Photolysis Of Ketene Research Articles

Acetylene combustion reactions. Rate constant measurements of HCCO with O 2 and C 2H 2

The second-order rate constant for the reaction of the ketenyl radical (HCCO) with O 2 was measured at room temperature by using infrared kinetic spectroscopy as 6.5 (6) × 10 −13 cm 3 molecule −1 s −1. An upper bound for the rate constant for the reaction between HCCO and C 2H 2 was determined as 1 × 10 −13 cm 3 molecule −1 s −1. HCCO was produced by 193 nm excimer laser photolysis of ketene and probed with a tunable infrared diode laser operating at 2014 cm −1.

Infrared flash kinetic spectroscopy of ν2 of ketenyl radical

The high-resolution infrared spectrum of the heavy atom antisymmetric stretch vibration of the ketenyl radical (HCCO) has been observed by means of infrared kinetic spectroscopy using 193-nm photolysis of ketene followed by diode laser probing of the transient absorption. The observed transitions were assigned and fitted with rotational, spin-rotational, and centrifugal distortion constants. The origin for this band is located at 2022.644 cm −1.

Infrared flash kinetic spectroscopy of the ketenyl radical

The high resolution infrared spectrum of the heavy atom antisymmetric stretch of the ketenyl radical (HCCO) has been observed by means of infrared kinetic spectroscopy producing the ketenyl by 193 nm photolysis of ketene with diode laser probing of the transient absorption. Individual rovibrational transitions have been identified near the band origin located at approximately 2023 cm −1. A preliminary kinetic study of the ketenyl radical's reaction with nitric oxide was conducted. A second order rate constant of 3.9(5) × 10 −11 cm 3 molecule −1 s −1 was obtained for this reaction by following the decay of ketenyl in the presence of excess NO.

The yield of CO in the reaction: CH 2(ã 1A 1) + CH 2CO

The yield of CO from CH 2(ã 1A 1) + CH 2CO has been measured by monitoring the transient IR absorption of carbon monoxide following pulsed laser photolysis of ketene (CH 2CO + hv(308 nm) → CH 2 (ã 1A 1) + CO). Analysis of the prompt CO production as a function of CH 2 (ã 1A 1) scavenger concentration ([ i-C 4H 8] and [N 2]) gives a CO yield of 0.63 ± 0.13, which suggests that collision-induced intersystem crossing (CH 2 (ã 1A 1) + CH 2 CO → CH 2 (X̃ 3B 1) + CH 2CO) is important in this reaction.

Bending overtones and barrier height of ã 1A1 CH2 by flash photolysis stimulated emission pumping

The J≤7 and Ka≤3 rotational levels of the bending vibrational overtones, (0,2,0), (0,3,0), and (0,4,0), and the (1,1,0) combination band of ã 1A1CH2 were characterized in detail by stimulated emission spectroscopy. The methylene was generated from ketene photolysis. Spectroscopic analyses using a rigid rotor Hamiltonian gave the vibrational term values and rotational constants. The latter were found to be strongly J and Ka dependent suggesting the inadequacy of the rigid rotor Hamiltonian for calculating the rotational energy to high accuracy. Effects of Coriolis, Fermi, and singlet–triplet perturbations were all observed in the spectra and discussed. Abnormally intense ΔKa=3 transitions were observed in the ã←b̃, 2162 and 2163 bands. From fitting the experimentally measured bending vibrational term values by a model calculation based on the WKB theory, the barrier height to linearity in the ã state was determined to be 9870 cm−1.

Transient vibrational spectroscopy of [formula omitted

The 2 ν 2 level of a ̃ 1A 1 CH 2, generated by flash photolysis of ketene, has been characterized by using both stimulated emission pumping and hot-band laser-induced fluorescence. Spectroscopic analyses yield a vibrational term value of 2667.74(11) cm −1 and rotational constants A = 23.617(50) B = 11.384(11), and C = 6.8444(89) cm −1. Abnormally intense Δ K a = 3 transitions have also been observed in the a ̃ ← b ̃ , 2 2 16 band.

