Molecular halogen elimination from halogen-containing compounds in the atmosphere.

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Molecular halogen elimination from halogen-containing compounds in the atmosphere King-Chuen Lin* and Po-Yu Tsai Atmospheric halogen chemistry has drawn much attention, because the halogen atom (X) playing a catalytic role may cause severe stratospheric ozone depletion. Atomic X elimination from X-containing hydrocarbons is recognized as the major primary dissociation process upon UV-light irradiation, whereas direct elimination of the X2 product has been seldom discussed or remained a controversial issue. This account is intended to review the detection of X2 primary products using cavity ring-down absorption spectroscopy in the photolysis at 248 nm of a variety of X-containing compounds, focusing on bromomethanes (CH2Br2, CF2Br2, CHBr2Cl, and CHBr3), dibromoethanes (1,1-C2H4Br2 and 1,2-C2H4Br2) and dibromoethylenes (1,1-C2H2Br2 and 1,2-C2H2Br2), diiodomethane (CH2I2), thionyl chloride (SOCl2), and sulfuryl chloride (SO2Cl2), along with a brief discussion on acyl bromides (BrCOCOBr and CH2BrCOBr).

Received 15th November 2013, Accepted 18th February 2014

The optical spectra, quantum yields, and vibrational population distributions of the X2 fragments have been characterized, especially for Br2 and I2. With the aid of ab initio calculations of potential energies

DOI: 10.1039/c3cp54828g

and rate constants, the detailed photodissociation mechanisms may be comprehended. Such studies are fundamentally important to gain insight into the dissociation dynamics and may also practically help to

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assess the halogen-related environmental variation.

1. Introduction Atmospheric halogen chemistry has drawn much attention, because the emission of halocarbons into the troposphere may potentially cause severe damage to the stratospheric ozone layer. There are several catalytic cycles involving reactive free radicals of Cl, ClO, Br, and BrO that cause chlorine and bromine to destroy effectively stratospheric ozone.1–4 Iodine chemically coupled with emissions of bromine and chlorine is expected to account for the widespread depletion of lower stratospheric ozone below 20 km altitude.5 A short tropospheric lifetime of halocarbons can essentially prevent transport of Department of Chemistry, National Taiwan University, Taipei, and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan. E-mail: [email protected]; Fax: +886-2-23621483

King-Chuen Lin is a distinguished professor of the Department of Chemistry at National Taiwan University and a distinguished research fellow of National Science Council, Taiwan. He received his BS degree in Chemistry from National Taiwan University, Taiwan, his PhD in Chemistry from Michigan State University, USA, and his postdoctoral degree from Cornell University. His research interests are photodissociation and reaction dynamics in gas and condensed phases, atmospheric chemistry, and single molecule spectroscopy.

7184 | Phys. Chem. Chem. Phys., 2014, 16, 7184--7198

these source gases to the stratosphere and thus reduce the amount of reactive free radicals formed. Regarding their tropospheric lifetimes, for instance, trihalomethanes including CHBr2Cl, CHBrCl2, and CHBr3 were reported to last for about 120 days by the reaction with OH, whereas their lifetimes with respect to photolysis near the earth’s surface (0–5 km) were found to be 20–60 days.6 The way to remove these pollutants is mainly through the UV photodissociation processes. Therefore, understanding the related UV photochemistry is extremely important for the assessment of their environmental impact. In the halogen photochemistry, atomic halogen (X) elimination is recognized as the major primary dissociation process, whereas direct elimination of molecular halogen (X2) has been seldom discussed. By calculating the ozone loss rate either per inorganic bromine or chlorine atom, Daniel et al. suggested that the bromine is about 45 times more effective than chlorine Po-Yu Tsai is a postdoctoral fellow of the Department of Chemistry at National Taiwan University. He received his BS degree in Chemistry from Tunghai University and his PhD in Chemistry from National Taiwan University, Taiwan. His research focuses on energy transfer reactions in the gas phase, photodissociation dynamics of oriented molecules, and atmospheric chemistry.

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in global ozone destruction.7 Photochemistry of bromine-containing hydrocarbons has received much less attention compared to that of chlorine-containing hydrocarbons. Thus, this account is intended to stress on detection, characterization, and dissociation mechanisms associated with the Br2 primary elimination channel, but discuss briefly the Cl2 and I2 elimination processes. It has been a controversial issue whether the X-containing hydrocarbons may release the X2 primary products upon UV-light irradiation. Since the early 1960s, the CF2 product has been observed in the photolysis of C2F2Br2 at 248 nm, and was ascribed to a process of the Br2 elimination.8,9 But the Br2 moiety has not been detected directly. In contrast, the same molecule at the photolysis wavelengths of 223–260 nm was studied using a crossed laser-molecular beam technique, suggesting that (1) a single C–Br bond fission is the only primary reaction channel, and (2) CF2 should be accompanied by two Br fragments.10,11 The photodissociation of SOCl2 is another example. The Cl2 + SO channel was verified following irradiation at 248 nm by observation of a mass/charge ratio of 70 using photofragment translational spectroscopy (PTS).12 In contrast, a recent investigation found no evidence of such a channel (SOCl2 - SO + Cl2) using three-dimensional imaging of photofragments performed at 235 nm.13 Even in the photolysis of halogen-substituted ethylenes, the molecular halogen elimination has been rarely characterized.14–20 For instance, Sato et al. have observed HCl, Cl, C2H2, C2HCl, and C2Cl2 fragments in dichloroethylenes and trichloroethylenes at 193 and 157 nm.14 He et al.18 and Chandra et al.19 have detected H, HCl, Cl(2P3/2), and Cl*(2P1/2) in 1,1- and 1,2-C2H2Cl2 at 193 nm and other UV wavelengths. But the prior studies have never found the Cl2 fragments. From a theoretical point of view, Morokuma and coworkers20 suggested that a high energy barrier might impede the release of the Cl2 product. In 1,2-dibromoethylene at 248 nm by PTS,21 Lin and coworkers claimed to have found two exclusive dissociation channels of Br2 + C2H2 and Br(fast) + Br(slow) + C2H2 with a branching ratio of B0.2 : 0.8. However, such a Br2 channel could not be observed using velocity ion imaging by our group,22 nor by the Lipson group over a different range of laser wavelengths.23 It is difficult to detect the neutral X2 by using the resonanceenhanced multiphoton ionization (REMPI) technique. For instance, the REMPI detection of the Br2 signal suffers from (1) competition of Br2+ interference resulting from dissociative ionization,24 e.g., RBr2 + 2hv - RBr2+ + hv - R + Br2+, where R denotes a hydrocarbon, and (2) partial Br2 dissociation that may be caused concomitantly by the UV light applied.25 Even using a laser-induced fluorescence (LIF) technique, it is inefficient to probe Br2 in the B3Pou+ ’ X1Sg+ transition, due to a low fluorescence quantum yield caused by predissociation with a repulsive C1P1u state.26 In contrast, as an emerging absorption technique, cavity ring-down absorption spectroscopy (CRDS) turns out to be superior to most spectroscopic techniques in detecting the halogen molecules.27 We have recently taken advantage of the CRDS method28 to probe the X2 (Cl2, Br2, and I2) fragments in a series of haloalkanes, haloalkenes, and acyl halides. Such investigation has


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an important impact on fundamental understanding of the dissociation dynamics and practical help with the photochemical assessment of halogen-related environmental issues. This account is intended to review the CRDS detection of the Br2, I2, and Cl2 primary products in the photolysis at 248 nm of a variety of halogen-containing compounds, focusing mainly on bromomethanes (CH2Br2, CF2Br2, CHBr2Cl, and CHBr3), dibromoethanes (1,1-C2H4Br2 and 1,2-C2H4Br2), dibromoethylenes (1,1-C2H2Br2 and 1,2-C2H2Br2), iodomethane (CH2I2), thionyl chloride (SOCl2), and sulfuryl chloride (SO2Cl2), along with a brief discussion on acyl bromides (BrCOCOBr and CH2BrCOBr). The optical spectra, quantum yields, and vibrational population distributions of the molecular halogen elimination will be characterized. With the aid of calculations of ab initio reaction pathways and rate constants, we can gain insight into the detailed photodissociation mechanisms.

