Formation of Hydroxyl Radical from the Photolysis of Salicylic Acid

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Formation of Hydroxyl Radical from the Photolysis of Salicylic Acid Can-Hua Zhou, Shi-Bo Cheng, Hong-Ming Yin,* and Guo-Zhong He State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ABSTRACT: Photodissociation dynamics of salicylic acid (SA) in the gas phase at different photolysis wavelengths (266, 315
317 nm) is investigated by probing the nascent OH photoproduct employing the single-photon laser-induced fluorescence (LIF) technique. At all the photolysis wavelengths it is found that the nascent OH radicals are produced mostly in a vibrationally ground state (υ00 = 0) and have similar rotational state distributions. The two spin
orbit and Λ-doublet states of the OH fragment formed in the dissociation are measured to have a nonstatistical distribution at each photolysis wavelength. The LIF signal of the OH could be observed upon photolysis at 317 nm but not at 317.5 nm. The threshold of OH formation from SA photodissociation is estimated to be 98.2 ( 0.9 kcal/mol. The effect of the phenolic OH group on the dissociation of SA is discussed.

1. INTRODUCTION The photochemical and photodissociation dynamics of polyatomic molecules is of considerable interest toward the fundamental understanding of the dynamics of the elementary reactions.1
5 In contrast to small molecules, photodissociation of polyatomic molecules may involve more complex dynamics, from which one can obtain the dynamical information on the dissociation pathways and the nature of the dissociative state potential energy surface by detecting the energy disposal in the photofragmentation process. The photochemistry and photodissociation of carboxylic acids have been investigated extensively6
15 because it generates the important OH radical. The hydroxyl radical is the most important chemical cleaning agent in the atmosphere. Pushpa et al.6 and Kumar et al.7 studied the photodissociation of saturated and unsaturated carboxylic acids at 193 and 248 nm. They observed an appreciable amount of energy being channeled into the relative translation of OH and its cofragment. Naik et al.8 and Kwon et al.9 studied the photolysis of acetic acid at 193 nm. They found that the dissociation takes place indirectly along the triplet surface via curve crossing with the reverse barrier in the exit channel. Li et al.10 and Fang et al.11 mapped the potential energy surfaces of the low-lying excited states of benzoic acid using the complete active space self-consistent field method. They proposed the C
OH bond cleavage occurs from the T2 state to generate the OH radical. The benzoic acid12,13 and o-nitrobenzoic acid14 may undergo dissociation to produce the OH radical from the excited state with a considerable exit barrier. Very recently, Dyakov et al.15 reported the photodissociation of benzoic acid in a molecular beam at 193 and 248 nm using multimass ion imaging techniques. They also observed the OH elimination channel at both photolysis wavelengths. Similar to other carboxylic acids, salicylic acid (SA), which is an important conjugated compound of biological, medicinal, and industrial interest, has attracted the attention of many experimental r 2011 American Chemical Society

and theoretical studies over the years. Most of the work related to SA are mainly focused on its excited state intramolecular proton transfer processes.16
18 The photodissociation study of 2-, 3-, and 4-hydroxybenzoic acid in a molecular beam at 193 nm using multimass ion imaging techniques has been reported recently, which shows the importance of intramolecular hydrogen bond in the excited state dynamics and provides an alternative molecular mechanism for the photostability of aromatic amino acids upon irradiation of ultraviolet photons.19 In continuation of our earlier works on benzoic acid12,13 and o-nitrobenzoic acid,14 we studied the photolysis of SA at different excitation wavelengths to understand the effect of another chromophore OH on its excitation and dynamics of SA dissociation. The phenolic OH substitution in SA may affect the dissociation threshold of OH formation comparing to benzoic acid. In the present study, we reported investigations on the photodissociation of SA in the gas phase producing the nascent OH radical by employing the single-photon LIF technique under collision-free conditions. The photolysis of SA was carried out at 266, 315, 316, and 317 nm to investigate the influence of excitation energy on the dynamics of OH. By analyzing the LIF spectrum, the internal state distribution of the nascent OH radical was obtained. The energy threshold for producing OH radical from the photodissociation of SA is estimated from the experimental results. Compared to the photolysis of benzoic acid, the effect of the phenolic OH group on the dissociation of SA is discussed. The dissociative state and the probable mechanism of OH formation have also been suggested. Received: March 23, 2011 Revised: April 20, 2011 Published: May 05, 2011 5062