Rate constants for triplet methylene (~X3B1) removal by oxygen, nitric oxide and acetylene from infrared diode laser flash kinetic spectroscopy

Removal rates of ground-state triplet methylene ({tilde X}{sup 3}B{sub 1}, CH{sub 2}) due to the reaction with O{sub 2}, NO, and C{sub 2}H{sub 2} have been measured by laser flash kinetic spectroscopy. Excimer laser photolysis of ketene produced {sup 3}CH{sub 2}. The subsequent time behavior of the {sup 3}CH{sub 2} transient was recorded by directly monitoring CW diode laser absorption due to {sup 3}CH{sub 2}. From analysis of {sup 3}CH{sub 2} decay curves under pseudo-first-order conditions, the following rate constants were determined (298 {plus minus} 2 K): k{sub O{sub 2}} = (3.24 {plus minus} 0.26) {times} 10{sup {minus}12} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}, k{sub NO} = (2.91 {plus minus} 0.24) {times} 10{sup {minus}11} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}, and k{sub C{sub 2}H{sub 2}} < 1.4 {times} 10{sup {minus}14} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. The results are in good agreement with recent, direct measurements made using flow tube techniques but are in poor agreement with earlier, indirect measurements.

Multiphoton ionization spectra of radical products in the F(2P)+ketene system: Spectral assignments and formation reaction for CH2F, observation of CF and CH

The reactions of F(2P)+ketene and F(2P)+ketene-d2 were studied in a flow reactor. Spectra of the radical products CH2F, CD2F, CH, CF, and atomic carbon were detected between 292–395 nm by resonance enhanced multiphoton ionization (REMPI) mass spectrometry. Fluoromethyl radicals were identified as a major product of the F+ketene elementary reaction. REMPI spectra of fluoromethyl radicals originated from two-photon preparation of 3p, 4p, and 5p Rydberg states (quantum defect ∼0.6). Absorption of a third laser photon ionized the radicals, a 2+1 REMPI mechanism. Rydberg band origins were observed in CH2F at ν00=52 863, 63 275, and 67 265 cm−1 and in CD2F at ν00=52 786, 63 195, and 67 186 cm−1. A normal mode analysis revealed the vibrational frequencies of the C–F stretch, CH2 scissors, and out-of-plane bending modes in the Rydberg states. The ground state out-of-plane bending frequency in CH2F is 260(30) cm−1 and in CD2F it is 170(30) cm−1. CH radicals were generated by the photolysis of ketene and observed at ∼311 nm by two-photon excitation through the D 2Π(v′=2) ←←X 2Πr band. The reaction mechanism that generated the CF radicals was not determined. The REMPI CF radical spectrum generated by a 2+1 photon mechanism appeared as a series of bandheads described by the constants ν00=∼52 572 cm−1, ωe =1820 cm−1, and ωexe =−18.2 cm−1.

ChemInform Abstract: Kinetic Study of Chemically Activated Fluorocyclopropane.

AbstractThe photolysis of ketene at 313 nm was studied in the presence of fluoroethene and oxygen.

Visible absorption and magnetic-rotation spectroscopy of 1CH2: The analysis of the b̃ 1B1 state

The b̃1B1←ã 1A1 spectrum of CH2 radical has been recorded with Doppler-limited resolution in the 15 600–18 650 cm−1 wavelength region by laser flash-kinetic absorption spectroscopy. Singlet methylene is produced by photolysis of ketene at 308 nm. Assignments for 477 transitions originating from 1A1(0,0,0) and (0,1,0) levels with J≤8 and Ka≤5 are reported from some 10 000 lines observed. Term values are given for these 1A1 levels and 1B1(0,v2,0) levels with 13≤v2≤17. Magnetic-rotation signals were observed for 60% of the 424 lines studied. This anomalous Zeeman effect is explained by singlet–triplet mixing in both ã and b̃ states. Extensive singlet–triplet mixing occurs for 1B1 levels with Ka≠0; this mixing is allowed in second order by Renner–Teller coupling of b̃ 1B1 with ã 1A1 and by spin-orbit coupling via ã 1A1 to X̃ 3B1. The consequent shifts prevent determination of accurate rovibrational structural parameters for the 1B1 state.

