Formation of Polyynes C4H2, C6H2, C8H2, and C10H2 from

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Formation of Polyynes CH, CH, CH, and C H from Reactions of CH, CH, CH, and CH Radicals with CH 2

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Yi-Lun Sun, Wen-Jian Huang, and Shih-Huang Lee J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 2, 2015

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The Journal of Physical Chemistry Letters

Formation of Polyynes C4H2, C6H2, C8H2, and C10H2 from Reactions of C2H, C4H, C6H, and C8H Radicals with C2H2 Yi-Lun Sun, Wen-Jian Huang, and Shih-Huang Lee* Scientific Research Division, National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Some of the polyynes (HC2n+2H, 1 ≤ n ≤ 4) are observable in planetary atmospheres, interstellar space, and flames. Polyynes are proposed to play an important role in synthesis of large carbonaceous molecules. We explore the dynamics of reactions of C2nH (n = 1 – 4) radicals with C2H2 by interrogating time-of-flight spectra and photoionization-efficiency spectra of products C2n+2H2. The reactions of n = 2 – 4 were investigated for the first time. The translational-energy release is biased to low energy but extends to the energetic limit of product HC2n+2H + H, corresponding to a fraction of 0.34 – 0.36 on translational energy. Product C2n+2H2 has a deconvoluted ionization threshold in good agreement with the ionization energy of polyynes. The quantum-chemical calculations support the experimental observations. This work verifies that the title reaction is an important source for formation of polyynes that have been observed in interstellar/circumstellar media and combustion processes.

TOC GRAPHICS

KEYWORDS crossed molecular beams, time of flight, synchrotron radiation, photoionization, reaction dynamics


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Polyynes (HC2n+2H, sometimes called polyacetylenes) were observed in planetary atmospheres, interstellar space, and flames. Diacetylene HC4H, the smallest polyyne, resides in Titan’s atmosphere1 and the circumstellar envelope of protoplanetary nebula CRL 618.2 Besides, triacetylene HC6H also exists in CRL 618.2 HC2n+2H (n = 1 – 4) were detected in fuel-rich flames of hydrocarbons.3 Polyynes are proposed to be common intermediates in hydrocarbon combustion and to play an important role in synthesis of large carbonaceous molecules such as polycyclic aromatic hydrocarbons (PAHs), fullerenes, and soot. Therefore, the understanding on the formation mechanisms of polyynes is beneficial for exploring the synthesis processes of large carbonaceous molecules. C2nH is producible from C2nH2 by photolysis (or pyrolysis) and chemical reactions in planetary atmospheres and interstellar/circumstellar media (or combustion processes). C2H (ethynyl), C4H (butadiynyl), C6H (hexatrinyl), and C8H (octatetranyl) were detected in the line of sight towards the Taurus Molecular Cloud (TMC-1)4,5 and the circumstellar envelope of carbon star IRC+10216.6 C2H, C4H, and C6H were observed also towards CRL 618.7,8 C2nH is elusive to be probed in a flame due to its low concentration. Nonetheless, C2nH is suggested to be an important combustion intermediate because of its high reactivity. Ethyne (acetylene, C2H2) is well known as an abundant interstellar species and an important combustion intermediate. Therefore, reactions (1) are proposed to be the important sources for formation of polyynes in extremely cold interstellar space (10 – 20 K) and in hot combustion environments (1000 – 2000 K). However, the experiments on reactions (1) by product detections are limited to the reaction of C2D + C2H2 → C4HD + H / C4D + H2.9 C2nH + C2H2 → C2n+2H2 + H

(1)


