C3H Radical Research Articles

Formation of C9H2 and C10H2 from Reactions C3H + C6H2 and C4H + C6H2.

The reactions of C3H and C4H radicals with C6H2 were investigated for the first time. Reactants C3H, C4H, and C6H2 were synthesized in two beams of C2H2 diluted with helium by pulsed high-voltage discharge. We measured translational-energy distributions, angular distributions, and photoionization-efficiency spectra of C9H2 and C10H2 produced from the title reactions in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet photoionization. The C3H (C4H) + C6H2 reaction releases 42% (33%) of available energy into the translational degrees of freedom of product C9H2 (C10H2) + H and scatters products into a nearly isotropic angular distribution. The photoionization-efficiency spectrum of C9H2 (C10H2) is in good agreement with that of C9H2 (C10H2) produced from the C7H (C8H) + C2H2 reaction. The ionization threshold, after deconvolution, was determined as 8.0 ± 0.1 eV for C9H2 and 8.8 ± 0.1 eV for C10H2. The combination of measurements of product translational-energy release and photoionization-efficiency spectra indicates productions of 3HC9H/c-1HC3(C)C5H/c-1HC7(C)CH + H and 1HC10H + H in the two title reactions, which are supported also by quantum-chemical calculations. Ratios branching to the three isomers of C9H2 remain unknown. This work demonstrates that long carbon-chain molecules (e.g., C9H2 and C10H2) can be synthesized from reactions of CmH (e.g., m = 3 and 4) radicals with polyynes (e.g., HC6H) and gives some valuable implications to planetary, interstellar, and combustion chemistry.

Symmetry breaking and spectral considerations of the surprisingly floppy c-C3H radical and the related dipole-bound excited state of c-C3H.

The C3H radical is believed to be prevalent throughout the interstellar medium and may be involved in the formation of polycyclic aromatic hydrocarbons. C3H exists as both a linear and a cyclic isomer. The C2v cyclopropenylidenyl radical isomer was detected in the dark molecular cloud TMC-1, and the linear propenylidenyl radical isomer has been observed in various dark molecular clouds. Even though the c-C3H radical has been classified rotationally, the vibrational frequencies of this seemingly important interstellar molecule have never been directly observed. Established, highly accurate quartic force field methodologies are employed here to compute useful geometrical data, spectroscopic constants, and vibrational frequencies. The computed rotational constants are consistent with the experimental results. Consequently, the three a1 (ν1, ν2, and ν3) and one b1 (ν6) anharmonic vibrational frequencies at 3117.7 cm-1, 1564.3 cm-1, 1198.5 cm-1, and 826.7 cm-1, respectively, are reliable predictions for these, as of yet unseen, observables. Unfortunately, the two b2 fundamentals (ν4 and ν5) cannot be treated adequately in the current approach due to a flat and possible double-well potential described in detail herein. The dipole-bound excited state of the anion suffers from the same issues and may not even be bound. However, the trusted fundamental vibrational frequencies described for the neutral radical should not be affected by this deformity and are the first robustly produced for c-C3H. The insights gained here will also be applicable to other structures containing three-membered bare and exposed carbon rings that are surprisingly floppy in nature.

UV-induced hydrogen-atom transfer and hydrogen-atom detachment in monomeric 7-azaindole isolated in Ar and n-H2 matrices.

Photochemical transformations were investigated for monomers of 7-azaindole isolated in low-temperature Ar and normal-H2 (n-H2) matrices. The most stable N1H tautomer was the only form of the compound populated in Ar and n-H2 matrices before any irradiation. Upon exposure of Ar matrices to UV (λ > 270 nm) light, two higher-energy tautomers N7H and C3H were photoproduced. Additionally, spectral signatures of the photogenerated 7-azaindolyl radical were also found. All of these photoproducts were experimentally observed for the first time. So far, the N7H tautomer had been known only as a transient species, appearing upon relaxation of photoexcited hydrogen-bonded dimers or complexes. For 7-azaindole isolated in an n-H2 matrix and irradiated at λ > 270 nm, only the C3H tautomer and the 7-azaindolyl radical were photogenerated, whereas the N7H tautomer was not photoproduced at all. The drastic dependence of photogeneration of the N7H form on the matrix environment (solid Ar or solid n-H2) is as a characteristic feature of a specific class of UV-induced hydrogen-atom-transfer processes occurring in matrix-isolated heterocycles.