Kinetic Study of Chemically Activated Fluorocyclopropane

AbstractThe photolysis of ketene at 313 nm was studied in the presence of fluoroethene and oxygen. The product formation was observed as a function of total pressure between 12 and 1080 mbar. Chemically activated fluorocyclopropane was formed by addition of singlet methylene to the double bond, and subsequent isomerization to the fluoropropenes as well as decomposition of these primary products to allene, propyne and HF was observed. The product yield was described by RRKM calculations; a stepladder deactivation model was used.

Rotational state distribution of CO photofragments from triplet ketene

The nascent rotational state distribution of CO(v″=0,J″) following excimer laser photolysis of ketene at 351 nm has been determined under collisionless conditions in a flow cell. At this low excitation energy dissociation can only take place on the triplet potential surface leading to CH2(X̃ 3B1) and CO(X̃ 1Σ+). The available energy permits only the vibrational ground state of CO to be populated. The observed rotational distribution of CO(v″=0,J″) deviates drastically from a phase space theory statistical distribution as well as from a thermal one. A Boltzmann plot of this distribution exhibits a population inversion for J″&amp;lt;13. The nonstatistical behavior is attributed to a barrier along the dissociation path. The fragments are repelled too rapidly for energy to be randomized between them. Thus the photofragmentation dynamics of triplet ketene contrasts markedly with dissociation on the singlet surface which has no barrier and gives a statistical CO rotational state distribution. An impulsive model calculation for the ab initio transition state geometry is in surprisingly good agreement with the experimental energy partitioning among the fragment degrees of freedom. This suggests that the CCO bond angle is strongly bent at the top of the barrier and that the barrier height is a substantial fraction of available energy.

ChemInform Abstract: KINETIC STUDY OF CHEMICALLY ACTIVATED CHLOROCYCLOPROPANE

AbstractAus Keten photolytisch bei 313 und 366 nm erzeugtes Carben CH; im (′A,)‐ und (3B,)‐Zustand addiert sich an Chlorethen zu Chlorcyclopropan, das dann einen Energieinhalt zwischen 378 und 427 kJ/mol hat.

The formation of C3H3 (propynyl) radicals in the reaction of CH2(1A1) with acetylene

AbstractResults obtained from the photolysis of ketene with acetylene strongly support the formation of C3H3 radicals in the title reaction. Stationary state studies are interpreted in terms of the reaction with a rate constant (109.8 s−1) which is compared to RRKM predictions. In pulsed laser induced decomposition experiments, recombination products involving C3H3 have been detected (some for the first time) and their formation modeled using step (3) with the same rate constant.

Kinetic study of chemically activated chlorocyclopropane

AbstractChlorocyclopropane has been produced by addition of CH2(1A1) and CH2(3B1) to chloroethene. CH2 was generated by the photolysis of ketene at 313 and 366 nm. Chlorocyclopropane was formed in a chemically activated state, had an energy content between 378 and 427 kJ/mol, and reacted in three parallel channels to 3‐chloropropene, cis‐ and trans‐1‐chloropropene. As secondary reactions elimination of HCl from the chemically activated primary products occurred to form allene and propyne. The apparent rate constants for the isomerization and elimination reactions are reported. The results of RRKM calculations including distribution functions for the activated chlorocyclopropane and a stepladder model for the deactivation support the proposed reaction scheme.