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2. EXPERIMENTAL SECTION The experimental setup used in the present work has been described in detail elsewhere,13,14,20 and only a brief description is as follows. The second-harmonic output (532 nm) of a seeded Nd:YAG laser (Spectra-Physics, GCR-170) was used to pump a dye laser (Lumonics, HD-500). The output of the dye laser was then introduced into a harmonic generator (Lumonics, HT-1000) to produce the frequency-doubled laser pulses in the 310
320 nm region, which were used as the photolysis laser beam. Another second-harmonic output (532 nm) of a seeded Nd:YAG laser (Spectra-Physics, PRO190) was used to pump a dye laser (Coherent, SCANMATE-PRO), which generated the wavelength-tunable laser pulses. The dye laser was operated with DCM dye, of which the corresponding fundamental wavelength tuning range 600
640 nm is used. The output of the dye laser was checked by a laser wavelength meter (Coherent) and then introduced into a harmonic generator (Coherent, ScanmateSHG) to produce the frequency-doubled UV laser pulses in the 300
320 nm region, which were used as the probe laser beam. Moreover, the second-harmonic output (532 nm) of the seeded Nd: YAG laser (Spectra-Physics, GCR-170) was converted into 266 nm by a KD*P crystal, which was also used as the photolysis laser beam. Both the photolysis and the probe beams were collinearly counter propagated through the center of the photolysis cell. The probe laser was delayed 15 ns with respect to the photolysis laser, which was controlled by a generator (SRS, DG535). This delay was sufficient to separate the two laser pulses and was short enough to avoid collision effects under the pressure (typically 200 mTorr) used during the experiments. The OH fragment was probed by exciting the A2Σþ (υ0 = 0) r 2 X Π (υ00 = 0) transition of OH and monitoring the subsequent A f X fluorescence. The fluorescence of the OH fragment was collected by a photomultiplier tube (PMT, Hamamatsu CR161). A suitable band filter (λcenter = 310 nm, full width at half-maximum (fwhm) = 15 nm) was placed between the collecting lens and the PMT to cut off scattering from the photolysis laser and the probe laser. The signal was then averaged by a boxcar (SRS, SR250), A/D converted by a homemade interface, and stored into a personal computer. In the experiments, multiphoton process and saturation effect were avoided by keeping the laser beam energies low enough. The measured LIF signal was found to be linearly dependent on the photolysis as well as the probe laser power, indicating that photodissociation of SA is a single-photon process and that the fluorescence is not saturated. Typically, the pulse energies used in the experiments were about 2.8
4 and 0.03
0.05 mJ/cm2 for the photolysis and probe laser beams, respectively. Additionally, SA is known to exist as a dimer in the solid state, in certain nonpolar solvents, and also in the gas phase under moderate and high concentrations.16
18 In the present experimental condition, the pressure of the gaseous sample and the helium carrier was maintained to be low (typically 200 mTorr); therefore, the contribution from the dimer of SA could be negligible. SA (g99.5%, Aldrich) was purchased and used without further purification. During the experiments, the sample was heated to 440 K, helium carrier gas was bubbled through the sample, and the gas mixture was continuously expanded into the center of the photolysis cell. 3. EXPERIMENTAL RESULTS 3.1. LIF Spectra of the OH Photofragment. The LIF spectra of the OH fragment from SA photolysis at different excitation

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Figure 1. A2Σþ (υ0 = 0) r X2Π (υ00 = 0) band of the one-photon LIF spectrum of the nascent OH fragment from the photodissociation of SA at 315 nm: (a) experiment and (b) simulation. The starred rotational line Q1 (4.5) of the OH radical was used as the pointer for the PHOFEX spectrum of SA.

