Letter pubs.acs.org/JPCL
Is Nitrate Anion Photodissociation Mediated by Singlet−Triplet Absorption? Ondřej Svoboda and Petr Slavíček* Department of Physical Chemistry, Institute of Chemical Technology, Technická 5, 16628 Prague 6, Czech Republic S Supporting Information *
ABSTRACT: Photolysis of the nitrate anion is involved in the oxidation processes in the hydrosphere, cryosphere, and stratosphere. While it is known that the nitrate photolysis in the long-wavelength region proceeds with a very low quantum yield, the mechanism of the photodissociation remains elusive. Here, we present the quantitative modeling of singlet− singlet and singlet−triplet absorption spectra in the atmospherically relevant region around 300 nm, and we argue that a spin-forbidden transition between the singlet ground state and the first triplet state contributes non-negligibly to the nitrate anion photolysis. We further propose that the nitrate anion excited into the first singlet excited state relaxes nonradiatively into its ground state. The full understanding of the nitrate anion photolysis can improve modeling of the asymmetric solvation in the atmospheric processes, e.g., photolysis on the surfaces of ice or snow. SECTION: Spectroscopy, Photochemistry, and Excited States
N
intensity is strongly controlled by solvation and intermolecular interactions.20−26 The very small quantum yield is attributed to the so-called cage effect, i.e., it is expected that the solvent cage prevents the geminate fragments to separate.27 In this work, we propose a new mechanism of the nitrate dissociation. We suggest that the nitrate anion photodissociation is mediated via a direct spin-forbidden absorption from the singlet ground state into the first triplet state. The photolysis experiments indicate that the dissociation leads to the formation of the atomic oxygen in its triplet state (channel 1) or the oxygen anion (channel 2).28 Both photoproducts can be formed within the triplet manifold of the nitrate anion. The dissociation to the oxygen anion can also proceed on the singlet electronic state. Let us first inspect various reaction channels from the energetic point of view. Upon the absorption, the nitrate anion gains an excess energy of approximately 4 eV (see Table 1). In
itrate anion belongs among the most important trace species in the hydrosphere and atmosphere.1 Nitrate anion impurities and condensation nuclei are also present in stratospheric clouds.2,3 While the nitrate anion was for a long time considered the final product in the NOx oxidation chain, it has recently been recognized as a key chromophore in Antarctic snow, where it can photodissociate and then emit reactive particles into the atmosphere. This process clearly demonstrates a close interaction between the cryosphere and the atmosphere.4−11 Upon the solar light (λ > 290 nm) absorption, the nitrate anion dissociates with rather low quantum yields ϕ into the nitrite anion and triplet oxygen (1) or into the nitrogen dioxide and oxygen anion (2):12 NO−3 + hv (305 nm) → NO−2 + 3O
ϕ = (1.1 ± 0.1) × 10−3 (1)
NO−3 + hv (305 nm) → NO2 + O−
ϕ = (9.2 ± 0.4) × 10−3 (2)
Table 1. Excitation Energies (with Respect to the Ground State Minimum) of the Nitrate Anion in the Gas Phase and in Solutiona
−
The resulting O anion immediately reacts with water, forming hydroxyl radical (OH•).13 The OH• radical is the most important oxidative species in the atmosphere. In the hydrosphere, the OH• radical contributes to cleaning polluted waters via oxidation of organic molecules.14−16 Although the nitrate anion dissociation at ∼300 nm has been the subject of many experimental studies,12,17−19 its molecular mechanism remains surprisingly poorly understood. It is generally believed that the photodissociation is mediated by the absorption through a symmetrically forbidden n → π* transition into the first singlet excited state (S1). The n → π* transition around 310 nm is rather weak (f ∼ 10−4), and its © 2014 American Chemical Society
ΔE/gas phase
ΔE/aqueous phase
4.08 eV 3.88 eV
4.17 eV 3.96 eV
NO3− (S1) NO3− (T1) a
Calculated at the EOM-CCSD/aug-cc-pVQZ level. Polarizable continuum model was used to account for the solvent effects.
Received: April 9, 2014 Accepted: May 19, 2014 Published: May 19, 2014 1958
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the gas phase, the nitrate anion can only dissociate into NO2− and 3O (see Table 2). However, the solvation also opens the channel producing the oxygen anion and the NO2 radical.
