Photoisomerization Reaction Mechanisms of o-Nitrophenol Revealed

Article pubs.acs.org/JPCA

Photoisomerization Reaction Mechanisms of o‑Nitrophenol Revealed by Analyzing Intersystem Crossing Network at the MRCI Level Chao Xu,†,‡ Le Yu,‡,§ Chaoyuan Zhu,*,‡ and Jianguo Yu† †

College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China Institute of Molecular Science, Department of Applied Chemistry and Center for Interdisciplinary Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan § Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, The College of Chemistry & Materials Science, Shaanxi key Laboratory of Physico-Inorganic Chemistry, Northwest University, Xi'an 710069, P. R. China ‡

S Supporting Information *

ABSTRACT: 6SA-CASSCF(10, 10) /6-31G (d, p) and MRCI/cc-pVDZ methods were performed to probe photoisomerization reaction mechanisms of o-nitrophenol. Two low-lying singlet electronic states (S0 and S1) and two low-lying triplet electronic states (T1 and T2) were found to weave an intersystem crossing network in which a dominant stepwise photoisomerization provides a very efficient reaction pathway; the reaction takes place in the wide region of crossing seam-surface woven by S1 and T1 states first, followed by T1 and S0 states. Both intersystem crossing regions show strong spin−orbital coupling in the order of 40 wavenumbers. All nitro and aci-nitro isomers and transition states on four electronic potential energy surfaces are calculated along with analysis of both dominant and subdominant relaxation pathways, especially weak spin−orbital coupling (∼10 wavenumbers) between T2 and S1 states and effective conical intersection between T2 and T1 states opening a new relaxation pathway S1 → T2→ T1.

■. INTRODUCTION Nitrophenols, acting as one of the most prototypical representative nitroaromatics, attract extensive attention owing to their important effects on both clean and polluted atmosphere.1−3 Primary sources of nitrophenols in atmosphere include raw materials from pharmaceutical industry, combustion processes, and traffic activities.4 Nitrophenols in atmosphere are involved in the photochemical reaction with aerosol or radical species such as OH, NO3, and so on.5,6 It was found that the HONO and OH in the atmosphere come from the photolysis of nitrophenols. However, whether nitrophenols in atmosphere could be fully responsible for the formation of HONO and OH, and how the photochemical processes happen, are still elusive. Strong intramolecular hydrogen bonding between OH and NO2 groups in the ortho position of nitrophenol (o-nitrophenol) reveals spectacular chemical and physical properties different from the other nitrophenol isomers. The acinitrophenol can be formed via the initial intramolecular hydrogen transfer from hydroxyl group to nitro group in o-nitrophenol. © XXXX American Chemical Society

Bejan et al.,7 for the first time, observed the formation of HONO in the gas phase in flow tube photoreactor upon irradiation of o-nitrophenol. For the formation mechanism of HONO from o-nitrophenol in the gas, Bejan et al. excluded the heterogeneous NO2 photochemistry but proposed that the reaction is initiated by an intramolecular hydrogen transfer from the phenolic OH group to the nitro group; energy transfer forms aci-nitrophenol in the excited state, followed by the subsequent elimination of HONO. Following Bejan et al.’s hypothesis of hydrogen transfer on the photolysis of o-nitrophenol as the first step, Nagaya et al.8 proposed, based on their Fourier transform infrared (FTIR) experiment with an aid of density functional theory (DFT) calculations, that the all the generated isomers of the aci-nitrophenol were produced by vibrational relaxation after electronic excitation via UV and they Received: June 27, 2015 Revised: September 2, 2015

A

DOI: 10.1021/acs.jpca.5b06166 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A confirmed the aci-nitrophenol as the photoreaction intermediate. Wang et al.9 studied a resonance Raman spectrum of o-nitrophenol in cyclohexane solution plus the DFT calculations, and they concluded that photodissociation dynamics in the Franck−Condon region is essentially multidimensional character involved with 15 vibrational normal modes. Wei et al.10 employed the laser-induced-fluorescence technique to analyze the dynamics channel of the o-nitrophenol photolysis, and they concluded that OH-produced involved multistep reactions including aci-nitrophenol isomers via photoisomerization. Cheng et al.11 investigated the photodissociation of nitrophenols in gas phase and analyzed the formation of OH radicals; they selected different photolysis wavelengths in the range of 361−390 nm to measure the nascent OH state distributions combined with DFT calculations, and then they concluded that electronically excited nitrophenol mostly relaxed to the lowest triplet state via intersystem crossing followed by the intramolecular hydrogen transfer to form an aci-nitrophenol isomer. It is well-known that most nitroaromatic compounds are nonfluorescent12 because strong spin−orbital coupling exists between the singlet and triplet states,13−15 and efficient intersystem crossing (ISC) has been measured in a number of nitroarene compounds.16−19 Theoretical calculations at the level of (time-dependent) DFT have been mainly performed for the singlet ground state S0 and triplet excited state T1.8,11 Actually, the intramolecular hydrogen transfer can take place from both the singlet-excited state S1 and T1, and intersystem crossing between these two states and higher triplet excited state T2 participation can greatly complicate photoisomerzation reaction mechanisms of o-nitrophenol. An explicit mechanism accurately reflecting the excited-state intramolecular hydrogen transfer of o-nitrophenol must be investigated at high-level ab initio electronic structure method plus quantitative information on spin−orbital coupling. This is the purpose of the present work. We first present a brief description of the complete-activespace self-consistent field (CASSCF) method20,21 and followed by energy correction of the multireference configuration interaction (MRCI) with the Davidson correction (+Q),22−24 which is the highest level of ab initio electronic structure methods. Then, we report how two singlet electronic states (ground-state S0 and excited-state S1) and two triplet electronic states (excited states T1 and T2) weave complicated intersystem crossings with strong spin−orbital couplings, which are responsible for photoisomerzation reaction of o-nitrophenol. We explore the most efficient photoisomerzation reaction pathways through the intersystem crossing network.