2. Experimental The CRDS apparatus used for the photodissociation study is described elsewhere.29–31 The radiation sources are composed of a 20 ns-pulsed excimer laser emitting at 248 nm for photolysis of precursors and a 5–8 ns-pulsed Nd:YAG laser-pumped dye laser (482–524 nm) used to probe the released X2 fragment in the B3Pou+ ’ X1Sg+ transition.29–33 The energies of photolysis and probe lasers were controlled in the range of 8–28 and 0.5–1 mJ, respectively. The photolysis laser beam was focused with a 30 cm focal-length cylindrical lens onto a four-armed stainless steel ring-down cell at right angle to the cavity, while the probe laser beam was injected along the cavity axis after a 20–30 ns delay (Fig. 1). The two beams were overlapped in the center of the flow cell. The volume of the overlapping region was evaluated by multiplication of the beam width and height of the photolysis laser and the beam diameter of the probe laser, corresponding to (18 1 3) 5 mm3. In order to remain mostly in the TEM00 mode, the probe beam was guided through a spatial filter made of a pair of lenses of 10 cm and 5 cm focal length and a pinhole with 70 mm diameter, prior to injection through the front mirror of the ring-down cell.

Fig. 1 Schematic diagram of the stainless steel reaction chamber adopted for cavity ring-down absorption spectroscopy.

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A photomultiplier tube was positioned behind the rear mirror to detect the intensity of the light pulse leaking out of the mirror. The temporal profile of the ring-down signal was recorded on a transient digitizer. The ring-down time constant for each laser pulse was determined by a best fit to the acquired exponential decay. The precursors were purified by repeated freeze–pump–thaw cycles at 77 K and then flowed through the ring-down cell at a pressure of 10–500 mTorr monitored by an MKS pressure gauge. The X2 absorption spectra, with a spectral resolution of 0.1 cm1, were acquired with the aid of a lab-developed program based on a Matlab environment. For the temperaturedependence measurements, the whole chamber was wrapped with a heating tape and the varied temperature was monitored by a thermocouple positioned nearby the center region.

3. Results and discussion 3.1.

Photodissociation of Br-substituted methanes

3.1.1. Br2 production. CH2Br2 is one of the major brominecontaining organic compounds, contributing up to 20% of Br atoms into the upper troposphere upon UV-light irradiation.34 But the work on its photodissociation dynamics is limited.35–37 CF2Br2 (Halon 1202) is anticipated to yield three major dissociation channels of CF2Br + Br, CF2 + 2Br, and CF2 + Br2 that are energetically attainable in the ultraviolet photolysis.11 But it has not been confirmed whether a primary dissociation pathway can produce CF2 + Br2. By using femtosecond pump– probe spectroscopy, Dantus and coworkers investigated the UV-multiphoton dissociation of several dihalomethanes,36,37 obtaining the halogen products lying in the excited states. The related mechanisms essentially differ from those found in the nanosecond domain. Bromoform is another long-lived source of atmospheric bromine, the UV photodissociation of which results in the fragments of HBr, CBr, CHBr, CHBr2, Br, and Br2.38 North and coworkers38 using the PTS technique demonstrated that the Br2 product originates from a secondary dissociation of CHBr2, whereas Xu et al. using a velocity ion imaging technique claimed the observation of the Br2 primary channel in the 234 and 267 nm photolysis.39 But they measured indirectly the CHBr and CHBr2 intensities that were used to determine the branching ratios of Br2 and Br elimination. This account reports the first finding of the Br2 primary product in the CRDS experiments. For comparison, another three-halogensubstituted methane CHBr2Cl was selected for the CRDS experiments. Since the bond dissociation energy of C–Cl is 302 kJ mol1 forming CHBr2 + Cl with respect to the C–Br dissociation energy of 247 kJ mol1 into CHBrCl + Br,29,40 the Cl-atom loss should be significantly less than the Br-atom loss at 248 nm with the A band excited and thus the interference from the secondary photodissociation of CHBr2 is minimized while detecting the Br2 molecular product. However, it should be noted that when the B band is excited, the efficiency of Cl- and Br-atom loss might be inverted. For instance, for photodissociation of CH2BrI, excitation at 248 nm within the A absorption band preferentially leads to the C–I bond cleavage,


Fig. 2 A portion of Br2 spectra acquired in the photolysis of BrCOCOBr at 248 nm. (a) Trace acquired experimentally for the bands of v = 0, 1, and 2 contained in the 534–535 nm region, (b) the simulated counterpart with the population ratio of Br2(v = 1)/Br2(v = 0) fixed at 0.65 and Br2(v = 2)/ Br2(v = 0) optimized to 0.34, (c) simulated counterpart with the transition involving only v = 0, (d) simulated counterpart with the transition involving only v = 1, and (e) simulated counterpart with the transition involving only v = 2.

while excitation at 210 nm within the B band leads predominantly to the C–Br bond cleavage.41,42 The CRDS spectra of Br2 are acquired at room temperature,29–31 containing the Br2 v = 0 transition from the bands (35,0) to (44,0) in the 515.5–518 nm range, and both v = 0 and 1 transitions from (30,0) to (33,0) and from (36,1) to (44,1) in the 522–524 nm range. The Br2 signals, free from interference of other species, disappear when the photolysis laser is off. The CRDS spectra for v = 2 were not observed for these molecules, but acquired for BrCOCOBr in the 534–535 nm region, as shown in Fig. 2.31 The spectral assignment is made according to the report by Barrow et al.43 The Br2 (v = 0, 1, and 2) spectral intensities were simulated27,31 and the results for the vibrational population ratios are listed in Table 1. The Br2 fragment obtained in each molecule is essentially vibrationally hot, according to the Boltzmann law. The details of spectral simulation are described in the following. The rotational and vibrational constants of the 79Br2 isotopic variant were determined by Gerstenkorn and Luc.44 The molecular constants of 79,81Br2 and 81Br2 are then evaluated based on the isotope ratio. Given these molecular constants of the X1Sg+ and B3Pou+ states for all the isotopic variants, the Br2 CRDS spectra may be assigned accurately. The Br2 (v = 0) spectral

Table 1 Quantum yields and vibrational population ratios of various multi-Br-substituted hydrocarbons

Quantum yield CHBr3 CF2Br2 CH2Br2 CHBr2Cl 1,2-C2H2Br2 1,1-C2H2Br2 1,2-C2H4Br2 1,1-C2H4Br2

0.23 0.04 0.2 0.05 0.12 0.07 0.36 0.05

0.05 0.01 0.1 0.03 0.1 0.04 0.18 0.03

v = 1/v = 0 ratio 0.8 0.4 0.7 0.5 0.7 0.55 0.8 0.5

0.2 0.2 0.2 0.2 0.2 0.05 0.1 0.2


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intensities I can be further simulated according to the following equation:


I ¼k

ðFCFÞðHLFÞ NJ 00 riso f 2J 00 þ 1

(1)

where k is a factor associated with instrument and experimental ¨nl– conditions, FCF the Franck–Condon factor,45 HLF the Ho London factor, J 00 the rotational quantum number of the ground state Br2, NJ 00 the Boltzmann distribution of the rotational population for which 300 K is assumed, because the rotational population has been thermally equilibrated during a long ringdown time. riso the ratio of isotopic variants, 79Br2 : 79,81Br2 : 81Br2 equal to 0.2569 : 0.4999 : 0.2431, and f the intensity ratio of strong and weak rotational lines. Because the nuclear spin of Br is 3/2, f becomes 5/3 for the intensity ratio of odd to even rotational quantum number.46 Given BrCOCOBr as an example, while taking into account all the band transitions involved with the same line width of 0.1 cm1 and the related spectral intensities by eqn (1), a spectral simulation for the Br2 (v = 0) population is obtained in the 515.5–518 nm range. Then, a theoretical counterpart is analogously obtained in the 522–524 nm range by adjusting the Br2(v = 1)/Br2(v = 0) population ratio. As the Br2(v = 1)/Br2(v = 0) ratio is fixed, a simulated spectrum in the 534–535 nm region is optimized by adjusting the population ratio of Br2(v = 2)/Br2(v = 0). In this manner, the theoretical counterpart in Fig. 2 is obtained, showing consistency with the experimental findings. The quantum yield of the Br2 channel can be determined as follows: f¼