On the absolute photoionization cross section and dissociative photoionization of cyclopropenylidene.

We report the determination of the absolute photoionization cross section of cyclopropenylidene, c-C3H2, and the heat of formation of the C3H radical and ion derived by the dissociative ionization of the carbene. Vacuum ultraviolet (VUV) synchrotron radiation as provided by the Swiss Light Source and imaging photoelectron photoion coincidence (iPEPICO) were employed. Cyclopropenylidene was generated by pyrolysis of a quadricyclane precursor in a 1 : 1 ratio with benzene, which enabled us to derive the carbene's near threshold absolute photoionization cross section from the photoionization yield of the two pyrolysis products and the known cross section of benzene. The cross section at 9.5 eV, for example, was determined to be 4.5 ± 1.4 Mb. Upon dissociative ionization the carbene decomposes by hydrogen atom loss to the linear isomer of C3H(+). The appearance energy for this process was determined to be AE(0K)(c-C3H2; l-C3H(+)) = 13.67 ± 0.10 eV. The heat of formation of neutral and cationic C3H was derived from this value via a thermochemical cycle as Δ(f)H(0K)(C3H) = 725 ± 25 kJ mol(-1) and Δ(f)H(0K)(C3H(+)) = 1604 ± 19 kJ mol(-1), using a previously reported ionization energy of C3H.

A LABORATORY STUDY OF C 3 H + AND THE C 3 H RADICAL IN THREE NEW VIBRATIONALLY EXCITED 2 Σ STATES USING A PIN-HOLE NOZZLE DISCHARGE SOURCE

Rotational lines of the positive molecular ion C3H+ and of the neutral C3H radical in three new vibrationally excited states with 2Σ symmetry have been detected in a supersonic molecular beam in the centimeter-wave band. The fundamental rotational line of the ion is quite weak, but is observed with similar intensity in a dc discharge through several different hydrocarbon gases when helium is the buffer gas. Under these conditions, the fractional abundance of C3H+ relative to C3H is estimated to be of order 10−4, i.e., toward the lower end of the ratio (10−3–10−4) found for protonated ions using the same discharge nozzle. For each new 2Σ state of the C3H radical, spectroscopic constants, including those describing hydrogen hyperfine structure, have been determined to high precision. Lines of one 2Σ state (B = 11271 MHz) are particularly intense in our molecular beam; for this state and a second one (B = 11306 MHz), millimeter-wave transitions have also been observed between 180 and 340 GHz using a long path dc glow absorption spectrometer. On the basis of intensity measurements with this spectrometer, the inferred rotation–vibration constant α, and theoretical calculations, the state with B = 11271 MHz is tentatively assigned to the ν5 bending mode, predicted to lie ∼300 cm−1 above ground.

Unraveling the deposition mechanism in a-C:H thin-film growth: A molecular-dynamics study for the reaction behavior of C3 and C3H radicals with a-C:H surfaces

In this molecular-dynamics study, we present the simulated growth of thin a-C:H films using the Brenner [(Phys. Rev. B 42, 9458 (1990)] potential. These simulations are relevant for the growth of thin films, grown using low-energy hydrocarbons. In this work, we investigate the reaction mechanisms of both the linear and the cyclic isomers of C3 and C3H on an a-C:H surface. We found that the cyclic species are always more reactive as compared to the linear species, due to their lower stability. The C3 species are found to be more reactive than the C3H species, due to steric hindrance of the H atom, shielding the C atom from the surface. The different mechanisms are discussed. The resulting film properties for different flux ratios of C3 and C3H have also been investigated. It is shown that films as deposited from C3 and C3H have a low density and show low cross-linking. A clear change in microstructure is observed as the ratio between the cyclic and the linear species changes. These simulations provide insights into the reaction behavior of the investigated species, and how this influences the resulting film properties.