level; all frequencies in stationary points are positive, whereas there is only one imaginary frequency in transition states, and then the intrinsic reaction coordinate (IRC) calculation is performed to confirm the corresponding reactant and product. Intramolecular hydrogen transfer of o-nitrophenol actively involves two low-lying singlet states S0 and S1 and two low-lying triplet states T1 and T2. Therefore, we tested with four-state averaged 4SA-CASSCF(10,10) first, and then 5SA-CASSCF(10,10) and 6SA-CASSCF(10,10). The main issues are natural characters of excited states and smooth evolution of molecular orbitals (related to four electronic states mentioned above) along the IRC path and at intersystem crossings. We also test basis sets 6-31G (d, p) and 6-31+G (d, p), and even try including more active orbitals and active electrons. Finally, three low-lying singlet excited-states and three lowlying triplet excited-states are included in the state-averaged CASSCF method with equal weight for the each state, namely, 6SA-CASSCF (10, 10) /6-31G (d, p) method is confirmed to be accurate to describe the potential energy surfaces of the S0, S1, T1, and T2 states; it is employed throughout all calculations in the present work. Energy corrections are followed by the MRCI+Q method with cc-pVDZ basis set. All calculations are performed by using MOLPRO2009 program package.26 Linear Interpolations and Spin−Orbital Couplings. To properly describe photoisomerization pathways in terms of intersystem crossing network, we propose one-dimensional curved potential energy surfaces for these four coupled electronic states by using the linear interpolations in terms of internal coordinates27−31 (LIIC) between two important saddle points (minima, transition states, or intersystem points). The N-1 linear interpolations in terms of the variable internal coordinate i can be expressed as

■. COMPUTATIONAL DETAILS Complete Active Space Self-Consistent Field Method. The selection of the active space is the key step in the CASSCF calculation. By following Björn’s rules,25 we choose a suitable active space under such conditions that the active space consists of 10 electrons in 10 orbitals; including the two π, π* pairs in the benzene ring, one π, π* pair and lone pair in the nitro group, and the rest of the orbitals are σ and σ* involved in N−O bond. The nature of the CASSCF orbitals is checked for all relevant local minima and transition states to make sure that the chosen active space is appropriate to describe the reaction process. Geometries of minima and transition states are fully optimized without imposing any constraint (C1 point group). The character of these critical points is determined by analyzing the calculated harmonic vibrational frequencies at the CASSCF

where k (0, 1) and l (1, 2) are numbering for singlet- and triplet-excited states, respectively.

Ω(i) = Ω(0) +

i (Ω(N ) − Ω(0)), i = 0, 1, 2, ⋯ , N N (1)

where Ω(0) and Ω(N) denote as one of the internal coordinate variables given at two saddle points with equal stepsize interpolations to determine the ith internal coordinate Ω(i). Along one-dimensional curved LIIC potential energy surfaces, we compute spin−orbit couplings (SOC) by using multireference configuration interaction (MRCI) with the full Breit−Pauli Hamiltonian.32 The strength of SOC between singlet and triplet electronic states are obtained from27 SOClk =

∑ |⟨Tl ,u|ĤSO|Sk⟩|2 u

where u = x , y , z (2)

■. RESULTS AND DISCUSSION Nitro and Aci-Nitro Isomers in the S0 State and Vertical Excitations. An optimized global minimum of the ground state for nitrophenol has a planar geometry and shows strong intramolecular hydrogen-bonding interaction between the hydroxyl group and the nitro group, referred to as S0-NP, which is taken as energy reference to be zero. While H atom transfers from hydroxyl group to nitro group, the O15H12 single bond is rotatable as shown in Figure 1; the different local minima of the aci-nitro isomers could be shown. We found three aci-nitro isomers, which are denoted as S0-isomer1, S0-isomer2, and S0-isomer3, respectively (the same as DFT B


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The Journal of Physical Chemistry A

Figure 1. Atom numbering for o-nitrophenol. All key geometries of the optimized isomers of nitro and aci-niro in the S0 state at 6SA-CASSCF (10, 10) /6-31G (d, p) level.

Figure 2. Vertical excitation energies of nitro and aci-nitro isomers at all sudden points of the S0 states, with geometries optimized by 6SA-CASSCF (10, 10) /6-31G (d, p) level, followed by MRCI+Q/cc-pVDZ energy corrections.

