Dynamics of the reaction of C2 with C6H2: An implication for the formation of interstellar C8H Yi-Lun Sun, Wen-Jian Huang, Chih-Hao Chin, and Shih-Huang Lee
Citation: The Journal of Chemical Physics 141, 194305 (2014); doi: 10.1063/1.4901981 View online: http://dx.doi.org/10.1063/1.4901981 View Table of Contents: http://aip.scitation.org/toc/jcp/141/19 Published by the American Institute of Physics
THE JOURNAL OF CHEMICAL PHYSICS 141, 194305 (2014)
Dynamics of the reaction of C2 with C6 H2 : An implication for the formation of interstellar C8 H Yi-Lun Sun, Wen-Jian Huang, Chih-Hao Chin, and Shih-Huang Leea) National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan
(Received 20 September 2014; accepted 5 November 2014; published online 20 November 2014) The reaction C2 + C6 H2 → C8 H + H was investigated for the first time. Reactant C2 (C6 H2 ) was synthesized from 1% C3 F6 /He (5% C2 H2 /He) by pulsed high-voltage discharge. We measured the translational-energy distribution, the angular distribution, and the photoionization spectrum of product C8 H in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet ionization. This reaction released average translational energy of 8.5 kcal mol−1 corresponding to a fraction of 0.37 in translation. C8 H was identified as octatetranyl based on the maximal translational-energy release 23 ± 2 kcal mol−1 and the ionization threshold 8.9 ± 0.2 eV. Kinematic constraints can qualitatively account for the nearly isotropic angular distribution. The quantum-chemical calculations indicate that the exothermic reactions C2 (X 1 g + /a 3 u ) + HC6 H → C8 H + H can proceed without entrance and exit barriers, implying the importance in the cold interstellar medium. This work verifies that interstellar C8 H can be formed through the C2 + C6 H2 reaction. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4901981] I. INTRODUCTION
In addition to atomic carbon, small carbon clusters C2 , C3 , and C5 were discovered in interstellar space.1, 2 In contrast, interstellar C4 has not been identified unambiguously.3 Dicarbon C2 is a ubiquitous interstellar species that has been observed toward over 50 stars.4 The first triplet state a 3 u of C2 (hereafter designated as 3 C2 ) lies merely 1.74 kcal mol−1 above its ground state X 1 g + (hereafter designated as 1 C2 ).5 C2 radicals are reactive with unsaturated hydrocarbons (e.g., C2 H2 , C2 H4 , C3 H4 , and C3 H6 ) typically without entrance barriers.6, 7 Therefore, dicarbon might play an important role in carbon chemistry of interstellar environments, planetary atmospheres, and hydrocarbon combustion. Polyynes (polyacetylenes, HC2n H) are hydrocarbons with alternating single and triple bonds. The two smallest polyynes, diacetylene (HC4 H) and triacetylene (HC6 H), were identified first in the circumstellar envelopes of CRL 618 and CRL 2688; C4 H2 (C6 H2 ) is surprisingly 0.6 (0.3) times the abundance of ethyne (acetylene, C2 H2 ).8 Polyynes were proposed to be precursors or intermediates in the synthesis of large hydrocarbons like polycyclic aromatic hydrocarbons (PAH), fullrenes, and soots. The isomers cumulene carbenes (H2 C2n ), e.g., butatrienylidene (H2 C4 ) and hexapentaenylidene (H2 C6 ), that have three or more cumulative double bonds have been identified in the Taurus Molecular Cloud (TMC-1)9 and in the circumstellar envelope of carbonrich star IRC+10216.10 Polyynic radicals (C2n H), e.g., C2 H, C4 H, C6 H, and C8 H, were detected in hydrocarbon-rich interstellar and circumstellar environments.10 Their abundances typically decrease a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: +886-3-578-0281. Fax: +886-3-578-3813.
0021-9606/2014/141(19)/194305/8/$30.00
with the increase of the carbon-chain length. C2n H is reactive with unsaturated hydrocarbons typically without any entrance barrier. It is suggested that the reaction C2n H + C2m H2 is responsible for the growth of large polyynes. Interstellar C2n H could be produced from photodissociation of C2n H2 via hydrogen-atom elimination and from chemical reactions like C2 + C2n-2 H2 → C2n H + H. Only the reactions of dicarbon with acetylene and diacetylene were investigated in kinetics and/or dynamics. The reactions of 1 C2 and 3 C2 with C2 H2 had rate coefficients (2.4– 4.8) × 10−10 and (0.8–1.6) × 10−10 cm3 molecule−1 s−1 , respectively, in the temperature range 24–300 K, indicative of no entrance barriers; 1 C2 is about three times the reaction cross section of 3 C2 .11 The reactions C2 + C2 H2 → C4 H + H and C2 + C4 H2 → C6 H + H were investigated in crossed-molecular beams by interrogating translationalenergy distributions and angular distributions of products C4 H and C6 H.12–14 The quantum-chemical calculations12, 13 indicated that 1 C2 adds to the C≡C bond of acetylene (or diacetylene) to form a cyclic complex without any entrance barrier. Subsequently, the complex rearranges to diacetylene (or triacetylene) followed by decomposition to C4 H + H (or C6 H + H) without an exit barrier. In contrast, the reaction of 3 C2 with acetylene (or diacetylene) occurs on the triplet potentialenergy surface without an entrance barrier but possibly with an exit barrier, depending on the decomposition path leading to C4 H + H (or C6 H + H). Octatetranyl (C8 H) is the largest polyynic radical that has been detected in the interstellar medium;10 however, its formation mechanism is still unclear. The synthesis of unstable reactants is the bottleneck for the study of reactions of dicarbon with large polyynes. To the best of our knowledge, there is no literature report on the reactions of C6 H2 with any molecular species. In the present work, reactants C2 and