Threshold ionization mass spectrometry of reactive species in remote Ar∕C2H2 expanding thermal plasma

Triple stage threshold ionization mass spectrometry is successfully implemented in an Ar∕C2H2 expanding thermal plasma setup. More than 20 hydrocarbon radicals and molecules formed in the plasma, including for example the C2H, C3, C3H, and C3H2 radicals or C3H4 or C4H2 molecules, are measured and their absolute densities are determined. Thanks to a careful design, a high sensitivity of the instrument is achieved and species with densities as low as 1×1016m−3 can be detected. Issues related to the absolute density calibration procedure are considered. The proper determination of the background signal by means of a beam chopper and the influence of the chopper on the measurement are discussed and the possible composition distortion of the sampled beam due to the collisions in the sampling orifice is checked. Furthermore, reported values of electron impact ionization cross sections of hydrocarbon species are compared and, based on their similarity in the near threshold energy region, the C2H2 electron impact ionization cross section is proposed as a reasonable approximation for other hydrocarbon radicals. The results for C, CH, and C2 radicals are compared with previous cavity ringdown absorption spectroscopy measurements and a good agreement is obtained.

Rotational spectrum and carbon-13 hyperfine structure of the C3H, C5H, C6H, and C7H radicals

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we have detected the singly substituted carbon-13 isotopic species of C(5)H, C(6)H, and C(7)H. Hyperfine structure in the rotational transitions of the lowest-energy fine structure component ((2)Pi(12) for C(5)H and C(7)H, and (2)Pi(32) for C(6)H) of each species was measured between 6 and 22 GHz, and precise rotational, centrifugal distortion, Lambda-doubling, and (13)C hyperfine coupling constants were determined. In addition, resolved hyperfine structure in the lowest rotational transition (J = 32-->12) of the three (13)C isotopic species of C(3)H was measured by the same technique. By combining the centimeter-wave measurements here with previous millimeter-wave data, a complete set of (13)C hyperfine coupling constants were derived to high precision for each isotopic species. Experimental structures (r(0)) have been determined for C(5)H and the two longer carbon-chain radicals, and these are found to be in good agreement with the predictions of high-level coupled-cluster calculations. C(3)H, C(5)H, and C(7)H exhibit a clear alternation in the magnitude and sign of the (13)C hyperfine coupling constants along the carbon-chain backbone. Because the electron spin density is nominally zero at the central carbon atom of C(3)H, C(5)H, and C(7)H, and at alternating sets of carbon atoms of C(5)H and C(7)H, owing to spin polarization, almost all of the (13)C coupling constants at these atoms are small in magnitude and negative in sign. Spin-polarization effects are known to be important for the Fermi-contact (b(F)) term, but prior to the work here they have generally been neglected for the hyperfine terms a, c, and d.

Radical-molecule reaction C3H+H2O: A mechanistic study

Despite the importance of the C(3)H radical in both combustion and interstellar space, the reactions of C(3)H toward stable molecules have never been studied. In this paper, we report our detailed mechanistic study on the radical-molecule reaction C(3)H+H(2)O at the Becke's three parameter Lee-Yang-Parr-B3LYP6-311G(d,p) and coupled cluster with single, double, and triple excitations-CCSD(T)6-311G(2d,p) (single-point) levels. It is shown that the C(3)H+H(2)O reaction initially favors formation of the carbene-insertion intermediates HCCCHOH (1a,1b) rather than the direct H- or OH-abstraction process. Subsequently, the isomers (1a,1b) can undergo a direct H- extrusion to form the well-known product propynal HCCCHO (P(5)). Highly competitively, (1a,1b) can take the successive 1,4- and 1,2-H-shift interconversion to isomer H(2)CCCHO(2a,2b) and then to isomer H(2)CCHCO(3a,3b), which can finally take a direct C-C bond cleavage to give product C(2)H(3) and CO (P(1)). The other products are kinetically much less feasible. With the overall entrance barrier 10.6 kcal/mol, the title reaction can be important in postburning processes. Particularly, our calculations suggest that the title reaction may play a role in the formation of the intriguing interstellar molecule, propynal HCCCHO. The calculated results will also be useful for the analogous C(3)H reactions such as with ammonia and alkanes.