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The kinetics of reactions of C2H and C4H with C2H2 were explored by interrogating reactants C2H and C4H. The rate coefficient of reaction C2H + C2H2 was determined as (2.3– 1.1)×10-10 cm3 molecule-1 s-1 in the range 15–295 K with a slightly inverse dependence on the temperature.10,11 The rate coefficient of reaction C4H + C2H2 was determined as (3.9–1.5)×10-10 cm3 molecule-1 s-1 in the range 39–300 K with a maximum at 52 K.12,13 The non-Arrhenius behavior is indicative of the absence of an entrance barrier. The dynamics of the C2D + C2H2 reaction that leads to products C4HD + H and C4D + H2 were explored in a crossed-molecularbeam quadrupole-mass apparatus by interrogating products C4D and C4HD at a mass-to-charge ratio m/z = 50 u with electron-impact ionization.9 The atomic (molecular) hydrogen-loss channel had an angular distribution enhanced at the sideway (forward and backward) direction. Theoretical calculations indicated that the terminal carbon atom of C2H (C4H) adds to a carbon atom of C2H2 to form a complex HC2CHCH (HC4CHCH) that subsequently decomposes to HC4H (HC6H) + H.9,14 Besides, the reaction C2H + C2nH2 → C2n+2H2 + H is also proposed to be a source for formation of polyynes. The dynamics of reaction C2D + C4H2 → C6HD + H was explored with crossed-beam experiments and quantum-chemical calculations.15 Unfortunately the dynamics of reactions of C2nH (n ≥ 2) radicals with C2H2 remain unknown, which drives us to investigate the C2nH + C2H2 reactions with n = 1 – 4. In the present work, the title reactions were investigated in a crossed-molecular-beam quadrupole-mass apparatus16-18 by measuring time-of-flight (TOF) and photoionizationefficiency (PIE) spectra of products with synchrotron vacuum-ultraviolet (VUV) light. One source chamber served to generate a pulsed beam of C2nH radicals from 1% C2H2/He by discharge19 and the other source chamber served to generate a pulsed beam of 5% C2H2/He. The yield of a discharge product that has a mass larger than 100 u or contains more than three


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hydrogen atoms is negligibly small (Figure S1 in the Supporting Information). The reactant PIE spectrum is in good agreement with that of electronically-ground linear-C2n+2H produced from the reaction C2 + C2nH2 → C2n+2H + H (Figure S2 in the Supporting Information). Furthermore, the deconvoluted ionization threshold coincides with the ionization energy of C2n+2H. Most of the excited-state reactants could be quenched to the ground state by supersonic expansions at a stagnation pressure 105 psia and by radiative transitions during a flight of ~ 50 µs to the reaction region. The remnant excited-state reactants, if any, cannot compete with the abundant highlyreactive ground-state reactants. Figure 1 presents the Newton diagrams superimposed with twodimensional velocity-distribution contours of products C2n+2H2 for the reactions C2nH (n = 1 – 4) + C2H2. The travel direction of C2nH (C2H2) is defined as Θ = 0° (90°) in the laboratory frame. ΘCM denotes the flight direction of the center of mass (CM) of a reaction system; ΘCM = 43.8°, 26.1°, 18.2° and 13.9° for n = 1 – 4, respectively. The reactant collision energies (Ec) are 9.9, 13.1, 14.8 and 15.9 kcal mol-1 for n = 1 – 4.

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Figure 1. Newton diagrams superimposed with two-dimensional product velocity-distribution contours for the reactions C2nH (n = 1 – 4) + C2H2. VC2nH and VC2H2 denote the velocities of C2nH and C2H2. Dashed lines denote the detection axes at several laboratory angles.


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We measured angle-specific TOF spectra of products C4H2 at m/z = 50 u, C6H2 at 74 u, C8H2 at 98 u, and C10H2 at 122 u at 15 – 23 laboratory angles using photon energy 11.6 eV. Figure 2 presents four selected TOF spectra and their simulations for each product. Each TOF spectrum was partitioned into a reactive (red line) and a nonreactive (green line) component. The top row of Fig. 3 depicts the laboratory angular distributions P(Θ) along with the simulations that have the same color codes as in Fig. 2 for the four C2n+2H2 products. The nonreactive part arising from the elastic and inelastic collisions of C2n+2H2 + C2H2 → C2n+2H2 + C2H2 was simulated with a product translational-energy distribution biased to the energetic limit (i.e., the reactant collision energy) 13.2, 14.9, 15.9 and 16.6 kcal mol-1 for n = 1 – 4 and with a product angular distribution highly peaked at the forward direction. Because the nonreactive part is viewed as a background and not detected entirely in the laboratory angles, the employed translational-energy and angular distributions might not be accurate enough and thus are omitted here. The uncertainty on the laboratory angular distribution of the nonreactive part may result in an uncertainty ~ 10% on the magnitude of the reactive part.

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Figure 2. Angle-specific TOF spectra of products C4H2 (first), C6H2 (second), C8H2 (third), and C10H2 (forth row) recorded at m/z = 50, 74, 98 and 122 u, respectively, with photoionization energy 11.6 eV. Open circles denote the experimental data. Red (green) curves denote the simulations of the reactive (nonreactive) part. Black curves are the sums of the red and green curves. Each panel shows the corresponding laboratory angle.

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Figure 3. Top: Laboratory angular distributions of products C2n+2H2. Open circles denote the experimental data. Red (green) curves denote the simulations of the reactive (nonreactive) part. Black curves are the sums of the red and green curves. Middle: CM product translational-energy distributions for the reactions C2nH + C2H2 → C2n+2H2 + H. Arrows indicate the energetic limits of the polyynic products. Bottom: PIE spectra of products C2n+2H2. Arrows indicate the ionization energies of polyynes. Columns (a) – (d) are for n = 1 – 4, separately.