½Br2 ; Np

(2)

where [Br2] indicates the Br2 concentration produced in the beam-crossed region of photolysis/probe lasers and Np is the photon number density absorbed in the same region. The Br2 concentration is evaluated by a ratio of a/sBr2, in which a is the absorption coefficient and sBr2 the absorption cross section.27,47 a is determined by the ring-down time measurements, d 1 1 ; (3) a¼ cl t t0

addition to vibrational population ratios. The Br2 yield in CHBr3 equals 0.23 0.05 which is overestimated,48 containing partial contribution from the secondary dissociation of CHBr2.38 In comparison, the quantum yield in CHBr2Cl is 0.05 0.03.29 While considering the structural analog, this value may well be taken as the lower limit of the Br2 yield in CHBr3. Between these two compounds, a higher photolyzing laser energy is required to decompose CHBr3, implying that a large fraction of the Br2 product comes from multiphoton absorption processes. In CF2Br2, the quantum yield is 0.04 0.01. Troe and coworkers have used the IR multiphoton absorption technique to probe the Br2 channel with a branching ratio of 0.10–0.25, depending on the IR laser energy.49 This range may be considered as an upper limit of the Br2 yield at 248 nm. 3.1.2. Verification of the primary product of Br2. To inspect the possibility of atomic Br recombination during the acquisition period with a ring-down time o1 ms, a single-Br-containing substitute CCl3Br or CH2ClBr with a comparable absorption cross section was photolyzed.50 None of the Br2 fragments can be detected, even with increased pressure and prolonged photolysisprobe delay time (Fig. 3). To verify the primary product of Br2, two further experiments involving laser power and pressure dependence were performed. A straight line is obtained in a plot of the rotational intensity versus the incident laser energy, indicative of a singlephoton involvement in the molecular elimination. Each plot for CH2Br2, CF2Br2, and CHBr2Cl has taken into account the origin point in the linear regression fit, thereby ruling out the effect of optical saturation or partial saturation. The related plots for CH2Br2 as an example are shown in Fig. 4. However, for CHBr3 the line cannot be extrapolated through the origin; a partial contribution from the CHBr2 secondary dissociation should be significant, as described above.38 The pressure-dependence measurements also yield a straight line, indicative of the Br2 production from a single molecular photodissociation. The linearly dynamic range of the pressure dependence differs for each molecule studied. When the pressure keeps increasing, the Br2 product intensity becomes level-off and rapidly tends to decrease. The complicated high pressure phenomena might be caused by quenching of the excited state population of the

where d is the distance between two reflective mirrors; l the optical length of the absorber; c the light speed, and t and t0 are the ring-down times of Br2 as a result of the probe laser wavelength in-resonance and off-resonance, respectively. Meanwhile, the absorbed photon number density can be evaluated by using the following equation, Np ¼

Ein Eout ; hvDV

(4)

where h denotes Planck’s constant, n is the radiation frequency, DV is the volume of the beam-crossed region, and Ein and Eout indicate the beam intensity of incoming and outgoing radiation for photolysis. Accordingly, the Br2 quantum yields for the molecules studied are evaluated and the results are listed in Table 1, in


Fig. 3 Detection of CRDS spectra of Br2 following 248 nm photolysis of (a) CCl3Br with a pressure of 2.2 Torr and the photolysis-probe delay time of 80 ns and (b) CF2Br2 with a pressure of 0.5 Torr and a delay time of 20 ns.


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Fig. 4 (a) Line intensity of Br2 rotation at 519.68 nm eliminated from CH2Br2 versus the incident laser energy. The origin point is taken into account for the linear regression fit. The same plot with the corresponding number density of Br2 against the absorbed photon density yields a slope, indicative of the quantum yield of 0.2 0.1 for the channel of molecular elimination. (b) Line intensity of Br2 rotation at 519.68 nm versus the CH2Br2 pressure from 0.7 to 3.1 Torr.

precursor prior to photodissociation, quenching of the ro-vibrational population of the produced Br2, and the possibility of secondary reactions. The Br2 fragment may be alternatively produced by the dissociated Br atoms in reactions with the precursor. A single atomic halogen elimination has been well known to dominate the UV-decomposition channels of the precursors studied.51–53 Thus, the resultant Br yields should be so large that the pseudofirst order reaction cannot be applied to the above reactions. In this sense, the Br2 production requires two-precursor involvement, which is against the linear pressure-dependence measurements. Furthermore, such a reaction is endothermic with a slow reaction rate, because a large C–Br dissociation energy is accompanied by a small Br2 formation energy in the bond breaking/formation process. Alternatively, a further experiment may be carried out by adding a Br scavenger in the ring-down cell. Given BrCOCOBr as an example,31 the Br reaction with


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diethyl ether acting as the Br scavenger has a larger reaction rate constant than with BrCOCOBr. If the added amount of diethyl ether exceeds the precursor, then the generated Br fragment must be largely consumed with little chance to proceed via the Br + precursor reaction. The resulting Br2 spectra do not show any suppression by the Br scavenger, thus indicative of an insignificant Br2 contribution from such a secondary reaction. However, it is not easy to find an appropriate Br scavenger, because it may concomitantly quench the excited precursor rapidly to reduce the efficiency of Br2 elimination.27 There is one more possibility to generate Br2 through dissociation of adducts which may be formed between halogen atoms and haloalkanes. Enami et al.54 have claimed the observation of adducts in the reaction of Cl with XCH2I (X = H, CH3, Br, Cl, and I) in 25–125 Torr of N2 diluent at 250 K. Sehested et al.55 also found two reaction pathways in the reaction of F with CH3Br, yielding (1) CH2Br and HF, and (2) the adduct CH3Br F. The F atoms were produced using pulse radiolysis of 1 atm of SF6. If the adducts further undergo secondary decomposition, the halogen molecule is possibly produced. However, under our experimental conditions, Br2 could not be generated via dissociation of such adducts. First, the adduct formation requires a high pressure matrix added for stabilization by termolecular collisions.54,55 Second, the subsequent elimination of Br2 has a very small rate, because of endothermicity. Third, if the adducts require one additional photon to release Br2, two-photon absorption is necessary to form one Br2 molecule that is against our observation of laser energy dependence. 3.1.3. Photodissociation mechanisms. How does the Br2 elimination occur in the photolysis of these brominecontaining molecules? It has been well recognized that the first UV-absorption band (n, s*) of bromo-alkanes at 248 nm readily dissociates a single Br atom.51,52 It is hard to eliminate a Br2 molecule from this excited state. For instance, the excited state of CH2Br2 promoted at 248 nm has a 11,3B1 symmetry.56 Such excited molecules are symmetry-forbidden to eliminate Br2, without undergoing nonadiabatic transition to the ground potential surface. As the C–Br bond is elongated, the CH2Br2 structure changes from a C2v to Cs symmetry and the subse˜ 1A 0 coupling becomes strengthened to facilitate quent 11A 0 –X the internal conversion (IC). A small fraction of population may have a chance to transit to the higher vibrational levels of ˜1A 0 prior to the C–Br bond fission. The the ground state X experiments of the temperature effect on Br2 elimination may support the above photodissociation mechanism. For the molecules studied, the intensities were enhanced from 5% to 16% within the temperature increment from 20 to 50 1C.29,56,57 As the temperature increases, higher levels of excited states are populated such that the increased density of states may facilitate the total rate of the level-to-level coupling, resulting in a more efficient IC process. The detailed photodissociation mechanisms can be understood with the aid of potential energy surface (PES) calculations. The Br2 and probable dissociation channels on the adiabatic singlet ground-state (or first triplet state for some cases) PES for each compound are characterized. The geometries


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and the harmonic frequencies of reactants, transition states (TS), and products are obtained at the level of the hybrid density functional theory, B3LYP/6-311G(d,p); the energies are further refined by the coupled cluster CCSD(T)/cc-pVTZ with B3LYP/6-311G(d,p) zero-point energy corrections. The vertical energies of the lowest excited states with respect to the ground state are determined by the Davidson-corrected multi-reference configuration interactions with single and double excitation (MRDCI) calculations, in which the cc-pVTZ basis functions are employed. The core electron correlation is not involved, because of its minor effect on the relative energy.58,59 1,1-dibromoethylene is examined herein via a calculation level of full-electron CCSD(T)/ Sapporo-DZP-201260 showing a relative energy deviation less than 1.3 kcal mol1 with respect to a frozen core approximation, whereas the consumed CPU time is four times more. For simplicity, the spin–orbit splitting of the Br atom is not taken into account for the ab initio calculations, but this would not have any effect on the mechanism proposed for the Br2 elimination. In the photodissociation of CHBr2Cl,29 the TS correlates to the CHCl + Br2 products, following a reaction coordinate of the ground state PES. For the optimized TS structure, the C–Br bond lengths are elongated asymmetrically and the Br–Br distance is largely stretched. Thus, a mechanism of asynchronous photodissociation is favored and the Br2 population is vibrationally hot. CH2Br2 follows a similar mechanism to produce vibrationally hot Br2 fragments.56 In contrast, a sequential photodissociation mechanism is favored in CF2Br2.57 That is, a single C–Br bond breaks first, and then the free Br atom moves to form a Br–Br bond in the TS, followed by the Br2 elimination. Reid and coworkers61 have extensively calculated the dissociation pathways for multi-halocarbons including CF2Cl2, CF2Br2, and CHBr3, and demonstrated that the molecular products are dominated by the pathway through such an isomerization TS carrying a halogen–halogen bond. For instance, the decomposition of CF2Cl2 via an isomerization TS CF2Cl Cl gives rise to a 3% Cl2 yield in appreciable agreement with the findings in infrared multiphoton dissociation reported by Lee and coworkers.62 In CHBr3, the three-center TS reported previously48 lies at B139 kJ mol1 higher than the calculated isomerization barrier HBrCBr Br.61 Thus, the pathway to the Br2 product mainly proceeds via this lower isomerization TS. A similar isomerization/photodissociation process has been found in dihalomethanes in the condensed phase. Phillips and co-workers demonstrated that iso-CH2I–Br is mainly responsible for the transient absorption spectrum following either A-band or B-band excitation of bromoiodomethane in cyclohexane solution in the nanosecond resonance Raman experiments.63 The UV excitation leads to a fast C–I or C–Br bond fission, followed by recombination to form iso-CH2Br–I or iso-CH2I–Br within the solvent cage. But only the latter photoproduct can be detected during the nanosecond period, because iso-CH2I–Br species is stable with a longer lifetime than iso-CH2Br–I. In contrast, the former photoproduct was mainly observed under solid matrix at 12 K following the A-band excitation,64,65 since the elongated C–I bond breaks rapidly into CH2Br + I, followed by recombination due to the cage effect.