An ab initio MO study on structures and energetics of C3H−, C3H, and C3H+

The geometrical structures of cyclic and linear conformers in C3H−, C3H, and C3H+ are optimized using the full-valence CASSCF and CCSD(T) levels of theory. The atomic natural orbital ([5s3p2d1f/3s2p1d]) and correlation consistent basis (aug-cc-pVTZ) sets are used. The relative stability between the cyclic and linear conformers is investigated using the CCSD(T) and multireference CI levels of theory with the aug-cc-pVTZ basis set. The basis set dependency is checked with the 6-311+G(3d2 f,2pd) basis set. The most stable conformer in C3H− and C3H is C2v cyclic with a C3-ring, and that in C3H+ C∞v linear. The energy difference between the cyclic and linear in C3H radical is really small, being around 1.0 kcal/mol. The π-electron population on the C3-ring in cyclic C3H− is 2.00, which is a typical value predicted on the basis of the 4n+2 (n=0) aromatic-rule. The π-electron population on the C3-ring decreases in the order of C3H−, C3H, and C3H+, consistent with the order of the stability of the cyclic conformers. The adiabatic electron affinity and ionization potential in C3H are calculated to be 2.01 and 9.06 eV, respectively. The excitation energies from the most stable isomer are also calculated at the multireference CI level of theory with the aug-cc-pVTZ basis set.

A combined experimental and theoretical study on the formation of interstellar C3H isomers.

The reaction of ground-state carbon atoms with acetylene was studied under single-collision conditions in crossed beam experiments to investigate the chemical dynamics of forming cyclic and linear C3H isomers (c-C3H and l-C3H, respectively) in interstellar environments via an atom-neutral reaction. Combined state-of-the-art ab initio calculations and experimental identification of the carbon-hydrogen exchange channel to both isomers classify this reaction as an important alternative to ion-molecule encounters to synthesize C3H radicals in the interstellar medium. These findings strongly correlate with astronomical observations and explain a higher [c-C3H]/[l-C3H] ratio in the dark cloud TMC-1 than in the carbon star IRC+10216.

Molecular structure of the linear C3H radical: Microwave spectrum of the 13C substituted species

The rotational spectra of three 13C isotopic species of the linear C3H radical in the 2Πr ground vibronic state and the 2Σμ vibronic state of the ν4 (HCC bending) vibrational mode were observed by using a source-modulated spectrometer. The radical was produced in a free-space cell by a glow discharge in a mixture of C2H2, He, and 13CO. The rotational, spin-rotation interaction, Λ-type doubling, and hyperfine interaction constants were determined from the analysis of the observed spectra, where vibronic interaction between the 2Πr and 2Σμ states was taken into account. The rs structure of C3H, denoted by CαCβCγH, was derived from the rotational constants for the normal, deuterated, and three 13C species: rs(Cα–Cβ)=1.3263(1) Å, rs(Cβ–Cγ)=1.2539(2) Å, and rs(Cγ–H)=1.0171(1) Å. The short C–H distance is interpreted in terms of a large amplitude motion of the ν4 mode, whose vibrational energy is very low due to the Renner–Teller effect. An ab initio molecular orbital calculation was carried out, and the structure in which the HCC angle is bent was found to be slightly more stable than the linear structure. The spin densities of three carbon atoms are derived from the hyperfine interaction constants, and they are discussed in connection to the quasilinear nature of the molecule.