The reactive part arising from the title reaction was simulated with a product translational-energy distribution P(Et) and a product angular distribution P(θ) in the CM frame. The P(Et) distributions are presented in the middle row of Fig. 3. The P(θ) distributions are nearly isotropic for the four title reactions and thus omitted here for brevity. The distributions


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P(Et) and P(θ) were employed also to construct the product velocity-distribution contours exhibited in Fig. 1. Et includes translational energies of two momentum-matched products C2n+2H2 and H. C2nH and C2H2 counter propagated to each other with a relative velocity (vrel) 2544 m s-1 in the CM frame. θ = 0° (180°) is defined as the incidence direction of C2nH (C2H2) in the CM frame. The P(Et) distribution has a maximal probability at 5 – 8 kcal mol-1 and has a maximal energy release extending to the energetic limit (i.e., the available energy) 38.1, 43.7, 43.3 and 47.4 kcal mol-1 for the reactions of n = 1 – 4. The four reactions have average translational-energy releases 13.9, 14.7, 15.2 and 16.9 kcal mol-1 corresponding to translational fractions 0.36, 0.34, 0.35 and 0.36 for n = 1 – 4. Because the counter product, a hydrogen atom, carries no internal energy, the internal-energy distribution of C2n+2H2 is derivable straightforward from the translational-energy distribution. The bottom row of Fig. 3 exhibits the PIE spectra of products C4H2, C6H2, C8H2 and C10H2 recorded at Θ = 44°, 26°, 18° and 15°, separately, in the energy range 7.6 – 11.6 eV. The small variation of photon flux versus photon energy was not corrected. The baseline was shifted to zero. After deconvolution from the photon-energy bandwidth ~ 0.42 eV, the ionization threshold was determined as 10.0 eV for C4H2, 9.5 eV for C6H2, 9.1 eV for C8H2, and 8.8 eV for C10H2 with an uncertainty of ± 0.1 eV. The present experimental values are in good agreement with the literature-reported ionization energy 10.03 eV of HC4H, 9.45 eV of HC6H, 9.08 eV of HC8H, and 8.75 eV of HC10H.20 The combination of the maximal translational-energy release and the photoionization threshold identifies the C2n+2H2 products as polyynes exclusively. Normalized to the same reactant ion signals, the four C2n+2H2 products have an integralion-signal ratio 5.7:1.7:1:1.3 for n = 1 – 4. Reactants C2nH were ionized with 12.5 eV and


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products C2n+2H2 with 11.6 eV. Because the four reactions have the same reactant collision velocity vrel, the product ratio might reflect the ratio of cross sections of the four C2nH/H exchange reactions with specific reactant collision energy. Regardless of the difference on reactant/product detection efficiency (e.g., ionization cross sections and dissociative ionization), the dependence of product ratios on the C2nH radicals is attributed possibly to the following reasons. More exit channels open and become more significant as the carbon number of C2nH increases from n = 1 to 4 and the corresponding collision energy increases from 9.9 to 15.9 kcal mol-1. Besides, the decomposition back to the reactant also becomes more significant as reactant collision energy increases. HC10H (HC6H) is the largest polyyne that has been discovered in flames (circumstellar envelopes), which might pertain to the dependence of production efficiency of polyynes on the reactant C2nH. Furthermore, the low gas density, typically 103 – 105 cm-3 in a molecular cloud,21 is the key factor to slow down the synthesis rate of large polyynes in interstellar space.

Figure 4. Potential-energy surfaces of the reactions C2nH + HC2H → HC2n+2H + H. The molecular structures of the n = 3 reaction were exemplified here. The potential energies (in kcal mol-1) listed from top to bottom are for the reactions of n = 1 – 4.


