The reactions of the methylidyne radical (CH) with ethylene, acetylene, allene, and methylacetylene are studied at room temperature using tunable vacuum ultraviolet (VUV) photoionization and time-resolved mass spectrometry. The CH radicals are prepared by 248 nm multiphoton photolysis of CHBr 3 at 298 K and react with the selected hydrocarbon in a helium gas flow. Analysis of photoionization efficiency versus VUV photon wavelength permits isomer-specific detection of the reaction products and allows estimation of the reaction product branching ratios. The reactions proceed by either CH insertion or addition followed by H atom elimination from the intermediate adduct. In the CH + C 2 H 4 reaction the C 3 H 5 intermediate decays by H atom loss to yield 70(±8)% allene, 30(±8)% methylacetylene and less than 10% cyclopropene, in agreement with previous RRKM results. In the CH + acetylene reaction, detection of mainly the cyclic C 3 H 2 isomer is contrary to a previous RRKM calculation that predicted linear triplet propargylene to be 90% of the total H-atom co-products. High-level CBS-APNO quantum calculations and RRKM calculation for the CH + C 2 H 2 reaction presented in this manuscript predict a higher contribution of the cyclic C 3 H 2 (27.0%) versus triplet propargylene (63.5%) than these earlier predictions.Extensive calculations on the C 3 H 3 and C 3 H 2 D system combined with experimental isotope ratios for the CD + C 2 H 2 reaction indicate that H-atom assisted isomerization in the present experiments is responsible for the discrepancy between the RRKM calculations and the experimental results. Cyclic isomers are also found to represent 30(±6)% of the detected products in the case of CH + methylacetylene, together with 33(±6)% 1,2,3-butatriene and 37(±6)% vinylacetylene. The CH + allene reaction gives 23(±5)% 1,2,3-butatriene and 77(±5)% vinylacetylene, whereas cyclic isomers are produced below the detection limit in this reaction. The reaction exit channels deduced by comparing the product distributions for the aforementioned reactions are discussed in detail.3

The gas-phase valence binding energy spectrum of isolated ion-pairs of the commonly used [1-ethyl-3-methylimidazolium][bis(trifluoromethylsulfonyl)imide)] room-temperature ionic liquid is obtained by photoionization of a molecular beam of ionic liquid vapor by extreme ultraviolet light. The isolated ion-pair nature of the ionic liquid vapor is corroborated by single photon ionization mass spectroscopy, complemented by computed vaporization energetics of ion-pairs and clusters of ion-pairs. The valence binding energy spectrum of the isolated ion-pairs is discussed in comparison with available liquid-phase data and theoretical density functional theory calculations.

The reactions of the ethynyl radical (C(2)H) with propyne and allene are studied at room temperature using an apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. The C(2)H radical is prepared by 193-nm photolysis of CF(3)CCH and the mass spectrum of the reacting mixture is monitored in time using synchrotron-photoionization with a dual-sector mass spectrometer. Analysis using photoionization efficiency curves allows the isomer-specific detection of individual polyynes of chemical formula C(5)H(4) produced by both reactions. The product branching ratios are estimated for each isomer. The reaction of propyne with ethynyl gives 50-70% diacetylene (H-C[triple bond]C-C[triple bond]C-H) and 50-30% C(5)H(4), with a C(5)H(4)-isomer distribution of 15-20% ethynylallene (CH(2)=C=CH-C[triple bond]CH) and 85-80% methyldiacetylene (CH(3)-C[triple bond]C-C[triple bond]CH). The reaction of allene with ethynyl gives 35-45% ethynylallene, 20-25% methyldiacetylene and 45-30% 1,4-pentadiyne (HC[triple bond]C-CH(2)-C[triple bond]CH). Diacetylene is most likely not produced by this reaction; an upper limit of 30% on the branching fraction to diacetylene can be derived from the present experiment. The mechanisms of polyynes formation by these reactions as well as the implications for Titan's atmospheric chemistry are discussed.

The reaction of the methylidyne radical (CH) with acetaldehyde (CH(3)CHO) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of CHBr(3) and react with acetaldehyde in an excess of helium and nitrogen gas flow. Five reaction exit channels are observed corresponding to elimination of methylene (CH(2)), elimination of a formyl radical (HCO), elimination of carbon monoxide (CO), elimination of a methyl radical (CH(3)), and elimination of a hydrogen atom. Analysis of the photoionization yields versus photon energy for the reaction of CH and CD radicals with acetaldehyde and CH radical with partially deuterated acetaldehyde (CD(3)CHO) provides fine details about the reaction mechanism. The CH(2) elimination channel is found to preferentially form the acetyl radical by removal of the aldehydic hydrogen. The insertion of the CH radical into a C-H bond of the methyl group of acetaldehyde is likely to lead to a C(3)H(5)O reaction intermediate that can isomerize by β-hydrogen transfer of the aldehydic hydrogen atom and dissociate to form acrolein + H or ketene + CH(3), which are observed directly. Cycloaddition of the radical onto the carbonyl group is likely to lead to the formation of the observed products, methylketene, methyleneoxirane, and acrolein.

Product channels for the self-reaction of the resonance-stabilized allyl radical, C3H5 + C3H5, have been studied with isomeric specificity at temperatures from 300-600 K and pressures from 1-6 Torr using time-resolved multiplexed photoionization mass spectrometry. Under these conditions 1,5-hexadiene was the only C6H10 product isomer detected. The lack of isomerization of the C6H10 product is in marked contrast to the C6H6 product in the related C3H3 + C3H3 reaction, and is due to the more saturated electronic structure of the C6H10 system. The disproportionation product channel, yielding allene + propene, was also detected, with an upper limit on the branching fraction relative to recombination of 0.03. Analysis of the allyl radical decay at 298 K yielded a total rate coefficient of (2.7 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1), in good agreement with previous experimental measurements using ultraviolet kinetic absorption spectroscopy and a recent theoretical determination using variable reaction coordinate transition state theory. This result provides independent indirect support for the literature value of the allyl radical ultraviolet absorption cross-section near 223 nm.

The CH(X2Π) + propene reaction is studied in the gas phase at 298 K and 4 Torr (533.3 Pa) using VUV synchrotron photoionization mass spectrometry. The dominant product channel is the formation of C4H6 (m/z 54) + H. By fitting experimental photoionization spectra to measured spectra of known C4H6 isomers, the following relative branching fractions are obtained: 1,3-butadiene (0.63 ± 0.13), 1,2-butadiene (0.25 ± 0.05), and 1-butyne (0.12 ± 0.03) with no detectable contribution from 2-butyne. The CD + propene reaction is also studied and two product channels are observed that correspond to C4H6 (m/z 54) + D and C4H5D (m/z 55) + H, formed at a ratio of 0.4 (m/z 54) to 1.0 (m/z 55). The D elimination channel forms almost exclusively 1,2-butadiene (0.97 ± 0.20) whereas the H elimination channel leads to the formation of deuterated 1,3-butadiene (0.89 ± 0.18) and 1-butyne (0.11 ± 0.02); photoionization spectra of undeuterated species are used in the fitting of the measured m/z 55 (C4H5D) spectrum. The results are generally consistent with a CH cycloaddition mechanism to the C═C bond of propene, forming 1-methylallyl followed by elimination of a H atom via several competing processes. The direct detection of 1,3-butadiene as a reaction product is an important validation of molecular weight growth schemes implicating the CH + propene reaction, for example, those reported recently for the formation of benzene in the interstellar medium ( Jones, B. M. Proc. Natl. Acad. Sci. U.S. A. 2011, 108, 452−457 ABSTRACTThe CH (X 2 Π) + propene reaction is studied in the gas phase at 298 K and 4 Torr (533.3 Pa) using VUV synchrotron photoionization mass spectrometry. The dominant product channel is the formation of C 4 H 6 (m/z 54) + H. By fitting experimental photoionization spectra to measured spectra of known C 4 H 6 isomers, the following relative branching fractions are obtained: 1,3-butadiene (0.63 ± 0.13), 1,2-butadiene (0.25 ± 0.05) and 1-butyne (0.12 ± 0.03) with no detectable contribution from 2-butyne. The CD + propene reaction is also studied and two product channels are observed that correspond to 2011, 108, 452-457).KEYWORDS: radical, gas-phase, photoionization, synchrotron, propene, methylidyne, mass spectrometry. 3 INTRODUCTIONThe highly reactive methylidyne (CH) radical affects the chemistry of energetic gasphase environments including combustion 1-3 , interplanetary atmospheres 4-6 and the interstellar medium. 7 In order to accurately model these systems, detailed chemical data are needed in the form of reaction rate coefficients and product branching fractions. Our purpose here is to determine total product branching fractions and provide mechanistic details for the CH (X 2 Π) + propene reaction. The present work builds on a series of previous product detection studies of the CH radical with small, unsaturated hydrocarbons: acetylene (C 2 H 2 ), ethylene (C 2 H 4 ), allene (C 3 H 4 , CH 2 CCH 2 ) and propyne (C 3 H 4 , CH 3 CCH). 8 Recent works have also investigated the CH reaction with the carbonyl-conta...

Combined data of photoelectron spectra and photoionization efficiency curves in

Low-temperature rate coefficients are measured for the CN + benzene and CN + toluene reactions using the pulsed Laval nozzle expansion technique coupled with laser-induced fluorescence detection. The CN + benzene reaction rate coefficient at 105, 165, and 295 K is found to be relatively constant over this temperature range, (3.9-4.9) × 10 -10 cm 3 molecule -1 s -1 . These rapid kinetics, along with the observed negligible temperature dependence, are consistent with a barrierless reaction entrance channel and reaction efficiencies approaching unity. The CN + toluene reaction is measured to have a rate coefficient of 1.3 × 10 -10 cm 3 molecule -1 s -1 at 105 K. At room temperature, nonexponential decay profiles are observed for this reaction that may suggest significant back-dissociation of intermediate complexes. In separate experiments, the products of these reactions are probed at room temperature using synchrotron VUV photoionization mass spectrometry. For CN + benzene, cyanobenzene (C 6 H 5 CN) is the only product recorded with no detectable evidence for a C 6 H 5 + HCN product channel. In the case of CN + toluene, cyanotoluene (NCC 6 H 4 CH 3 ) constitutes the only detected product. It is not possible to differentiate among the ortho, meta, and para isomers of cyanotoluene because of their similar ionization energies and the ∼40 meV photon energy resolution of the experiment. There is no significant detection of benzyl radicals (C 6 H 5 CH 2 ) that would suggest a H-abstraction or a HCN elimination channel is prominent at these conditions. As both reactions are measured to be rapid at 105 K, appearing to have barrierless entrance channels, it follows that they will proceed efficiently at the temperatures of Saturn's moon Titan (∼100 K) and are also likely to proceed at the temperature of interstellar clouds (10-20 K).