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C6 H2 were synthesized separately from C3 F6 (hexafluoropropene) and C2 H2 by discharge. We investigated the reaction C2 + C6 H2 → C8 H + H in crossed-molecular beams by interrogating product C8 H with synchrotron vacuum-ultraviolet (VUV) ionization. The translational-energy distribution, the angular distribution, and the photoionization spectrum of C8 H were revealed. II. EXPERIMENTS
The crossed molecular-beam apparatus that comprises two source chambers, a reaction chamber, and a detection chamber has been described in Ref. 15. Both reactants C2 and C6 H2 were synthesized in situ. Source chamber #1 (#2) was equipped with an Even-Lavie valve and a two-electrode discharge device16 to generate a pulsed molecular beam of C2 (C6 H2 ), hereafter designated as B1 (B2), from a mixture of 1% C3 F6 /He (5% C2 H2 /He) at a stagnation pressure of 105 psia. While a gas pulse passed through the discharge region, a high-voltage pulse (−1 kV, 20 μs) was applied to the cathode to ignite a plasma along with production of the desired molecular species. The current was set at 40 mA for the C3 F6 discharge and 20 mA for the C2 H2 discharge. The C2 (C6 H2 ) pulse had a mean speed 2150 (1780) m s−1 and a speed ratio better than 6. Both the collimated molecular beams B1 and B2 intercepted each other at 90◦ with collision energy (Ec ) 16.9 kcal mol−1 between C2 and C6 H2 . After a free flight along a path of length 100.5 mm, reaction products were ionized with tunable synchrotron radiation that had a photon flux ∼ 1016 photons s−1 and an energy resolution ∼4% in the VUV region. The product cations that had a specific mass-to-charge (m/z) ratio were selected with a quadrupole-mass filter and then detected with a Daly-type ion counter. A multi-channel scaler sampled the ion signals into 4000 bins of width 1 μs. After subtraction of the ion flight interval from the total flight duration, a neutral-product time-offlight (TOF) spectrum can be obtained. The source-chamber assembly is rotatable, which allows us to record TOF spectra at various laboratory angles (); = 0◦ and 90◦ are defined as the propagating directions of B1 and B2, respectively. To yield a laboratory angular distribution, product TOF spectra were measured with a appropriate VUV photon energy by rotating the source-chamber assembly. Besides, TOF spectra were measured at a fixed laboratory angle by scanning the VUV photon energy to yield a photoionization spectrum. The laboratory angle (or photon energy) was scanned back to back to avoid a long-term drift in the measurement of laboratory angular distributions (or photoionization spectra). The TOF spectra recorded at the same experimental condition were summed together to yield a good signal-to-noise ratio. Components of the experimental apparatus were synchronized at 200 Hz with two pulse generators. III. RESULTS AND ANALYSIS
The upper (lower) panel of Fig. 1 exhibits the photoionization spectrum of reactant C2 (C6 H2 ) recorded at = 0◦ (90◦ ) in the energy range 9.1–14.5 (7.6–11.6) eV. A small pinhole was employed as the entrance of the ionizer to avoid sig-
FIG. 1. Photoionization spectra of reactants C2 (upper) and C6 H2 (lower) produced from 1% C3 F6 /He and 5% C2 H2 /He, respectively, by discharge. Arrows indicate the literature-reported ionization energies 11.4 eV of C2 and 9.45 eV of HC6 H. The photoionization spectrum of C2 produced from the discharge of 10% CO/He was also shown in the upper panel. The photoionization spectrum of HC6 H produced from the C4 H + C2 H2 reaction was also shown in the lower panel.
nal saturation. The small variation of photon flux versus photon energy was not corrected. Baselines of the photoionization spectra were shifted to zero. Arrows indicate the literaturereported ionization energy 11.4 eV of C2 and 9.45 eV of HC6 H.17, 18 The upper panel also shows the photoionization spectrum of C2 produced from the discharge of 10% CO/He for comparison.7 The lower panel also shows the photoionization spectrum of HC6 H produced from the reaction of C4 H + C2 H2 → HC6 H + H. The good agreement in photoionization spectra indicates that C2 and HC6 H were successfully synthesized in the present work. Figure 2 depicts the Newton diagram superimposed with the two-dimensional velocity-distribution map of product C8 H for the reaction C2 + C6 H2 → C8 H + H. VC2 (VC6H2 ) denotes the velocity of reactant C2 (C6 H2 ). CM (= 68.6◦ ) represents the flight direction of the center of mass (CM) of the reaction system. Because product C8 H had small velocity in the CM frame, the product was concentrated near CM . The panel above the Newton diagram presents the laboratory angular distribution P(), i.e., the integral ion signal versus the laboratory angle, along with the simulation for the C8 H product. Figure 3 exhibits the angle-specific TOF spectra along with the simulation of product C8 H (m/z = 97 u) recorded at 12 laboratory angles from 63◦ to 74◦ with photon energy 11.6 eV. Some molecular species coming mainly from the molecular beam B2 by nonreactive scattering appeared in the
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FIG. 2. (Lower) Newton diagram superimposed with the two-dimensional velocity-distribution map of product C8 H for the reaction C2 + C6 H2 → C8 H + H. VC2 (VC6H2 ) denotes the velocity of reactant C2 (C6 H2 ). CM represents the flight direction of the center of mass of the reaction system. Dashed lines denote the detection axes at three laboratory angles. (Upper) Laboratory angular distribution of product C8 H recorded at m/z = 97 u with photoionization energy 11.6 eV. Open circles denote the experimental data and the solid curve denotes the simulation.