calculations in ref 10). All nitro and aci-nitro isomers in the S0 state show the planar geometry within the aromatic ring as shown in Figure 1; the bond lengths and dihedral angles involving hydroxyl group and nitro group have significant geometrical differences reflecting isomerization on the groundstate potential energy surface. Among these aci-nitro isomers in the S0 state, the S0-isomer1 is the only one that exhibits intramolecular hydrogen bond with ortho oxygen atom of benzene ring, and the energy is 32.0 kcal/mol above global minimum S0-NP state; however, it is unstable, and it easily leads back to the S0-NP state by just overcoming 3 kcal/mol energy barrier as shown in Figure 2, which represents local minima and transition states for the ground-state S0 at which vertical excitation energies of all excited states are estimated. Because of the influence of oxygen atom of the HONO group, the S0-isomer2 energy (31.8 kcal/mol) is slightly higher than that of S0-isomer3 (29.7 kcal/mol). Three transition states on S0 potential energy surfaces are found as shown in Figure3; there is the planar hydrogen transfer transition state S0-TS1 between the S0-NP and S0-isomer1 (in S0-TS1, the O11H12 bond length is elongated by 0.329 Å, and the bond of O15H12 is shortened by 0.764 Å compared with in S0-NP), there is the noncoplanar transition state S0-TS2 between the S0-isomer1 and S0-isomer2 (in S0-TS2, dihedral angle C5C4N13O15 rotates −13.9° in which the HONO group is out of the aromatic skeleton compared with in S0-NP), and there is migration of planarity transition state S0-TS3 between the S0-isomer2 and S0-isomer3 (in which the H12 atom can be migrated from O15 atom of HONO group to the atom O14). The S0-isomer2 needs overcoming 11 kcal/mol energy barrier to reach the S0-isomer1, while S0-TS3 has pretty high potential energy barrier ∼36 kcal/mol higher than that of

Figure 3. Key geometries of the optimized transition states for the S0, T1, and S1 states at 6SA-CASSCF (10, 10) /6-31G (d, p) level.

both S0-isomer2 and S0-isomer3, as shown in Figure 2 in which vertical excitation energy orders are lined up with S1 state C


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Figure 4. Key geometries of the optimized isomers of nitro and aci-niro in the S1, T1, and T2 states at 6SA-CASSCF (10, 10) /6-31G (d, p) level.

to sp3, which follows the photoexcitation from the ground state to the excited state in forming a pyramid structure. The O15H12 bond length is 0.950 Å in S0-isomer2 and 0.948 Å in S1-isomer2, and it is 0.949 Å in S0-isomer3 and 0.945 Å in both S1-isomer3 and T1-isomer3. Furthermore, the pyramid electronic structure of the N13 atom turns the C5C4N13O15 dihedral angle into 41.1° in S1-isomer2, 86.1° in S1-isomer3, 66.6° in T1-isomer3, and 31.4° T2-isomer2 in compared to 0.0° in the S0 isomers. However, both the T2-isomer3 and S0-isomer3 have planar geometries, and the O15H12 distance in T2 state is 2.153 Å longer than that in the S0 state. The optimized transition state T1-TS1 (see Figure 3) that separates the reactant T1-NP and product T1-isomer1 by twisting the C4N13O15H12 dihedral angle and expanding the O15N13O14 angle has discrepant phenomena; the O11H12 bond increases to 1.158 Å accompanied by the bond fracture, and O15H12 decreases to 1.238 Å. However, the optimized transition state T1-TS2 that separates the T1-isomer1 and T1-isomer3 is very different from T1-TS1 state; the O14N13O15H12 dihedral angle reverses to 2.9°from the 126.8° of T1-isomer1, and the H12 atom is closer to the O14 atom instead of the atom O15. The hydrogen transfer from the hydroxyl group to the nitro group is exothermic from the T1-TS1, in contrast to the S0-TS1, and the energy profile exhibits a barrier of 19.1 kcal/mol. In the case of this reaction, the barrier height of T1-TS is easy for excited nitro to overcome; therefore, this reaction path seems to be energetically feasible. For the reaction from T1-isomer1 to T1-isomer3, the barrier height of T1-TS2 is ∼71.3 kcal/mol. The T1-TS2 has a tendency to form a four-member ring with striking structural variation so that an unexpected high potential energy is obtained. It is quite difficult for T1-isomer1 to go over such high barrier; thus, the process is considered to be less feasible. We conclude that the final probable photo product should be T1-isomer1 rather than T1-isomer3, and the latter cannot compete with the former due to the high barrier.

highest, followed by T2 state, T1 state, and S0 state. These indicate that intersystem crossing points are taking places in between those sudden points. Excited Singlet States, Triplet States, and Intersystem Crossings. We found the corresponding nitro and aci-nitro isomers in the excited states; S1 state (S1-NP, S1-isomer1, S1-isomer2, and S1-isomer3), T1 state (T1-NP, T1-isomer1, and T1-isomer3), and T2 state (T2-NP, T2-isomer1, T2-isomer2, and T2-isomer3), respectively. The optimized key geometries of the nitro and aci-nitro isomers in the S1, T1, and T2 excited states are shown in Figure 4. The key geometries of S1-NP is similar to the above S0-NP; for example, the O11H12 bond lengths are 0.946 Å in S0-NP and 0.948 Å in S1-NP, while the O15H12 bond lengths are 1.877 Å in S0-NP and 1.855 Å in S1-NP. However, the S1-NP, the T1-NP and T2-NP states are nonplanar structures, such as the C5C4N13O15 dihedral angle is rotated by 51.0°(−10.9°) and C5C4N13O14 dihedral angle is rotated by −175.6°(137.5°) for T1-NP (T2-NP), and the O11H12 bond length is 0.943 Å (0.948 Å) in T1-NP (T2-NP). In the T1 (T2) state, the O15···H12 hydrogen bond length is 2.095(1.923) Å in comparison with the intramolecular hydrogen bond 1.877 Å in S0-NP. Because of the twisting of HONO group, the aci-nitro isomer geometries in the S1 and T1 states have C1 group symmetry, which is different from corresponding geometries in S0 state. For instance, the O15H12 bond length in the S1-isomer1 is shortened by 0.01 Å compared to the S0-isomer1, while O15H12 bond in the T1-isomer1 is the same as in S0-isomer1. The key dihedral angle the C5C4N13O15 is 33.0° and 60.5° in S1-isomer1 and T1-isomer1 compared to 0.0° in S0-isomer1; C5C4N13O14 is −139.6° and −162.0° in S1-isomer1 and T1-isomer1 compared to 180.0° in S0-isomer1, while the O15H12 bond length is shortened by 0.016 Å compared to the S0-isomer1. All changes can be interpreted as the translocation of the hydrogen from the hydroxyl group to the nitro group, and the hybridization form of electronic structure of the N13 atom changes from sp2 D