wavelengths were recorded by scanning the OH A2Σþ (υ0 = 0) r X2Π (υ00 = 0) transition in the 306
315 nm range. Photolysis wavelengths of 266, 315, 316, 317, and 317.5 nm were used to investigate the dependence of the photochemistry of SA on the excitation energy. In the experiments, the nascent OH radicals could be observed upon photolysis at 266, 315, 316, and 317 nm but not at 317.5 nm. At those photolysis wavelengths except 317.5 nm the OH radicals were found to be formed in the vibrational ground state. The experimental LIF excitation spectrum of the (0, 0) band of the nascent OH is shown in Figure 1a, formed upon photolysis at 315 nm. The intensities of the observed LIF rotational lines were normalized with respect to the energies of the photolysis and probe laser pulses as well as the pressure in the photolysis cell. The profiles of the LIF spectra formed at other photolysis wavelengths were found to be similar to that obtained at 315 nm after careful inspection. It is worthwhile to note that the probe wavelengths are shorter than some photolysis wavelengths used in the present experiment. Once the intensity of the probe pulses is too high, the signals of the OH radical can be observed with the probe laser alone via a multiphoton process. Hence, the occurrence of the multiphoton process should be avoided by keeping the intensity of the probe laser low enough. There is no OH LIF signal by the probe laser alone during the experiment. To precisely assign the J00 levels of OH populated in the dissociation process, it is necessary to compare the experimental spectrum with the simulated one calculated from the known spectroscopic constants.21 The rotational state population N (J00 , υ00 ) in the ground electronic state could be obtained from the intensities of the observed LIF rotational lines. The relationship between the intensities of the fluorescence signal (IF) and the rovibrational state populations N (J00 , υ00 ) is expressed by IF µ

NðJ 00 , υ00 Þqυ0 υ00 SJ 0 J 00 f ðν, ν0 Þ ð2J 00 þ 1Þ

ð1Þ

where qυ0 υ0 0 are the known Franck
Condon factors for the OH A
X transition,22 SJ0 J0 0 are the line strengths for the OH A
X one-photon rotational transitions,23 and f(ν,ν0) = F(ν0)exp(
R(ν
ν0)2) is the laser intensity function. 5063

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Figure 2. (Top) Corrected PHOFEX spectrum that monitors the LIF intensity of the OH Q1 (4.5) rotational line as a function of photolysis wavelength. (Bottom) Photolysis laser energy curve of photolysis wavelengths.

The OH transitions are labeled following Hund's case (a). The P, Q, or R branches are for cases in which ΔJ =
1, 0, or 1, and the subscript 1 or 2 represents the different spin
orbit states 2 Π3/2 or 2Π1/2, respectively. Due to the parity selection rule (þ T
), the Q branch only corresponds to the Π(A00 ) state while the P and R branches are attributed to the Π(A0 ) state. The simulated spectrum of the OH (0,0) band corresponding to the photolysis wavelength of 315 nm is plotted in Figure 1b. The experimental LIF excitation spectra at other photolysis wavelengths were also simulated. Scalar properties of the nascent OH product in the photodissociation can be obtained from the LIF spectrum. Detailed analysis of the quantum state distributions of the OH fragment will be presented later. 3.2. Photofragment Excitation Spectrum. The photofragment excitation (PHOFEX) spectrum was obtained by monitoring the LIF intensity for a single OH (υ00 = 0, J) rotational line while the photolysis laser wavelength was scanned. The upper plot in Figure 2 displays the LIF intensity of the OH Q1 (4.5) rotational line as a function of the continuously scanned photolysis wavelength from 310.45 to 317.5 nm, and the lower plot is the energy curve of photolysis wavelengths in this region. The PHOFEX spectrum was corrected with respect to the variations in photolysis laser energy with wavelengths and the pressure in the photolysis cell. The LIF intensities of several other intense rotational lines against the photolysis wavelengths were also obtained. All rotational lines investigated show a similar profile. As illustrated in Figure 2, no distinct structure is observed but the intensity of the OH Q1 (4.5) rotational line gradually decreases with the decreasing excitation energy and disappears at 317.5 nm, indicating a reduction in the relative quantum yield of OH formation. The LIF intensity of OH from the photodissociation of SA at 317 nm is very weak, and the noise level is the same compared to the photolysis laser energy curve at 317.5 nm. 3.3. State Distribution of the Nascent OH Photofragment. 3.3.1. Rotational State Distribution. The OH rotational distribution at 315 nm photolysis of SA is determined by analyzing the P1 and P2 rotational lines, because these lines are mostly free of interference from other lines. On the basis of the normalized experimental data, the rotational state distribution of