TOF spectra; the species might be C8 H produced by the discharge of C2 H2 . To get the nonreactive part alone, we turned off the B1 discharge and took the angle-specific TOF spectra in the same angular range. Because the velocity of B1 becomes a little slow as the discharge is off, the TOF distribution of the nonreactive part differs somewhat from that taken as the B1 discharge is on. After removal of the nonreactive part, the TOF spectra of the reactive part can be obtained as presented in Fig. 3. Some artifacts near 45 μs and some backgrounds behind 100 μs retain in the TOF spectra. The CM translational-energy distribution P(Et ) and the CM angular distribution P(θ ), employed to simulate the TOF spectra and the laboratory angular distribution, are presented in the top and middle panels of Fig. 4. Solid lines are the bestfit functions and dotted lines are acceptable fits within the error limits. Here, Et includes the translational energies of both products C8 H and H. θ = 0◦ (180◦ ) is defined as the incidence direction of reactant C2 (C6 H2 ) in the CM frame. Arrows indicate the energetic limits 21.3 and 23.0 kcal mol−1 of the reactions 1,3 C2 + HC6 H → C8 H + H. The P(Et ) and P(θ ) distributions were also employed to construct the velocity-distribution map shown in Fig. 2. The bottom panel of Fig. 4 exhibits the photoionization spectrum of product C8 H recorded at = 69◦ in the energy range 8.3–11.6 eV. An arrow indicates the ionization energy 8.9 eV of C8 H.
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FIG. 3. Angle-specific TOF spectra of product C8 H recorded at m/z = 97 u with photoionization energy 11.6 eV. Open circles denote the experimental data and solid curves denote the simulations. Each panel shows the corresponding laboratory angle.
FIG. 4. CM translational-energy distribution (top), CM angular distribution (middle), and photoionization spectrum (bottom) of product C8 H. In the upper two panels, solid lines are the best-fit functions and dotted lines are acceptable fits within the error limits. In the bottom panel, C8 H was recorded at = 69◦ . Arrows indicate the energetic limits 21.3 and 23.0 kcal mol−1 of the reactions 1,3 C2 + HC6 H → C8 H + H and the ionization energy 8.9 ± 0.2 eV of product C8 H.
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FIG. 5. Potential-energy surface of the reaction 1 C2 + HC6 H → C8 H + H calculated at the level of CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ. Molecular structures optimized are shown along with their energy levels. Relative energies are given in kcal mol−1 .
Figures 5 and 6 exhibit the potential-energy surfaces of reactions 1,3 C2 + HC6 H → C8 H + H, respectively, calculated with the method CCSD(T)/aug-cc-pVTZ//B3LYP/augcc-pVDZ. The zero-point energy correction was included at the level of B3LYP/aug-cc-pVDZ. Molecular structures optimized at the level of B3LYP/aug-cc-pVDZ were shown therein. The connection of each transition structure (TS) with its reactant and product was confirmed with the calculation of intrinsic reaction coordinate (IRC). The reaction enthalpy H at 0 K and the available energy (Eava = Ec – H) were calculated to be −4.4 kcal mol−1 and 21.3 kcal mol−1 , respectively, for the 1 C2 + HC6 H → C8 H + H reaction. In the
present computation, 3 C2 lies above 1 C2 by 3.8 kcal mol−1 that is 2.06 kcal mol−1 larger than the experimental value 1.74 kcal mol−1 ;5 the difference is attributed to the computational uncertainty. Thus, the available energy of the 3 C2 + HC6 H → C8 H + H reaction is determined as 23.0 kcal mol−1 . The energy difference between 1 C2 and 3 C2 cannot be distinguished in the present experiments. Figures 7(a)–7(d) present the potential-energy surfaces scanned from intermediates Is1, Is2, Is3, and Is4, respectively, to 1 C2 + C6 H2 with the method B3LYP/aug-cc-pVDZ in order to verify the absence of entrance barriers. The zeropoint energy is not included and the potential energy of
FIG. 6. Potential-energy surface of the reaction 3 C2 + HC6 H → C8 H + H calculated at the level of CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ. Molecular structures optimized are shown along with their energy levels. Relative energies are given in kcal mol−1 .
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to 3 C2 + C6 H2 with a step increment 0.1 Å or 0.2 Å. The potential energy of asymptote 3 C2 + C6 H2 is set to zero. The potential energy goes up to the asymptote without a maximum higher than the asymptotic limit, i.e., there is no barrier on these four reaction paths. Fig. 8(b) also presents a potential-energy-surface scan from It4 toward the asymptote (open circle). The surface has an energy maximum located at 2.3 Å. Using the corresponding molecular structure as an initial guess for transition-state optimization, this structure is optimized to TSt2 shown in Fig. 6.