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Figure 5. Key geometries of the optimized intersystem crossing points at 6SA-CASSCF (10, 10) /6-31G (d, p) level.

Key optimized geometries related to hydrogen transfer are also summarized in Table 1. The MRCI energies for equilibrium structures, transition states, and intersystem crossings on potential energy surfaces of the S0, S1, T1, and T2 states are summarized in Table 2 and depicted in Figure 6. Full geometry information in Cartesian coordinates is given in Supporting Information (S1 to S6 sections). Photoisomerization Pathways. Now, we analyze the photoinduced reaction processes of nitro and propose possible photoisomerization pathways. Let us start from Franck− Condon region of the ground-state S0-NP with vertical excitation up to S1 state, which is 100.3 kcal/mol in energy, in comparison with the lowest minimum S1-NP state (planar geometry; 78.8 kcal/mol) followed by S1-isomer1 (83.2 kcal/mol), S1isomer2 (53.3 kcal/mol), and S1-isomer3 (61.9 kcal/mol). The lowest transition state on S1 state is S1-TS1 (103.4 kcal/mol). Thus, there is a potential well ∼24.6 kcal/mol deep where molecule can be trapped for a long time after photoexcitation up to S1 state. Vertical excitation energy from S0-NP to T1 state is up to 90.03 kcal/mol, the lowest local minimum T1-isomer1 has energy of 47.3 kcal/mol followed by T1-NP state (72.0 kcal/mol) and T1-isomer3 (73.1 kcal/mol). The lowest transition state on T1 state is T1-TS1 with energy of 91.1 kcal/mol, so that there is a potential well ∼20 kcal/mol deep almost same as the potential well on S1 state. Vertical excitation energy from S0-NP to T2 state is up to 93.9 kcal/mol, the lowest local minimum T2-isomer3 has energy 52.4 kcal/mol followed by T2 -isomer2 (52.9 kcal/mol), T2-NP state (56.1 kcal/mol), and T2-isomer1 (89.7 kcal/mol). The lowest transition state on T2 state is T2-TS1 with energy up to 120 kcal/mol; so that there is a potential well ∼64 kcal/mol deep and molecule cannot adiabatically escape such a deep well once it is trapped in T2 state. In brief, the photoreaction of nitro can be interpreted as a two-step process. In the first step, the nitro minimum achieves an aci-nitro minimal via the hydrogen transfer transition state. In the second step, the original aci-undergoes isomerization to form the other isomers. To get further insights into the nature of the potential energy surface,

Determining the transition state in the initial hydrogen transfer process of the system in the S1 state is also very important for understanding reaction mechanism. So we locate the transition state at the same calculated level as denoted S1-TS in Figure 3. Harmonic vibrational frequency analysis further affirms that it is the transition state for the hydrogen transfer between the hydroxyl and nitro groups in the S1 state. Actually, the optimized S1-TS structure is coplanar with the aromatic skeleton, and the C5C4N13O15 dihedral angle is still same as that of S1-NP. These changes indicate that whole molecule is not out of the plane temporarily. At the same time, the O11−H12 bond length is 1.076 Å, while that of S1-NP is 0.948 Å, and the distances between O15 and H12 shorten to 1.370 from 1.855 Å in S1-NP, accompanied with the hydrogen migration. For this process, the barrier height is found to be 103.4 kcal/mol refer to S0-NP at the MRCI level with the zeropoint energy correction. The intersystem crossing between the S1 and T2 in the Franck−Condon region of nitro is designated as S1T2-IC1, and the other S1/T2 crossing is denoted as S1T2-IC2. S1T2-IC1 and S1T2-IC2 are structurally similar to their optimized geometries in the S1 state as shown in Figures 5 and 4. For example, both S1T2-IC1 and S1-NP have the planar structures; the O11H12 bond length in S1T2-IC1 is 0.955 Å, just 0.007 Å longer than that of S1-NP; the C5C4N13O15 dihedral angle is 62.2° in S1T2-IC2 compared to the 41.1° in S1-isomer2, and the O15H12 bond length is 0.946 Å, which is very close to 0.948 Å in S1-isomer2. The two intersystem crossings between the S0 and T1 are referred to as S0T1-IC1 and S0T1-IC2 according to the relative position of transferred hydrogen atom with respect to the nitro group. The structure of S0T1-IC1 is close to T1-isomer1, which is a nonplanar structure, while S0-isomer1 is planar. We can see in Figure 4 that in S0T1-IC1, the O15H12 bond length is 0.950 Å, the dihedral angle of C5C4N13O15 is 52.9°, and C5C4N13O14 is −91.0°. Similar situation for S0T1-IC2 can be seen from the torsion of the nitro group. As shown in Figure 5 in S0T1-IC2, the C5C4N13O15 dihedral angle is 71.3°, C5C4N13O14 is −158.9°, and the O15H12 bond length is 0.945 Å. E