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Note that the isomerization photoproducts detected in the condensed phase have a mechanism different from the isomerization TS along the dissociation pathway in the gas phase.61 3.2.

Photodissociation of Br-substituted ethanes

The isomeric forms of 1,1- and 1,2-dibromoethanes are photolyzed at 248 nm, both yielding the Br2 fragments on the ground state PESs. Nevertheless, their photodissociation processes differ distinctly from each other.30 The Br2 yield and branching ratio of Br2(v = 1)/Br2(v = 0) in 1,2-dibromoethane is 0.36 and 0.8 0.1 in contrast to 0.05 and 0.5 0.2 in 1,1-dibromoethane. In contrast, the Br2 number density increases by a factor of 35% versus 190%, when the temperature is increased from 34 to 52 1C. The discrepancy can be interpreted with the aid of the PES calculations. As shown in Fig. 5 and 6, the TS energies lie at 334.1 and 492.4 kJ mol1 for 1,2- and 1,1-dibromoethanes, respectively. The former state is well below the excitation energy at 483 kJ mol1 (248 nm), whereas the latter is slightly above. Their excited density of states and the IC efficiency are approximately equal. The production rates are thus determined mainly by the transition barrier. Because a small energy barrier impedes the photodissociation of the ground state 1,1-dibromoethane, the Br2 yield becomes relatively low, but rises rapidly with the temperature according to Arrhenius theory. In 1,2-dibromoethane, Br2 is anticipated to eliminate via a four-center mechanism with the Br–Br bond distance of 2.904 Å in the optimized TS structure, whereas in 1,1-dibromoethane Br2 is via a three-center elimination with the bond distance of 2.543 Å. The difference of the Br–Br stretch bond might explain why the former isomer leads to a hotter Br2. 3.3.

Photodissociation of Br-substituted ethylenes

The prior studies of photodissociation mechanisms of haloethylenes,66–69 denoted as C2H4–nXn, yield at least three dissociation channels. They are (1) four-center elimination to release HX, H2, HCCH or HCCX, (2) three-center elimination to release HX, H2, HCCH or HCCX via vinylidene or halovinylidene intermediates, and (3) either H or X migration to form haloethylidene, followed by three-center elimination. Among these mechanisms, molecular halogen elimination has never been reported except for 1,2-dibromoethylene but yielding the skeptical results with the PTS technique.21 In contrast, as with the cases of bromoalkanes, the CRDS spectra of Br2 (v = 0 and 1) can be readily acquired at 248 nm in the isomeric forms of 1,1- and 1,2-dibromoethylenes.70,71 The spectrum obtained in 1,1-C2H2Br2 is given as an example (Fig. 7). All possible examinations were carried out to ensure that the obtained Br2 fragments stem from the primary photodissociation occurring on the ground state surface. As dibromoethylenes are excited to the (p, p*) state at 248 nm, the mutually twisted BrCBr (or HCBr) and HCH (or HCBr) groups may enhance the electronic coupling to facilitate the IC process.72 The ab initio potential energy calculations help understand the detailed photodissociation mechanisms. As shown in Fig. 8, 1,1-dibromoethylene follows essentially four


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Fig. 5 The dissociation channels of 1,2-C2H4Br2, in which the energies in kJ mol1 relative to 1,2-C2H4Br2 are computed with CCSD(T)/cc-pVTZ level of theory with B3LYP/6-311G(d,p) zero-point energy corrections at B3LYP/6-311G(d,p) optimized geometries. The vertical energies of 1,2-C2H4Br2 excited states are computed with MRDCI/cc-pVTZ at B3LYP/6-311G(d,p) optimized ground state geometry.

Fig. 6 The dissociation channels of 1,1-C2H4Br2, in which the energies in kJ mol1 relative to 1,1-C2H4Br2 are computed with the methods identical to the calculations for 1,2-C2H4Br2 in Fig. 5.

pathways to the products of HCCH + Br2.71 The route (a) proceeds along i4 (1,1-C2H2Br2) - TSi4-Br2 - HCCH + Br2. The route (b) follows i4 - TSi2-i4 - TSi3-Br2(b-2a) or TScis-Br2(b-2b) - HCCH + Br2,


in which TSi3-Br2(b-2a) is an isomerization TS lying at 105 kJ mol1 less than TScis-Br2(b-2b). The route (c) is along i4 - TSi3-i4 TSi3-Br2 - HCCH + Br2, sharing the same isomerization


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Fig. 7 The CRDS spectra of Br2 v = 0 and 1 obtained from photolysis of 1,1-C2H2Br2 at a pressure of 0.7 Torr. Spectral simulation takes into account the population (a) only in the v = 0 level, (b) only in the v = 1 level, and (c) in both the v = 1 and 0 levels with a branching ratio of 0.55, in comparison with (d) the experimental results.

Fig. 8 The Br2 dissociation channels of 1,1-C2H2Br2 on its adiabatic singlet ground state PES, in which the energies in kJ mol1 relative to 1,1-C2H2Br2 are computed with CCSD(T)/cc-pVTZ level of theory with B3LYP/6-311G(d,p) zero-point energy corrections.

TS with the route (b-2a). Accordingly, the three-center elimination of Br2 via TSi3-Br2 along the routes (b-2a) and (c) are energetically more favored. The Br–Br bond distance in the TSi3-Br2 structure is largely stretched, thereby Br2 may be produced vibrationally hot. Similarly, in 1,2-C2H2Br2 the three-center mechanism via isomerization TSi3-Br2 to produce vibrationally hot Br2 prevails over a four-center elimination from the cis-isomer.70 The former pathway shows a dissociation rate constant about 30 times faster than the latter calculated by the Rice–Ramsperger– Kassel–Marcus (RRKM) method.70,73 The RRKM rate constants were calculated on singlet ground state (or the first triplet state) PESs of the molecules studied. If the rate of the energy equilibration is faster than the reaction rate, the rate constant can be explained statistically. Given total


energy conservation throughout the reaction, for an unimolecular k

reaction A ! Aa ! P, where A* is the energized reactant, Aa the transition state, and P the products, the rate constant k(E) as a function of energy is expressed as kðEÞ ¼

s W a ðE E a Þ ; h rðEÞ

(5)

where s is the symmetry factor, Wa the number of states of the TS, and r the density of states of the reactant. The saddle-point method74,75 is applied to evaluate the number of states and the density of states; the molecule is viewed as a collection of harmonic oscillators. The geometries and the harmonic frequencies of the reactant, TS, and products were essentially


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obtained at the level of the hybrid density functional theory, B3LYP/6-311G(d,p); the energies are further refined by the coupled cluster CCSD(T)/cc-pVTZ with B3LYP/6-311G(d,p) zero-point energy corrections.71 For some cases, the Br2 production may not be sufficiently interpreted by the pathways constructed and the subsequent RRKM calculations based on the statistical evaluation. CH2BrCOBr as an example gave rise to the Br2 yield of 0.24 0.08 in photodissociation at 248 nm,27 about 10 times larger than that by the RRKM calculations while taking into account the other competitive dissociation channels. Therefore, an alternative mechanism might be proposed such as roaming pathway.76–84 In this photodissociation process, recoiling atoms or radicals, weakly bound to the other moiety, can meander and finally undergo intramolecular abstraction forming molecular products. Based on the dissociation pathways (Fig. 9),27 a fraction of CH2BrCOBr molecules may undergo a radical dissociation producing CH2BrCO + Br at 292.5 kJ mol1 via internal conversion. The barrierless radical channel lies in an energy state in proximity to the TS at 294.0 kJ mol1 associated with the route to Br2 + CH2CO; such a condition is essentially favored for the roaming occurrence. The Br atom, carrying a small translational energy released from this barrierless channel, may then be feasibly attracted around a wide flat region of the Br BrCH2CO PES prior to abstraction of another Br atom on CH2BrCO. Such a roaming saddle point Br BrCH2CO

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appears to have a structure similar to the isomerization TS that carries a low energy barrier to form the halogen molecules, as described above.61–65 For the Br2 product generated from this process, the related laser energy and pressure dependence each is expected to yield a slope of unity that agrees with our CRDS findings. Therefore, partial contribution of Br2 from this roaming pathway might account for a larger quantum yield observed. 3.4.