Identification of the Sequence CH(2Π) + C2H2 → C3H2 + H (and C3H + H2) Followed by C3H2 + O → C2H + HCO (or H + CO) as C2H Source in C2H2/O/H Atomic Flames

In this work, the novel reaction mechanism initiated by the fast CH(2Π) + C2H2 → C3H2 + H reaction (r13a) and followed by C3H2 + O → C2H + HCO (or H + CO) (r25a) was established as the dominant C2H formation pathway in low-pressure acetylene/atomic oxygen flames at 600 K. The C2H2/O/H flames were investigated in an isothermal discharge-flow reactor at a pressure of 2 Torr, with He as bath gas. Concentration vs reaction-time data were obtained by molecular beam sampling and threshold ionization mass spectrometry. The crucial role of CH(2Π) was evidenced by CH4-addition experiments on room-temperature C2H2/O/H systems, where CH(2Π) and CH2(1A1) are the sole intermediates that react rapidly with CH4. The observed strong reduction of [C3H2], [C2H], and [C4H2] upon CH4 addition could be correlated quantitatively with the known removal of CH(2Π) by CH4. The reaction channels r13a and r25a as sources of C3H2 and C2H, respectively, were each established by quasi-steady-state analyses of the pertaining radicals in C2H2/O/H mixtures at 600 K. By a similar method, the observed C3H radicals could be attributed to a minor channel of the CH(2Π) + C2H2 reaction (r13) parallel to that producing C3H2. At 600 K and 2 Torr, the following approximate product yields of reaction r13 were derived: C3H2 plus H, and C3H plus H2. Concomitantly with the identification of reaction r25a as dominant C2H source, the rate constant of the reaction C2H + O (r19) was determined relative to the well-known kinetic coefficients of C2H + C2H2 and C2H + O2: k19 = (9 ± 4) × 10-11 cm3 molecule-1 s-1 at 600 K. It is suggested that a sizeable fraction of the ethynyl radicals formed in fuel-rich hydrocarbon flames is produced likewise by oxidation of C3Hx radicals (x = 1−3) that arise in the fast reactions of CH(X2Π) and CH2(a1A1) with C2H2.

Microwave spectrum and molecular structure of the cyclic C3H radical

The rotational spectral lines of the cyclic C3H radical in the 2B2 ground electronic state are observed by microwave spectroscopy. The radical is produced in an absorption cell by discharging a mixture of C2H2, CO, and He. The spectral lines of two 13C isotopic species are also observed. The radical is a planar molecule with C2v symmetry, and the rs structure is determined as follows: rs(C–H)=1.0760 Å, rs(C–C)=1.3771 Å, and rs(C–CH)=1.3739 Å. From the observed inertial defect, the vibrational frequency of the C–C antisymmetric vibration is estimated to be about 500 cm−1, which is fairly low for the frequency of the C–C stretching mode. The low vibrational frequency arises from the vibronic interaction between the ground electronic state and the low-lying 2A1 electronic state. On the basis of the hyperfine coupling constants obtained for the 13C species, the unpaired electron is found to be almost evenly distributed among the three carbon atoms. Such delocalization of the unpaired electron is consistent with the observed geometrical structure.

Vibrational spectrum of linear C3H in solid argon

A Fourier transform infrared isotopic study has been made of the deuterium and carbon-13 substituted C3H radical produced by trapping in solid Ar the products of the vacuum ultraviolet photolysis of CH2CHCH3 (propylene) or a mixture of methane and acetylene. The 1824.7 and 1159.8 cm−1 absorptions, which in earlier work using only deuterium isotosopic data could be assigned to either the cyclic or linear isomer, can now be definitely identified on the basis of extensive 13C isotopic measurements as the ν2(σ+) and ν3(σ+) fundamentals of linear C3H. The frequencies of the fundamentals and D and 13C isotopic shifts are in good agreement with the predictions of preliminary ab initio calculations for the linear isomer.