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The Journal of Physical Chemistry A Table 1. Optimized Geometric Parametersa at the 6SA-CASSCF(10, 10) /6-31G (d, p) Level S0-NP

S0-isomer1

S0-isomer2

S0-isomer3

O11H12 O15H12 ∠C5C4N13O15 ∠C5C4N13O14 ∠O14N13O15H12

0.946 1.877 0.0 −180.0 180.0 S1-NP

1.610 0.974 0.0 180.0 −180.0 S1-isomer1

3.545 0.950 0.0 −180.0 0.0 S1-isomer2

4.542 2.097 0.0 −180.0 0.0 S1-isomer3


0.948 1.855 0.0 −180.0 180.0 T1-NP


0.943 2.095 51.0 −175.6 175.5 T2-NP


0.948 1.923 −10.9 137.5 −136.8 S0T1-IC1

O11H12 O15H12 ∠C5C4N13O15 ∠C5C4N13O14 ∠O14N13O15H12 a

1.834 0.964 33.0 −139.6 128.1 T1-isomer1 2.406 0.974 60.5 −162 126.8 T2-isomer1 1.784 0.958 8.8 −175.9 170.1 S0T1-IC2

2.219 0.950 52.9 −91.0 70.6

3.299 0.948 41.1 −169.9 46.2 T1-isomer3 4.341 2.638 66.6 −160.3 32.1 T2-isomer2 3.392 0.948 31.4 −174.1 31.0 S1T2-IC1

4.327 2.651 71.3 −158.9 32.1

0.955 1.794 0.0 180.0 −180.0

4.412 2.722 86.1 −163.9 30.3

T2-isomer3 4.528 2.153 0.0 180.0 0.0 S1T2-IC2

S1T1-IC

3.058 0.946 62.2 −158.7 78.6

1.361 1.105 41.0 −127.3 117.2

Bond lengths in angstroms and dihedral angles in degrees.

Table 2. Potential Energiesa of All Isomers, Transition States, and Intersystem Crossings Calculated at the MRCI/ cc-pVDZ Level NP TS1 isomer1 TS2 isomer2 TS3 isomer3 S0T1-IC1 S0T1-IC2 S1T2-IC1 S1T2-IC2 S1T1-IC

S0

S1

T1

T2

0.0(0.0) 35.5(1.54) 32.0(1.39) 42.7(1.85) 31.8(1.38) 67.5(2.93) 29.7(1.29) 48.0(2.08) 72.6(3.15)

78.83(3.41) 103.4(4.48) 83.2(3.61)

72.0(3.12) 91.1(3.95) 47.3(2.05) 118.6(5.14)

56.1(2.43) 120(5.20) 89.7(3.89)

53.3(2.31) 61.9(2.68)

95.1(4.12) 53.7(2.33) 102.2(4.43)

optimized intersystem crossing point between S1 and T1 states referred to as S1T1-IC at energy (102.2 + 100.8)/2 = 101.5 kcal/mol. There are two optimized intersystem crossing points between S1 and T2 states referred to as S1T2-IC1 and S1T2-IC2 that are located at energy (95.1 + 90.5)/2 = 92.8 kcal/mol and (53.7 + 53.8) = 53.75 kcal/mol, respectively. On the basis of potential energy landscape of intersystem crossings, we would conclude that there are four electronic states coupled together responsible for photoisomerization of aci-nitro (two singlet states S0 and S1, and two triplet T1 and T2 states). To analyze the seam surfaces around intersystem crossing points and photoisomerization pathways from curved onedimensional potential energy profiles, we choose four key saddle points, namely, the ground-state S0-NP, the first singlet excited state S1-NP, and intersystem crossings S1T1-IC and S0T1-IC1. Then we utilize eq 1 to plot the curved LIIC potential energy surfaces as shown in Figure 6. Along curved LIIC between the Franck−Condon point and the S1-NP, potential energies of both the first singlet excited state S1 and two triplet states the T1 and T2 are all monotonically decreasing; the two triplet states form the avoided crossing seam, while the first singlet excited state stays adiabatically. This indicates that after photoexcitation in Franck−Condon region, the system relaxes adiabatically to S1-NP state in the first step. Along curved LIIC between the S1-NP and intersystem crossing S1T1-IC, potential energies of the first singlet excited state S1 and two triplet states the T1 and T2 are monotonically increasing to the corresponding transition states first and then

52.9(2.29) 73.1(3.17) 48.2(2.09) 73.2(3.18)

52.4(2.27)

90.5(3.92) 53.8(2.33) 100.8(4.37)

a

In the units of kilocalories per mole (electronvolts). All geometries are optimized by 6SA-CASSCF (10, 10) /6-31G (d, p) method.

the hydrogen migration process from nitro to aci-nitro, and the intersystem crossings between S0 and T1, T1 state and S1 state, and T2 state and S1 state are essential for understanding photoisomerization reaction mechanisms. There are two optimized intersystem crossing points between S0 and T1 states referred to as S0T1-IC1 and S0T1-IC2 that are located at energy (48.0 + 48.2)/2 = 48.1 kcal/mol and (72.6 + 73.2)/2 = 72.9 kcal/mol, respectively. There is one F


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Figure 6. Potential energy profiles for all nitro and aci-nitro isomers, transition states, and intersystem crossings on the S0, T1, T2, and S1 states calculated at MRCI+Q/cc-pVDZ level.