IV. DISCUSSION
FIG. 7. Potential-energy-surface scans from intermediates Is1 (a), Is2 (b), Is3 (c), and Is4 (d) to asymptote 1 C2 + C6 H2 with the method B3LYP/aug-cc-pVDZ. The potential energy was calculated along the C–C bond marked in red with a step increment of 0.1 Å. In each step optimization, one or two CCC angles marked with arcs were fixed but the other degrees of freedom were relaxed.
asymptote 1 C2 + C6 H2 is set to zero. The potential energy was calculated along the C–C bond marked in red with a step increment of 0.1 Å. In each step optimization, one or two CCC angles marked with arcs were fixed to partly constrain the geometry but the other degrees of freedom were relaxed. Each potential-energy surface goes up to the asymptote 1 C2 + C6 H2 without a maximum higher than the asymptote, indicating that there is no barrier above the reactant 1 C2 + C6 H2 for the head-on and side-on additions of 1 C2 to a C≡C bond of C6 H2 . Note that the potential-energy surface scanned is not guaranteed to be the energy-minimum path. Two molecular structures optimized with the red C–C bond fixed at 1.5 Å and 5.5 Å were shown in each panel. Figures 8(a)–8(d) present the potential-energy surfaces scanned from intermediates It1, It2, It3, and It6, respectively,
FIG. 8. Potential-energy-surface scans from intermediates It1 (a), It2 (b), It3 (c), and It6 (d) to asymptote 3 C2 + C6 H2 with the method B3LYP/aug-ccpVDZ. The potential energy was calculated along the C–C bond marked in red with a step increment of 0.1 Å or 0.2 Å. In each step optimization, the CCC angles marked with arcs were fixed but the other degrees of freedom were relaxed. There is no constraint in the scan from It2 to the asymptote. Panel (b) also presents a potential-energy-surface scan from It4 towards the asymptote (open circle).
Despite the syntheses of a variety of fluorocarbon species (Cp Fq ) in B1 and hydrocarbon species (Cx Hy ) in B2 by discharge, the title reaction was conducted successfully in the present work based on the following facts. First, the combination of mass selectivity and soft photoionization allows us to detect the desired species without a fatal interference from other species. Provided that the desired reaction product has a mass different from those of Cp Fq and Cx Hy , the product detection will incur merely a small or negligible interference from both the molecular beams. The low discharge current and the short discharge duration limit the synthesis of large fluorocarbons and hydrocarbons. We inspected the ion signals of species Cx Hy that have 1 ≤ x ≤ 11 and 0 ≤ y ≤ 2x + 2 with ionizing photon energy 12.5 eV. Due to a large background from the precursor, the detection of species C2 Hy (0 ≤ y ≤ 3) was omitted. For each x value larger than 2, the ion signal of Cx Hy has a maximum at y = 2 and diminishes drastically as the y value is away from 2. The ion signal of Cx Hy that has x ≥ 9 or y ≥ 5 is negligibly small. The ion-signal ratio is 23:100:3:43:1:4 for Cx H2 with x = 3– 8. Of all the CHy species, CH3 is most abundant but has an ion signal as small as C7 H2 . The saturated species Cx H2x+2 , albeit thermodynamically stable and chemically inert, are almost absent in the present discharge condition. C8 H is about one order of magnitude less abundant than C8 H2 in B2 and gives rise to a background in the detection of product C8 H as mentioned in Sec. III. Second, the velocity of Cp Fq is ∼ 2150 m s−1 and of Cx Hy is ∼ 1780 m s−1 so that each Cp Fq + Cx Hy reaction has a distinct CM angle. For the H-loss or H2 -loss channel, the carbon-containing product is concentrated near the CM angle due to a small product velocity in the CM frame. Of all the Cp Fq + Cx Hy reactions, only the title reaction C2 + C6 H2 → C8 H + H has a product signal at m/z = 97 u with a maximum near its CM angle 68.6◦ . There is no evidence for the C2 + C6 H3 reaction that leads probably to C8 H2 + H or to C8 H + H2 because C6 H3 is one order of magnitude less abundant than C6 H2 in B2. Subtracting the contribution of isotopomer 13 12 C C5 H2 that has an abundance ratio [13 C12 C5 H2 ]/[12 C6 H2 ] ≈ 0.011 × 6 = 0.066, C6 H3 becomes ∼ 3% of C6 H2 in abundance. Further considering the possible mass leak from m/z = 74 u to 75 u, C6 H3 might become much less abundant. Therefore, the reaction C2 + C6 H3 → C8 H + H2 cannot compete with the rapid title reaction. Of all the possible products from the Cp Fq + Cx Hy reactions, C2 H16 F3 , C4 H11 F2 , and
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C6 H6 F have the same mass as C8 H. However, it is impossible to form C2 H16 F3 and C4 H11 F2 because the total number of H and F atoms is over saturation. Furthermore, the reaction Cp F (p = 0 – 6) + C6-p H7 → C6 H6 F + H can be omitted because the yield of C6-p H7 in B2 is negligibly small and the corresponding CM angle is away from the present value 68.6◦ ; for instance, 73.8◦ for p = 0, 60.8◦ for p = 1, 46.6◦ for p = 2, and 32.9◦ for p = 3. Third, C8 H is producible in stoichiometry from reactions Cp + C8–p H2 → C8 H + H that have CM angles at 80.4◦ , 68.6◦ , 55.0◦ , 40.8◦ , 27.7◦ , 16.6◦ , and 7.9◦ for p = 1–7, respectively. Nonetheless, only the title reaction C2 + C6 H2 can fit the experimental observations satisfactorily. The other reactions have CM angles away from the angular range 63◦ –74◦ of C8 H recorded in the present work and thus have minor or negligible contributions into this angular range. We inspected the ion signals of m/z = 97 u at the corresponding CM angles for p = 1–5 and found only the signal at 68.6◦ could have a better signal-to-noise ratio. The signal at 80.4◦ incurs a large background from B2 and the signals at 55.0◦ , 40.8◦ , and 27.7◦ are almost below the limit of detection sensitivity. The ion-signal ratio of C2 :C3 :C4 :C5 in B1 was determined as 12:11:
Citation: The Journal of Chemical Physics 141, 194305 (2014); doi: 10.1063/1.4901981 View online: http://dx.doi.org/10.1063/1.4901981 View Table of Contents: http://aip.scitation.org/toc/jcp/141/19 Published by the American Institute of Physics
THE JOURNAL OF CHEMICAL PHYSICS 141, 194305 (2014)
Dynamics of the reaction of C2 with C6 H2 : An implication for the formation of interstellar C8 H Yi-Lun Sun, Wen-Jian Huang, Chih-Hao Chin, and Shih-Huang Leea) National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan
(Received 20 September 2014; accepted 5 November 2014; published online 20 November 2014) The reaction C2 + C6 H2 → C8 H + H was investigated for the first time. Reactant C2 (C6 H2 ) was synthesized from 1% C3 F6 /He (5% C2 H2 /He) by pulsed high-voltage discharge. We measured the translational-energy distribution, the angular distribution, and the photoionization spectrum of product C8 H in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet ionization. This reaction released average translational energy of 8.5 kcal mol−1 corresponding to a fraction of 0.37 in translation. C8 H was identified as octatetranyl based on the maximal translational-energy release 23 ± 2 kcal mol−1 and the ionization threshold 8.9 ± 0.2 eV. Kinematic constraints can qualitatively account for the nearly isotropic angular distribution. The quantum-chemical calculations indicate that the exothermic reactions C2 (X 1 g + /a 3 u ) + HC6 H → C8 H + H can proceed without entrance and exit barriers, implying the importance in the cold interstellar medium. This work verifies that interstellar C8 H can be formed through the C2 + C6 H2 reaction. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4901981] I. INTRODUCTION
In addition to atomic carbon, small carbon clusters C2 , C3 , and C5 were discovered in interstellar space.1, 2 In contrast, interstellar C4 has not been identified unambiguously.3 Dicarbon C2 is a ubiquitous interstellar species that has been observed toward over 50 stars.4 The first triplet state a 3 u of C2 (hereafter designated as 3 C2 ) lies merely 1.74 kcal mol−1 above its ground state X 1 g + (hereafter designated as 1 C2 ).5 C2 radicals are reactive with unsaturated hydrocarbons (e.g., C2 H2 , C2 H4 , C3 H4 , and C3 H6 ) typically without entrance barriers.6, 7 Therefore, dicarbon might play an important role in carbon chemistry of interstellar environments, planetary atmospheres, and hydrocarbon combustion. Polyynes (polyacetylenes, HC2n H) are hydrocarbons with alternating single and triple bonds. The two smallest polyynes, diacetylene (HC4 H) and triacetylene (HC6 H), were identified first in the circumstellar envelopes of CRL 618 and CRL 2688; C4 H2 (C6 H2 ) is surprisingly 0.6 (0.3) times the abundance of ethyne (acetylene, C2 H2 ).8 Polyynes were proposed to be precursors or intermediates in the synthesis of large hydrocarbons like polycyclic aromatic hydrocarbons (PAH), fullrenes, and soots. The isomers cumulene carbenes (H2 C2n ), e.g., butatrienylidene (H2 C4 ) and hexapentaenylidene (H2 C6 ), that have three or more cumulative double bonds have been identified in the Taurus Molecular Cloud (TMC-1)9 and in the circumstellar envelope of carbonrich star IRC+10216.10 Polyynic radicals (C2n H), e.g., C2 H, C4 H, C6 H, and C8 H, were detected in hydrocarbon-rich interstellar and circumstellar environments.10 Their abundances typically decrease a) Author to whom correspondence should be addressed. Electronic mail:
[email protected]. Tel.: +886-3-578-0281. Fax: +886-3-578-3813.
0021-9606/2014/141(19)/194305/8/$30.00
with the increase of the carbon-chain length. C2n H is reactive with unsaturated hydrocarbons typically without any entrance barrier. It is suggested that the reaction C2n H + C2m H2 is responsible for the growth of large polyynes. Interstellar C2n H could be produced from photodissociation of C2n H2 via hydrogen-atom elimination and from chemical reactions like C2 + C2n-2 H2 → C2n H + H. Only the reactions of dicarbon with acetylene and diacetylene were investigated in kinetics and/or dynamics. The reactions of 1 C2 and 3 C2 with C2 H2 had rate coefficients (2.4– 4.8) × 10−10 and (0.8–1.6) × 10−10 cm3 molecule−1 s−1 , respectively, in the temperature range 24–300 K, indicative of no entrance barriers; 1 C2 is about three times the reaction cross section of 3 C2 .11 The reactions C2 + C2 H2 → C4 H + H and C2 + C4 H2 → C6 H + H were investigated in crossed-molecular beams by interrogating translationalenergy distributions and angular distributions of products C4 H and C6 H.12–14 The quantum-chemical calculations12, 13 indicated that 1 C2 adds to the C≡C bond of acetylene (or diacetylene) to form a cyclic complex without any entrance barrier. Subsequently, the complex rearranges to diacetylene (or triacetylene) followed by decomposition to C4 H + H (or C6 H + H) without an exit barrier. In contrast, the reaction of 3 C2 with acetylene (or diacetylene) occurs on the triplet potentialenergy surface without an entrance barrier but possibly with an exit barrier, depending on the decomposition path leading to C4 H + H (or C6 H + H). Octatetranyl (C8 H) is the largest polyynic radical that has been detected in the interstellar medium;10 however, its formation mechanism is still unclear. The synthesis of unstable reactants is the bottleneck for the study of reactions of dicarbon with large polyynes. To the best of our knowledge, there is no literature report on the reactions of C6 H2 with any molecular species. In the present work, reactants C2 and
