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Wavelength-Dependent Photodissociation of Benzoic Acid Monomer in r C-O Fission Qiu Fang†,‡ and Ya-Jun Liu*,† College of Chemistry, Beijing Normal UniVersity, 100875, Beijing, China and Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, AlbaNoVa, S-10691 Stockholm, Sweden ReceiVed: September 04, 2009; ReVised Manuscript ReceiVed: NoVember 26, 2009
In concert with the latest laser-induced fluorescence (LIF) experiment [Wei et al. J. Phys. Chem. A 2008, 112, 4727], we investigated the photodissociation mechanics of the benzoic acid monomer (BAM) with R C-O fission by means of state-of-the-art ab initio calculations. Complete active space self-consistent-field (CASSCF) and multireference CASSCF second-order perturbation theory (MSCASPT2) calculations were performed on the ground and a number of low-lying excited states of BAM. Our calculations indicated that R C-O fission from the S1 state is in competition with the fission from the T2 state upon the 266-284 nm wavelength photon. This differs from the conclusion of the previous theoretical investigation and clarified the vague experimental conclusion made earlier. According to our calculations, R C-O fission mainly occurs at the T2 state upon photoexcitation at 284-294 nm, and the photon with a wavelength longer than 294 nm is unable to present the R C-O fission. This conclusion agrees with the LIF experimental observation. Introduction Aromatic carboxylic acids are among the primary compounds of interest in the formation of fossil fuels and in determining the distribution of organic carbon in natural aqueous systems.1 As the simplest member of the aromatic carboxylic acid family, the benzoic acid monomer (BAM) has been investigated both experimentally and theoretically. Most of this research is related to the electronic structure and spectra of BAM.2-6 However, the photodissociation of BAM and the other carboxylic acids in UV plays more critical roles in atmospheric, combustion, and interstellar chemistry.7-15 Aromatic decarboxylation processes also play a part in different areas of organic synthesis and in biochemical reactions. Ruelle16,17 reported the first theoretical study of the decarboxylation of benzoic acid and salicylic acid. The results calculated indicated how water molecules can catalyze the reaction. Brill et al.1 investigated the effect of OH substitution on the rates and mechanisms of decarboxylation of BAM. BelBruno et al.18 studied the mechanisms of decarboxylation of ortho-substituted benzoic acids by density functional theory (DFT). Fang et al.19 examined the dissociation dynamics of three reaction channels by the complete active space self-consistent-field (CASSCF) method, as follows:
C6H5COOH f C6H6 + CO2
(1)
f C6H5CO + OH
(2)
f C6H5 + COOH
(3)
Among them, channel 2 lacked experimental information. Following this theoretical study, Yin et al. did the corresponding experimental investigation by LIF technique at 266 nm20 and 280-295 nm21 focusing on the detection of an OH fragment. Based on the theoretical calculations,19 the R C-O bond fission along the first singlet excited state (S1) pathway is not in * To whom correspondence should be addressed. E-mails: yajun.liu@ bnu.edu.cn. † Beijing Normal University ‡ Royal Institute of Technology
competition with the intersystem crossing (ISC) processes from S1 to the first or second triplet states (T1 or T2). The C-O bond cleavage begins at the T2 state and leads to the fragments of C6H5CO and OH in the ground state (S0), which is the most likely channel upon photoexcitation of the benzoic acid monomer at 270 nm or shorter wavelengths. The LIF experiment at 266 nm20 supported the conclusion as calculated above. It determined that when BAM is excited by a 266 nm laser photon, it has sufficient energy to pass the barriers of S1 and T2. Considering the theoretical calculation shows the barrier of T2 to be lower than that of S1, and that there is a three-surface intersection S1/T1/T2, the T2 state is dominant in producing an OH fragment from the photodissociation of BAM at 266 nm. The same research group recently investigated the photodissociation dynamics of BAM at 280-295 nm.21 It found that, taking into account the internal energy of BAM, if the photolysis wavelength is shorter than 284.5 nm, the total energy of photolysis is above the barriers of the S1 and T2 states. Thus, BAM has the ability to pass the S1 and T2 barriers to produce an OH radical. However, if the wavelength of the photolysis laser is longer than 284.5 nm, the total energy of photolysis is below the barrier of the S1 state but higher than the barrier of the T2 state: the OH is produced only from the T2 state. Hence, the T2 state is proposed to be the dissociative state for photodissociation of BAM at 280-294 nm. In brief, based on the experimental observation20,21 and theoretical calculations,19 the conclusion on channel 2 was drawn as below. At 266-280 nm, channel 2 could occur on the S1 and T2 potential energy surfaces (PES). Because the barrier of T2 is lower than that of S1, the path via T2 is dominant. At 280-294 nm, channel 2 can only occur on the T2 PES, and at wavelengths longer than 294 nm, channel 2 cannot occur. This conclusion was based on the previous calculations.19 However, these calculations can be substantially improved. First, these CASSCF calculated energies lack dynamic correlation, which can be included by CASPT2 calculations. Second, the selected active space, 10-electronsin-8-orbitals, only included π bond orbitals, which could be enough for the calculations of vertical excitation energy (Tv).