Photodissociation of I-substituted methane

Diiodomethane (CH2I2) as an example in photodissociation of I-substituted methane contains at least three dissociation channels of CHI + I(2P3/2), CHI + I*(2P1/2) and CH2 + I2 in the UV excitation. The lowest UV absorption spectra of CH2I2 split into five overlapping states with the 1B1, 2B1, 1B2, 1A1, and 2A1 symmetry, in the order of increasing energy.85–88 Among them, the 1B1 and 2A1 states are correlated with the dissociation channel of CH2I + I(2P3/2), while the remaining states are all correlated with CH2I + I*(2P1/2).88 CH2I2 is dominated by the dissociation channel of CH2I and I (or I*). Thus, its photodissociation dynamics is actively focused on determination of quantum yields of released iodine atoms,86,89 translational energy distribution,85,87,88 internal energy deposition in the radical,86 and anisotropy parameter measurements.85,87,88 Thus far, the I2 elimination from CH2I2 was reported in the vacuum ultraviolet region via either single- or multi-photon

Fig. 9 The dissociation pathways of CH2BrCOBr, in which the energies in units of kJ mol1 relative to the ground state are computed with CCSD(T)/ cc-pVTZ level of theory with B3LYP/cc-pVTZ zero-point energy corrections at B3LYP/cc-pVTZ optimized geometries.



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excitation, and only the excited states of I2 were observed in the emission measurements. By using a Kr(1165, 1236 Å) lamp, Okabe et al.90 obtained a primary fragment I2(3P2g) that was vibrationally excited up to v = 35 with a quantum yield of 0.006 at 1236 Å, but little vibrational excitation was found at 1306 Å. Donovan and coworkers91 observed fluorescence from F1Su+ and D1Su+ states by using multiphoton excitation at 248 and 193 nm. Marvet and Dantus92 verified a concerted elimination of I2(3P2g) in less than 100 fs by two-photon excitation at 310 nm using pump–probe femtosecond spectroscopy. The ground state I2 product with the ultraviolet excitation has never been observed, although this channel was suspected to probably exist by Leone and co-workers.86 The ground state I2 product and dissociation mechanism in a single-photon dissociation of CH2I2 at 248 nm was characterized for the first time by using CRDS.93 Since the CRDS experiments were carried out at room temperature, a cluster formation like (CH2I2)2 is negligible. Such a cluster-induced photochemistry was investigated in CH3I prepared in a jetcooled temperature, thereby leading to the I2 product following (CH3I)2 photodissociation.94 CH3I was also found to produce electronically excited B states of I2 on a MgO(100) surface, due to multi-photon excitation followed by the reaction of I* + I - I2(B) on the surface.95 As shown in Fig. 10, a portion of CRDS spectra of I2 (v = 0,1,2) in the B3Pou+ ’ X1Sg+ transition in photolysis of CH2I2 at 248 nm contains the v = 0, 1, and 2 bands corresponding to (39,0)–(56,0), (44,1)–(62,1), and (49,2)–(50,2).32 Because the I2 product is heavy and easily stick onto the chamber wall, such experiments were conducted with an increased pumping speed to make sure no memory effect from the residual I2. The CRDS spectrum is accompanied by a simulated counterpart based on a similar equation as in eqn (1) (Fig. 10). The spectral positions and rotational intensities are comparable between them, although the signal quality is worsened by the background noise. Accordingly, the vibrational population ratio of v(0) : v(1) : v(2) is determined to be 1 : (0.65 0.10) : (0.30 0.05), corresponding to a Boltzmannlike vibrational temperature of 544 73 K with a square of

Fig. 10 A portion of I2 spectra (v = 0, 1, and 2) acquired in the photolysis of CH2I2 at 248 nm. (a) denotes trace acquired experimentally, while (b) is a simulated spectrum.


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regression coefficient, R2 = 0.98. The I2 product was further verified to result from a primary dissociation channel following one-photon absorption of CH2I2 at 248 nm.93 The quantum yield f of the I2 elimination channel can be determined by comparison with a reference sample of CH2Br2 by the following equation:93 fI2 ½I2 sCH2 Br2 nCH2 Br2 ¼ : fBr2 ½Br2 sCH2 I2 nCH2 I2

(6)

Given the halogen concentrations determined by eqn (3), the quantum yield of Br2,56 the absorption cross sections of sCH2I2 and sCH2Br2 equal to 1.6 1018 (ref. 96) and 3.7 1019 cm2 (ref. 56) at 248 nm, respectively, and the intensity ratio of the halogen molecules along with the individual absorption cross section, the quantum yield of I2 dissociated from CH2I2 is evaluated to be 0.0040 0.0025. The obtained quantum yield happens to be as small as those of the excited state I2,90 but they have different dissociation pathways. Fig. 11 shows two pathways for the I2 elimination. Route (1) encounters a TS (tsI2) along the PES at 407.0 kJ mol1 above the ground state CH2I2, while route (2) goes through a TS (tsCI) at 191.5 kJ mol1 and an intermediate state at 179.3 kJ mol1 before reaching the products of I2 + CH2. The tsI2 structure shows the character of a C1 symmetry, with the I–C–I angle suppressed to 60.21 and the C–I bond lengths elongated asymmetrically to 2.578 and 3.017 Å, such that the associated I–I moiety may be released. Another tsCI structure has a Cs symmetry with the I–C–I angle of 70.81 and the two C–I bond lengths changed to 1.991 and 3.460 Å. The tsCI structure is similar to an isomerization TS in which a X–X bond is formed before reaching the products, like the cases of CF2Cl2 and CF2Br2.57,61 Note that in this work the geometries and the harmonic frequencies of the reactant, intermediate, TS, and products are obtained at the level of B3LYP/midix,97 instead of B3LYP/6-311G(d,p) adopted in the Br-related systems; the energies are further refined by the coupled cluster CCSD(T)/ midix with B3LYP/midix zero-point energy corrections. The attempts to locate the TS for I elimination by B3LYP were not successful, the multi-configuration approach, CASSCF/midix, was employed instead. The basis set midix can be appropriately utilized for a moderate size like I, and succeed to predict a reasonable I2 bond length. According to the RRKM method, the dissociation rate constants for the routes (1) and (2) are calculated to be 2.41 109 and 3.52 1011 s1, respectively. Thus, I2 is favored to eliminate via an asynchronous threecenter mechanism. As compared to the I2 equilibrium bond length of 2.727 Å, the dominant pathway via tsCI with a I I distance of 3.379 Å is anticipated to produce vibrationally excited I2, in agreement with the observation. As compared to CH2Br2, the excited states of CH2I2 are stabilized to cause a red shift in the excitation transition. Thus, CH2I2 is excited at 248 nm to the mixed state of 1B2 and 1A1, whereas CH2Br2 reaches a lower mixed state of B1 and B2.56,98,99 The 1A1 and 1B2 symmetries become A 0 and A00 , respectively, as the C–I bond is elongated and the CH2I2 structure changes to


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Fig. 11 The dissociation channels initiated from the ground state potential energy surface of CH2I2, in which the energies in kJ mol1 relative to CH2I2 are computed with CCSD(T)/midix level of theory with B3LYP/midix zero-point energy corrections at B3LYP/midix optimized geometries.