II. Absorption into the S1 state followed by intersystem crossing. The dissociation then takes place on the PES of the ground (T1) triplet state. III. Absorption through a spin-forbidden process into the T1 state and subsequent dissociation on the T1 potential energy surface. Here, we quantify the relative importance of the third channel initiated by the direct population of the triplet state. The singlet−triplet (S−T) absorption is typically negligible if a molecule does not possess a heavy atom. However, the singlet− singlet (S−S) absorption intensity around 300 nm is rather low and the spin-forbidden S−T absorption thus might become competitive. Using the time-dependent density functional theory (TDDFT) with two-component relativistic Hamiltonian obtained after the Barysz−Sadlej−Snijders transformation of the Dirac Hamiltonian (BSS),29 we calculated the absorption spectrum of the nitrate anion within the reflection principle approximation.30−32 Figure 2 shows the resulting photoabsorption spectrum. The difference curve between relativistic and nonrelativistic calculations shows the photoabsorption intensity increase gained from the singlet−triplet absorption. The figure clearly demonstrates that spin−orbit coupling effects are strong for the wavelengths above 300 nm for isolated nitrate anion. In other words, large part of the absorption in this region leads to population of the triplet state. Since water molecules greatly affect the singlet absorption cross-section,20 we also performed calculations for the simplest nitrate−water cluster, NO3−···H2O. Unlike the singlet absorption, the solvation does not enhance the absorption to triplet states (cf. the red (singlet−singlet) curves with the green (singlet−triplet absorption) curves). We have not calculated the whole spectrum for nitrate anion with more water molecules; yet single-point calculations indicate that the singlet−triplet contribution is not too much affected by the hydration. The total quantum yield for the nitrate anion photodissociation is roughly 10−2 12 (see also eqs 1 and 2). Assuming that the absorption cross section into the triplet state does not change upon solvation, the oscillator strength (obtained by integration of the absorption cross-section) due to the triplet absorption remains ∼10−6 ( f = 4 × 10−6 for NO3−). The experimental value of the oscillator strength of the fully hydrated nitrate anion is ∼10−4.33 The ratio of the triplet and singlet oscillator strengths therefore provides ∼10−2. The observed quantum yield could thus be explained entirely as a
Table 2. Gibbs Free Energies for a Formation of Various Reaction Products Both in the Gas Phase and in Watera NO3− (S0) NO2− + 1O NO2− + 3O NO2 + O−
ΔGr/gas phase
ΔGr*/aqueous phase
0 eV 6.43 eV 3.45 eV 4.83 eV
0 eV 5.97 eV 3.10 eV 3.60 eV
a
Calculated at the MP2/aug-cc-pVDZ level. Details of the calculations of the free energies are provided in the Supporting Information.
As a next step we calculated potential energy surfaces for the ground and two excited states in question (S1 and T1). Figure 1 shows the 2D scan of the potential energy surface. It confirms our expectations that only the dissociation into 3O and NO2− (on a triplet manifold state) is energetically accessible in the gas phase. Yet, the solvation can also open the O− and NO2 reaction channel (both within the singlet and triplet manifold). The dissociation of the nitrate anion is characterized by very low quantum yields and the nitrate anion does not exhibit any significant fluorescence. This indicates that an efficient deactivation mechanism exists for the molecules in the S1 state. The deactivation usually proceeds via the crossing between two electronic states, conical intersections. Indeed, we found a pyramidalized conical intersection structure with one N−O bond elongated to 1.35 Å close to the minimum on the excited state surface. The minimal energy conical intersection (MECI) is positioned 0.70 eV below the S1 energy in the Franck−Condon geometry (and 0.25 eV above the S1 state minimum; the energies were calculated at the MRCI(10,7)/aug-cc-pVDZ level). Thus, it can be expected that the majority of the S1 state population quickly funnels into the ground state. Note that the population can also be transferred into the ground state also via a surface crossing along the N−O dissociative coordinate. We might consider three different scenarios for the dissociation reaction: I. Absorption into the S1 state followed by oxygen anion dissociation on the PES of the S1 state.