Figure 7. Potential energy profiles along LIIC plotted with four saddle points (S0-NP, S1-NP, S1T1-IC, and S0T1-IC1). All energies are referenced to the S0-NP energy at the MRCI+Q/cc-pVDZ level. Magnification of region of intersystem crossing (upper inset) between S1 and T1 states and (lower inset) between S0 and T1 states.

decreasing slowly; the first singlet excited state stays adiabatically to the transition state S1-TS1 (∼24 kcal/mol energetic barrier with respect to the S1-NP state) and then enters the intersystem crossing region with quite large seam-surface area in which energy separation is less than 1.0 kcal/mol and the spin−orbital coupling is on the order of 40 cm−1. However, in this area spin−orbital coupling between the S1 and T2 appears as small as 10 cm−1. This indicates that in the second step the main channel of isomerization takes place from intersystem crossing of the S1 and T1 states with minor effect from intersystem crossing of the S1 and T2 states. In the vicinity region of S1T1-IC, the torsional dihedral angle around the C5C4N13O15 increases from 37.4° to 44.1°, while it decreases from −137.7° to −118.4° around the C5C4N13O14. Moreover, the O15H12 bond shortens from 1.17 to 1.05 Å, and O11H12 bond increases from 1.16 to 1.53 Å. Along curved LIIC between the S1T1-IC and the S0T1-IC1, potential energies of

the ground state S0 and the triplet state T1 are monotonically decreasing with very large seam-surface area of intersystem crossing in which energy separation is less than 1.0 kcal/mol and the spin−orbital coupling is on the order of 40 cm−1 as well. This indicates in the third step that isomerization takes place from T1 to S0 state very fast. Figure 7 indeed shows very clear stepwise isomerization pathways through the intersystem crossing network. Three steps of isomerization are analyzed from the previous discussion. The first step is that system relaxes adiabatically from the Franck−Condon region to S1-NP state. The second step is more complicated where the present potential energy along curved LIIC tells system to reach S1-TS1 transition state first, followed by an intersystem crossing process from S1 to T1 state. This determines important intramolecular hydrogen transfer reaction from O11 to O15 atom. However, in the multidimensional potential energy surfaces it could be that G


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Figure 8. One-slide summary of proposed photoisomerization reaction mechanisms of O-nitrophenol.

switching probability method,38 which is suitable for conical intersection case to include situation with spin−orbital couplings.

system takes place in an intersystem crossing process without going through S1-TS1 transition state and instead going through T1-TS1 transition state (which is lower in energy than S1-TS1). Two competing processes actually coexist to determine the hydrogen transfer reaction. During the hydrogen atom transfer, the geometrical changes are mainly described by changes of the nitro group and hydroxyl group, while the aromatic ring plays a minor role. Along curved LIIC as shown in Figure 7 starting from Franck−Condon region, there is a quite long region in which two triplet states T2 and T1 are energetically close together and form a conical intersection; besides, there is small spin−orbital coupling ∼10 cm−1 between T2 and S1 states. Therefore, it opens possible new isomerization pathway S1→T2→T1→S0 after long time adiabatic evolution along S1 state. The third step is intersystem crossing process from T1 to S0 state. When the system relaxes to the T1 state, one of the dominant photoproducts is T1-isomer1 (see Figure 6) followed by relaxation to S0 (S0-isomer1) state via S0T1-IC1 intersystem crossing, and this is dominant pathway. Another minor photoproduct is T1-isomer3 (see Figure 6) followed by relaxation to S0 (S0-isomer3) state via S0T1-IC2. This minor isomerization has actually been observed in the lowtemperature matrix-isolation IR spectroscopy experiment, which corresponds to the S0-isomer3.8 When system relaxes to the S0 state, efficient hydrogen back-transfer can take place from S0-isomer1 to S0-NP via S0-TS1 transition state (see Figure 6) just at 3.5 kcal/mol barrier height. For quantitatively analyzing competing processes of major and minor isomerization steps, we need to run ab initio on-the-fly nonadiabatic molecular dynamics based on full dimensional potential energy surfaces of S0, S1, T1, and T2 calculated from the present MRCI method. This will be a separate issue for another publication. Trajectory-based ab initio nonadiabatic molecular dynamics simulations have been successfully applied to photoisomerization and photoreaction processes involving conical intersections.33−38 Quite recently, it has been extended to include the processes involving in intersystem crossings.39,40 We are now on the way to extend our analytically global

■. CONCLUDING REMARKS We summarize the photoisomerzation reaction mechanism from the intersystem crossing network weaved by electronic potential energy surfaces S0, S1, T1, and T2 calculated at MRCI ab initio quantum level. We draw schematic plot as shown in Figure 8 to present simple and neat picture of photoisomerzation reaction mechanism; the ground state of nitrophenol is photoexcited to the Franck−Condon region of the first excited singlet state S1. The dominant reaction pathway is that system evolves from either S1-TS1 → S1T1-IC → T1 state or S1T1-IC → T1 state, and then from T1 state to S0 (S0-isomer1) via S0T1-IC1, finally back to original o-nitrophenol ground state S0-NP. One new reaction pathway is that system evolves from either S1T2-IC1 → T2 (T2-NP) state and then back to T1 state via conical intersection between T2 and T1 states or it possibly goes to S1-TS1 → S1T1-IC → T2 (T2-NP), and then back to T1-NP state via internal conversion followed by isomerization to T1-isomer1. One minor reaction pathway is that system evolves from T1-isomer1 to T1-isomer3 by isomerization, and then end up to S0-isomer3 via intersystem crossing S0T1-IC2.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b06166. Cartesian coordinates of all electronic structures of isomers (local minima, transition states, intersystem crossings) for the S0, S1, T1, and T2 states optimized at the 6SACASSCF(10, 10) /6-31G (d, p) level of method. (PDF) H