˜ 1A 0 coupling is the Cs symmetry. In the IC process, the 1A 0 –X strengthened to facilitate the level-to-level interaction, while the ˜ 1A 0 states with different interaction between the 1A00 and X symmetry is relatively weak. Accordingly, the population lying in the 1A 0 state is favored to transit to the high vibrational levels ˜1A 0 prior to direct photodissociation. The of the ground state X photodissociation of CH2I2 and CH2Br2 behaves differently in some aspects. For instance, the quantum yield of the I2 elimination in CH2I2 is 0.0040 0.0025, in contrast to a result of 0.2 0.1 for the Br2 elimination in CH2Br2. The reasons are yet to be known. However, the fraction of population undergoing internal conversion causes the difference between them. For CH2I2, only the 1A1 population is allowed to undergo IC, sharing a small fraction in the excitation band. As for CH2Br2, the population excited to the B1 state takes a larger fraction. Like the A1 state, B1 is efficient to undergo IC, as the Br–CH2Br bond is elongated. The temperature effect is another different behavior between these two structural analogs. The I2 intensity increases up to 220%, but the Br2 intensity increases only about 16% within a similar temperature increment of 24–30 K. The heavier iodine mass should result in a larger density of states for CH2I2 at the same excitation energy. The density of


vibrational states for CH2I2 and CH2Br2 at 248 nm were thus calculated to yield 1.15 106 and 5.61 105 cm1, respectively, in which the harmonic frequencies were calculated by B3LYP/ midix.93 As temperature increases, the density of states of CH2I2 increases more rapidly to make the IC process more efficient, with respect to CH2Br2. That might cause a large discrepancy in the temperature effect. 3.5. Photodissociation of thionyl chloride and sulfuryl chloride Thionyl chloride (SOCl2) and sulfuryl chloride (SO2Cl2) have become of atmospheric interest as they may eliminate both Cl and SO; the latter pollutant is readily oxidized to H2SO4, the acid rain source. The photochemistry of SOCl2 has been well investigated, producing three dissociation channels: (1) SOCl + Cl, (2) SO + Cl2, and (3) SO + 2Cl. The insight into photolysiswavelength dependence of dissociation channels with the related branch ratios,12,100–102 anisotropy parameter for each channel,12,13,103 dissociation characterization for the threebody production,12,104 and nascent vibrational and spin-state distribution of SO103,105 was mainly gained. The prior studies are essentially focused on probing of the Cl (Cl*) and SO


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Fig. 12 A portion of Cl2 spectra (v = 0) acquired in the photolysis of SOCl2 and SO2Cl2 at 248 nm. A 5% of Cl2 gas in Ne buffer gas and simulated counterpart are also included for comparison.

fragments for understanding the dynamical complexity especially for the channels (1) and (3). Nevertheless, whether the Cl2 fragment is produced is suspected by a recent investigation using threedimensional imaging to probe photofragments at 235 nm.13 Thus far, the Cl2 fragment was monitored using time-offlight mass spectrometer,12,13 but its optical spectrum has not been observed for understanding the internal state distributions. In this account, a photolysis laser beam at 248 nm is applied to excite SOCl2 in the (pSCl*, nS) transition with an absorption cross section of 7.1 1018 cm1.106 Fig. 12 shows the optical spectrum of Cl2 (v = 0) following photolysis of SOCl2 at a pressure of 175 mTorr that is detected for the first time using the CRDS method. The spectra of Cl2 (v = 0,1) and (v = 0,1,2) were also obtained in the region of 503–504 and 511–513 nm, respectively. When the photolysis laser is turned off, the Cl2 signal disappears. A pure Cl2 gas diluted to 5% in Ne buffer gas at a total pressure of 165 mTorr is compared in Fig. 12. The spectral consistency indicates that the Cl2 spectrum dissociated is not interfered with by other dissociation products. According to eqn (1), a simulated counterpart is calculated for comparison, in which the isotopic variants 37 37 Cl Cl and 35Cl37Cl are not taken into account. From the simulation, the vibrational population ratio at each state may be analyzed. To remove the possibility of Cl2 resulting from atomic Cl recombination, a single Cl-containing molecule is substituted such as acetyl chloride (CH3COCl) with a comparable absorption cross section, but no any Cl2 signal can be detectable even if the sample pressure is increased to 600 mTorr. The pressure and laser energy dependence of the Cl2 fragment are also measured, yielding a straight line with a slope of unity, indicative of a single-photon involvement in a single molecular elimination. Two photons are otherwise required for the secondary recombination process, if the source of Cl atoms is from two separate SOCl2 molecules. A concerted three-body dissociation (SO + Cl + Cl) was reported to be the main channel accounting for 480% of the overall process at 193 nm, whereas


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no contribution of such a three-body dissociation was observed when the wavelength was changed to 248 nm.103 The atomic Cl recombination from this dissociation channel is thus neglected. Further, the reaction of eliminated Cl with SOCl2 requires two SOCl2 molecules that are opposed to the pressure dependence measurement. Because the Cl yield was reported to be 96.5% at 248 nm,103 a pseudo-first order condition cannot be applied in this case. These experimental results may exclude the probable contributions from atomic Cl recombination, secondary reaction of energized fragments, or reactions between Cl and the precursor. Determination of quantum yield of the Cl2 channel is underway in our group. To the best of our knowledge, photochemistry of SO2Cl2 has never been investigated except for photolysis-wavelength dependence of absorption cross sections105 which turns out to be larger by about one order of magnitude than SOCl2. According to the estimate with heat of formation data and atomic energy levels,105 the following dissociation channels are anticipated depending on the photolysis wavelength: (1) O2S + Cl2, (2) O2SCl + Cl (Cl*), (3) O2S + 2Cl (Cl*), (4) OSCl + OCl, (5) OSCl2 + O(3P) (O(1D)), and (6) OSCl + O(3P) + Cl. None of them have been investigated. As shown in Fig. 12, this account reports the first case of the Cl2 optical spectra of the channel (1) in photolysis at 248 nm using the CRDS technique. The required experiments were also carried out to confirm that the Cl2 product is obtained via primary fragmentation. Further characterization of the photodissociation dynamics is still underway.

4. Conclusion Molecular halogen elimination has been seldom reported among the photodissociation channels of halogen-containing compounds ever studied. This account is intended to attract the attention of this molecular channel by reviewing the CRDS detection of the X2 (Br2, Cl2 and I2) fragments resulting from a variety of multi-X-containing molecules with characterization of the related elimination pathways. Such a channel has been verified to stem from primary dissociation of a single molecule on the ground state PES via internal conversion. Like the halogen products studied, some molecular channels are found to dissociate similarly from the ground-state PESs following photodissociation.107,108 According to the ab initio potential energy calculations, the parent molecules in the highly vibrational levels of the ground state are energetically accessible to surpass the transition states and break into the X2 products. Even for the simple halomethanes, their photodissociation behaves distinctly from three-center to four-center concerted and from asynchronous to sequential mechanisms. As compared to bromomethanes and bromoethanes, the photodissociation mechanisms of dibromoethylenes are more complicated; more than one route may be followed to release the Br2 fragment. With respect to X elimination, the obtained X2 yield is low ranging from 0.004 to 0.3. However, the small amount of halogen molecules might have a chance to become or induce a stable reservoir in the stratosphere. The ozone loss


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rate is often used as an indicator of the extent of stratospheric ozone depletion along specific altitudes and latitudes. This rate is defined as the ratio of reactive free radicals (such as Cl, ClO, Br, and BrO) to the gas sources and halogen-induced reservoirs (such as HX and XONO2) that reach the stratosphere.5,7 Understanding the X2 dissociation mechanism should contribute to assessing the issues of halogen-induced ozone destruction and halogen-related environmental change.

Acknowledgements This work is supported by National Taiwan University, Ministry of Education, and National Science Council of Taiwan, Republic of China, under contract no. NSC 99-2113-M-001-025-MY3.