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Figure 3. Boltzmann plot for the rotational state distributions of the nascent OH fragment generated from the photolysis of SA at 315 nm.

Figure 4. Rotational population distribution of the OH fragments generated from the photodissociation of SA at (a) 266 and (b) 315 nm.

the OH radical at all the excitation wavelengths could be characterized by a Boltzmann temperature from the expression of the Boltzmann population distribution ln

NðJ}Þ
εðJ}Þhc ¼ þ constant ð2J} þ 1Þ kTR

ð2Þ

The Boltzmann plot obtained from the 315 nm photodissociation of SA is shown in Figure 3. It is evident that the rotational population distribution is much scattered, and the Boltzmann plot shows clearly a curvature, indicating that the population of the nascent OH (υ00 = 0, J00 ) fragment is not an equilibrium over all the rotational levels J00 . Similar curvature was also seen in the photodissociation of o-nitrobenzoic acid.14 By analogy to the analysis on o-nitrobenzoic acid, in Figure 3 the initial points of lower energies up to 800 cm
1 lie on a straight line, with a lower rotational temperature of 700 K, and the later points at higher 5064

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Figure 5. Ratios of the rotational state distribution of 2Π3/2/2Π1/2 versus the rotational quantum number J00 of the nascent OH (X2Π, υ00 = 0) fragment after SA photolysis at 315 nm. Preferential population of the 2Π3/2 state of OH is observed.

Figure 6. Relative Λ-doublet population versus the rotational quantum number J00 of the nascent OH (X2Π, υ00 = 0) fragment from the photolysis of SA at 315 nm. The Π(A0 ) state is populated in preference to the Π(A00 ) state.

energies show a higher rotational temperature of 1100 K. The 2 corresponding rotational energy Erot v0 0 of the ground state X Π 00 (υ = 0) is calculated to be 1.6 ( 0.2 kcal/mol at 315 nm using 00 00 00 Erot v0 0 = ∑J0 0 P(J )ε(J ), where P(J ) is the relative state population 00 and ε(J ) is the energy of a given rotational state. Moreover, the rotational state distributions of OH upon photolysis at 266 and 315 nm are shown in Figure 4. The rotational state distributions of OH peaked at J00 = 3.5 and extended to J00 = 12.5 at both wavelengths. The similar trend was also found at other photolysis wavelengths. Obviously, the rotational state distribution of OH produced from SA photolysis is independent of the variation in experimental excitation energy for all photolysis wavelengths. The extremely similar behavior in the rotational state distribution of OH was also observed in the 280
294 nm photodissociation of benzoic acid.13 One reasonable explanation is that the dissociation occurs from a repulsive state or from a potential energy surface having an exit barrier, leaving the residual energy being distributed in the relative translation. In addition, the OH A2Σþ (υ0 = 1) r X2Π (υ00 = 1) transition was also scanned at each photolysis wavelength to see whether any OH is produced in the υ00 = 1 level. However, no LIF signal of the OH (X2Π, υ00 = 1) radical was observed under the present experimental conditions, even when higher photolysis laser power was used. This suggests that the OH fragment from the photodissociation of SA at all photolysis wavelengths is vibrationally cold. 3.3.2. Spin
Orbit State Distribution. Because of spin
orbit coupling between the electronic spin and the orbital angular momentum, the ground state (X2Π) of the OH radical is split into two spin
orbit components, 2Π3/2 and 2Π1/2. Populations of the two spin
orbit states, 2Π3/2 and 2Π1/2, can be obtained from the intensities of the P1(Q1) and P2(Q2) lines, respectively. The ratios of the population of the 2Π3/2 and 2Π1/2 states measured at 315 nm are plotted versus the rotational quantum number J00 in Figure 5. As can be seen from the plot, at low J00 , the ratios of the two spin
orbit states are close to unity, but the OH fragment exhibits a preferential population of the 2Π3/2 state at high J00 . Similar trends were also found at other photolysis wavelengths.