10.1021/jp908567m 2010 American Chemical Society Published on Web 12/18/2009
Photodissociation of Benzoic Acid Monomer
Figure 1. The schematic structure and atomic labels of BAM.
However, when investigating the dissociation channel 2 with C12-O14 bond breaking (see Figure 1), the C12-O14 σ and σ* must be included in the active space. Third, a better basis set for excited states should be employed. Our study used stateof-the-art calculations to further explain clearly the dissociation channel 2 of BAM below the 266 nm photon energy and especially discuss whether T2 is the exclusive state for channel 2 at 280-266 nm. Computational Methods All the optimizations and harmonic vibrational frequencies were done by the CASSCF method.22 The energies were improved by the CASSCF second-order perturbation theory (CASPT2) method23 or multireference CASPT2 (MSCASPT2) methods.24 The [He] cores for O and C were frozen in all (MS)CASPT2 computations. The selected active space of BAM comprises 14-electrons-in-12-orbitals (see Figure S1), which includes three π and three π* orbitals in the phenyl ring, the CdO π and π* orbitals, one nonbonding orbital for each oxygen, and the C-O σ and σ* orbitals. The selected active space of C6H5CO radical is 11-in-10, which includes three π and three π* orbitals in the phenyl ring, the CdO π and π* orbitals, one nonbonding orbital for the CdO oxygen, and one single-occupied C in CdO. For the OH radical, the 2s and three 2p orbitals of oxygen, the 1s of hydrogen, and two secondary orbitals were included in the active space, which is 7-in-7. The basis set employed was the relativistic atomic natural orbital type basis set, ANO-RCC,25 contracted to H/2s1p, C/3s2p1d, and O/3s2p1d (in principle a double-ζ type contraction or better, henceforth denoted ANO-RCC-VDZP). Based on a series of test calculations, we employed an IPEA modification of the zeroth-order Hamiltonian with a value of 0.15 on some important geometries for accurate energies. For the other calculations, the IPEA shift was employed in its 0.25 default value in the MOLCAS program.26 For BAM, the calculated adiabatic excitation energies (T0) of the second excited state (S2) by mean of CASPT2 calculations with a 0.25 IPEA shift is 0.17 eV higher than the experimental value, which is the only experimental value available. For the energy calculation of the T2 transition state, an imaginary shift of 0.05 was used to remove the intruder-state problem.27 All calculations were performed using the MOLCAS 6.4 quantum chemistry software.28 Results S0 Geometry and Tv Values. The previous CAS(10,8)/ccpVDZ optimizations19 located two stable conformers of the BAM ground state. They are all of planar structures but have different relative positions of OH and CdO. The one with OH
J. Phys. Chem. A, Vol. 114, No. 1, 2010 681 and CdO at trans-position (see Figure 1) is more stable in energy. The most stable conformer of the BAM ground state was currently optimized at CAS(14,12)/ANO-RCC-VDZP level. The main optimized geometric parameters are listed in Table 1, where they are compared with the previous CASSCF optimized ones,19 and those detected experimentally.29 The optimized S0 geometry here is closer to the experimental one and superior to the DFT optimized one30 (not shown in Table 1; for detailed geometry, see Supporting Information). The Tv values of S1-A′′, S2-A′, S3-A′, T1-A′, T2-A′′, and T3-A′ were calculated by the MS-CASPT2//CASSCF method and is listed in Supporting Information. The transition characteristics can be clearly