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Figure 4 depicts the potential-energy surfaces of the reactions C2nH + HC2H → HC2n+2H + H calculated with the method of CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ including the correction of zero-point energy (ZPE) at the level of B3LYP/aug-cc-pVDZ. For brevity only the molecular structures of the n = 3 reaction were presented but the potential energies were listed from top to bottom for the reactions of n = 1 – 4. The theoretical calculations indicate that C2nH can add to C2H2 via a head-on approach or a V-shape side-on approach at the entrance channel. The head-on collision initially forms a van-der-Waals (vdW) complex that in turn rearranges to a more-stable complex HC2nCHCH through a V-shape transition structure. The vdW complex and the transition structure lie below the reactant at the computational level of B3LYP/aug-cc-pVDZ without correction of ZPE. The vdW complex also correlates with a highly-lying transition state where a hydrogen atom transfers from C2H2 to C2nH. In contrast, the V-shape side-on collision incurs no energy barrier due possibly to a 2pπ-2pπ interaction between C2nH and C2H2. As both reactants approach closer to each other, the angle between both the carbon chains of C2nH and C2H2 increases gradually by the chemical force leading to the complex HC2nCHCH. At the exit channel, the complex either decomposes directly to HC2n+2H + H or undergoes hydrogen migration followed by hydrogen elimination to form the same product HC2n+2H + H. The production of cumulenic isomer H2C2n+2 that lies above HC2n+2H by ~ 44 kcal mol-1 (for n = 1) is energetically forbidden in the present work. The C2H + C2H2 reaction has a rate coefficient less than the C4H + C2H2 reaction by a factor of ~ 0.6 at temperatures below 300 K,[10–13] which is attributed partly to the energy barrier of 1.4 kcal mol-1 at the head-on approach and partly to a smaller dispersion force on the C2H + C2H2 reaction. Product C2n+2H2 has a nearly isotropic angular distribution that is qualitatively explainable based on the kinematic model.22,23 The total angular momentum J (= j + ℓ) is


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deposited mostly to the rotational angular momentum j’ of C2n+2H2 but minor to product orbital angular momentum ℓ’. j is reactant rotational angular momentum that is close to zero in a supersonic molecular beam. ℓ is reactant orbital angular momentum that is perpendicular to vrel based on ℓ = µ×b×vrel; here, µ is the reduced mass and b the impact parameter of two colliding reactants. On assumption of a triatomic reaction A + BC(j) → AB(j’) + C, ℓ’ is proportional to a mass factor cos2β that is 0.019, 0.025, 0.029 and 0.031 for the title reactions of n = 1 – 4. The product angular distribution will approach isotropic as ℓ’ is close to zero or its magnetic sublevels are equally populated. The exit (or dissociating) transition states have a planar structure with the principal axis c perpendicular to the molecular plane. The type-c rotation is most favorable for reaction proceeding and leads products into an angular distribution enhanced at the forward and backward directions if lifetime is much longer than rotational periods. Since the transition structures are near prolate tops, the type-b rotation is also energetically favorable and leads to a sideway-enhanced angular distribution. The combination of both b- and c-type rotations with some weighting factor results in the less anisotropic angular distribution for the hydrogen-loss channel. On the basis of classical capture theory,22 the maximal impact parameter (bmax) and the corresponding orbital angular momentum (ℓmax) in the reactions of C2nH (n = 1 – 4) + C2H2 are estimated as 3.6, 3.7, 4.0 and 4.2 Å and 183, 254, 305 and 348 ħ in terms of average polarizability and ionization energy of both reactants.17,24 The average polarizabilities are 4.72, 8.59, 14.7, 23.7 and 3.33 Å3 and the ionization energies are 11.6, 10.1, 9.3, 8.7 and 11.4 eV for C2H, C4H, C6H, C8H and C2H2, respectively; the polarizability (ionization energy) of C2nH was calculated at the level of B3LYP (CCSD(T)/B3LYP+ZPE). The rate coefficients of the C2nH + C2H2 reactions are predicted to be (3.7–6.6)×10-10, (3.9–6.8)×10-10, (4.3–7.5)×10-10, and (4.8–


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8.4)×10-10 cm3 molecule-1 s-1 for n = 1 – 4, respectively, in the temperature range 10–300 K based on the capture theory. The capture model overestimates the rate coefficients notably at high temperatures due to ignoring the short-range interaction. Though the capture theory cannot account accurately for the aforementioned experimental rate coefficients of the reactions of C2H and C4H with C2H2, the title reactions might have a rate coefficient on the order of 10-10 cm3 molecule-1 s-1 at temperatures below 300 K. This prediction is beneficial in chemical modeling as the experimental data is unavailable. The interferences of discharge side-products, except the C2n+2H2 species by nonreactive scattering (i.e., the green part in Fig. 2), on the title reactions can be ignored according to the following statements. First, the association of C2n + C2H2 → C2n+2H2 was not observed under the single-collision condition. Second, the product ion-signal ratios are 0.9, 0.3, 0.3 and 0.1 for m/z 49 to 50 u, 73 to 74 u, 97 to 98 u, and 121 to 122 u recorded at 45°, 27°, 19°, and 14°, respectively, with photon energy 11.6 eV. Based on the natural isotopic ratio 0.011 of 13C to 12C, the contribution of