References 1 M. J. Molina and F. S. Rowland, Nature, 1974, 249, 810–812. 2 J. Farman, B. Gardiner and J. Shanklin, Nature, 1985, 315, 207–210. 3 S. A. Montzka, J. H. Butler, R. C. Myers, T. M. Thompson, T. H. Swanson, A. D. Clarke, L. T. Lock and J. W. Elkins, Science, 1996, 272, 1318–1322. 4 S. Montzka, J. Butler, R. Myers, T. Thompson, T. Swanson, A. Clarke, L. Lock and J. Elkins, Science, 1996, 272, 1318–1322. 5 S. Solomon, R. R. Garcia and A. R. Ravishankara, J. Geophys. Res., 1994, 99, 20491–20499. 6 M. Bilde, T. J. Wallington, C. Ferronato, J. J. Orlando, G. S. Tyndall, E. Estupinan and S. Haberkorn, J. Phys. Chem. A, 1998, 102, 1976–1986. 7 J. S. Daniel, S. Solomon, R. W. Portmann and R. R. Garcia, J. Geophys. Res., 1999, 104, 23871–23880. 8 D. E. Mann and B. A. Thrush, J. Chem. Phys., 1960, 33, 1732–1734. 9 C. L. Sam and J. T. Yardley, Chem. Phys. Lett., 1979, 61, 509–512. 10 D. Krajnovich, Z. Zhang, L. Butler and Y. T. Lee, J. Phys. Chem., 1984, 88, 4561–4566. 11 M. R. Cameron, S. A. Johns, G. F. Metha and S. H. Kable, Phys. Chem. Chem. Phys., 2000, 2, 2539–2547. 12 G. Baum, C. S. Effenhauser, P. Felder and J. R. Huber, J. Phys. Chem., 1992, 96, 756–764. 13 A. Chichinin, T. S. Einfeld, K.-H. Gericke and J. Grunenberg, Phys. Chem. Chem. Phys., 2005, 7, 301–309. 14 K. Sato, S. Tsunashima, T. Takayanagi, G. Fujisawa and A. Yokoyama, J. Chem. Phys., 1997, 106, 10123–10133. 15 D. A. Blank, W. Sun, A. G. Suits, Y. T. Lee, S. W. North and G. E. Hall, J. Chem. Phys., 1998, 108, 5414–5425. 16 C. Y. Wu, C. Y. Chung, Y. C. Lee and Y. P. Lee, J. Chem. Phys., 2002, 117, 9785–9792. 17 S. H. Lee, W. K. Chen, C. Chaudhuri, W. J. Huang and Y. T. Lee, J. Chem. Phys., 2006, 125, 144315. 18 G. X. He, Y. A. Yang, Y. Huang and R. J. Gordon, J. Phys. Chem., 1993, 97, 2186–2193.


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19 M. Chandra, D. Senapati, M. Tak and P. K. Das, Chem. Phys. Lett., 2006, 430, 32–35. 20 J. F. Riehl, D. G. Musaev and K. Morokuma, J. Chem. Phys., 1994, 101, 5942–5956. 21 Y. R. Lee, C. C. Chou, Y. J. Lee, L. D. Wang and S. M. Lin, J. Chem. Phys., 2001, 115, 3195–3200. 22 L. Hua, W. B. Lee, M. H. Chao, B. Zhang and K. C. Lin, J. Chem. Phys., 2011, 134, 194312. 23 W. Shi, V. N. Staroverov and R. H. Lipson, J. Chem. Phys., 2009, 131, 154304. 24 L. Hua, W. B. Lee, M. H. Chao, B. Zhang and K. C. Lin, J. Chem. Phys., 2011, 134, 194312. 25 Y. J. Jee, Y. J. Jung and K. H. Hung, J. Chem. Phys., 2001, 115, 9739–9746. 26 J. Tellinghuisen, J. Chem. Phys., 2001, 115, 10417–14024. 27 H. Fan, P. Y. Tsai, K. C. Lin, C. W. Lin, C. Y. Yan, S. W. Yang and A. H. H. Chang, J. Chem. Phys., 2012, 137, 214304. 28 J. J. Scherer, J. B. Paul, A. O’Keefe and R. J. Saykally, Chem. Rev., 1997, 97, 25–51. 29 P. Y. Wei, Y. P. Chang, Y. S. Lee, W. B. Lee, K. C. Lin, K. T. Chen and A. H. H. Chang, J. Chem. Phys., 2007, 126, 034311. 30 H. L. Lee, P. C. Lee, P. Y. Tsai, K. C. Lin, H. H. Kuo, P. H. Chen and A. H. H. Chang, J. Chem. Phys., 2009, 130, 184308. 31 C. C. Wu, H. C. Lin, Y. B. Chang, P. Y. Tsai, Y. Y. Yeh, H. Fan, K. C. Lin and J. S. Francisco, J. Chem. Phys., 2012, 135, 234308. 32 Laboratoire Aime Cotton, http://www.lac.u-psud.fr/-Atlasde-l-iode-et-du-tellure-. 33 J. A. Coxon and R. Shanker, J. Mol. Spectrosc., 1978, 69, 109–122. 34 A. Mellouki, R. K. Talukdar, A.-M. Schmoltner, T. Gierczak, M. J Mills, S. Solomon and A. R. Ravishankara, Geophys. Res. Lett., 1992, 19, 2059–2062. 35 Y. R. Lee, C. C. Chen and S. M. Lin, J. Chem. Phys., 2003, 118, 10494–10501. 36 U. Marvet, E. J. Brown and M. Dantus, Phys. Chem. Chem. Phys., 2000, 2, 885–891. 37 U. Marvet, Q. Zhang and M. Dantus, J. Phys. Chem. A, 1998, 102, 4111–4117. 38 P. Zou, J. Shu, T. J. Sears, G. E. Hall and S. W. North, J. Phys. Chem. A, 2004, 108, 1482–1488. 39 D. Xu, J. S. Francisco, J. Huang and W. M. Jackson, J. Chem. Phys., 2002, 117, 2578–2585. 40 F. Taketani, K. Takahashi and Y. Matsumi, J. Phys. Chem. A, 2005, 109, 2855–2860. 41 L. J. Butler, E. J. Hintsa and Y. T. Lee, J. Chem. Phys., 1986, 84, 4104–4106. 42 L. J. Butler, E. J. Hintsa and Y. T. Lee, J. Chem. Phys., 1987, 86, 2051–2074. 43 R. F. Barrow, T. C. Clark, J. A. Coxon and K. K. Yee, J. Mol. Spectrosc., 1974, 51, 428–449. 44 S. Gerstenkorn and P. Luc, J. Phys., 1989, 50, 1417–1432. 45 J. A. Coxon, J. Quant. Spectrosc. Radiat. Transfer, 1972, 12, 639–650.


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46 G. Herzberg, Molecular Spectra and Molecular Structure: I. Spectra of Diatomic Molecules, van Nostrand Reinhold, Toronto, 1950, pp. 133–139. 47 R. C. Sharma, H. Y. Huang, W. T. Chuang and K. C. Lin, Spectrochim. Acta, Part A, 2005, 60, 2115–2120. 48 H. Y. Huang, W. T. Chuang, R. C. Sharma, C. Y. Hsu, K. C. Lin and C. H. Hu, J. Chem. Phys., 2004, 121, 5253–5260. 49 B. Abel, H. Hippler, N. Lange, J. Schuppe and J. Troe, J. Chem. Phys., 1994, 101, 9681–9690. 50 V. L. Orkin and E. E. Kasimovskaya, J. Atmos. Chem., 1995, 21, 1–11. 51 S. L. Baughcum and S. R. Leone, J. Chem. Phys., 1980, 72, 6531–6545. 52 A. T. J. B. Eppink and D. H. Parker, J. Chem. Phys., 1998, 109, 4758–4767. 53 L. J. Butler, Annu. Rev. Phys. Chem., 1998, 49, 125–171. 54 S. Enami, S. Hashimoto, M. Kawasaki, Y. Nakano, T. Ishiwata, K. Tonokura and T. J. Wallington, J. Phys. Chem. A, 2005, 109, 1587–1593. 55 J. Sehested, M. Bilde, T. Mogelberg, T. J. Wallington and O. J. Nielsen, J. Phys. Chem., 1996, 100, 10989–10998. 56 P. Y. Wei, Y. P. Chang, W. B. Lee, Z. Hu, H. Y. Huang, K. C. Lin, K. T. Chen and A. H. H. Chang, J. Chem. Phys., 2006, 125, 133319. 57 C. Y. Hsu, H. Y. Huang and K. C. Lin, J. Chem. Phys., 2005, 123, 134312. 58 F. Jensen, Introduction to computational chemistry, Wiley, New York, 2nd edn, 2007, p. 136. 59 J. Zheng, J. R. Gour, J. J. Lutz, M. Wloch, P. Piecuch and D. G. Truhlar, J. Chem. Phys., 2008, 128, 044108. 60 T. Noro, M. Sekiya and T. Koga, Theor. Chem. Acc., 2012, 131, 1124–1128 (basis set is available from http://setani.sci. hokudai.ac.jp/sapporo/Welcome.do). 61 A. Kalume, L. George and S. A. Reid, J. Phys. Chem. Lett., 2010, 1, 3090–3095. 62 D. Krajnovich, F. Huisken, Z. Zhang, Y. R. Shen and Y. T. Lee, J. Chem. Phys., 1982, 77, 5977–5989. 63 X. Zheng and D. L. Phillips, J. Chem. Phys., 2000, 113, 3194–3203. 64 G. Maier and H. P. Reisenauer, Angew. Chem., Int. Ed. Engl., 1986, 25, 819–822. 65 G. Maier, H. P. Reisenauer, J. Lu, L. J. Scaad and B. A. Hess, Jr., J. Am. Chem. Soc., 1990, 112, 5117–5122. 66 J. J. Lin, S. M. Wu, D. W. Hwang, Y. T. Lee and X. Yang, J. Chem. Phys., 1998, 109, 10838–10846. 67 Y. B. Huang and R. J. Gordon, J. Chem. Phys., 1997, 106, 1418–1420. 68 J. Y. Tu, J. J. Lin, Y. T. Lee and X. M. Yang, J. Chem. Phys., 2002, 116, 6982–6989. 69 G. E. Hall, J. T. Muckerman, J. M. Preses, R. E. Weston, G. W. Flynn and A. Persky, J. Chem. Phys., 1994, 101, 3679–3688. 70 Y.-P. Chang, P.-C. Lee, K.-C. Lin, C.-H. Huang, B. J. Sun and A. H.-H. Chang, ChemPhysChem, 2008, 9, 1137–1145. 71 P.-C. Lee, P.-Y. Tsai, M.-K. Hsiao, K.-C. Lin, C.-H. Huang and A. H.-H. Chang, ChemPhysChem, 2009, 10, 672–679.