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C6 H2 were synthesized separately from C3 F6 (hexafluoropropene) and C2 H2 by discharge. We investigated the reaction C2 + C6 H2 → C8 H + H in crossed-molecular beams by interrogating product C8 H with synchrotron vacuum-ultraviolet (VUV) ionization. The translational-energy distribution, the angular distribution, and the photoionization spectrum of C8 H were revealed. II. EXPERIMENTS
The crossed molecular-beam apparatus that comprises two source chambers, a reaction chamber, and a detection chamber has been described in Ref. 15. Both reactants C2 and C6 H2 were synthesized in situ. Source chamber #1 (#2) was equipped with an Even-Lavie valve and a two-electrode discharge device16 to generate a pulsed molecular beam of C2 (C6 H2 ), hereafter designated as B1 (B2), from a mixture of 1% C3 F6 /He (5% C2 H2 /He) at a stagnation pressure of 105 psia. While a gas pulse passed through the discharge region, a high-voltage pulse (−1 kV, 20 μs) was applied to the cathode to ignite a plasma along with production of the desired molecular species. The current was set at 40 mA for the C3 F6 discharge and 20 mA for the C2 H2 discharge. The C2 (C6 H2 ) pulse had a mean speed 2150 (1780) m s−1 and a speed ratio better than 6. Both the collimated molecular beams B1 and B2 intercepted each other at 90◦ with collision energy (Ec ) 16.9 kcal mol−1 between C2 and C6 H2 . After a free flight along a path of length 100.5 mm, reaction products were ionized with tunable synchrotron radiation that had a photon flux ∼ 1016 photons s−1 and an energy resolution ∼4% in the VUV region. The product cations that had a specific mass-to-charge (m/z) ratio were selected with a quadrupole-mass filter and then detected with a Daly-type ion counter. A multi-channel scaler sampled the ion signals into 4000 bins of width 1 μs. After subtraction of the ion flight interval from the total flight duration, a neutral-product time-offlight (TOF) spectrum can be obtained. The source-chamber assembly is rotatable, which allows us to record TOF spectra at various laboratory angles (); = 0◦ and 90◦ are defined as the propagating directions of B1 and B2, respectively. To yield a laboratory angular distribution, product TOF spectra were measured with a appropriate VUV photon energy by rotating the source-chamber assembly. Besides, TOF spectra were measured at a fixed laboratory angle by scanning the VUV photon energy to yield a photoionization spectrum. The laboratory angle (or photon energy) was scanned back to back to avoid a long-term drift in the measurement of laboratory angular distributions (or photoionization spectra). The TOF spectra recorded at the same experimental condition were summed together to yield a good signal-to-noise ratio. Components of the experimental apparatus were synchronized at 200 Hz with two pulse generators. III. RESULTS AND ANALYSIS
The upper (lower) panel of Fig. 1 exhibits the photoionization spectrum of reactant C2 (C6 H2 ) recorded at = 0◦ (90◦ ) in the energy range 9.1–14.5 (7.6–11.6) eV. A small pinhole was employed as the entrance of the ionizer to avoid sig-
FIG. 1. Photoionization spectra of reactants C2 (upper) and C6 H2 (lower) produced from 1% C3 F6 /He and 5% C2 H2 /He, respectively, by discharge. Arrows indicate the literature-reported ionization energies 11.4 eV of C2 and 9.45 eV of HC6 H. The photoionization spectrum of C2 produced from the discharge of 10% CO/He was also shown in the upper panel. The photoionization spectrum of HC6 H produced from the C4 H + C2 H2 reaction was also shown in the lower panel.
nal saturation. The small variation of photon flux versus photon energy was not corrected. Baselines of the photoionization spectra were shifted to zero. Arrows indicate the literaturereported ionization energy 11.4 eV of C2 and 9.45 eV of HC6 H.17, 18 The upper panel also shows the photoionization spectrum of C2 produced from the discharge of 10% CO/He for comparison.7 The lower panel also shows the photoionization spectrum of HC6 H produced from the reaction of C4 H + C2 H2 → HC6 H + H. The good agreement in photoionization spectra indicates that C2 and HC6 H were successfully synthesized in the present work. Figure 2 depicts the Newton diagram superimposed with the two-dimensional velocity-distribution map of product C8 H for the reaction C2 + C6 H2 → C8 H + H. VC2 (VC6H2 ) denotes the velocity of reactant C2 (C6 H2 ). CM (= 68.6◦ ) represents the flight direction of the center of mass (CM) of the reaction system. Because product C8 H had small velocity in the CM frame, the product was concentrated near CM . The panel above the Newton diagram presents the laboratory angular distribution P(), i.e., the integral ion signal versus the laboratory angle, along with the simulation for the C8 H product. Figure 3 exhibits the angle-specific TOF spectra along with the simulation of product C8 H (m/z = 97 u) recorded at 12 laboratory angles from 63◦ to 74◦ with photon energy 11.6 eV. Some molecular species coming mainly from the molecular beam B2 by nonreactive scattering appeared in the
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FIG. 2. (Lower) Newton diagram superimposed with the two-dimensional velocity-distribution map of product C8 H for the reaction C2 + C6 H2 → C8 H + H. VC2 (VC6H2 ) denotes the velocity of reactant C2 (C6 H2 ). CM represents the flight direction of the center of mass of the reaction system. Dashed lines denote the detection axes at three laboratory angles. (Upper) Laboratory angular distribution of product C8 H recorded at m/z = 97 u with photoionization energy 11.6 eV. Open circles denote the experimental data and the solid curve denotes the simulation.