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(15) Lower, S. K.; El-Sayed, M. A. The Triplet State and Molecular Electronic Processes In Organic Molecules. Chem. Rev. 1966, 66, 199− 241. (16) Takezaki, M.; Hirota, N.; Terazima, M. Nonradiative Relaxation Processes and Electronically Excited States of Nitrobenzene Studied by Picosecond Time-Resolved Transient Grating Method. J. Phys. Chem. A 1997, 101, 3443−3448. (17) Takezaki, M.; Hirota, N.; Terazima, M. Relaxation of Nitrobenzene from The Excited Singlet State. J. Chem. Phys. 1998, 108, 4685−4686. (18) Crespo-Hernández, C. E.; Burdzinski, G.; Arce, R. Environmental Photochemistry of Nitro-Pahs Direct Observation of Ultrafast Intersystem Crossing in 1-Nitropyrene. J. Phys. Chem. A 2008, 112, 6313−6319. (19) Zugazagoitia, J. S.; Almora-Díaz, C. X.; Peon, J. Ultrafast Intersystem Crossing in 1-Nitronaphthalene. An Experimental and Computational Study. J. Phys. Chem. A 2008, 112, 358−368. (20) Knowles, P. J.; Werner, H.-J. An Efficient Second-Order MCSCF Method for Long Configuration Expansions. Chem. Phys. Lett. 1985, 115, 259−267. (21) Werner, H.-J.; Knowles, P. J. A Second Order Multiconfiguration SCF Procedure With Optimum Convergence. J. Chem. Phys. 1985, 82, 5053−5063. (22) Langhoff, S. R.; Davidson, E. R. Configuration Interaction Calculations on The Nitrogen Molecule. Int. J. Quantum Chem. 1974, 8, 61−72. (23) Knowles, P. J.; Werner, H.-J. An Efficient Method for The Evaluation of Coupling Coefficients in Configuration Interaction Calculations. Chem. Phys. Lett. 1988, 145, 514−522. (24) Werner, H.-J.; Knowles, P. J. An Efficient Internally Contracted Multiconfiguration−Reference Configuration Interaction Method. J. Chem. Phys. 1988, 89, 5803−5814. (25) Veryazov, V.; Malmqvist, P.-Å.; Roos, B. O. How to Select Active Space for Multiconfigurational Quantum Chemistry? Int. J. Quantum Chem. 2011, 111, 3329−3338. (26) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; Celani, P.; Korona, T.; Lindh, R.; Mitrushenkov, A.; Rauhut, G.; et al. MOLPRO, version 2009.1, a package of ab initio programs; 2009. See http://www.molpro.net. (27) Merchán, M.; Serrano-Andrés, L.; Robb, M. A.; Blancafort, L. Triplet-State Formation along The Ultrafast Decay of Excited Singlet Cytosine. J. Am. Chem. Soc. 2005, 127, 1820−1825. (28) Chen, H.; Li, S. Theoretical Study Toward Understanding Ultrafast Internal Conversion of Excited 9H-Adenine. J. Phys. Chem. A 2005, 109, 8443−8446. (29) Quenneville, J.; Martínez, T. J. Ab Initio Study of Cis−Trans Photoisomerization in Stilbene and Ethylene. J. Phys. Chem. A 2003, 107, 829−837. (30) Merchán, M.; Serrano-Andrés, L. Ultrafast Internal Conversion of Excited Cytosine via The Lowest ππ* Electronic Singlet State. J. Am. Chem. Soc. 2003, 125, 8108−8109. (31) Gobbo, J. P.; Borin, A. C. On The Population of Triplet Excited States of 6-Aza-2-Thiothymine. J. Phys. Chem. A 2013, 117, 5589− 5596. (32) Berning, A.; Schweizer, M.; Werner, H.-J.; Knowles, P. J.; Palmieri, P. Spin-Orbit Matrix Elements for Internally Contracted Multireference Configuration Interaction Wavefunctions. Mol. Phys. 2000, 98, 1823−1833. (33) Toniolo, A.; Ciminelli, C.; Persico, M.; Martínez, T. J. Simulation of The Photodynamics of Azobenzene on Its First Excited State: Comparison of Full Multiple Spawning And Surface Hopping Treatments. J. Chem. Phys. 2005, 123, 234308−234318. (34) Neukirch, A. J.; Shamberger, L. C.; Abad, E.; Haycock, B. J.; Wang, H.; Ortega, J.; Prezhdo, O. V.; Lewis, J. P. Nonadiabatic Ensemble Simulations of cis-Stilbene and cis-Azobenzene Photoisomerization. J. Chem. Theory Comput. 2014, 10, 14−23. (35) Xu, X.; Zheng, J.; Yang, K. R.; Truhlar, D. G. Photodissociation Dynamics of Phenol: Multistate Trajectory Simulations including Tunneling. J. Am. Chem. Soc. 2014, 136, 16378−16386.

AUTHOR INFORMATION

Corresponding Author

*Phone: +886-3-5131224. Fax: +886-3-5723764. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by Ministry of Science and Technology of the Republic of China under Grant No. 103-2113-M-009− 007-MY3. L.Y. is thankful for support from postdoctoral fellowship by Ministry of Science and Technology of the Republic of China under Grant No. 103-2811-M-009−048. C.X. is thankful for support from visiting student fellowship by National Chiao Tung Univ. C.Z. thanks the MOE-ATU project of the National Chiao Tung Univ. for support.