PCCP

72 I. Ohmine, J. Chem. Phys., 1985, 83, 2348–2362. 73 A. H. H. Chang, D. W. Hwang, X. M. Yang, A. M. Mebel, S. H. Lin and Y. T. Lee, J. Chem. Phys., 1999, 110, 10810–10820. 74 A. H. H. Chang, A. M. Mebel, X. M. Yang, S. H. Lin and Y. T. Lee, J. Chem. Phys., 1998, 109, 2748–2761. 75 H. Eyring, S. H. Lin and S. M. Lin, Basic Chemical Kinetics, Wiley, New York, 1980. 76 D. Townsend, S. A. Lahankar, S. K. Lee, S. D. Chambreau, A. G. Suits, X. Zhang, J. Rheinecker, L. B. Harding and J. M. Bowman, Science, 2004, 306, 1158–1161. 77 A. G. Suits, Acc. Chem. Res., 2008, 41, 873–881. 78 H. M. Yin, S. H. Kable, X. Zhang and J. M. Bowman, Science, 2006, 311, 1443–1446. 79 P. L. Houston and S. H. Kable, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 16079–16082. 80 B. R. Heazlewood, M. J. T. Jordan, S. H. Kable, T. M. Selby, D. L. Osborn, B. C. Shepler, B. J. Braams and J. M. Bowman, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 12719–12724. 81 S. Maeda, K. Ohno and K. Morokuma, J. Phys. Chem. Lett., 2010, 1, 1841–1845. 82 M. P. Grubb, M. L. Warter, A. G. Suits and S. W. North, J. Phys. Chem. Lett., 2010, 1, 2455–2458. 83 C. Chen, B. Braams, D. Y. Lee, J. M. Bowman, P. L. Houston and D. Stranges, J. Phys. Chem. Lett., 2010, 1, 1875–1880. 84 M. H. Chao, P. Y. Tsai and K. C. Lin, Phys. Chem. Chem. Phys., 2011, 13, 7154–7161. 85 M. Kawasaki, S. J. Lee and R. Bersohn, J. Chem. Phys., 1975, 63, 809–814. 86 S. L. Baughcum and S. R. Leone, J. Chem. Phys., 1980, 72, 6531–6545. 87 K. W. Jung, T. S. Ahmadi and M. A. El-Sayed, Bull. Korean Chem. Soc., 1997, 18, 1274–1280. 88 H. Xu, Y. Guo, S. Liu, X. Ma, D. Dai and G. Sha, J. Chem. Phys., 2002, 117, 5722–5729. 89 J. B. Koffend and S. R. Leone, Chem. Phys. Lett., 1981, 81, 136–141. 90 H. Okabe, M. Kawasaki and Y. Tanaka, J. Chem. Phys., 1980, 73, 6162–6166. 91 C. Fotakis, M. Martin and R. J. Donovan, J. Chem. Soc., Faraday Trans. 2, 1982, 78, 1363. 92 U. Marvet and M. Dantus, Chem. Phys. Lett., 1996, 256, 57–62. 93 S. Y. Chen, P. Y. Tsai, H. C. Lin, C. C. Wu, K. C. Lin, B. J. Sun and A. H. H. Chang, J. Chem. Phys., 2011, 134, 034315. 94 Y. B. Fan and D. J. Donaldson, J. Chem. Phys., 1992, 97, 189–196. 95 M. E. Vaida and T. M. Bernhardt, Faraday Discuss., 2012, 157, 437–449. 96 J. C. Mossinger, D. E. Shallcross and R. A. Cox, J. Chem. Soc., Faraday Trans., 1998, 94, 1391–1396. 97 R. E. Easton, D. J. Giesen, A. Welch, C. J. Cramer and D. G. Truhlar, Theor. Chim. Acta, 1996, 93, 281–301.


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PCCP

98 G. C. Causley and B. R. Russell, J. Chem. Phys., 1975, 62, 848–857. 99 H. Y. Xiao, Y. J. Liu, J. G. Yu and W. H. Fang, J. Comput. Chem., 2008, 29, 2513–2519. 100 H. Okabe, J. Am. Chem. Soc., 1971, 93, 7095–7096. 101 H. Kanamori, E. Tiemann and E. Hirota, J. Chem. Phys., 1988, 89, 621–624. 102 A. Rakhymzhan and A. Chichinin, J. Phys. Chem. A, 2010, 114, 6586–6593. 103 M. Roth, C. Maul and K.-H. Gericke, Phys. Chem. Chem. Phys., 2002, 4, 2932–2940.


Perspective

104 H. Wang, X. Chen and B. R. Weiner, J. Phys. Chem., 1993, 97, 12260–12268. 105 X. Chen, F. Asmar, H. Wang and B. R. Weiner, J. Phys. Chem., 1991, 95, 6415–6417. 106 A. P. Uthman, P. J. Demlein, T. D. Allston, M. C. Withiam, M. J. McClements and G. A. Takacs, J. Phys. Chem., 1978, 82, 2252–2257. 107 W. M. Gelbart, Annu. Rev. Phys. Chem., 1977, 28, 323–348. 108 J. M. Bowman and B. C. Shepler, Annu. Rev. Phys. Chem., 2011, 62, 531–553.


Halogen bonding molecular capsules.

Organochlorine compounds and their reactions in the atmosphere.

Determination of tetraalkyl lead compounds in the atmosphere.

Characterization of halogen···halogen interactions in crystalline dihalomethane compounds (CH2Cl2, CH2Br2 and CH2I2): a theoretical study.

Charge calculations in molecular mechanics. IX. A general parameterisation of the scheme for saturated halogen, oxygen and nitrogen compounds.

Molecular Evidence for Metabolically Active Bacteria in the Atmosphere.

On the mechanism of 2-enoate reductase. Elimination of halogen hydracids from 3-halogeno-2-enoates during reduction with NADH.

Addressing Methionine in Molecular Design through Directed Sulfur-Halogen Bonds.

Validated scoring of halogen bonding in molecular design.

Reduced sulfur compounds in the atmosphere of sewer networks in Australia: geographic (and seasonal) variations.

F compounds in the atmosphere of Istanbul.

Photochemical formation of organic compounds from mixtures of simple gases simulating the primitive atmosphere of the earth.

Type II halogen···halogen contacts are halogen bonds.

Formation of prebiochemical compounds in models of the primitive earth's atmosphere. II: CH4 - H2S atmospheres.

Emissions from pre-Hispanic metallurgy in the South American atmosphere.

The effects of some halogen-containing compounds on Bacillus subtilis endospores.

Steering Self-Assembly of Amphiphilic Molecular Nanostructures via Halogen Exchange.

Halogen Bonding versus Hydrogen Bonding: A Molecular Orbital Perspective.

Ocular ultraviolet exposure from halogen lamps.

Trace elements in the atmosphere.

Carbonyl compounds and dissolved organic carbon in rainwater of an urban atmosphere.

The cell competition-based high-throughput screening identifies small compounds that promote the elimination of RasV12-transformed cells from epithelia.

The determination and identification of molecular lead pollutants in the atmosphere.

Gas chromatographic-mass spectrometric determination of volatile organic compounds in an urban atmosphere.