(COCl)2 Probed with Cavity Ring-Down Absorption ... - ACS Publications

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Cl Elimination in 248 nm Photolysis of (COCl) Probed with Cavity Ring-Down Absorption Spectroscopy Ting-Kang Huang, Bo-Jung Chen, King-Chuen Lin, Lin Lin, Bing-Jian Sun, and Agnes H. H. Chang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12810 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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The Journal of Physical Chemistry

Cl2 Elimination in 248 nm Photolysis of (COCl)2 Probed with Cavity Ring-down Absorption Spectroscopy

Ting-Kang Huang, Bo-Jung Chen, and King-Chuen Lin* Department of Chemistry, National Taiwan University, Taipei, and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan and Lin Lin, Bing-Jian Sun, and A. H. H. Chang Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan

Pages: 33 Tables: 2 Figures: 5

*To whom correspondence should be addressed. KCL: [email protected]

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ACS Paragon Plus Environment


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Abstract

Cavity ring-down absorption spectroscopy (CRDS) is employed to investigate one-photon dissociation of (COCl)2 at 248 nm obtaining a primary Cl2 elimination channel.

A

ratio

of

vibrational

population

is

estimated

to

be

1:(0.12±0.03):(0.011±0.003) for the v=0, 1, and 2 levels. The quantum yield of Cl2 molecular channel is obtained to be 0.8 ±0.4 initiated from the X% 1Ag ground state surface (COCl)2 via internal conversion. The obtained total quantum yield is attributed to both primary ((COCl)2 + hν 2CO + Cl2) and secondary reactions (dominated by Cl+ COCl Cl2 + CO). The former is estimated to share a yield >0.14, while the latter contributes up to 0.66. The photodissociation pathway to the molecular products is calculated to proceed via a four-center transition state (TS) from which Cl2 is eliminated synchronously. Installation of the mirrors with reflectivity of 99.995% in the CRDS apparatus prolongs the ringdown time to 70 µs, thus allowing for the contribution from 17% up to 66% of the total Cl2 yield from secondary reaction depending on the reaction temperature. Despite uncertainty in determining the product yield, it is for the first time to observe the primary Cl2 dissociation channel eliminated from (COCl)2.

2


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I. Introduction

Oxalyl chloride (COCl)2 has been adopted as a potential precursor of a Cl source with a very large quantum yield for kinetic and photochemical studies.1-3 This liquid compound at room temperature is also a useful reagent frequently used in organic synthesis,4

such

as

the

preparation

of

acyl

chlorides

from

the

corresponding carboxylic acids. The generated gaseous by-products are easily separated. However, oxalyl chloride may be readily photolyzed in the atmosphere to release chlorine-related pollutants. It is thus worthwhile to understand the photochemical processes of its gaseous phase. Thus far, the electronic structure and conformational properties of (COCl)2 have been widely studied,5-8 but the related photochemistry is less reported. In the photochemical studies, Schroeder et al.9 reported Ar and Xe-matrix isolated experiments of (COCl)2 using Fourier-transform infrared spectroscopy (FTIR). They found a cis-conformer formation and cis-gauche interconversion in the Ar matrix isolation, and confirmed a rapid dissociation to phosgene (Cl2CO) and carbon monoxide in the xenon-matrix experiments.9 Suits and coworkers found that oxalyl chloride dissociates to 2Cl + 2CO upon laser irradiation at 235 and 193 nm.10,11 They observed an impulsive three-body dissociation yielding fast components of Cl*(2P1/2), Cl(2P3/2), CO and COCl at ∼235 nm,10 followed by dissociation of most of COCl to produce slow components. Lee and coworkers12 later analyzed rovibrational IR emission spectra of CO (v≤3) at 248 nm. They obtained the fast component of CO, but did not observe the slow component which mostly lay in the v=0 level following secondary decomposition of COCl. They did not detect the COCl(v=1) either, and proposed that this primary product dissociated rapidly. Burkholder and coworkers13 recently studied the pressure dependence of COCl at the wavelength of 351, 248, and 3



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193 nm and concluded that the stabilized COCl product was produced with a quantum yield of 0.2 and 1.0 at 248 and 351 nm, respectively. In addition, they determined the total Cl quantum yield to be about 2 at both 248 and 193 nm. The studies on photodissociation of (COCl)2 are mainly focused on the chlorine atomic elimination pathways. The production of Cl2 channel was seldom reported. In a study of (COCl)2 photolysis at 193 nm, this molecular product was found on a millisecond scale by the recombination of Cl atoms under a high pressure of He buffer gas.14 Is it possible to find out oxalyl chloride undergoing a primary chlorine molecular elimination in the UV photolysis? That is, (COCl)2 + hν 2CO + Cl2. Note that the primary Cl2 is defined as the product directly eliminated in the photodissociation of (COCl)2, in contrast to a secondary product which indicates the same product but obtained by the reaction sources other than the direct photolysis. For instance, the following reactions, (1) Cl+ COCl Cl2 + CO, (2) Cl + (COCl)2 Cl2 + CO + COCl, and (3) Cl + Cl + M Cl2 + M, are likely to generate secondary Cl2 products. Thus far, photodissociation of oxalyl chloride emphases the Cl atomic elimination, but ignores the possibility of the Cl2 production. From a fundamental point of view, it is of importance to understand complete dissociation channels, especially for the chlorine-related processes which may potentially cause hazardous pollution. For the practical application, the Cl and Cl2 products stem from different pathways. Cl2 may have chance to decompose to Cl atoms. Therefore, investigating Cl2 pathway and quantum yield should help assess the accuracy of theoretical modelling for the Cl amount evaluation. It is not trivial to probe the Cl2 fragment. As reported previously, FTIR and laser-induced fluorescence (LIF) technique cannot be appropriately applied for the detection of halogen molecules.15,16 By using (2+1) resonance-enhanced multiphoton 4


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ionization (REMPI) technique coupled with ion and photoelectron imaging, Parker and co-workers17 promoted the Cl2 molecule to a super-excited state via different vibrational levels (v=0-15) of the 1Πg(4s) Rydberg state. This prepared state may undergo autoionization, either forming Cl2+(X2Πg) or dissociating to electronically excited neutral chlorine atoms. However, they did not obtain any Cl2+ fragment directly. They then inferred that this molecular ion is likely to completely dissociate into Cl+ and Cl by absorbing one successive photon. It is apparently difficult to quantify the Cl2 fragment by REMPI technique. As demonstrated with cavity ring-down absorption spectroscopy (CRDS),18-21 we have successfully monitored the Br2 and I2 fragments eliminated from a series of halogen-containing hydrocarbons. We also observed a primary product of Br2 in one-photon dissociation of (COBr)2 at 248 nm and proposed a plausible reaction mechanism for the products Br2 + 2CO.22 By taking advantage of CRDS merits, we attempt to probe analogously the Cl2 channel in (COCl)2 as photolyzed at 248 nm. However, it is more difficult and challenging to probe the Cl2 fragment with an absorption cross section 100 times smaller (480 – 520 nm) than the Br2 molecule.21,23 In this work, we aim to characterize the ro-vibrational population distribution of the Cl2 fragment, to confirm a primary product Cl2 thus obtained, and then to evaluate the quantum yield of the molecular channel. Finally, we will discuss the probability of Cl2 contribution from the secondary reactions and elucidate the photodissociation pathway with the aid of theoretical calculations. In this manner, we may gain deep insight into the chlorine molecular pathways to supplement the prior reports concerning the chlorine-related processes.

II. Experimental 5



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The CRDS apparatus has been described elsewhere.20,24,25 A 20 ns-pulsed excimer laser emitting at 248 nm was used to photolyze the (COCl)2 molecule, while a 5-8 ns-pulsed Nd:YAG laser-pumped dye laser (482-513 nm) was used to probe the + released Cl2 fragment in the B 3 Π ou ← X 1 Σ +g transition.26,27 Both pulsed lasers

operated at 10 Hz were synchronized in a brief delay of 30 ns. The corresponding pulse energies were controlled in the range of 15-50 and 0.5-1 mJ/pulse, respectively. With the same geometric configuration adopted,21 the two laser beams were overlapped in the center of the flow cell with an interactive volume of (20x1x2)±5 mm3. The ring-down cell has the long arms sealed by two mirrors with high reflectance of 99.995% at 500 nm, a diameter of 25.4 mm and a radius of curvature of 1 m. The flow rate of the cell was controlled to replenish reactant sample for each laser shot. A spatial filter was used to guide the probe beam to remain mostly the TEM00 mode.21 A photomultiplier tube was located behind the rear mirror to monitor the light pulse leaking out of the mirror, followed by a transient digitizer used to record the temporal profile of the ring-down signal. After purification by repeated freeze-pump-thaw cycles at 77 K, the precursors were flowed through the ring-down cell at a pressure of 10-200 mTorr monitored by an MKS pressure gauge. The Cl2 absorption spectra, with a spectral resolution of 0.1 cm-1, were acquired with the aid of a lab-developed program based on a Matlab environment.

III. Theoretical methods A. Ab initio electronic structure calculations As with the work reported for Br2 previously,19,24,25 the Cl2 and Cl dissociation 6


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channels on the adiabatic ground state potential energy surface (PES) of (COCl)2 are calculated at the level of CCSD(T)/cc-pVTZ. The PES presented is optimized geometries at energy minima for reactants (r), intermediates (i), and products (p), and saddle points for transition states (ts) except for the tsi2p2, which is obtained by minimum flux (the minimum of number of states). The geometries are verified through harmonic frequencies, except for tsi2p2. Their optimized geometries and harmonic frequencies are obtained at the level of the hybrid density functional theory, B3LYP/cc-pVTZ; the energies are further refined by the coupled cluster CCSD(T)/cc-pVTZ with B3LYP/cc-pVTZ zero-point energy corrections. The GAUSSIAN09 programs are utilized in the electronic structure calculations.

B. RRKM rate constant and branch ratio calculations

The Rice-Ramsperger-Kassel-Marcus (RRKM) rate constants28 for Cl2 and Cl elimination pathways on the singlet ground state PES of (COCl)2 are predicted, upon irradiation at 248 nm. The saddle-point method28,29 is applied to evaluate the number of states and the density of states; the molecule is viewed as a collection of harmonic oscillators, of which the harmonic frequencies and energy states are obtained as described above. The transition state for Cl elimination along a barrierless pathway is located by applying variational RRKM theory. Utilizing RRKM rate constants, the rate equations of reaction mechanism at 248 nm are solved with Runge-Kutta method. The solutions give the concentrations as a function of time, whose asymptotic values yield the branching ratio of Cl2 and Cl.

IV. Results and Discussion A. Optical spectrum of primary Cl2 product 7



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The UV/vis spectra of (COCl)2 were reported to be continuous from 200 to 450 nm with absorption cross sections decreasing monotonically from 2x10-18 to 2x10-22 cm2.13,30 The spectrum was attributed to the electronic transition (π*CO, nO) which arises mainly from gauche and partially from trans conformers. These two conformers with a small energy difference of 300 cm-1 may co-exist at room temperature.31,32 In this work, the probe laser beam was applied to detect the CRDS spectrum of + Cl2 (v=0, 1, and 2) product in the B 3 Π ou ← X 1 Σ +g transition at room temperature,

following photolysis of (COCl)2 at 248 nm. The small absorption cross sections of Cl2 required the installation of a pair of cavity mirrors with reflectivity of 99.995% at 500 nm. Accordingly, three segments of Cl2 spectrum were acquired, including the region of 485-486.5 nm for the transition from the vibrational bands (v’, v”)=(19-26, 0), 501-502.5 nm from the bands (12-16, 0) and (18-23, 1), and 511.5-512.5 nm from the bands (9-11, 0), (14-17, 1), and (20-25, 2). An example of the Cl2(v=0) spectrum is given in Fig.1a. The spectral assignment is referred to the report by Coxon.27,33-35 The Cl2 signals disappear when the photolysis laser is off. As referred to the pure Cl2 spectrum,21 the obtained spectral regions are free from interference of other species. We conducted three additional experiments to confirm that the Cl2 fragment is a primary product in the photolysis. The rotational lines at 501.89 nm dominated by P(11) and R(13) of the (12,0) band was selected to examine the dependence of Cl2 absorption on photolysis laser pulse energy and on (COCl)2 pressure. As shown in Fig.2a, a plot of rotational intensity as a function of the photolysis laser energy up to 47 mJ/pulse yields a straight line passing through the origin, indicating that a single-photon is involved in molecular elimination. The effect of multi-photon dissociation contribution to Cl2 formation is minimized. Fig.2b shows a linear plot of 8


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the Cl2 absorption as a function of pressure up to 90 mTorr beyond which deviation from linearity is found. Such deviation might be caused by self-quenching of the precursor or the Cl2 quenching effect. The maximum pressure allowed for linear dependence varies for different precursors. For instance, the maximum pressure of 500 mTorr was reached in the case of (COBr)2.22 The self-quenching rate constant might be an influential factor. In this work, we prevent the (COCl)2 vapor pressure from this complicated nonlinear region throughout the experiments. These above-mentioned

experiments

confirm

that

Cl2

is

produced

from

the

photodissociation of a single (COCl)2 molecule following absorption of a single photon at 248 nm. It is necessary to inspect the probable contribution of Cl2 fragment from the Cl atomic recombination during the µs regime of ring-down time constant. A single Cl-containing molecule of acetyl chloride (CH3COCl) was used for examination which had a slightly smaller absorption cross section of 1.15x10-19 cm2 at 248 nm,36 but no Cl2 signal could be detected even when the laser energy was increased to 55 mJ/pulse and the sample pressure was enlarged to 500 mTorr, at least five times larger than using (COCl)2 as precursor. Another Cl-containing molecule propionyl chloride with absorption cross section ~6×10-20 cm2 at 248 nm was similarly used.37 The pressure and laser energy were set at 700 mTorr and 54 mJ/pulse, but no Cl2 signal was detected, even though 400 mTorr Ar gas was added to stabilize possible Cl2 product. It usually requires a third body to stabilize the energized Cl2 product in the reaction, Cl + Cl Cl2. A high pressure environment is necessary for atomic recombination, no matter which source of Cl atom, one or two (COCl)2 molecules. The applied vapor pressure and laser energy for the single Cl-containing molecules in examining the possibility of atomic recombination have largely exceeded the experimental conditions for (COCl)2. Further, the Ar pressure at 400 mTorr, added in 9



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the reaction chamber to slow down the Cl velocity in the three-body reaction, is much larger than the (COCl)2 pressure used. The above-mentioned experiments indicate the Cl atomic recombination to Cl2 formation negligible.

B. Vibrational

population

distribution

and

quantum

yield

determination

Once with the SOCl2 case,21 we applied a method of spectral simulation to obtain the vibrational branching for each level which is seriously spectrally congested. A spectral simulation for the Cl2(v=0) population is thus obtained, appearing essentially consistent with the experimental findings (Fig.1a). Then, a theoretical counterpart is analogously obtained in the 501.3-502.3 nm range by adjusting the Cl2(v=1)/Cl2(v=0) population ratio to be 0.12±0.03 (Fig.1b). As the Cl2(v=1)/Cl2(v=0) ratio is fixed, a simulated spectrum in the 511.5-512.5 nm region is optimized by adjusting the population ratio of Cl2(v=2)/Cl2(v=0) to be 0.011±0.003. The population ratio of v(0):v(1):v(2) is then estimated to be 1:(0.12±0.03):(0.011±0.003), roughly corresponding to a Boltzmann vibrational temperature of 360±60 K. The quantum yield φ of the Cl2 fragment produced in the 248 nm photolysis of (COCl)2 can be evaluated by comparing with that of Cl2 in SOCl2 as a reference sample.21 In this manner, the following formula is obtained,

φs [Cl 2 ]s [Cl 2 ]r = / φr σ s Es ns σ r Er nr

(1)

where the subscript s and r denote (COCl)2 and SOCl2, respectively; [Cl2], the Cl2 concentration produced in the region where the photolysis and probe lasers overlap; E, the photolysis laser energy for parent molecule; σ, the absorption cross section of 10


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precursor, and n, the number density in the cell. The absorption cross section of SOCl2 was determined to be (6.77±0.34)x10-18 cm2 at 248 nm.21 The absorption cross section of (COCl)2 was determined herein to be (2.7±0.1)x10-19 cm2 in a 10-cm long gas cell using a UV/vis absorption spectrometer, in which the instrument factor was calibrated with three reference samples of acetyl bromide,38 acetyl chloride,36 and acetone.39 The result determined is consistent with 248 nm (COCl)2 absorption cross section of 2.7x10-19 cm2 reported previously.13,14 Given the quantum yield 0.40±0.13 of Cl2 produced in SOCl2 at 248 nm,21 the σ values of (COCl)2 and SOCl2 at 248 nm, the photolysis laser energies and pressures adopted each for (COCl)2 and SOCl2, along with the individual measurements of a relative line intensity of Cl2 at 501.89 nm, the Cl2 quantum yield was thus obtained to be 0.8±0.4. Note that the quantum yield of Cl2 from SOCl2 photolysis was determined by both direct and relative methods;21 the detail of former method was described previously.19 The result obtained on either direct or relative method was consistent with each other. Therefore, the relative method adopted in this work is reliable. As compared to the quantum yield 0.11±0.06 of Br2 elimination from (COBr)2, the result obtained in (COCl)2 is likely to be overestimated, because of a long ringdown time of 70 µs during which the secondary reactions may happen to contribute the Cl2 amount. The probable secondary reactions are proposed in the following. (COCl)2 was verified to dissociate via a two-step mechanism at 235 nm as follows,10 (COCl)2+ hvCl (or Cl*)+CO+COCl*

(2)

The obtained energetic COCl* fragment is thermally unstable and subsequently decomposes to COCl*CO + Cl

(3)

Photolysis of (COCl)2 gives three-body dissociation (eq.2) with a photodissociation 11



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rate constant depending on the laser energy and the absorption cross section, while the resultant COCl* further decomposes thermally up to a 82% extent (eq.3) which is confirmed later by Burkholder and coworkers.13 The following secondary reactions may probably produce the additional Cl2, apart from the primary product in the photodissociation of (COCl)2, Cl+ COCl Cl2 + CO

(4)

Cl + (COCl)2 Cl2 + CO + COCl

(5)

Cl + Cl + M Cl2 + M

(6)

These reaction schemes (eq.4, 5, and 6) have the rate constants of 1.6x10-10 cm3molecule-1s-1(ref.14,40),

0.14, while the latter contributes

(COCl)2 Probed with Cavity Ring-Down Absorption ... - ACS Publications

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