Figure 1. Two-dimensional cut through the potential energy surfaces of the ground (S0) and two excited states (S1 and T1). Calculated using the MRCI(10,7)/aug-cc-pVDZ method. The position of the minimum energy conical intersection (MECI) is also indicated. FC point stands for Franck−Condon point. 1959
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Figure 2. Photoabsorption spectrum of the NO3− anion and NO3−···H2O complex calculated using both relativistic and nonrelativistic Hamiltonians. The black curves represent the total photoabsorption cross section, while the red curves refer to the absorption arising from singlet−singlet absorption and the green ones refer to the singlet−triplet absorption. The path integral molecular dynamics simulations on the B97-D/aug-cc-pVDZ PES was used for the initial sampling of geometries. Transition dipole moments were calculated at the TDDFT/CAM-B3LYP/aug-cc-pVDZ level, whereas the EOM-CCSD/aug-cc-pVDZ method was used for excitation energies. The reflection principle30−32 was used to evaluate the spectrum.
(using the QCHEM 4.0 program package41) was used for the calculation of energies, whereas the CAM-B3LYP/aug-ccpVDZ method with a two-component relativistic Hamiltonian obtained after the Barysz−Sadlej−Snijders transformation of the Dirac Hamiltonian in the finite basis set42 as is implemented in the Dirac 1229 program was used for evaluating the oscillator strengths. We used the reflection principle approximation to evaluate the spectrum. In the Supporting Information section, we provide the justification of the choice of our method. The reaction Gibbs energies were calculated in the gas phase and in water. The hydration free energies were calculated using dielectric continuum models (SMD)43 for the neutral molecules and experimental hydration free energies for the anions.44 The hydration Gibbs energies for the O− anion was estimated within the cluster-continuum approach, using the Gibbs energy of the fluoride anion as a reference.45 The Gaussian 09 program package46 was used for these calculations. More on the calculation of the reaction Gibbs energies can be found in the Supporting Information.
consequence of the S−T absorption followed by a subsequent dissociation on the triplet PES. Consequently, we argue that the S−T absorption contributes significantly to the nitrate photodissociation, even though the reaction channels I and II cannot be completely ruled out. In this publication, we offer a new perspective on the photodissociation dynamics of the nitrate anion. We show that the direct absorption into the triplet state dominantly accounts for the observed photodissociation quantum yields. Such an important role of the S−T transition in a molecule without heavy atoms is rather unusual. Here, the S−T transitions are competitive only because of the symmetry restrictions on the S−S transition. We also suggest that the nitrate anion in the first singlet excited state deactivates nonradiatively via a pyramidalized conical intersection. Our findings could be used in modeling of nitrate photodissociation in various molecular environments,27,34 e.g., for understanding properties of the nitric acid adsorbed on ice where the nitrate anion coexists with the nondissociated nitric acid.35−37
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COMPUTATIONAL METHODS Potential energy surfaces of the nitrate anion in the gas phase were calculated with the multi-reference configuration interaction (MRCI) method, using the active space of 10 electrons in 7 orbitals and considering six electronic states and aug-ccpVDZ basis set (MRCI(10,7)/aug-cc-pVDZ). The Molpro 2010 package38 was used for these calculations. The conical intersection search was performed using the penalty Lagrange multiplier technique.39 The procedure takes into account only energies and gradients on upper and lower states, all of which were calculated at the MRCI(10,7)/6-31g* level. Subsequently, we have recalculated the energies for the MECI structure using the same active space and larger aug-cc-pVDZ basis. The initial set of geometries (900 randomly selected points) for the photoabsorption spectrum simulation40 was obtained from the path integral simulation in ref 20. The combined approach was used to evaluate the excited states energies and oscillators strengths; the EOM-CCSD/aug-cc-pVDZ method
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information comprises the following: Optimized geometries: global minimum, S1 state minimum, S1/S0 conical intersection Interpolation between Franck−Condon point and the S1 minimum; benchmark of the electronic structure methods; details on the Gibbs Free energies calculations. 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. 1960
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ACKNOWLEDGMENTS We gratefully acknowledge the support of the Grant Agency of the Czech Republic (Project No. P208/10/1724). Some of the calculations were performed at the computer cluster of Jaroslav Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic. O.S. is a student of the Max Planck international school “Dynamical Processes in Atoms, Molecules and Solids.”
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