility of the two photon process. The faster internal conversion favors more energy transfer from the electronically excited state to the highly vibrationally excited X̃ 2A1 states thus leading to the formation of OH radical if NO2 gets energy above the barrier of 51.6 kcal/mol.15 In conclusion, the reaction of the two photon absorbed NO2 with H2O makes negligible contributions to the formation of OH radicals. In contrast, single photon absorption at 420 nm.6−10,15,16,25,32,37 The minimum structure of the third excited C̃ 2A″ state was also optimized to exhibit an elongated N1−O2 bond (1.338 Å) and decreased O−N−O angle (109.6°) in comparison with those in the ground state (see the section of Supporting Information). The electronic population analysis clearly shows that the singly occupied electrons around the O2 atom have different spin quantum numbers. This indicates that the population in the third excited C̃ 2A″ state leads to the fission of the N1−O2 bond producing O (3P) rather than high energy O (1D), which undergoes numerous investigations at λ < 420 nm.32 In other words, N−O bond fission occurs in the third excited state leading to the ground state O (3P), while water decomposition or the hydrogen abstraction reaction computed by Minaev et al. should take place in the ground state rather than C̃ 2A″ excited state that has been explicitly described in our previous work.15 The barrier calculated by Minaev et al. should be compared with the zero level of the ground state minimum rather than that of the C̃ 2A″ state. Interestingly, the barrier is estimated to be ∼57.0 kcal/mol with respect to the zero level of the ground state minimum, which is very close to our computed values at the CASSCF/CASPT2 level.15

■

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was financially supported by FANEDD 200932, NCET-11-0030, and NSFC20973025. REFERENCES

(1) Monks, P. S. Chem. Soc. Rev. 2005, 34, 376. (2) Crutzen, P. J. Ann. Rev. Earth Planet. Sci. 1979, 7, 443. (3) Ehhalt, D. H. Science 1998, 279, 1002. (4) Ehhalt, D. H.; Rohrer, F. J. Geophys. Res. 2000, 105, 3565. (5) 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.; et al. Science 1998, 279, 49. (6) Li, S. P.; Matthews, J.; Sinha, A. Science 2008, 319, 1657. (7) Carr, S.; Heard, D. E.; Blitz, M. A. Science 2009, 324, 336. (8) Li, S. P.; Matthews, J.; Sinha, A. Science 2009, 324, 336. (9) Crowley, J. N.; Carl, S. A. J. Chem. Phys. A 1997, 101, 4178. (10) Blitz, M. A. J. Chem. Phys. A 2010, 114, 8016. (11) Amedro, D.; Parker, A. E.; Schoemaecker, C.; Fittschen, C. Chem. Phys. Lett. 2011, 513, 12. (12) Takahashi, K.; Hayashi, S.; Suzuki, T.; Matsumi, Y. J. Phys. Chem. A 2004, 108, 10497. (13) Yasuhiro, S.; Ayako, Y.; Shungo, K.; Yoshizumi, K. Environ. Sci. Technol. 2005, 39, 8847. (14) Sörgel, M.; Regelin, E.; Bozem, H.; Diesch, J.-M.; Drewnick, F.; Fischer, H.; Harder, H.; Held, A.; Hosaynali-Beygi, Z.; Martinez, M.; et al. Atmos. Chem. Phys. 2011, 11, 10433. (15) Fang, Q.; Han, J.; Jiang, J. L.; Chen, X. B.; Fang, W. H. J. Phys. Chem. A 2010, 114, 4601. (16) Chen, X. B.; Fang, W. H. J. Phys. Chem. A 2010, 114, 8017. (17) Minaev, B. F.; Zakharov, I. I.; Zakharova, O. I.; Tselishtev, A. B.; Filonchook, A. V.; Shevchenko, A. V. ChemPhysChem 2010, 11, 4028. (18) Luo, G. F.; Chen, X. B. J. Phys. Chem. Lett. 2012, 3, 1147. (19) Wine, P. H.; Ravishankara, A. R. Chem. Phys. Lett. 1981, 77, 103. (20) Levy, H. Science 1971, 173, 141. (21) Matsumi, Y.; Comes, F. J.; Hancock, G.; Hofzumahaus, A.; Hynes, A. J.; Kawasaki, M.; Ravishankara, A. R. J. Geophys. Res. 2002, 107, 4204. (22) Lou, S.; Holland, F.; Rohrer, F.; Lu, K.; Bohn, B.; Brauers, T.; Chang, C. C.; Fuchs, H.; Häseler, R.; Kita, K.; et al. Atmos. Chem. Phys. 2010, 10, 11243. (23) Dominé, F.; Shepson, P. B. Science 2002, 297, 1506. (24) Wennberg, P. O.; Dabdub, D. Science 2008, 319, 1624. (25) Wilkinson, I.; Whitaker, B. J. Annu. Rep. Prog. Chem., Sect. C: Phys. Chem. 2010, 106, 274. (26) (a) Fang, W. H.; Liu, R. Z. J. Am. Chem. Soc. 2000, 122, 10866. (b) Ding, L. N.; Chen, X. B.; Fang, W. H. Org. Lett. 2009, 11, 1495. (27) (a) Chen, X. B.; Fang, W. H. J. Am. Chem. Soc. 2003, 125, 9689. (b) Chen, X. B.; Ma, C. S.; Phillips, D. L.; Fang, W. H. Org. Lett. 2009, 12, 5108. (28) (a) Chen, X. B.; Fang, W. J. Am. Chem. Soc. 2004, 126, 8976. (b) Xu, Y. C.; Chen, X. B.; Fang, W. H.; Phillips, D. L. Org. Lett. 2011, 13, 5472. (29) (a) Fang, W. Acc. Chem. Res. 2008, 41, 452. (b) Han, J.; Chen, X. B.; Shen, L.; Chen, Y.; Fang, W. H.; Wang, H. B. Chem.Eur. J. 2011, 17, 13971. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;