TOF spectra; the species might be C8 H produced by the discharge of C2 H2 . To get the nonreactive part alone, we turned off the B1 discharge and took the angle-specific TOF spectra in the same angular range. Because the velocity of B1 becomes a little slow as the discharge is off, the TOF distribution of the nonreactive part differs somewhat from that taken as the B1 discharge is on. After removal of the nonreactive part, the TOF spectra of the reactive part can be obtained as presented in Fig. 3. Some artifacts near 45 μs and some backgrounds behind 100 μs retain in the TOF spectra. The CM translational-energy distribution P(Et ) and the CM angular distribution P(θ ), employed to simulate the TOF spectra and the laboratory angular distribution, are presented in the top and middle panels of Fig. 4. Solid lines are the bestfit functions and dotted lines are acceptable fits within the error limits. Here, Et includes the translational energies of both products C8 H and H. θ = 0◦ (180◦ ) is defined as the incidence direction of reactant C2 (C6 H2 ) in the CM frame. Arrows indicate the energetic limits 21.3 and 23.0 kcal mol−1 of the reactions 1,3 C2 + HC6 H → C8 H + H. The P(Et ) and P(θ ) distributions were also employed to construct the velocity-distribution map shown in Fig. 2. The bottom panel of Fig. 4 exhibits the photoionization spectrum of product C8 H recorded at = 69◦ in the energy range 8.3–11.6 eV. An arrow indicates the ionization energy 8.9 eV of C8 H.
J. Chem. Phys. 141, 194305 (2014)
FIG. 3. Angle-specific TOF spectra of product C8 H recorded at m/z = 97 u with photoionization energy 11.6 eV. Open circles denote the experimental data and solid curves denote the simulations. Each panel shows the corresponding laboratory angle.
FIG. 4. CM translational-energy distribution (top), CM angular distribution (middle), and photoionization spectrum (bottom) of product C8 H. In the upper two panels, solid lines are the best-fit functions and dotted lines are acceptable fits within the error limits. In the bottom panel, C8 H was recorded at = 69◦ . Arrows indicate the energetic limits 21.3 and 23.0 kcal mol−1 of the reactions 1,3 C2 + HC6 H → C8 H + H and the ionization energy 8.9 ± 0.2 eV of product C8 H.
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FIG. 5. Potential-energy surface of the reaction 1 C2 + HC6 H → C8 H + H calculated at the level of CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ. Molecular structures optimized are shown along with their energy levels. Relative energies are given in kcal mol−1 .
Figures 5 and 6 exhibit the potential-energy surfaces of reactions 1,3 C2 + HC6 H → C8 H + H, respectively, calculated with the method CCSD(T)/aug-cc-pVTZ//B3LYP/augcc-pVDZ. The zero-point energy correction was included at the level of B3LYP/aug-cc-pVDZ. Molecular structures optimized at the level of B3LYP/aug-cc-pVDZ were shown therein. The connection of each transition structure (TS) with its reactant and product was confirmed with the calculation of intrinsic reaction coordinate (IRC). The reaction enthalpy H at 0 K and the available energy (Eava = Ec – H) were calculated to be −4.4 kcal mol−1 and 21.3 kcal mol−1 , respectively, for the 1 C2 + HC6 H → C8 H + H reaction. In the
present computation, 3 C2 lies above 1 C2 by 3.8 kcal mol−1 that is 2.06 kcal mol−1 larger than the experimental value 1.74 kcal mol−1 ;5 the difference is attributed to the computational uncertainty. Thus, the available energy of the 3 C2 + HC6 H → C8 H + H reaction is determined as 23.0 kcal mol−1 . The energy difference between 1 C2 and 3 C2 cannot be distinguished in the present experiments. Figures 7(a)–7(d) present the potential-energy surfaces scanned from intermediates Is1, Is2, Is3, and Is4, respectively, to 1 C2 + C6 H2 with the method B3LYP/aug-cc-pVDZ in order to verify the absence of entrance barriers. The zeropoint energy is not included and the potential energy of
FIG. 6. Potential-energy surface of the reaction 3 C2 + HC6 H → C8 H + H calculated at the level of CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVDZ. Molecular structures optimized are shown along with their energy levels. Relative energies are given in kcal mol−1 .
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to 3 C2 + C6 H2 with a step increment 0.1 Å or 0.2 Å. The potential energy of asymptote 3 C2 + C6 H2 is set to zero. The potential energy goes up to the asymptote without a maximum higher than the asymptotic limit, i.e., there is no barrier on these four reaction paths. Fig. 8(b) also presents a potential-energy-surface scan from It4 toward the asymptote (open circle). The surface has an energy maximum located at 2.3 Å. Using the corresponding molecular structure as an initial guess for transition-state optimization, this structure is optimized to TSt2 shown in Fig. 6.