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C12C3H to m/z = 50 u has a ratio of ~ 0.011×4×0.9 = 0.04. In the same

manner, the ratios are 0.02, 0.03, and 0.01 for the contributions of 13C12C5H to 74 u, 13C12C7H to 98 u, and 13C12C9H to 122 u, respectively. However, these ratios might be overestimated because the dissociative ionization C2n+2H2 → C2n+2H+ + H is ignored here. Third, the closed-shell reaction C2nH2 + C2H2 incurs an entrance barrier expectedly much higher than the reactant collision energy. Forth, the species C2nHy with y ≥ 3 in the radical beam are scarce. Moreover, the C2nHy + C2H2 reaction needs eject y pieces of hydrogen atoms to produce C2n+2H2 with a negligibly small branching ratio. Fifth, the dissociation C2n+x+2Hy+2+ → C2n+2H2+ + CxHy (x ≥ 1) was not observed with ionizing photon energy below 12 eV. Finally, the experimental results can be explained satisfactorily with the title reactions.


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In the circumstellar envelope of CRL 618, the column densities of C2H, C4H and C6H were determined as 2.0×1015, 1.2×1015 and ≤ 2.5×1013 cm-2, and of HC2H, HC4H and HC6H as 2.0×1017, 1.2×1017 and 6.0×1016 cm-2, respectively.2,7,8,25 The successive C2H-addition process, C2H + HC2nH → HC2n+2H + H,26 has been considered as a source for the growth of polyynes in CRL 618 whereas the reactions (1) attract less attention in the modeling of chemistry on CRL 618.25 We suggest that the type (1) reactions also play an important role in the formation of polyynes due to the large rate coefficients (a few 10-10 cm3 molecule-1 s-1) at low temperatures and the relatively high concentrations on C2nH and C2H2. Though only HC4H and HC6H have been identified in CRL 618, we suggest that larger polyynes like HC8H and HC10H are producible through the reactions (1) and to be discovered in the future with novel technology. C2H, C4H, C6H, C8H and C2H2 have been observed in TMC-14,5,27 and IRC+10216.6,28 Thus, the reactions (1) might take place also in these two interstellar media. Hansen et al. detected HC2n+2H (n = 1 – 4) in fuel-rich flames of allene (H2CCCH2), propyne (HCCCH3), and cyclopentene (cyclo-C5H8) with flame-sampling molecular-beam VUV photoionization mass spectrometry.3 Polyynes are hard to oxidize under the fuel-rich (or oxygenlean) condition such that their concentrations can be high enough to be detected. The mole fraction of HC2n+2H was about one order of magnitude less than that of HC2nH.3 C2H2 is an abundant intermediate in combustion of hydrocarbons and has a peak mole fraction ~ 0.05 in the experiments of Hansen et al.3 Highly reactive radicals like C2nH in flames typically cannot be probed due to their low concentrations. Nonetheless, we suggest that the series of reactions (1) are responsible for the growth of polyynes in flames based on the current work. In the present work, we also observed the synthesis of C2n+2H2 (n = 1 – 4) from the discharge of 1% C2H2/He in


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a molecular beam. Therefore, the title reactions play a crucial role in formation of polyynes in combustion and discharge processes of hydrocarbons. In summary, the dynamics of reactions of C2H, C4H, C6H and C8H radicals with C2H2 were explored in crossed molecular beams with product TOF and PIE spectroscopy and with quantum-chemical calculations. The C2nH (n = 1 – 4) radical adds to a carbon atom of C2H2 to form a complex HC2nCHCH that subsequently decomposes to HC2n+2H + H. The translationalenergy release is biased to low energy but extends to the energetic limit of product HC2n+2H + H. The kinematic constraint can account qualitatively for the less anisotropic angular distribution of the hydrogen-loss channel. The measurement of product photoionization threshold confirms the production of polyynes HC2n+2H. The efficiency of HC2n+2H production declines as the carbon number increases. This work verifies that the barrierless exothermic title reactions are important sources for the formation of polyynes not only in hot combustion processes but also in extremely cold interstellar media.

ASSOCIATED CONTENT Supporting Information Mass spectrum of hydrocarbon species synthesized from 1% C2H2/He by pulsed high-voltage discharge. Photoionization-efficiency spectra of C4H, C6H, and C8H. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank the National Synchrotron Radiation Research Center (NSRRC) and the Ministry of Science and Technology (MOST), Taiwan (grant No. MOST103-2113-M-213-003-MY3) for supports.

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Formation of Polyynes C4H2, C6H2, C8H2, and C10H2 from

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