This phenomenon of preferential population of the 2Π3/2 state has also been observed for the OH radicals produced from the photodissociation of several other molecular systems, such as thiolactic acid6 and benzoic acid.12,13 The distinct deviation of the measured spin
orbit ratio from the statistical value could be explained partly based on the energy difference between these two states: the 2Π1/2 state lies higher in energy than the 2 Π3/2 state at the same J00 . Moreover, the consensus is that the preferential population of the 2Π3/2 state stems from the coupling between the initially prepared excited singlet state with a nearby triplet state.24,25 3.3.3. Λ-Doublet State Distribution. Each spin
orbit state of OH is further split into two Λ-doublet states, Π(A0 ) and Π(A00 ), resulting from the interaction between the orbital angular momentum and the nuclear rotation. These two states in the OH radical are created by different orientations of the Π lobes with respect to the plane of rotation. In the Π(A0 ) state, the pπ lobe lies in the plane of rotation, whereas in the Π(A00 ) state, the pπ lobe is perpendicular to the plane of rotation.26 As mentioned earlier, the Q branch is used for probing the Π(A00 ) state, whereas the R and P branches are used for probing the Π(A0 ) state. Thus, the R/Q (or P/Q) intensity ratios represent the ratios of the Π(A0 ) and Π(A00 ) states. In Figure 6, the Λ-doublet ratios for the 2Π3/2 state obtained are plotted as a function of rotational quantum number J00 for the photolysis of SA at 315 nm. Obviously, the Π(A0 ) state is preferentially populated with a Λ-doublet population ratio (Π(A0 )/Π(A00 )) of 1.5 ( 0.3. Similarly, the Λ-doublet ratios are all more than unity at other photolysis wavelengths. This result suggests that the pπ electronic orbital for the unpaired electron is predominantly in its plane of rotation of OH. Furthermore, the relative population of the Λ-doublet of OH provides the exit channel dynamics in the bond cleavage process. As the ratio Π(A0 )/Π(A00 ) is similar at all wavelengths studied, the exit channel dynamics of OH formation seem to be similar. The preferential Λ-doublet population also suggests that the transition state for the OH formation channel might be planar in nature in the photodissociation of SA at all excitation wavelengths. 5065

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4. DISCUSSION 4.1. Dominant Photochemical Path of OH Formation. Due to the existences of the phenolic OH group and the carboxylic group in SA, on photoexcitation of SA, three possible pathways are responsible for OH formation and can be depicted as follows