seen from Table S1 and the CASSCF orbital graphs in Figure S1. Note that the ordering numbers of these excited states were based on their T0 values. CASPT2//CASSCF Calculated PECs with the OH Group Leaving under Cs Symmetry. Our main purpose in this article is to explain the photodissociation mechanism of BAM with the OH group leaving. The experimentally employed photon energy is less than 266 nm (4.66 eV),20,21 which can at most reach doorway state S2-A′. States higher than S2-A′ are not our concern. The states below S2-A′ are S0-A′, S1-A′′, T1-A′, T2A′′, and T3-A′ (see Tables 1 and S1). First, for saving CPU time, we ruled out the T1 state as the dissociating state for producing an OH fragment. The T1 state is a (π,π*) transition, which is localized in the aromatic ring and correlates with channel 3, rather than 2 of BAM dissociation.19,31 BAM shows phosphorescence alone with a high quantum yield.3 Since phosphorescence is not our concern in this article, we only calculated the potential energy curves (PECs) of S0-A′, S1-A′′, S2-A′, T2-A′′, and T3-A′, as presented in Figure 2. As the previous calculations19 showed, most of the excited states are of planar structures, as the ground state is. However, the hydroxyl of the S1 and T2 states rotate from the molecular plane. Because the rotation barrier of hydroxyl is very little, especially when the C-O σ bond is stretched longer, the nonplanar geometries can be roughly replaced by their corresponding planar ones with a slight loss of calculation accuracy (generally less than 3 kcal/mol). This approximation saves much CPU time and lowers the difficulty of calculating of the PECs. For such a calculation, the dissociation coordinate in every state was constructed. That is, for a given C-O distance ranging from 1.3 to 5.0 Å, all the remaining degrees of freedom were optimized at the CASSCF level. The CASPT2 method was then employed to calculate the energy of the CASSCF partially optimized geometry of each point on the PECs of the six states. As shown in Figure 2, with OH leaving, the dissociation of BAM leads to two groups of products. When the C-O distance was stretched longer than 4.13 Å, the difference between the two groups became nearly the same, namely, 0.79 eV. The CASPT2//CASSCF calculated the T0 value of the first doublet excited state of the C6H5CO radical to be 0.80 eV (the detailed optimized geometries of C6H5CO radical ground and excited states are given in Supporting Information). According to the PECs in Figure 2, S2-A′ and T3-A′ will be dissociated to C6H5CO* + OH, whereas the other three states will be dissociated to C6H5CO + OH if the requisite energy is available. The experimentally observed products are in ground states, that is, C6H5CO + OH. According to Figure 2, the crucial states connected to the photodissociation of BAM under 266-295 nm should be S0, S1, S2, and T2. The crossing points between S2 and S1 (S2/S1), S2 and T2 (S2/T2), and S1 and T2 (S1/T2) were located; their selected geometrical parameters are listed in Table
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Figure 2. The CASPT2//CASSCF calculated PECs of the ground and low-lying excited states along the coordinate of the OH group leaving under Cs symmetry of BAM. The inset shows the schematic PECs of the S1 and T2 states. Values marked in the inset were the relative energies (kcal/mol) at the S0 minimum, corrected by ZPE on the corresponding fully optimized geometries. The relative position of the S1 and T2 PECs are qualitatively correct for clearance.