Laboratory microwave spectroscopy of the linear C3H and C3D radicals and related astronomical observation

Laboratory observations of rotational spectral lines of the C{sub 3}H and C3D radicals in their ground and vibrationally excited states are compared with astronomical observations with the Nobeyama Radio Observatory's 45-m radio telescope carried out toward IRC + 10216 in May 1988. The C{sub 3}H radical is found to have an extremely low vibrationally excited state, nu4(2Sigma mu), caused by the Renner-Teller effect in the nu4 (CCH bending) state. The radio telescope observations failed to detect the lines of the vibrationally excited C3H at the rms noise level of 24 mK. The upper limit to the column density of the nu4(2Sigma mu) is estimated to be 3 x 10 to the 12th/sq cm. 27 refs.

Laboratory and astronomical detection of the cyclic C3H radical

Laboratory and astronomical detections of the rotational spectrum of the cyclic C/sub 3/H radical are reported, and the formation process by ion-molecular reactions is discussed. Laboratory production of the radical involved the discharging of a mixture of C/sub 2/H/sub 2/, CO and He, following which 49 lines were assigned and the molecular constants determined. The astronomical search was conducted using the 2(12)-1(11) transition at 91.5 GHz whose four fine and hyperfine components have been detected toward TMC-1. A column density estimation of 6 x 10 to the 12th/sq cm has been obtained for the cyclic C/sub 3/H. 21 references.

The rotational spectrum of the C3H radical

The rotational spectrum of the recently discovered C3H radical in both fine-structure ladders of its electronic ground state (X2Pi) with lambda doubling and hyperfine structure has been determined to high accuracy, and the strongest transitions at frequencies less than 376 GHz are tabulated. Recently published spectroscopic constants derived from astronomical and laboratory data (Gottlieb et al., 1985) allowed calculation of the spectrum to a radial velocity of 0.4 km/s at frequencies less than 200 GHz, the uncertainties above being somewhat larger. The hyperfine constants of the sequence of four hydrocarbon radicals CH, C2H, C3H, and C4H are briefly described.

Astronomical identification of the C3H radical.

The C3H radical, a linear carbon chain with a 2 pi electronic ground state, has been identified in the millimeter-wave spectra of two astronomical sources, IRC +10216 and TMC-1, and conclusively confirmed (accompanying Letter) in a laboratory glow discharge. In IRC +10216 four rotational transitions have been observed, three in the lower fine-structure ladder (2 pi 1/2) and one in the upper (2 pi 3/2), each a resolved or partially resolved lambda-doublet. In TMC-1, both lambda components of the lowest lying 3/2 --> 1/2 transition of the 2 pi 1/2 ladder have been observed, each with well-resolved hfs. In IRC +10216 the excitation of C3H is similar to that of SiCC: the rotational temperature Trot within the 2 pi 1/2 ladder is low (8.5 K), owing to rapid radiative decay, while Trot across the ladders is high (approximately 52 K) because interconnecting far-IR radiative transitions are only weakly permitted. The column density of C3H in IRC +10216 averaged over the estimated source diameter of 84" is 2.8 x 10(13) cm-2, an order of magnitude less than that of C2H and C4H. A determination of the spectroscopic constraints of C3H that permitted the entire radio spectrum of this molecule to be calculated to high accuracy has been derived from analysis of the combined astronomical and laboratory data presented in the accompanying Letter.

Laboratory detection of the C3H radical

Millimeter-wave lines of the C3H radical, including six of the lines observed in space, were detected in a laboratory glow discharge through a flowing mixture of C2H2, He, and CO. Each of the 10 rotational transitions measured, five between 98 and 185 GHz in the 2Pi1/2 ladder and five between 80 and 194 GHz in the 2Pi3/2 ladder, is split by lambda-type doubling, and all but three possess resolved hyperfine structure. The excellent agreement between the fine-structure, rotation, lambda-doubling, and hyperfine constants derived from the laboratory data and from the astronomical observations conclusively confirms the identifications in IRC +10216 and TMC-1 by Thaddeus and colleagues (1985). An accurate set of spectroscopic constants, which allow calculation of the entire radio spectrum of C3H to a radial velocity of 0.1 km/s, was derived from a simulataneous fit to the laboratory frequencies and to the well-resolved hfs observed in the narrow-line astronomical source TMC-1.