SA f C6 H4 COOH þ OH, ΔH ¼ 113:1 kcal=mol

ð3Þ

SA f HOC6 H4 CO þ OH, ΔH ¼ 103:7 kcal=mol

ð4Þ

SA f HOC6 H4 þ HOCO f HOC6 H4 þ OH þ CO, ΔH ¼ 134:3 kcal=mol ð5Þ These reaction enthalpies are calculated at the B3LYP/6311þG(d,p) level of theory with the Gaussian 03 program package.27 Reactions 3 and 4 are direct dissociation of the C
OH bond in the phenolic OH group and the carboxylic group, respectively, while the reaction 5 is indirect dissociation involving the C
C single-bond cleavage resulting in HOC6H4 þ HOCO and a secondary reaction to form OH and CO. Tseng et al.28 pointed out that no dissociation occurs upon excitation of phenol at 266 nm because the photon energy is insufficient for direct fission of the OH bond. On the basis of the thermochemical data,29 the bond energy of C
OH in phenol (110.8 kcal/mol) is higher than that of O
H (90.4 kcal/mol), so the OH radical of SA photolysis cannot be produced from the direct dissociation of the phenolic OH group. In addition, like other carboxylic acids,6,12,13 we can exclude the possibility of OH as a secondary product from HOCO. According to the thermochemical data, the C
C bond and the HO
CO bond cleavage were estimated to be endothermic by 105.210 and 36.06 kcal/mol, respectively. Thus, the needed minimal energy for the second reaction to produce the OH fragment is 141.2 kcal/mol, which is much higher than the energy used in the present experiment even including the internal energy (vide infra) of SA. Therefore, the observed OH product in the photodissociation of SA at all excitation wavelengths is dominantly due to the reaction 4, namely, direct C
OH bond cleavage from the carboxylic group. 4.2. Threshold of OH Formation. Establishment of the energy threshold of OH formation from SA photolysis is of interest toward the fundamental understanding of the dynamics of its photochemical process. In the present experiments, the sample and the stainless-steel pipe were heated and maintained at 440 K, so the internal energy of SA should be considered. Due to the heated sample/He mixture being continuously expanded into the center of the photolysis cell, it was assumed that SA had not relaxed to lower than 390 K. Therefore, the internal energy of SA is calculated to be about 8.9
7.1 kcal/mol (440
390 K) at the B3LYP/6-311þG(d,p) level of theory. We get the threshold of OH formation from SA photolysis by comparing the LIF signal at the photolysis wavelength from 310.45 to 317.5 nm. The LIF intensity at these photolysis wavelengths is normalized with respect to the energies of the photolysis and probe lasers as well as the pressure in the photolysis cell. The strong LIF rotational line Q1 (4.5) (starred in Figure 1) of OH radical was chosen as the pointer for the PHOFEX spectrum of SA. As depicted in Figure 2, the LIF intensity of the pointer rotational line decreases slowly from 310.45 to 317 nm and disappears at 317.5 nm. The region between 317 and 317.5 nm is important for the photolysis of SA. The OH LIF signal was checked carefully from photolysis

of SA at 317 and 317.5 nm, and the LIF signal was very weak at 317 nm photolysis of SA. No significant OH LIF signals were observed with different photolysis laser energy density at 317.5 nm photolysis of SA. The lack of the OH LIF signal at 317.5 nm is possible due to the reduction in absorption coefficients of SA. The UV absorption spectrum of SA30 was examined carefully; however, the absorption coefficient is almost the same at both 317 and 317.5 nm photolysis wavelengths. Actually, the LIF signal of OH vanished when the photolysis wavelength turned to 317.5 nm in our experiments. Thus, 317 nm (90.2 kcal/mol) is attributed to the limit of OH formation from the photodissociation of SA. By adding the internal energy 8.9
7.1 kcal/mol to the photon energy of 317 nm, the energy threshold for producing OH radical from the photodissociation of SA is estimated to be about 98.2 ( 0.9 kcal/mol. For molecules like this size, the dissociation rate could be slow, especially when the photolysis photon energy is just slightly above the dissociation threshold. Therefore, for the given short delay time (15 ns) between pump and probe laser pulses in this work, the dissociation threshold represents the upper limit of OH formation from the photodissociation of SA. 4.3. Effect of the Phenolic OH Group on the Dissociation of SA. As the simplest member of the aromatic carboxylic acid family, benzoic acid has been investigated both experimentally and theoretically in recent years.10
15 Li et al.10 examined the potential energy surfaces of the low-lying excited states and proposed that the C
O bond cleavage begins at the T2 state and leads to the fragments of C6H5CO and OH, which is the most likely channel upon photoexcitation of benzoic acid at 270 nm or shorter wavelengths. Recently, our group did the corresponding experimental investigation by LIF technique12,13 with focusing on detection of the OH fragment. In the subsequent LIF experiment at 280
295 nm, we found that if the photolysis wavelength is shorter than 284.5 nm, benzoic acid has enough energy to pass the calculated S1 and T2 barriers10 to produce the 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: OH is produced only from the T2 state. In a very recent work, Fang et al.11 revisited the OH dissociation channel of benzoic acid photodissociation and especially discussed whether T2 is the exclusive state for the OH channel at 280
266 nm. Briefly, based on the experimental observation13 and theoretical calculations,11 it can be concluded that the R C
O fission from the S1 state is in competition with the fission from the T2 state upon photolysis at 266
284 nm while it mainly occurs at the T2 state at 284
294 nm, and the photon with a wavelength longer than 294 nm is unable to present the R C
O fission. Absorption of experimental excitation photons excites the parent molecule SA to the calculated S state16
18 upon photolysis wavelengths of 266, 315, 316, and 317 nm. As mentioned above, the observed OH product in SA photolysis at all the excitation wavelengths is dominantly due to the direct C
OH bond cleavage from the carboxylic group. By analogy to benzoic acid, it is reasonable to suggest that in the photodissociation of SA the excited triplet state T2 populated from the initial S1 state via intersystem crossing with an exit barrier is likely the most dominant electronic state to produce OH. In addition, the LIF signal of the OH Q1 (4.5) rotational line could be observed upon photolysis at 317 nm but not at 317.5 nm, which gives us reason to predict that the exit barrier height on the T2 state of SA is around 317 nm. Taking the internal energy of SA (8.9
7.1 kcal/mol) into account, the barrier height on the T2 state is estimated to be about 98.2 ( 0.9 kcal/mol. 5066