TABLE 1: Selected Geometric Parameters (In Å and Degrees) of the Present CAS(14,12)/ANO-RCC-VDZP (level a) and Previous CAS(10,8)/cc-pVDZ (Level b from Ref 19) Optimizationsa states
level
C1-C2
C1-C6
C1-C12
C12-O13
C12-O14
S0
a b exp.29 a b a b a b a b a b a b a a a
1.393 1.401
1.391 1.385
1.477 1.489
1.416 1.395 1.430 1.440 1.476 1.485 1.416 1.410 1.432 1.391 1.434 1.391 1.427 1.426 1.452
1.423 1.395 1.432 1.441 1.462 1.480 1.421 1.406 1.428 1.391 1.428 1.391 1.428 1.428 1.452
1.396 1.472 1.451 1.468 1.440 1.453 1.389 1.446 1.362 1.480 1.359 1.485 1.413 1.420 1.438
1.215 1.196 1.24 1.386 1.403 1.220 1.204 1.225 1.204 1.384 1.365 1.222 1.236 1.217 1.235 1.275 1.265 1.369
1.331 1.337 1.29 1.344 1.351 1.335 1.332 1.336 1.342 1.343 1.354 1.798 1.784 1.841 1.783 1.349 1.347 1.269
S1 S2 T1 T2 TS- S1 TS- T2 S2/S1 S2/T2 S1/T2
H15O14C12O13 0.0 0.0 -74.8 -65.5 0.0 0.0 0.0 0.0 -73.5 -71.5 -49.8 -52.5 -48.9 -63.1 0.0 0.0 0.0
a TS-S1 and TS-T2 are the transition states along the S1 and T2 PECs, respectively, and S1/S2 and T2/S2 are the crossing points between S1 and S2, and T2 and S2 PESs, respectively. See Figure 1 for atomic labels.
1. Below we will confine our calculations to some key points connected to these five states Accurate Calculations of Some Key Points on the PECs. We fully optimized the geometries of S0, S1, S2, T1, and T2 in C1 symmetry. The results indicted that S0, S2, and T1 are of planar structure, but S1 and T2 have nonplanar structures. The vital geometric parameters are listed and compared with previous
CASSCF optimized results19 in Table 1. (For their detailed Cartesian coordinates, see Supporting Information.) The currently optimized geometries mostly agree with the previous ones,19 although some of them have differences that must be reckoned with, since the present optimizations have larger active space and a better basis set. Frequency analyses have indicated that the currently located geometries of S0, S1, T1, and T2 are
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TABLE 2: The (MS-)CASPT2//CASSCF Calculated T0 Values (eV) of Some Key Points
a
state
calculated
calculateda
S1 S2 T2 TS- S1 TS-T2 S2/S1 S2/T2 S1/T2
4.13 4.41 3.99 5.12 5.08 4.57 4.48 4.55
3.90
Exp.b 4.46
3.85 4.92 4.81
T0 with ZPE correction. b Experimental value from ref 5
the minima. The corresponding vibrational frequencies that have been calculated are listed in Table S2. The T0 values of S0, S1, S2, T1, and T2 were calculated by the CASPT2//CASSCF method and are listed in Table 2. The experimentally detected S2 T0 value was found to be 4.46 eV,5 which is the only experimental value available and in agreement with the one currently calculated at 4.41 eV. The saddle-point on the PECs of S1 (TS-S1) and T2 (TS-T2) were also located, and their selected geometrical parameters were listed in Table 1. Frequency analyses indicate that they are first-order saddle points (see details in Supporting Information). The energy of C6H5CO + OH at the C-O distance of 4.13 Å was calculated with zero point energy (ZPE) correction. The energies of the related minima and saddle points were also corrected by ZPE. The key relative energies were described in the inset of Figure 2. Discussion According to the Tv and f values in Table S1, the S2 state should be the doorway state for the experimentally employed 266 to 294 nm photon.20,21 Because the experiment was performed at room temperature, about 6.5 kcal/mol (0.28 eV) internal energy should be considered added. Moreover, the CASPT2 calculated energies typically have errors less than 0.3 eV.32 The experimentally employed 266-294 nm photon could reach the CASPT2 calculated states having 4.80 to 5.24 eV T0 values at most. According to Figure 2, the photo doorway state S2 cannot dissociate itself. However, it could dissociate to the ground state products through S1 or T2 after the internal conversion (IC) or ISC, if energy is available. The crossing point S2/S1 (S2/T2) is 4.57 eV (4.48 eV) above the S0 minimum (see Table 2). The photoexcitation in the wavelength range of 266-294 nm can reach the two crossing points. Thus, if sufficient energy is available, BAM will dissociate via S1 or T2 following the IC S2/S1 or ISC S2/T2, respectively. According to the inset of Figure 2, the transition