IV. DISCUSSION
FIG. 7. Potential-energy-surface scans from intermediates Is1 (a), Is2 (b), Is3 (c), and Is4 (d) to asymptote 1 C2 + C6 H2 with the method B3LYP/aug-cc-pVDZ. The potential energy was calculated along the C–C bond marked in red with a step increment of 0.1 Å. In each step optimization, one or two CCC angles marked with arcs were fixed but the other degrees of freedom were relaxed.
asymptote 1 C2 + C6 H2 is set to zero. The potential energy was calculated along the C–C bond marked in red with a step increment of 0.1 Å. In each step optimization, one or two CCC angles marked with arcs were fixed to partly constrain the geometry but the other degrees of freedom were relaxed. Each potential-energy surface goes up to the asymptote 1 C2 + C6 H2 without a maximum higher than the asymptote, indicating that there is no barrier above the reactant 1 C2 + C6 H2 for the head-on and side-on additions of 1 C2 to a C≡C bond of C6 H2 . Note that the potential-energy surface scanned is not guaranteed to be the energy-minimum path. Two molecular structures optimized with the red C–C bond fixed at 1.5 Å and 5.5 Å were shown in each panel. Figures 8(a)–8(d) present the potential-energy surfaces scanned from intermediates It1, It2, It3, and It6, respectively,
FIG. 8. Potential-energy-surface scans from intermediates It1 (a), It2 (b), It3 (c), and It6 (d) to asymptote 3 C2 + C6 H2 with the method B3LYP/aug-ccpVDZ. The potential energy was calculated along the C–C bond marked in red with a step increment of 0.1 Å or 0.2 Å. In each step optimization, the CCC angles marked with arcs were fixed but the other degrees of freedom were relaxed. There is no constraint in the scan from It2 to the asymptote. Panel (b) also presents a potential-energy-surface scan from It4 towards the asymptote (open circle).
Despite the syntheses of a variety of fluorocarbon species (Cp Fq ) in B1 and hydrocarbon species (Cx Hy ) in B2 by discharge, the title reaction was conducted successfully in the present work based on the following facts. First, the combination of mass selectivity and soft photoionization allows us to detect the desired species without a fatal interference from other species. Provided that the desired reaction product has a mass different from those of Cp Fq and Cx Hy , the product detection will incur merely a small or negligible interference from both the molecular beams. The low discharge current and the short discharge duration limit the synthesis of large fluorocarbons and hydrocarbons. We inspected the ion signals of species Cx Hy that have 1 ≤ x ≤ 11 and 0 ≤ y ≤ 2x + 2 with ionizing photon energy 12.5 eV. Due to a large background from the precursor, the detection of species C2 Hy (0 ≤ y ≤ 3) was omitted. For each x value larger than 2, the ion signal of Cx Hy has a maximum at y = 2 and diminishes drastically as the y value is away from 2. The ion signal of Cx Hy that has x ≥ 9 or y ≥ 5 is negligibly small. The ion-signal ratio is 23:100:3:43:1:4 for Cx H2 with x = 3– 8. Of all the CHy species, CH3 is most abundant but has an ion signal as small as C7 H2 . The saturated species Cx H2x+2 , albeit thermodynamically stable and chemically inert, are almost absent in the present discharge condition. C8 H is about one order of magnitude less abundant than C8 H2 in B2 and gives rise to a background in the detection of product C8 H as mentioned in Sec. III. Second, the velocity of Cp Fq is ∼ 2150 m s−1 and of Cx Hy is ∼ 1780 m s−1 so that each Cp Fq + Cx Hy reaction has a distinct CM angle. For the H-loss or H2 -loss channel, the carbon-containing product is concentrated near the CM angle due to a small product velocity in the CM frame. Of all the Cp Fq + Cx Hy reactions, only the title reaction C2 + C6 H2 → C8 H + H has a product signal at m/z = 97 u with a maximum near its CM angle 68.6◦ . There is no evidence for the C2 + C6 H3 reaction that leads probably to C8 H2 + H or to C8 H + H2 because C6 H3 is one order of magnitude less abundant than C6 H2 in B2. Subtracting the contribution of isotopomer 13 12 C C5 H2 that has an abundance ratio [13 C12 C5 H2 ]/[12 C6 H2 ] ≈ 0.011 × 6 = 0.066, C6 H3 becomes ∼ 3% of C6 H2 in abundance. Further considering the possible mass leak from m/z = 74 u to 75 u, C6 H3 might become much less abundant. Therefore, the reaction C2 + C6 H3 → C8 H + H2 cannot compete with the rapid title reaction. Of all the possible products from the Cp Fq + Cx Hy reactions, C2 H16 F3 , C4 H11 F2 , and
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C6 H6 F have the same mass as C8 H. However, it is impossible to form C2 H16 F3 and C4 H11 F2 because the total number of H and F atoms is over saturation. Furthermore, the reaction Cp F (p = 0 – 6) + C6-p H7 → C6 H6 F + H can be omitted because the yield of C6-p H7 in B2 is negligibly small and the corresponding CM angle is away from the present value 68.6◦ ; for instance, 73.8◦ for p = 0, 60.8◦ for p = 1, 46.6◦ for p = 2, and 32.9◦ for p = 3. Third, C8 H is producible in stoichiometry from reactions Cp + C8–p H2 → C8 H + H that have CM angles at 80.4◦ , 68.6◦ , 55.0◦ , 40.8◦ , 27.7◦ , 16.6◦ , and 7.9◦ for p = 1–7, respectively. Nonetheless, only the title reaction C2 + C6 H2 can fit the experimental observations satisfactorily. The other reactions have CM angles away from the angular range 63◦ –74◦ of C8 H recorded in the present work and thus have minor or negligible contributions into this angular range. We inspected the ion signals of m/z = 97 u at the corresponding CM angles for p = 1–5 and found only the signal at 68.6◦ could have a better signal-to-noise ratio. The signal at 80.4◦ incurs a large background from B2 and the signals at 55.0◦ , 40.8◦ , and 27.7◦ are almost below the limit of detection sensitivity. The ion-signal ratio of C2 :C3 :C4 :C5 in B1 was determined as 12:11:
