- Removal Rate Constants for Singlet Methylene with Oxygen-Containing Organic Species
-
The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, 1CH2 ( 1A1) with various oxygen-containing organic species.Removal rate constants for some 27 alcohols, ethers, ketones, aldehydes, carboxylic acids, and esters are reported for the first time.The removal rate constants for H2O and CH3OH have been remeasured and found to be in excellent agreement with values determined by other researchers.Improved removal rate constants for C2H5OH, n-C3H7OH, CH3OCH3, CH3CHO, CH3COCH3, CH3COOH, HCOOCH3, and CH3OCOOCH3 are also presented.In all cases the removal rate constants are large, indicating that reaction is the dominant process leading to loss of 1CH2.Comparisons are drawn between the reactivities of the various functional groups and between them and their hydrocarbon analogues.Because of the large data base provided by these measurements, mechanistic information can be inferred in a number of instances.
- Gutsche, Greg J.,Lawrance, Warren D.,Staker, Warren S.,King, Keith D.
-
-
Read Online
- RATE-CONSTANTS FOR REMOVAL OF METHYLENE (1B1) PREPARED IN THE (0,13,0), (0,14,0) AND (0,18,0) OVERTONES BY HELIUM AND SEVERAL HYDROCARBONS
-
Methylene (1B1) was prepared by a two-step process: photodissociation of ketene at 308 nm (XeCl excimer laser) gave "cold" methylene 1A1 and CO .Subsequent excitation of the 1A1 electronic state using a dye laser produced the 1B1 state .Bending overtones v2 = 13, 14 and 18 could be selected and their time resolved fluorescence studied in the presence of several gases.Very little dependence of overtone radiative lifetimes are found.Quenching rate constants are in all cases very fast, they range from 5*10-11 cm3 molec-1 s-1 for quenching of overtone v2 = 14 by helium to 1.1*10-9 cm3 molec-1 s-1 for quenching of the same overtone by trans-2-butene.Chemical forces seem to be involved in the quenching by hydrocarbons.
- Castillejo, M.,Figuera, J. M.,Garcia-Moreno, I.,Medina, J. J.,Rodriguez, J. C.
-
-
Read Online
- Photodissociation dynamics of the CH2Cl radical: Ion imaging studies of the Cl+CH2 channel
-
Photodissociation of the chloromethyl radical, CH2Cl to chlorine atom and methylene, which can serve as a prototype for halogenated methyl radicals, was described. Ch2Cl was produced in a molecular beam using pulsed pyrolysis. Cl and CH2 products were detected by laser ionization and their velocity and angular distributions were determined using the ion imaging technique. The available energy was partitioned into the translational degrees of freedom. It was found that the CH2Cl generated in a molecular beam via pulsed pyrolysis exhibits continuous, unstructured absorption throughout the investigated region, with Cl atoms as major products.
- Dribinski,Potter,Demyanenko,Reisler
-
-
Read Online
- The correlated product state distribution of ketene photodissociation at 308 nm
-
The correlated product state distribution for ketene photodissociation (CH2CO→CH2+CO) at 308 nm has been measured by using quantum-state-specific metastable time-of-flight (TOF) spectroscopy. This distribution is a matrix whose elements are the probability that if CO is produced in the dissociation with quantum-state |nCO〉, CH2 will be produced with quantum-state |nCH2〉. It was found that ketene photodissociation yields CH2 in three resolved states; the 1A1(000), and 1A1(010) states of CH2 are the major channels, while the 3B1 state is a minor channel. In addition to this scalar distribution, the vector correlations between the recoil velocity and the angular momentum of the CO fragment (v·j correlation), expressed by theβ00(22) bipolar moment, have also been obtained as a function of the kinetic energy release of the photoreaction. The correlated product state distribution was found not to follow the predictions of phase space theory, suggesting that dynamic hindrances exist in the photoreaction that have not been previously observed. A phase space theory calculation with restricted impact parameter values was also performed and compared to experiment. The impact parameter restricted phase space theory more accurately reproduced all of the correlated product state information obtained in this work as well as previous uncorrelated product state distributions for CH2 and CO. Both the ranges and the values of the allowed impact parameters obtained from these restricted calculations increase as the rotational energy of CO increases. Also, the values of the allowed impact parameters for 1A1(010) CH2 are larger than for 1A1(000) CH2. This strongly suggests that C-C-O bending modes are hindered at the transition state and therefore play an important role in the photodissociation.
- Morgan, Christopher G.,Drabbels, Marcel,Wodtke, Alec M.
-
-
Read Online
- Branching ratio of the C2H2 + O reaction at 290 K from kinetic modelling of relative methylene concentration versus time profiles in C2H2/O/H systems
-
In earlier work on the room temperature oxidation of C2H2 by O atoms, two distinct sources of methylene radicals have been identified: (i) direct, primary production via channel 1b of the C2H2 + O reaction, and (ii) delayed formation via the secondary reaction 3 involving the products HCCO and H of the other primary channel 1a. Presently, it was confirmed by a detailed sensitivity analysis that the precise shapes of the resulting total methylene concentration-versus-time profiles in C2H2/O systems depend strongly on the k1a/k1b branching ratio. Along that line, the important parameter k1a/k1b was determined from relative CH2 concentration-versus-time profiles measured in a variety of C2H2/O/H systems using Discharge Flow-Molecular Beam sampling Mass Spectrometry techniques (DF-MBMS). The data analysis was carried out by deductive kinetic modelling; the method, as applied to profile shapes, is discussed at length. Via this novel, independent approach, the CH2(3B1) yield of the two-channel C2H2 + O reaction was determined to be k1b/k1 = 0.17 ± 0.08. The indicated 2σ error includes possible systematic errors due to uncertainties in the rate constants of other reactions that influence the shapes of the CH2 profiles. The present result, which translates to an HCCO yield k1a/k1 = 0.83 ± 0.08, is in excellent agreement with other recent determinations. The above mechanism, with the subsequent reactions that it initiates, also reproduces the measured absolute [C2H2], [O], and [H] profiles with an average accuracy of 5%, thus validating the consistency of the C2H2/O/H reaction model put forward here.
- Peeters,Boullart,Langhans
-
-
Read Online
- Spectroscopic Observation of the CH2(1A1) Radical in the Reaction of C2H2 with O Atoms
-
Direct spectroscopic observation of the CH2(a1A1) radical in C2H2/O/H systems (T = 295 K) is reported for the first time.Characteristic rotational features of the 1B1(0,14,0) 1A1(0,0,0)
- Peeters, J.,Vanhaelemeersch, S.,Hoeymissen, J. Van,Borms, R.,Vermeylen, D.
-
-
Read Online
- Femtosecond dynamics of photoinduced molecular detachment from halogenated alkanes. II. Asynchronous concerted elimination of I2 from CH2I2
-
The photoinduced molecular detachment dynamics of CH2I2 have been investigated with femtosecond time resolution. Upon multiphoton excitation of CH2I2 with 312 nm femtosecond pulses, weak fluorescence in the 260-290 nm region was observed in addition to the I2 fluorescence in the 290-345 nm region studied in the previous paper. The weak fluorescence has also been interpreted as due to emission from I2, where I2 was produced from the photodissociation process CH2I2→CH2+I2*. In order to investigate the detailed dynamics of this reaction, femtosecond time-resolved data have been obtained by selective detection of the I2 fluorescence at 272 and 285 nm. From these transients, it has been found that the dissociation process takes place within the temporal width (50 fs) of the laser pulse and that the I2 photofragments exhibit coherent vibrational motion. The 272 nm transients also exhibit clear, fast decaying rotational anisotropy, quantitative analysis of which reveals a distribution of rather high rotational levels of I2. This permits us to conclude that the I2 detachment is an asynchronous concerted process; while breaking of the two CI bonds and formation of the II bond happen in a single kinetic step, one of the CI bonds breaks faster than the other. In addition, energy partitioning between the CH2 and I2 photofragments has also been explored based on the experimental observations. Since this study involves a multiphoton transition, a theoretical formulation for the time dependent rotational anisotropy is presented for the general case of multiphoton pump and multiphoton probe transitions.
- Zhang, Qingguo,Marvet, Una,Dantus, Marcos
-
-
Read Online
- Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet detection of the products
-
Vacuum ultraviolet (VUV) laser photoionization is combined with time-of-flight (TOF) mass spectrometry to determine the photofragments produced from the laser photodissociation of allene and propyne in a molecular beam. Detection of C3H+3 confirms that atomic hydrogen elimination is the primary process for both of these molecules. A hydrogen molecule elimination channel and a low mass carbon fragmentation channel of allene to produce C3H2+H2 and CH2+C2H2, respectively, have also been identified. Different ratios of various dissociation channels from these two molecules suggest that the dissociation mechanisms of these two isomers are different. Dissociation must occur before complete isomerization. These results are discussed in terms of recent theoretical calculations on the ground and excited states of these molecules. Secondary photodissociation of the products has been observed, even though the laser energies that have been used are less than 8 mJ/cm2 and the photolysis laser is not focused. Therefore, the present results show how important it is to determine product distributions as a function of the laser energy.
- Ni, Chi-Kung,Huang,Chen, Yit Tsong,Kung,Jackson
-
-
Read Online
- Direct Measurement of the Reaction CH3 + OH at Ambient Temperature in the Pressure Range 0.3 - 6.2 mbar
-
The falloff behavior of the CH3 + OH recombination reaction CH3 + OH -> CH3OH (1a) has been quantitatively investigated for the first time.Methyl decay profiles were measured in a flow reactor at 300 K and in a pressure range between 0.3 and 6.2 mbar.The experimental conditions were such that a possible channel to 1CH2 + H2O did not contribute strongly to the CH3 profiles.Rate coefficients were extracted from the data by comparison of the experimental profiles with compouter simulations.The results are in accord with the limiting rate coefficient suggested by Hochanadel et al., k1a(298 K) = 9.3 * 10-11 cm3 molecule-1 s-1.The experimental falloff curve is described by use of this value for K, together with an interpolation formula given by Troe, from which an approximate value for the low-pressure limit K01a = (2.5 +/- 1.0) * 10-27 cm6 molecule-2 s-1 has been derived (bath gas helium).For a quantitative assessment of the possible channel to 1CH2 + H2O, reaction 1d measurements of H2O were carried out, yielding an estimated upper limit for the rate coefficient of K1d(300 K) -12 cm3 molecule-1 s-1.
- Oser, H.,Stothard, N. D.,Humpfer, R.,Grotheer, H. H.
-
-
Read Online
- Photodissociation dynamics of the methyl radical 3s Rydberg state
-
The photodissociation dynamics of methyl radical have been investigated at 193.3 nm using photofragment translational spectroscopy.The formation of CH2 and H(2S) was the only dissociation pathway observed.Although it is not possible to assign the spin state of the methylene unambiguously, we believe the methylene is produced predominately in the 1A1 excited state.The translational energy distribution of the products is peaked at ca. 13 kcal/mole which is consistent with the magnitude of the exit barrier on the excited state potential energy surface.The breadth of the distribution suggests that the methyl radicals dissociate from a wide range of geometries.From the photofragment angular distribution an anisotropy parameter of β = -0.9+/-0.1 was determined.
- North, Simon W.,Blank, David A.,Chu, Pamela M.,Lee, Yuan T.
-
-
Read Online
- Cobalt carbene ion: Reactions of Co+ with C2H4, cyclo-C3H6, and cyclo-C2H4O
-
An ion beam apparatus is employed to study the formation of the cobalt carbene ion, CoCH2+.This ion is produced in the endothermic reaction of cobalt ions with ethene and cyclopropane and in an exothermic reaction with ethylene oxide.A model is proposed to account for the dependence of experimental cross sections on relative kinetic energy for the endothermic reactions.Using this model to interpret the experimental results, a bond dissociation energy D deg(Co+-CH2) = 3.7 +/- 0.3 eV is derived.
- Armentrout, P. B.,Beauchamp, J. L.
-
-
Read Online
- Direct formation of CH2 (b 1B1) in the near-UV photodissociation of diazirine
-
A prompt, long-lived, and red fluorescence has been observed in the photodissociation of the jet-cooled diazirine (H2CN2) excited at the origin (322.96 nm) of the S1←S0 transition. The fluorescence decays (~16 μs) and range (>575 nm) indicate that the emitting species is the electronically excited singlet methylene, CH2 (b 1B1). The linear dependence of the fluorescence intensity on the photolysis laser power strongly supports that the fluorescing CH2 (b 1B1) is produced directly from the first excited singlet state of diazirine. The observation that the fluorescence appears in the visible wavelength region indicates that the CH2 (b 1B1) fragments are highly vibrationally excited.
- Lim, Soon-Mi,Kim, Taek-Soo,Lim, Goo-Il,Kim, Sang Kyu,Choi, Young S.
-
-
Read Online
- Evidence for the primary decomposition of propylene oxide to singlet methylene
-
Using laser-induced fluorescence as a probe for 1CH2, we have determined that infrared multiphoton dissociation of propylene oxide yields singlet methylene as a primary product.Experiments with specifically labeled 1-d2 propylene oxide confirm, by showing only 1CD2, that the carbene is ejected exclusively from the terminal position of the heterocycle and that fragmentation is faster than any competing hydrogen migration process.
- Chou, Jim-Son J.,Adams, Thomas E.,Grant, Edward R.
-
-
Read Online
- Gas-phase reactions of rhenium-oxo species ReOn+, n = 0, 2 6, 8, with O2, N2O, CO, H2O, H2, CH4 and C2H4
-
The reactions of ReOn+, n = 0, 2-6, 8, with the small molecules O2, N2O, CO, H2O, H2, CH4 and C2H4, are studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry under single collision conditions on the timescale of seconds. The ReOn+ species are produced by laser vaporization of solid rhenium and pulsed supersonic expansion in a helium-oxygen mixture into high vacuum. A wide variety of reactions are observed, including methane activation and epoxidation reactions. Re+ reacts with ethylene by sequential dehydrogenation. ReO2+ and ReO4+ exhibit the most diverse reaction pathways, while ReO5+ almost exclusively undergoes ligand exchange. ReO6+ and ReO8+ are largely unreactive, the only efficient reactions are observed are with ethylene and water. Both molecules seem to be able to directly attack the dioxygen ligands. The observed chemistry is governed by a fine interplay between available coordination sites and thermochemistry.
- Beyer,Berg,Bondybey
-
-
Read Online
- Femtosecond dynamics of photoinduced molecular detachment from halogenated alkanes. I. Transition state dynamics and product channel coherence
-
The direct observation of the photoinduced molecular detachment of halogens X2 from halogenated alkanes RCHX2 is presented. Three-photon excitation at 312 nm produces molecular halogens and a carbene; the halogen products are formed predominantly in the D′ state. Femtosecond pump-probe spectroscopy of the reaction reveals a fast (τa prompt dissociation without intermediates. The experimental results demonstrate vibrational coherence in the halogen product, which requires that the reaction proceed by a concerted mechanism.
- Marvet, Una,Zhang, Qingguo,Brown, Emily J.,Dantus, Marcos
-
-
Read Online
- The Ketyl Radical in the Oxidation of Ethyne by Atomic Oxygen at 300-600 K
-
The reaction of ethyne with atomic oxygen was investigated in the temperature range 300-600 K, at a pressure of 2 torr.With molecular beam mass spectrometry, both methylene and ketyl radical were shown to be important primary products.An absolute measurement was made of the rate constant of reaction 7 of HCCO with O at T = 535 K: k7 = (1.10 +/- 0.10) X 1014 cm3 mol-1 s-1.The activation energy E7 was found to be E7 = 0.6 +/- 0.3 kcal mol-1.Reaction 2 of HCCO with H atoms is even faster; in Stern-Volmer experiments the ratio k2/k7 was determined to be 1.4 +/- 0.4 at T = 535 K and 1.3 +/- 0.2 at T = 285 K.
- Vinckier, C.,Schaekers, M.,Peeters, J.
-
-
Read Online
- High-temperature shock tube study of the reactions CH3 + OH → products and CH3OH + Ar → products
-
The reaction between methyl and hydroxyl radicals has been studied in reflected shock wave experiments using narrow-linewidth OH laser absorption. OH radicals were generated by the rapid thermal decomposition of tert-butyl hydroperoxide. Two different species were used as CH3 radical precursors, azomethane and methyl iodide. The overall rate coefficient of the CH3 + OH reaction was determined in the temperature range 1081-1426 K under conditions of chemical isolation. The experimental data are in good agreement with a recent theoretical study of the reaction. The decomposition of methanol to methyl and OH radicals was also investigated behind reflected shock waves. The current measurements are in good agreement with a recent experimental study and a master equation simulation.
- Vasudevan, Venkatesh,Cook, Robert D.,Hanson, Ronald K.,Bowman, Craig T.,Golden, David M.
-
p. 488 - 495
(2008/12/21)
-
- Experimental and modeling study of shock-tube oxidation of acetylene
-
Nine mixtures of acetylene and oxygen diluted in argon were studied behind reflected shock waves at temperatures of 1150-2132 K and pressures of 0.9-1.9 atm. Initial compositions were varied from very fuel-lean to moderately fuel-rich, covering equivalence ratios of 0.0625-1.66. Two more mixtures with added ethylene were used to boost the sensitivity to reactions of vinyl oxidation. The progress of reaction was monitored by laser absorption of CO molecules. The collected experimental data were subjected to extensive detailed chemical kinetics analysis. The initial kinetic model was assembled based on recent literature data and then optimized using the solution mapping technique. The analysis was extended to include recent experimental observations of Hidaka and co-workers (Combust Flame 1996, 107, 401). The derived model reproduces closely both sets of experimental data, the result obtained by modifying nine rate coefficients and three enthalpies of formation of intermediate species. The identified parameter tradeoffs and justification for the changes are discussed.
- Eiteneer, Boris,Frenklach, Michael
-
p. 391 - 414
(2007/10/03)
-
- Detection of1CH2 radicals in hydrocarbon pyrolysis behind shock waves using FM spectroscopy
-
Singlet methylene radical (1CH2) concentrations were measured for the first time in the pyrolysis of methane (CH4) and ethane (C2H6) behind shock waves. The very sensitive frequency modulation (FM) spectroscopy, already established for sensitve detection of amino radicals (NH2)[1,2], was used for that purpose. Applying computer simulations using a complex reaction mechanism the experimental 1CH2 signals were fitted and rate coefficients of different reaction channels were obtained. For the reaction channel (1a) CH4 + M → CH3 + H + M an extented Arrhenius expression of k1a = 6.5·1018-(T/298)-1.70·exp[-366kJ mol-1/RT] cm3mol-1s-1(±50%), and futhermore rate coefficients for (7) 3CH2+CH3 → H+C2H4 with k7 = 3.2 · 1013cm3mol-1 s-1 (±40%), (11) 3CH2 + H → CH + H2 with ku = 7.9 · 1013 cm3 mol-1 s-1 (±40%), and for the intersystem crossing via (6)1CH2+M → 3CH2+M with k6 = 1.40 · 1010 · (T/K)0.9cm3mol-1s-1 (±40%) were determined. The experimental conditions ranged from 1900 to 4000 K with corresponding pressures between 0.23 to 0.54 bar. by Oldenbourg Wissenschaftsverlag, Muenchen.
- Deppe, Joachim,Wagner, Heinz Gg.
-
p. 1501 - 1525
(2007/10/03)
-
- Investigation of the thermal decomposition of ketene and of the reaction CH2 + H2 ? CH3 + H
-
Using frequency modulation (FM) spectroscopy singlet methylene radicals have been detected for the first time behind shock waves. The thermal decomposition of ketene served as source for metylene radicals at temperatures from 1905 to 2780 K and pressures around 450 mbar. For the unimolecular decomposition reaction, (1) CH2CO+M → CH2 +CO+M, the rate constants obtained are: k1 = (9.5±5.7) · 1015 · exp[(-244±25) kJ mol-1/RT] cm3mol-1 s-1. As a first study of a methylene reaction at high temperatures by diretly tracing methylene the reaction of methylene with hydrogen, (8+9) 1.3CH2 + H2 → CH3 + H, was investigated at temperatures from 1930 to 2455 K and pressures around 500 mbar. For the total rate constant of the singlet and triplet methylene reaction a temperature independent value was obtained: log(kg+9/(cm3mol-1s-1)) = 13.89±0.26. A comparison with low temperature literature data and the systematics of activation energies of triplet methylene reactions allowed a consistent description of singlet and triplet contributions and of the forward and reverse reaction. by Oldenbourg Wissenschaftsverlag, Muenchen.
- Friedrichs, Gernot,Wagner, Heinz Gg.
-
p. 1601 - 1623
(2007/10/03)
-
- Shock tube study on the reaction of Si atoms with CH3 with respect to SiC formation
-
The reaction kinetics of ground state Si atoms was studied behind reflected shock waves in the presence of excess CH4 and CH3, respectively. The very fast Si atom formation by the thermal decomposition of SiH4 and Si2H6 was used as a reliable Si atom source at temperatures T > 1550K. CH3 was formed by the thermal decomposition of different source gases like CH3Cl and CH3I. All test gases were highly diluted in argon. The atomic resonance absorption spectroscopy (ARAS) was applied for time-resolved measurements of Si and H atoms and it was found that the reaction of Si atoms with CH4 (R1) is much slower than the reaction of Si atoms with CH3 (R2) which seems to be an important reaction leading to the formation of very reactive intermediate species on its way to form SiC. The product channels are discussed and a temperature independent mean rate coefficient for (R2) was determined: Si + CH3 k2a ? SiCH + H2 (R2a) k2a ? SiCH2 + H (R2b) with k2b/k2a a mean value for k2 = 2.0±0.69- 1014cm3mol-1s-1. A reaction mechanism including the established GRI (Gas Research Institute) mechanism is presented which explains all measured Si and H concentration profiles obtained during this study. by Oldenbourg Wissenschaftsverlag, Muenchen.
- Kunz,Bhaskaran,Roth
-
p. 811 - 824
(2007/10/03)
-
- Vacuum ultraviolet photochemistry of CH4 and isotopomers. II. Product channel fields and absorption spectra
-
Methyl and methylene fragments are achieved using a complete set of quantum yields for different photodissociation channels. The results depicted that the hydrogen (H) atoms are more abundant in photofragments than deuterium (D) atoms. The ultraviolet absorption spectrum of methane is identical at different temperatures. The quantum yields of H atoms are determined by the measurement of the ratio of areas under the laser-induced fluorescence excitation curves.
- Wang, Jen-Han,Liu, Kopin,Min, Zhiyuan,Su, Hongmei,Bersohn, Richard,Preses, Jack,Larese, John Z.
-
p. 4146 - 4152
(2007/10/03)
-
- A direct investigation of the reaction CH3+OH: Overall rate constant and CH2 formation at T=298 K
-
The rate constant and the product distribution of the reaction CH3+OH→ products (1) were studied at room temperature over a range of pressures (0.7 mbar ≤p≤467 mbar) with M = He as inert bath gas. At pressures of 45 to 467 mbar, towards the high pressure range of the reaction, experimental determinations were performed using the laser flash photolysis technique with transient UV absorption spectrometry (LFP/TAS) for direct monitoring of the CH3 and OH radicals. In this range, the overall rate constant for the reaction was found to be practically independent of pressure at k1 (298 K)=(4.4±0.8)×1013 cm3 mol-1 s-1. At pressures of 0.7 mbar to 4.0 mbar, the reaction was studied using the discharge flow (DF) technique with laser magnetic resonance (LMR) for the direct detection of the reactants CH3 and OH and the reaction product CH2. The measured overall rate constants at the low pressures were found to increase slightly with increasing pressure and approach the value at "high" pressure from the LFP/TAS experiments. From simulations of the measured concentration-versus-time profiles of the CH3, OH, and CH2 radicals, the branching ratio for the reaction channel CH3+OH → CH2+H2O (1.1) was determined at p=1.33 mbar, T=298 K to be k1.1/k1 = (0.89±0.09). Implications of the results are discussed considering the fall-off curve for the reaction, the reaction mechanism and the product distribution, and the available thermochemical data. WILEY-VCH Verlag GmbH, 1998.
- Deters,Otting,Wagner,Temps,Laszlo,Dob,Berces
-
-
- Photoinduced reactions of methyl radical in solid parahydrogen
-
Photolysis of methyl iodide in solid parahydrogen (p-H2) at about 5 K is studied with ultraviolet light at 253.7 and 184.9 nm. It is found that the light at 253.7 nm produces only methyl radical, whereas the light at 184.9 nm yields both methyl radical and methane. The mechanism of the formation of the photoproducts is elucidated by analyzing the temporal behavior of the observed vibrational absorption. It is concluded that methyl radical in the ground state does not react with p-H2 molecules appreciably but that the radical in the electronic excited state of B(2A1′), accessible by reabsorption of 184.9 nm photons by the radical, decomposes to a singlet methylene CH2 a(1A1) and a hydrogen atom (2S) and that the singlet methylene reacts with a p-H2 molecule to give methane.
- Fushitani, Mizuho,Sogoshi, Norihito,Wakabayashi, Tomonari,Momose, Takamasa,Shida, Tadamasa
-
p. 6346 - 6350
(2007/10/03)
-
- Photodissociation dynamics of 1,1-difluoroethylene at 157 nm excitation
-
Photodissociation of 1,1-difluoroethylene (F2CCH2) at 157 nm has been investigated using photofragment translational spectroscopy. Five dissociation channels have been experimentally observed; molecular HF elimination, H atom elimination, molecular hydrogen (H2) elimination, F atom elimination, and double bond breaking. Molecular HF elimination and H atom elimination channels are found to be the two major dissociation pathways in photodissociation of F2CCH2 at 157 nm excitation. Molecular hydrogen (H2) elimination and double bond cleavage are also significant, while F atom elimination is a minor process. Product translational energy distributions for all dissociation channels have also been measured. All translational energy releases are peaked at energies away from zero, indicating that the dissociation of F2CCH2 at 157 nm excitation most likely occurs with exit barriers on the ground electronic potential surface through internal conversion from the initially excited electronic state. Branching ratios and averaged energy partitions for different channels have also been estimated.
- Lin,Wu,Hwang,Lee,Yang
-
p. 10838 - 10846
(2007/10/03)
-
- Photofragment imaging of methane
-
The channels leading to the formation of atomic and/or molecular hydrogen in the photolysis of methane are studied. The H-atom elimination is investigated following photolysis at Lyman-α. The H2 elimination is investigated following two-photon
- Heck,Zare,Chandler
-
p. 4019 - 4030
(2007/10/03)
-
- Elementary Reactions in the C-H-F-System
-
The elementary reactions in the C-H-F-system F + CH3F --> CH2F + HF (1); F + CH2F --> CHF2 + H (2a); --> CHF + HF (2b) and CH2F + CH2F --> products (3), as well as CH3 + F --> CH2F + H (4a); --> CH2 + HF (4b) were studied in an isothermal flow reactor at room temperature and a nozzle reactor (T = 220 K) with mass spectrometric and laser induced detection devices.The rate constants k1 (T) = 2.1E13 (T/300 K)2.4 cm3/mol*s, k2a (300 K) = 5E13 cm3/mol*s, k2b (300 K) = 5E13 cm3/mol*s, k3 (300 K) = 7.5E12 cm3/mol*s, and k4 (300 K) = 6.8E13 cm3/mol*s were determined.The products of the reaction (3) were found to be C2H4F2 and C2H3F + HF.The pathway leading to C2H3F + HF was detected as the main product channel (>50percent).The rate and product distribution of reaction (3) was determined to be independent of pressure in the range 1 /= p (He)/mbar /= 7 and with quenching gases He, CO2 and SF6.In the reaction (4) the pathway (4a) contributes 12percent, and the H atom abstraction (4b) is with 88percent the main pathway. - Key words: Elementary reactions / Reaction kinetics / Mass spectrometry / Radical reactions (CH2F, CH3) / Atom reactions (F)
- Beiderhase, Th.,Hack, W.,Hoyermann, K.
-
p. 227 - 242
(2007/10/02)
-
- A Shock Study of Methyl-Methyl Reactions between 1200 and 2400 K
-
The methyl-methyl reaction was studied in a shock tube using uv narrowline laser absorption to measure time-varying concentration profiles of CH3.Methyl radicals were rapidly formed initially by pyrolysis of various precursors, azomethane, ethane, or methyl iodide, dilute in argon.The contributions of the various product channels, C2H6, C2H5 + H, C2H4 + H2, and CH2 + CH4, were examined by varying reactant mixtures and temperature.The measured rate coefficients for recombination to C2H6 between 1200 and 1800 K are accurately fit using the unimolecular rate coefficients reported by Wagner and Wardlaw (1988).The rate coefficient for the C2H5 + H channel was found to be 2.4 (+/- 0.5) x 1013 exp(-6480/T) 3/mol-s> between 1570 and 1780 K, and is in agreement with the value reported by Frank and Braun-Unkhoff (1988).No evidence of a contribution by the C2H4 + H2 channel was found in ethane/methane/argon mixtures, although methyl profiles the these mixtures should be particularly sensitive to this channel.An upper limit of approximately 1011 3/mol-s> over the range 1700 to 2200 K was inferred for the rate coefficient of the C2H4 + H2 channel.Between 1800 and 2200 K, methyl radicals are also rapidly removed by CH3 + --> 1CH2 + H2.In this temperature range, the reverse reaction was found to have a rate coefficient of 1.3 (+/- 0.3) x 1014 3/mol-s>, which is 1.8 times the room-temperature value.
- Davidson, D. F.,Rosa, M. D. Di,Chang, E. J.,Hanson, R. K.,Bowman, C. T.
-
p. 1179 - 1196
(2007/10/03)
-
- Transition state vibrational level thresholds foe the dissociation of triplet ketene
-
Rate constants for the unimolecular dissociation of ketene (CH2CO) and deuterated ketene (CD2CO)have been measured at the threshold for the production of CH2 ( 3B1) or CD2 ( 3B1) and CO ( 1Σ+) by photofragmentation in a cold jet.The rate constant increases in a stepwise manner as energy increases.This is in accord with the long-standing premise that the rate of a unimolecular reaction is controlled by flux through quantized transition-state thresholds at each energy level for vibrational motion orthogonal to the reaction coordinate.The firststep in rate constant and/or photofragment excitation (PHOFEX) spectrum gives accurate values for the barrier to dissociation above the zero-point energy of the products, 1281+/-15 cm-1 for CH2CO and 1071+/-40 cm-1 for CD2CO.The measured rate constants are fit by Rice-Ramsperger-Kassel-Marcus (RRKM) theory.The vibrational frequencies at the transition state obtained from the fits are compared with ab initio results.Vibrational motions at the transition state orthogonal to the reaction coordinate are also revealed in CO product rotational distributions.Calculations using an impulsive model which includes vibrational motions at the transition state reproduce the experimental dependence of the PHOFEX spectra on the CO J state quite well.The small dependence of rate constant on jet temperature (4-30 K) indicates that the Ka quantum number for rotation about its symmetry axis is conserved in the energized ketene molecule.
- Kim, Sang Kyu,Lovejoy, Edward R.,Moore, C. Bradley
-
p. 3202 - 3219
(2007/10/02)
-
- Photodissociation of ketene: CH2(1A1)(0,0,0) rotational state distributions
-
Ketene (CH2CO) cooled in a supersonic free jet is photodissociated by a tunable pulsed laser in the energy range from the threshold for production of CH2( 1A1)(0,0,0)+CO( 1Σ+) to 2900 cm-1 above.By scanning the 1CH2 probe laser wavelength, the CH2 laser-induced fluorescence spectrum is recorded and the 1CH2 product state distributions determined.The appearance thresholds and yield curves of individual 1CH2 rotational states are obtained by scanning the photolysis laser frequency with a fixed 1CH2 probe laser frequency.The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values.PHOFEX yield curves combined with the distribution data demonstrate that the ketene absorption cross section is constant within 10percent over the 0-1000 cm-1 range of the PHOFEX spectra.PHOFEX curves up to 200 cm-1 and rotational state distributions for singlet methylene at 50, 100, and 170 cm-1 above its threshold are in good agreement with phase spece theory (PST) calculations.The lowest energy methylene rotational states of + and - symmetry are formed in the ratio predicted by phase space theory at all energies.States of mixed singlet/triplet parentage are populated with equal probability for triplet characters ranging from 30percent to 70percent.For energies greater than 300 cm-1 above the threshold, the 1CH2 rotational distribution is substantially colder than the statistical distribution given by PST.These differences increase with excess energy above the singlet threshold.In this energy range, the dynamics of motion between the transition state and separated product fragments is in neither the adiabatic nor the statistical limit though the data do suggest that motion may well be adiabatic for molecules fragmenting to yield the lowest energy rotational states of ortho methylene.
- Garcia-Moreno, I.,Lovejoy, Edward R.,Moore, C. Bradley
-
p. 8890 - 8901
(2007/10/02)
-
- Photodissociation of ketene: Vibrationally excited CH2( 1A1)
-
Ketene (CH2CO) cooled in a supersonic free jet is photodissociated by a tunable pulsed laser in the energy range from 1460 to 2900 cm-1 above the threshold for singlet methylene CH2( 1A1) production.By scanning the 1CH2 probe laser wavelength, the CH2 laser-induced fluorescence (LIF) excitation spectrum is recorded and the 1CH2(0,1,0), (0,2,0), and (1,0,0) product state rotational distributions determined.As observed previously for the (0,0,0) state, the rotational state distributions of vibrationally excited methylene are in good agreement with phase space theory (PST) for excess energies less than 200 cm-1 above the appearance threshold of each vibrational state probed.For higher excess energies, 1CH2(0,1,0) and (0,2,0) rotational distributions like those for (0,0,0) are substantially colder than the statistical distribution given by PST.The quantum yields of vibrationally excited states are determined at several excess energies above the singlet threshold.These values are larger than predicted by PST and match values predicted by variational Rice-Ramsperger-Kassel-Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method.
- Garcia-Moreno, I.,Lovejoy, Edward R.,Moore, C. Bradley
-
p. 8902 - 8906
(2007/10/02)
-
- Diffuse Reflectance Infrared Fourier-transform Study of the Plasma Hydrogenation of Diamond Surfaces
-
Plasma hydrogenation of diamond surfaces was investigated using a diffuse reflectance infrared Fourier-transform (DRIFT) technique.Hydrogenation was carried out under microwave plasma conditions similar to those used for the chemical vapour deposition of diamond.Surface species chemisorbed on the diamond surface were characterized by DRIFT spectroscopy.The number of hydrogen-carbon bonds increased and the structure of the chemisorbed species on the diamond surfaces changed on increasing the temperature of the plasma hydrogenation.
- Ando, Toshihiro,Ishii, Motohiko,Kamo, Mutsukazu,Sato, Yoichiro
-
p. 1383 - 1386
(2007/10/02)
-
- Product Distributions of the C2H2 + O and HCCO + H Reactions. Rate Constant of CH2(X3B1) + H
-
The branching ratios of the two dominant methylene sources in C2H2/O/H systems C2H2 + O -> CH2(3B1) + CO (r2a) or HCCO + H (r2b) and HCCO + H -> CH2(1A1) + CO (r3a) or CH2(3B1) + CO (r3b) have been determined, at 285 K, using discharge-flow/molecular beam mass spectrometry techniques (D-F/MBMS).The ratios were derived from the observed decrease of MBMS CH2 signals upon substituting 25percent of the helium bath gas by 0.5 Torr of methane, which selectively scavenges CH2(1A1); in the absence of CH4 the singlet CH2 is mainly collisionally converted to CH2(3B1).In this way, the ratios of the total formation rates of singlet and triplet CH2 are obtained.As the rate of reaction r3 is linked to the rate of reaction r2b by the known fraction of HCCO reacting with H, in competition with O, both the ratios k2a/k2b and k3a/k3b can be extracted from the data.Thus, including also probably systematic errors, the HCCO yield of C2H2 + O is found to be k2b/k2 = 85 +4-9 percent and the CH2(1A1) yield of HCCO + H, k3a/k3 = 92 +/- 15percent (95percent confidence intervals).In addition, the rate constant of CH2(3B1) + H -> CH(2Π) + H2 (r1) was derived relative to the known K(3CH2+O) from CH2 signals at different / ratios; k1 = (1.6 +/- 0.6) * 1014 cm3 mol-1 s-1.
- Boullart, Werner,Peeters, Jozef
-
p. 9810 - 9816
(2007/10/02)
-
- Photodissociation of Methylacetylene at 193 nm
-
The photolysis of methylacetylene (MA) at 193 nm was studied using Fourier transform infrared spectroscopy for product analysis.Main primary processes are CH3C2H + h*ν -> CH3C2 + H and CH3C2H + h*ν -> CH2 + C2H2 with a quantum yield of 0.7 +/- 0.1 and 0.11 +/- 0.01, respectively.Cl2 was used as H atom scavenger.The CD3C2H photolysis was used to confirm that acetylene-d1 formed is a primary product and is independent of reactant pressures.The H atom addition to the center carbon atom of CD3C2H produces CD3 + C2H2 with a yield of 0.10 +/- 0.01; C2H2 is reduced to almost zero above 5 Torr of MA.The remaining H atom addition to the end carbon would produce allene and propylene.The photolysis of CD3C2H and Cl2 mixtures produces only HCl and DCl is not found, indicating that H atoms are dissociated immediately from the triple-bonded carbon, although the C-D bond in CD3C2H is much weaker.
- Seki, Kanekazu,Okabe, Hideo
-
p. 3345 - 3349
(2007/10/02)
-
- Isomers and reactivity of C3N+: An experimental study
-
The C3N+ ion, generated by electron impact on HC3N and C4N2 and as a product in several ion-molecue reactions, was found to exist in two isomeric forms: CCCN+ and cylic C3N+. These forms were distinguished by their different reactivities with a range of neutral reagents in a selected-ion How tube (SIFT). Isomeric identification was made by reference to existing ab initio calculations. The most reactive isomer, CCCN+, was the major form (≥90%) of the C3N+ ion from all sources of production examined and was found to undergo collision-rate reactions with most of the neutral molecules studied c-C3N+ was much less reactive, which implies an activation barrier in its reactions as it is the higher energy form. Product distributions are reported for the reactions of CCCN+, and rate coefficients for the reactions of both isomers with H2, CH4, NH3, M2O, N2, O2, CO, C2H2, HCN, CO2, and C2N2 at 300 ± 5 K are also given.
- Petrie, Simon,McGrath, Kathryn M.,Freeman, Colin G.,McEwan, Murray J.
-
p. 9130 - 9136
(2007/10/02)
-
- Determination of the singlet/triplet branching ratio in the photodissociation of ketene
-
The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm-1 below, 56, 110, 200, 325, 425, 1107, 1435, 1720, and 2500 cm-1 above the singlet threshold (30 116.2 cm-1), are measured in a supersonic free jet of ketene.The CO(v'' = 0) rotational distributions at 56, 110, 200, 325, and 425 cm-1 are bimodal.The peaks at low J's, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST).For CO(v'' = 0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm-1), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm-1 below the singlet threshold.The singlet yield shows a rapid increase in the low excess energy region (0-300 cm-1), and a slower increase above.The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields.The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region.The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory.At 2500 cm-1 excess energy, the CO(v'' = 1) rotational distribution is obtained, and the ratio of CO(v'' = 1) to CO(v'' = 0) products for the singlet channel is measured to be 0.045 +/- 0.017.This ratio is close to the variational Rice-Ramsberger-Kassel-Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016.
- Kim, Sang Kyu,Choi, Young S.,Pibel, Charles D.,Zheng, Qi-Ke,Moore, C. Bradley
-
p. 1954 - 1960
(2007/10/02)
-
- Bond-breaking without barriers. II. Vibrationally excited products
-
Ketene is photolyzed in a supersonic jet, and the vibrationally excited singlet methylene CH2 (a 1A1), produced is detected by laser-induced fluorescence.The appearance thresholds and yield curves of individual methylene rovibrational states are obtained by scanning the photolysis laser wavelength.As observed previously by probing the (0,0,0) state at lower photolysis energies, there are no barriers to dissociation and nuclear spin is conserved.Sharp steps are observed just above the energetic threshold in each of these photofragment excitation (PHOFEX) curves.This suggests that the rotational state distributions are given by phase space theory (PST).The quantum yield of the (0,1,0)101 rovibrational state is measured and the quantum yield for (0,1,0) inferred.These values are larger than predicted by PST, and are close to values predicted by variational Rice-Ramsberger-Kassel-Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method.This indicates that near the (0,1,0) energy threshold the (0,0,0) yield is constrained, as by a tight transition state.The appearance of steps spaced by the energies of a free CO rotor in the PHOFEX curves close to the thresholds of each vibrational state probed indicates that the near threshold flux of vibrationally ecited products is controlled by a loose "transition state" on a vibrationally adiabatic surface.These observations are consistent with the variational RRKM theory for dissociations without barriers in which each product vibrational state evolves on its own vibrationally adiabatic potential surface and has its own transition state.As the energy increases above the threshold for a vibrational state, its transition state move s in along the reaction coordinate and tightens.Thus total rates increase less rapidly with energy than in PST and vibrational distributions are skewed towards higher levels than in PST.
- Green, William H.,Mahoney, Arthur J.,Zheng, Qi-Ke,Moore, C. Bradley
-
p. 1961 - 1969
(2007/10/02)
-
- An Investigation of the Methanol Decomposition Behind Incident Shock Waves
-
The pyrolysis of methanol was investigated behind incident shock waves at temperatures and densities between 1400 and 2200 K and 1*10-6 and 5*10-6 mol/cm3, respectively.Narrow band-width laser adsorption for OH radicals and ARAS technique for H atoms was used to determine the decomposition channels.For the experimental conditions described above the direct OH-formation is found to be the main channel of about 80percent of the decomposition rate.The channel leading to H and CH2OH is found to be less than 5percent.There remains the possibility of channels leading from methanol to 1CH2 + H2O or to CH2O + H2. Keywords: Chemical Kinetics / Methanol / Ring dye Laser / Shock Waves / Spectroscopy, Ultraviolet
- Dombrowsky, Ch.,Hoffmann, A.,Klatt, M.,Wagner, H. Gg.
-
p. 1685 - 1687
(2007/10/02)
-
- Photofragmentation of Ketene to CH2(3B1) + CO. 1. Barrier Height and Dissociation Rate Constant
-
Ketene cooled in a supersonic free jet is photolyzed in the ultraviolet range.The appearance rate fragment CO from triplet ketene as a function of ultraviolet photon energy is measured by vacuum-ultraviolet laser-induced fluorescence.A tunneling-corrected RRKM theory is used to fit the observed dissociation rate of ketene using the ab initio vibrational frequencies of the transition state.The density of states inferred from the RRKM fit is a factor of 2 greater than the density of states of the groud electronic state (S0) and 300 times greater than that of T1.Tunneling corrections are necessary to fit the dissociation rate below the top of the exit barrier.The experimental rates show a sharp change in slope at 28290 cm-1 photolysis frequency.The height of the exit barrier is determined to be 3.79 +/- 0.06 kcal/mol above the ground-state fragments.The imaginary frequency that fits the data is (150 +/- 25)i cm-1, much lower than the ab initio frequency of 523i cm-1.At higher energies, the RRKM theory predicts faster rates than are observed.It is possible that the intersystem crossing rate is relatively independent of energy and becomes the rate-determining step for formation of triplet products at higher energies.
- Chen, I-Chia,Moore, C. Bradley
-
p. 263 - 269
(2007/10/02)
-
- Photofragmentation of Ketene to CH2(3B1) + CO. 2. Rotational-State Distribution of Product CO
-
Ketene ccooled in a supersonic jet is photolyzed at 1120, 190, and 30 cm-1 above and 50 cm-1 below the top of the barrier to dissociation on the triplet surface.The barrier is 1330 cm-1 above CH2(3B1) + CO.The rotational-state distributions of CO produced from triplet ketene are measured by vacuum-UV laser-induced fluorescence.These distributions cannot be described adequately by a Boltzmann temperature or by phase space statistical models.An average of 22.4 +/- 1percent of the total available energy goes to CO rotation as predicted by an impulsive model using the ab initio transition-state geometry.This indicates that the energy partitioning is determined by the geometry of the transition state with the repulsive force directed along the breaking bond.The fraction of energy going to CO rotation does not depend on the photolysis wavelenght.The Gaussian shape of the CO rotational-state distribution can be understood quantitatively in terms of zero-point motion of the C-C-O bending vibration of the transition state.Even when the excess energy above the top of the barrier is comparable to the barrier height itself, the energy release appears to be completely dynamically controlled.
- Chen, I-Chia,Moore, C. Bradley
-
p. 269 - 274
(2007/10/02)
-
- Kinetics of the Reaction of CH2(a1A1) with CH3C2H, HCN, CO2, N2O and COS
-
The reactions of CH2(a1A1) with CH3C2H, HCN, CO2, N2O and COS are investigated at room temperature.CH2(a1A1) is generated by pulsed laser photolysis of CH2CO.Overall removal rate constants are derived from concentration profiles under first order reaction conditions using direct, time resolved LIF detection of CH2(a1A1).The second order rate constants are found in units of 1E13 cm2/mol s to be 24, 18, 2.0, 3.8 and 20, respectively. - The contribution of physical quenching to the removal of CH2(a1A1) are determined by monitoring directyl the formation of CH2(X3B1) with LMR absorption technique.The branching ratios of collision induced intersystem crossing versus total consumption of 1CH2 are 0.14, 0.32, 0.67, 0.46 and 0.29 for the five reactants.
- Koch, M.,Temps, F.,Wagener, R.,Wagner, H. Gg.
-
p. 645 - 650
(2007/10/02)
-
- Elementary Reactions of N2(A3Σu+) with H2, F2 and H2C2O in the Gas Phase
-
Overall rate constants for the removal of N2(A,υ') by H2(X), F2(X) and H2C2O() were measured at T = 298 K in a fast flow reactor at constant injector position: N2(A) molecules in the vibrational states υ'= 0,1,2,3 were produced in the reaction of Ar(3P0,2) with N2(X) and detected by laser induced fluorescence (LIF) using the excitation of the first positive system of N2, B(3Πg) 3Σu+).The measured rate constants are: (1) k1 = 3.5 * 1E9 cm3/mol * s (υ'= 0) to 1.2 * 1E10 cm3/mol * s (υ' = 3). (2) k2 = 1.2 * 1E11 cm3/mol * s (υ' = 0) to 1.0 * 1E12 cm3/mol * s (υ'= 3). (3) k3 = 3.9 * 1E10 cm3/mol * s (υ' = 0) to 9.8 * 1E10 cm 3/mol * s (υ' = 3).The rates of reactions (1), (2) and (3) and the reaction mechanisms are discussed. - Keywords: Chemical Kinetics / Elementary Reactions / Energy Transfer / Photoelectrochemistry
- Boehmer, E.,Hack, W.
-
p. 170 - 176
(2007/10/02)
-
- Direct Determination of CH2(1A1) Removal Rates by Olefinic and Aromatic Hydrocarbons
-
Absolute rates of CH2(1A1) removal by the olefins ethene (C2H4), propene (C3H6), isobutene (i-C4H8), butadiene (C4H6), allene (C3H4) and the aromatic hydrocarbons toluene (C6H5CH3), p-xylene (C6H4(CH3)2), ethylbenzene (C6H5C2H5) and cumene (C6H5-(i-C3H7)) have been determined at low pressures and room temperature.CH2(1A1) was produced by pulsed laser photolysis of CH2CO.Concentration profiles of CH2(1A1) were recorded using laser induced fluorescence with a variable time delay between the probe laser and the photolysis laser. - The contributions of physical deactivation to the removal of CH2(1A1) in the reactions with C2H4, C3H6, C3H4 and C6H5CH3 were investigated quantitatively via the formation of CH2(3B1), detected by LMR. - 1CH2 is removed very efficiently by all the reactants, with second order rate constants in units of 1E14 cm3/mol s k(C2H4) = 1.4, k(C3H6) = 2.0, k(i-C4H8) = 2.0, k(C4H6) = 2.1, k(C3H4) = 1.9 and k(C6H5CH3) = 2.3, k(C6H4(CH3)2) = 2.2 k(C6H5C2H5) = 2.5, k(C6H5(i-C3H7)) = 2.6.The branching ratios of physical deactivation versus total removal for C2H4, C3H6, C3H4, C6H5CH3 were 0.20, 0.24, 0.30, 0.29.The dependence of reaction rates on alkylic substitution of the olefinic and aromatic Π bonding systems is small. - Keywords: Chemical Kinetics / Elementary Reactions / Photochemistry / Radicals
- Hack, W.,Koch, M.,Wagener, R.,Wagner, H. Gg.
-
p. 165 - 170
(2007/10/02)
-
- A solvent effect in reactions of singlet methylene
-
The effect of several solvents on the selectivity of singlet methylene (1:CH2) was investigated. It was found that product ratios from reactions in pentane, ethyl ether and cyclohexene solutions were identical; however, product ratios from reactions carried out in benzene were slightly different.
- Neugebauer,DeLuca
-
p. 7169 - 7172
(2007/10/02)
-
- A Study of the Reaction of CH2(1A1) with H2O in the Gas Phase
-
The reaction: CH2(1A1) + H2O() --> products was investigated in a quasistatic flash photolysis system at room temperature in the pressure range 0.5 1A1) produced in laser photolysis of CH2CO at λL = 308 nm, and the absolute concentration of the reaction product OH were monitored by laser induced fluorescence.In addition the population of the rotational states of the initial product OH (X, v = 0, K) was measured.The time delay between photolysis and probe pulse delivered the time resolution. - The overall 1CH2 depletion rate determined from Stern-Volmer plots is k1 (298 K) = (1.3 +/- 0.2)E14 cm3/mol s.The channel: 1CH2 + H2O --> CH3 + OH is determined to be the dominant reaction path in the range 0.5 1a/k1 1 in agreement with previous measurements.The bearing of these results on the reaction CH3 + OH and on the thermal CH3OH dissociation is briefly discussed. - Key words: Chemical Kinetics / Elementary Reactions / Photoelectrochemistry
- Hack, W.,Wagner, H. Gg.,Wilms, A.
-
p. 620 - 627
(2007/10/02)
-
- Bond breaking without barriers: Photofragmentation of ketene at the singlet threshold
-
Ketene ( CH2CO ) in a supersonic free jet was photodissociated by a tunable pulsed laser in the frequency range just above the threshold for production of singlet methylene, CH2 (1Α1).CH2 was detected by laser-induced fluorescence (LIF).The appearance threshold and yield curve of individual 1CH2 rotational states were obtained by scanning the photolysis laser frequency with a fixed LIF probe laser frequency.The dissociation occurs on the ground electronic state potential energy surface.The threshold for CH2CO1CH2 + CO is found to be 30116.2 +/- 0.4 cm-1.By varying the delay between the photolysis and probe pulses, a lower bound of 7 * 107 s-1 was set for the dissociation rate on the triplet surface at the singlet energy threshold.The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values.The experimental data provide a rigorous test of theoretical models of photofragment dynamics.The data clearly show that nuclear spin is conserved through the photodissociation.PHOFEX curves calculated from phase space theory (PST) are in excellent agreement with the experiment and show that there is no barrier along the reaction coordinate.The singlet/triplet branching ratio as a function of photolysis laser frequency is inferred from PST fits to the PHOFEX data.Comparisons with the statistical adiabatic channel model ( SACM ) are also presented.The data show that the only dynamical constraints on product state rotational energy distributions are conservation of energy, angular momentum, and nuclear spin.
- Chen, l.-Chia,Green, William H.,Moore, C. Bradley
-
p. 314 - 328
(2007/10/02)
-
- Understanding Unimolecular Dissociations with Loose Transition States: Photofragmentation Dynamics of Ketene at the Singlet Threshold
-
Photofragment excitation (PHOFEX) spectra of ketene (CH2CO) were taken by probing rovibronic transitions of the singlet methylene fragment while scanning the photolysis laser frequency.CH2 (1A1) was detected by laser-induced fluorescence in a supersonic free jet.The dissociation threshold of the reaction, CH2CO --> 1CH2 + CO was determined to be 30116.2 +/- 0.4 cm-1.The nuclear spin is conserved during the dissociation.Sharp rises in the 1CH2 PHOFEX spectra are observed whenever a new CO rotational state becomes energetically accessible.Phase Space Theory (PST) is in excellent agreement with the experimental data.The Statistical Adiabatic Channel Model (SACM) and the related SACM-PST theory fail to fit the experimental PHOFEX curves except in the limit where they become indistinguishable from PST (i.e. α/Sb>0.7).The data show that there are no barriers to dissociation and that essentially every accessible product quantum state is equally likely to be formed.The implications of these findings for the theory of unimolecular dissociation are discussed. - Keywords: Chemical Kinetics / Elementary Reactions / Photochemistry / Radicals / Spectroscopy, Ultraviolet
- Green, William H.,Chen, I-Chia,Moore, C. Bradley
-
p. 389 - 396
(2007/10/02)
-
- REACTIONS OF TRIFLUOROMETHYL AND TRIPLET METHYLENE RADICALS WITH SOME ALKYLSILANES
-
The recombination of CF3 and CH2SiF3 radicals was used to prepare CF3CH2SiF3 by collisional stabilization of the initially formed hot molecule, CF3CH2SiF3*.One fate of this hot species, not so far observed, is its decomposition to give CF3CH2 and SiF3 radicals, a process which leads to additional products.In separate studies of the reactions of 3CH2 with CF3CH2SiF3, the formation of CH3SiF3 is observed, and this is suggested to be through a 5-coordinate radical exchange process.
- Bell, T. N.,Marciniak, B.,Sherwood, A. G.
-
p. 3818 - 3820
(2007/10/02)
-
- Hydrocarbon Activation by Gas-Phase Lanthanide Cations: Interaction of Pr+, Eu+, and Gd+ with Small Alkanes, Cycloalkanes, and Alkenes
-
We describe ion beam studies of the interaction of gas-phase lanthanide ions, praseodymium (Pr+), europium (Eu+), and gadolinium (Gd+), with small alkanes, cycloalkanes, alkenes, and several oxygen-containing compounds.Only Gd+ is seen to activate C-H and C-C bonds of alkanes.The ground-state electronic configuration of Gd+ (4f75d16s1) is different from those of Pr+ (4f36s1) and Eu+ (4f76s1), leading to the conclusion that the f electrons play little part in the metal ion reactivity.Gd+ can be thought of as having two valence electrons, and indeed it reacts similarly to Sc+ and the other group 3 metal ions Y+ and La+, yielding products corresponding to elimination of hydrogen, alkanes, and alkenes.The elimination of neutral alkenes in the reaction of Gd+ with alkanes results in the formation of metal dialkyl or hydrido-alkyl complexes.This finding leads to estimates for the sum of two Gd+ ? bond dissociation energies of between 110 and 130 kcal/mol.Gd+ and Pr+ react readily with alkenes, yielding mostly dehydrogenation products along with smaller amounts of C-C bond cleavage products.Reactions of Gd+ and Pr+ with oxyen-containing species such as nitric oxide, formaldehyde, acetaldehyde, and acetone yield primarily the metal oxide ions and provide a lower limit for D(M+-O) of 179 kcal/mol, in good agreement with literature values of D(Pr+-O) = 188.4 +/- 5.2 kcal/mol and D(Gd+-O) = 181.0 +/- 4.4 kcal/mol.In keeping with the strong metal ? bonds, Gd+ is also seen to readily react with formaldehyde to eliminate CO and form GdH2+.
- Schilling, J. Bruce,Beauchamp, J. L.
-
-
- Quenching of CHF 1A'' (0,02 after Single Vibronic Level Excitation by the Diatomic Molecules H2, O2 and CO
-
The elementary deactivation processes of CHF(, v'2) by H2(X1Σg+), O2(X3Σg-, and CO(X1Σ+) were studied in a quasistatic photolysis chamber.Determination of time and spectrally resolved laser-induced fluorescence of CHF (1A'', v'2) in single vibronic excited levels (SVL) (0 2 1A' (0,0,0)> was generated in the fluorescence cell by multiphoton dissociation (MPD) of CH2F2.The overall CHF(, v'2)-depletion rate constants: .Asignificant increase of the rate constants for higher vibronic states was observed.The contribution of vibration deactivation to the overall depletion of the initially excited state was found to be small (2 = 1 and Δv'2 = 2 respectively.The reaction mechanisms are discussed. - Keywords: Elementary Reactions / Energy Transfer / Photochemistry
- Dornhoefer, G.,Hack, W.
-
p. 485 - 490
(2007/10/02)
-
- Infrared Laser Techniques to Study the Structure and Dynamics of Transient Species
-
Infrared multiple photon laser induced chemistry has moved over the past ten years between the extremes of heralding truly intramolecular selective chemistry and being merely a laboratory curiosity.The maturing of infrared multiple photon excitation (IRMPE) now finds IRMPE used routinely in a variety of experiments including spectroscopy, chemical kinetics, thermodynamics and laser driven synthesis.Continuously tunable infrared laser sources now allow a much wider range of molecules to be excited, and the photophysics of IRMPE to be probed in greater detail, than was possible with only limited line tunability.Examples of both the pfotophysics of IRMPE and applications to kinetics and spectroscopy are discussed, including vibrational photodissociation spectra of isolated metal clusters.
- Woodin, R. L.,Kaldor, A.
-
p. 269 - 276
(2007/10/02)
-
- A Direct Study of the Reaction CH2(3B1) + C2H4 in the Temperature Range 296 K = T = 728 K
-
The reaction between CH2-radicals in the triplet electronic ground state 3B1(3CH2) and C2H4 3CH2 + C2H4 --> products was studied in the gas phase at temperatures between 296 K 3CH2 in the presence of a large excess of C2H4 was found to be k1exp = 1E(12.69+/-0.12)exp(-(2810+/-140)K/T)cm3/mols.Correcting for the small contribution to the depletion of 3CH2 due to collisional excitation to the singlet first excited state 1A1(1CH2) followed by consecutive reactions of 1CH2, the rate constant for the direct reaction of 3CH2 with C2H4 was obtained to be k1 = 1E(12.50+/-0.10)exp(-(22.1+/-1.0)kj mol-1/RT)cm3/mols.A reaction mechanism describing both the present data as well as the results of earlier related studies in the CH2 + C2H4 system is proposed.The mechanism and product yields are discussed in terms of unimolecular rate theory.- Chemical Kinetics / Elementary Reactions / Laser Magnetic Resonance / Radicals
- Boehland, T.,Temps, F.,Wagner, H. Gg.
-
p. 468 - 475
(2007/10/02)
-