ASSOCIATED CONTENT

S Supporting Information *

Absolute energies (a.u.) and Cartesian coordinates for NO2 and NO2−H2O complex along N1−O3 distances in the highly 6899



Article

Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.02; Gaussian, Inc.: Wallingford, CT, 2003. (31) Andersson, K.; Barysz, M.; Bernhardsson, A.; Blomberg, M. R. A.; Carissan, Y.; Cooper, D. L.; Cossi, M.; Fleig, T.; Fülscher, M. P.; Gagliardi, L.; et al. Molcas, version 7.1; University of Lund: Lund, Sweden, 2008. (32) (a) Sander, S. P.; Friedl, R. R.; Abbatt, J. P. D.; Barker, J. R.; Burkholder, J. B.; Golden, D. M.; Kolb, C. E.; Kurylo, M. J.; Moortgat, G. K.; Wine, P. H.; et al. Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies. NASA Jet Propulsion Laboratory: Pasadena, CA, 2011, Evaluation No. 17, JPL Pubication 10-6. (b) Au, J. W.; Brion, C. E. Chem. Phys. 1997, 218, 109. (c) Schneider, W.; Moortgat, G. K.; Tyndall, G. S.; Burrows, J. P. J. Photochem. Photobiol. A 1987, 40, 195. (d) Hall, J. T. C.; Blacet, F. E. J. Chem. Phys. 1952, 20, 1745. (e) Merienne, M. F.; Jenouvrier, A.; Coquart, B.; Lux, J. P. J. Atmos. Chem. 1997, 27, 219. (f) Vandaele, A. C.; Hermans, C.; Simon, P. C.; Carleer, M.; Colin, R.; Fally, S.; Mérienne, M. F.; Jenouvrier, A.; Coquart, B. J. Quant. Spectrosc. Radiat. Transfer 1998, 59, 171. (g) Douglas, A. E.; Huber, K. P. Can. J. Phys. 1965, 43, 74. (h) Roehl, C. M.; Orlando, J. J.; Tyndall, G. S.; Shetter, R. E.; Vazquez, G. J.; Cantrell, C. A.; Calvert, J. G. J. Phys. Chem. 1994, 98, 7837. (i) Douglas, A. E. J. Chem. Phys. 1966, 45, 1007. (33) Zeeman, P. Nature 1897, 55, 347. (34) Uselman, W.; Lee, E. K. A. J. Chem. Phys. 1976, 65, 1948. (35) Johnston, H. S. Annu. Rev. Phys. Chem. 1992, 43, 1. (36) Ma, J. Q.; Liu, P.; Zhang, M.; Dai, H. L. J. Chem. Phys. 2005, 123, 154306. (37) Wine, P. H. J. Phys. Chem. Lett 2010, 1, 1749.

6900


Two Photon Dissociation Dynamics of NO2 and ... - ACS Publications

Recommend Documents