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REFERENCES

(1) Herterich, R.; Herrmann, R. Comparing The Distribution of Nitrated Phenols in The Atmosphere of Two German Hill Sites. Environ. Technol. 1990, 11, 961−972. (2) Belloli, R.; Barletta, B.; Bolzacchini, E.; Meinardi, S.; Orlandi, M.; Rindone, B. Determination of Toxic Nitrophenols in The Atmosphere by High-Performance Liquid Chromatography. J. Chromatogr., A 1999, 846, 277−281. (3) Luttke, J.; Scheer, V.; Levsen, K.; Wunsch, G.; Cape, J. N.; Hargreaves, K. J.; Storeton-West, R. L.; Acker, K.; Wieprecht, W.; Jones, B. Occurrence and Formation of Nitrated Phenols In and Out of Cloud. Atmos. Environ. 1997, 31, 2637−2648. (4) Harrison, M.; Barra, S.; Borghesi, D.; Vione, D.; Arsene, C.; Iulianolariu, R. Nitrated Phenols in The Atmosphere: A Review. Atmos. Environ. 2005, 39, 231−248. (5) Bejan, I.; Barnes, I.; Olariu, R.; Zhou, S.; Wiesen, P.; Benter, T. Investigations on The Gas-Phase Photolysis and OH Radical Kinetics of Methyl-2-Nitrophenols. Phys. Chem. Chem. Phys. 2007, 9, 5686− 5692. (6) Han, K.-L.; He, G.-Z. Photochemistry of Aryl Halides: Photodissociation Dynamics. J. Photochem. Photobiol., C 2007, 8, 55−66. (7) Bejan, I.; Abd El Aal, Y.; Barnes, I.; Benter, T.; Bohn, B.; Wiesen, P.; Kleffmann, J. The Photolysis of Ortho-Nitrophenols: A New Gas Phase Source of HONO. Phys. Chem. Chem. Phys. 2006, 8, 2028− 2035. (8) Nagaya, M.; Kudoh, S.; Nakata, M. Infrared Spectrum and Structure of The Aci-Nitro Form of 2-Nitrophenol in A LowTemperature Argon Matrix. Chem. Phys. Lett. 2006, 427, 67−71. (9) Wang, Y. Q.; Wang, H. G.; Zhang, S. Q.; Pei, K. M.; Zheng, X.; Phillips, D. L. Resonance Raman Intensity Analysis of The Excited State Proton Transfer Dynamics of 2-Nitrophenol in The ChargeTransfer Band Absorption. J. Chem. Phys. 2006, 125, 214506−214517. (10) Wei, Q.; Yin, H.-M.; Sun, J.-L.; Yue, X.-F.; Han, K.-L. The Dynamics of OH Channel in The 266 And 355nm Photodissociation of 2-Nitrophenol. Chem. Phys. Lett. 2008, 463, 340−344. (11) Cheng, S. B.; Zhou, C. H.; Yin, H. M.; Sun, J. L.; Han, K. L. OH Produced from O-Nitrophenol Photolysis: A Combined Experimental and Theoretical Investigation. J. Chem. Phys. 2009, 130, 234311− 234319. (12) Ohtani, H.; Kobayashi, T.; Suzuki, K.; Nagakura, S. Picosecond Spectroscopy Studies of The Intersystem Crossing of Aromatic Carbonyl and Nitro Compounds in Solution. Bull. Chem. Soc. Jpn. 1980, 53, 43−47. (13) El-Sayed, M. A. The Radiationless Processes Involving Change of Multiplicity in The Diazenes. J. Chem. Phys. 1962, 36, 573−574. (14) El-Sayed, M. A. SpinOrbit Coupling and The Radiationless Processes in Nitrogen Heterocyclics. J. Chem. Phys. 1963, 38, 2834− 2838. I


Article

The Journal of Physical Chemistry A (36) Spörkel, L.; Cui, G.; Koslowski, A.; Thiel, W. Nonequilibrium H/D Isotope Effects from Trajectory-Based Nonadiabatic Dynamics. J. Phys. Chem. A 2014, 118, 152−157. (37) Weingart, O.; Lan, Z.; Koslowski, A.; Thiel, W. Chiral Pathways and Periodic Decay in cis-Azobenzene Photodynamics. J. Phys. Chem. Lett. 2011, 2, 1506−1509. (38) Yu, L.; Xu, C.; Lei, Y.; Zhu, C.; Wen, Z. Trajectory-based Nonadiabatic Molecular Dynamics Without Calculating Nonadiabatic Coupling in the Avoided Crossing Case: Trans ↔ Cis Photoisomerization in Azobenzene. Phys. Chem. Chem. Phys. 2014, 16, 25883−25895. (39) Martinez-Fernandez, L.; Corral, I.; Granucci, G.; Persico, M. Competing Ultrafast Intersystem Crossing and Internal Conversion: A Time Resolved Picture for the Deactivation of 6-thioguanine. Chem. Sci. 2014, 5, 1336−1347. (40) Cui, G.; Thiel, W. Generalized Trajectory Surface-Hopping Method for Internal Conversion and Intersystem Crossing. J. Chem. Phys. 2014, 141, 124101−124113.

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Photoisomerization Reaction Mechanisms of o-Nitrophenol Revealed

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