Chemical activation study of the reactions of methylene with oxetan and 3,3-dimethyloxetan

Methylene (singlet) formed by the photolysis of ketene (313 nm) reacts with oxetan to yield 2-methyl- and 3-methyloxetans by insertion in the C—H bonds and probably also tetrahydrofuran. These products are formed chemically activated and undergo unimolecular decomposition unless collisionally stabilized. Using perfluoropropane as the bath gas, the results obtained with 2-methyloxetan have been interpreted using RRKM theory and a step-ladder model with a most probable step size of ca. 9 kJ mol–1. Using 206 nm radiation yields methylene with excess energy, some of which is still present when it reacts, and results in the production of 2-methyloxetan with an average lifetime approximately one-half that of the energised molecule produced when 313 nm radiation is used.

High‐energy kinetic study of chemically activated 1,1‐dichlorocyclopropane

Abstract1,1‐Dichlorocyclopropane has been produced by addition of CH2(1A1) to 1,1‐dichloroethylene. CH2(1A1) was generated by the photolysis of ketene at 277–334 nm. The 1,1‐dichlorocyclopropane was formed in a chemically activated state, had an energy content between 386 and 400 kJ/mol, and reacted in two parallel channels to 2,3‐dichloropropene and 1,1‐dichloropropene. 1,1‐Dichloropropene was also formed directly by insertion of CH2(1A1) into the CH bond of 1,1‐dichloroethylene. As secondary reactions elimination of HCl from chemically activated 2,3‐dichloropropene occurred with 3‐chloropropyne and chloroallene as products. In some of the experiments perfluoropropane was added as an inert gas. The apparent rate constants for the isomerization and elimination reactions are reported. The results of RRKM calculations including distribution functions for the activated 1,1‐dichlorocyclopropane and a step‐ladder model for the deactivation verify the proposed reaction scheme.

ChemInform Abstract: THE REACTION OF METHYLENE RADICALS WITH METHYL ISOCYANIDE

AbstractDas durch Photolyse von Keten erzeugte Carben reagiert mit Me‐NC unter Einschiebung in die C‐N‐ Bindung zum schwingungsangeregten Ethylisocyanid, das sich hauptsächlich zu ET‐CN stabilisiert.

Chemically activated methylcyclobutane from the 337.1 nm photolysis of ketene in the presence of cyclobutane. Dependence of S/D on extent of photolysis

The 3371 A photolysis of ketene/cyclobutane/oxygen mixtures gave stabilization/decomposition ratios for chemically activated methylcyclobutane that varied with the extent of photolysis. Experiments designed to circumvent this complication gave a 10 ± 2 kcal/mole collisional deactivation step size for cyclobutane.

Mechanism of product formation during the photolysis of ketene

The photolysis of ketene has been studied with both /sup 14/CH/sub 2/CO and CHTCO, and a new mechanism is suggested for the reactions of triplet methylene in pure ketene. The isotopic experiments are consistent with the set of reactions, (6) plus (8) to (11), /sup 3/CH/sub 2/ + /sup 3/CH/sub 2/ ..-->.. C/sub 2/H/sub 2/ + 2H (8), H + CH/sub 2/CO ..-->.. CH/sub 3/ + CO (9), /sup 3/CH/sub 2/ + CH/sub 3/ ..-->.. C/sub 2/H/sub 4/ + H (10,11), CH/sub 3/ + CH/sub 3/ ..-->.. C/sub 2/H/sub 6/ (6); and not with the earlier proposal of (3) to (6), /sup 3/CH/sub 2/ + CH/sub 2/CO ..-->.. CH/sub 3/ + CHCO (3), CHCO + CHCO ..-->.. C/sub 2/H/sub 2/ + 2CO (4), CH/sub 3/ + CHCO ..-->.. C/sub 2/H/sub 4/ + CO (5). The formation of atomic hydrogen rather than molecular hydrogen in (8) has important consequences for interpretation of the flash photolysis of ketene.