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The Journal of Physical Chemistry A The photochemical dissociation threshold of OH formation from SA is about 5 kcal/mol lower than that from benzoic acid photolysis. The phenolic OH obviously affects the threshold of OH formation in the aromatic carboxylic acid. The SA molecule has a strong intramolecular hydrogen bond between the phenolic hydroxy group and the nearby proton-accepting group. SA exists in several conformers with respect to the internal rotation of the carboxyl group and the orientation of hydroxyl.19,31,32 The intramolecular hydrogen bonding affects the stability. Considering two main rotational isomers identified by the experimental infrared spectra,31 the phenolic OH acts as a proton donor to the carbonyl group in rotamer I while the carboxylic OH is a proton acceptor in rotamer II. The intramolecular hydrogen bond in rotamer I will weaken the C
OH bond in the carboxylic group and produce a more stable 6-ring product if the C
OH bond was cleaved. The presence of H bonding in SA might play an important role in the photochemical dissociation threshold of OH formation. Considering the theoretical calculation by Yang19 et al., six rotamers are possible to exist in the present experimental condition (heated to 440 K) and it is difficult to distinguish the OH produced from which one. There is more than one structure in the excited state giving rise to OH formation, which may explain the curvature in the Boltzmann plot.

5. CONCLUSIONS The atmospherically important OH radical was detected from the photodissociation of SA in the gas phase at 266, 315, 316, and 317 nm employing the state-selective single-photon LIF technique. The relative quantum yield of OH formation was found to be dependent on the excitation energy. At all photolysis wavelengths, the nascent OH radicals were found to be vibrationally cold and their rotational state distributions did not show a Boltzmann population behavior. Similar rotational state distributions of the OH radicals from photolysis at different wavelengths were detected. Preferential population of the 2Π3/2 spin
orbit state of OH was observed, and the population of the Π(A0 ) Λdoublet state was predominant at each photolysis wavelength. Moreover, the LIF signal of the OH could be observed upon photolysis at 317 nm but not at 317.5 nm. Considering the internal energy of SA (8.9
7.1 kcal/mol), the threshold of OH formation from SA photodissociation is estimated to be about 98.2 ( 0.9 kcal/mol, which is about 5 kcal/mol lower than that from benzoic acid. Finally, it is concluded that the electronically excited SA mostly relaxes to the T2 state via intersystem crossing at all photolysis wavelengths, from which OH formation occurs via direct cleavage of the C
OH bond in the carboxylic group with an exit barrier in the reaction channel most likely. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by NSFC (21073185). C.-H.Z. sincerely wishes to express thanks to Dr. Ju-Long Sun for assistance in the experiments. ’ REFERENCES (1) Huber, J. R.; Schinke, R. J. Phys. Chem. 1993, 97, 3463.

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