states TS-S1 and TS-T2 are 113.5 kcal/mol (4.90 eV) and 110.9 kcal/mol (4.81 eV) above the S0 minimum. Considering the internal energy caused by room temperature, the typical CASPT2 calculated error, and the tunneling effect, the C-O bond cleavage along the S1 and T2 pathways is energetically accessible upon photoexcitation of BAM at 266-284 nm. Since the S2/S1 IC process is much easier than the S2/T2 ISC process, the S1 C-O bond cleavage might be a dominant channel for the BAM photodissociation at 266-284 nm. However, it should be pointed out that once the S1 state is populated, BAM can decay to the T2 state, due to a high barrier on the S1 pathway. From the above discussion, it can be concluded that the S1 and T2 C-O bond cleavages are a pair of competition pathways for the BAM photodissociation at 266 to 284 nm. This differs from the previous view based on the CASSCF calculations,19 which claimed that the C-O bond cleavage starts from the T2 state and is the most
possible dissociation channel upon photoexcitation at 270 nm or shorter wavelengths. Theoretical calculations and analyses of the photodissociation dynamics of BAM at 280-295 nm and longer wavelength are sorely lacking. The latest experiment concluded that T2 is the dissociative state for R C-O fission of BAM at 280 to 294 nm, and that at wavelengths longer than 294 nm this dissociation cannot occur.21 According to the current calculations, when the photon wavelength is between 284 and 294 nm, it cannot pass TS-S1, even if the biggest experimental and theoretical errors are taken into account. However, TS-T2 (4.81 eV) is 0.1 eV lower than TS-S1 in energy, although the difference is much smaller than the previous results calculated by CASSCF.19 The 284-294 nm photon could reach TS-T2 when both the internal energy (0.28 eV) and the largest CASPT2 calculated errors (0.3 eV) were considered. Therefore, the experimentally observed dissociation at 284-294 nm takes place exclusively through the T2 state. A photon with a wavelength longer than 294 nm cannot possibly reach both TS-T2 and TS-S1. For this reason the experiment could not observe the dissociation with an OH product at 295 nm. The conclusion drawn here on the R C-O fission of BAM at 280-294 nm and longer wavelengths agrees with the experimental oberservations.21 Conclusion The photodissociation mechanics of BAM with the R C-O fission was investigated by the CASSCF/CASPT2 calculated PECs of the ground and a number of low-lying excited states of BAM. The results of our calculations indicate that upon the 266-284 nm wavelength photon, the S1 and T2 C-O bond cleavages are a pair of competitive pathways. Upon photoexcitation at 284-294 nm, the R C-O fission mainly occurs in the T2 state. It is impossible for a photon with a wavelength longer than 294 nm to bring about the dissociation of the OH product. Acknowledgment. We would like to thanks Hong-Ming Yin for the valuable discussions on experiments. This study was supported by grants from the National Natural Science Foundation of China (Grants 20873010, 20673012, and 20720102038), the Major State Basic Research Development Programs (Grant 2007CB815206), and the project-sponsored by SRF for ROCS, SEM. Supporting Information Available: Structures, Tv values, and corresponding transition characteristics, and calculated vibronic frequencies for somel stationary points reported in the present work. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Li, J.; Brill, T. B. J. Phys. Chem. A 2003, 107, 2667. (2) Kumar, A.; Naik, P. Chem. Phys. Lett. 2006, 422, 152. (3) Baba, H.; Kitamura, M. J. Mol. Spectrosc. 1972, 41, 302. (4) Kamei, S. I.; Abe, H.; Mikami, N.; Ito, M. J. Phys. Chem. 1985, 89, 3636. (5) Meijer, G.; Vries, M. S.; Hunziker, H. E.; Wendt, H. E. J. Phys. Chem. 1990, 94, 4394. (6) Bakker, J. M.; MacAleese, L.; Helden, G. V.; Meijer, G. J. Chem. Phys. 2003, 119, 11180. (7) Hunnicutt, S. S.; Waits, L. D.; Guest, J. A. J. Phys. Chem. 1989, 93, 5188. (8) Hunnicutt, S. S.; Waits, L. D.; Guest, J. A. J. Phys. Chem. 1991, 95, 562. (9) Arendt, M. F.; Browning, P. W.; Butler, L. J. J. Chem. Phys. 1995, 103, 5887. (10) Kitchen, D. C.; Forde, N. R.; Butler, L. J. J. Phys. Chem. A 1997, 101, 6603.
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