Unimolecular Reaction Dynamics of CH3COCl and FCH2COCl: An Infrared Chemiluminescence and ab Initio Study (original) (raw)
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Canadian Journal of Chemistry, 1994
The reactions of H atoms with CCl3, CF2Cl, and CH2CH2Cl radicals have been studied in a flow reactor at 300 and 475 K by observation of the infrared emission from the HCl and HF products. These reactions were observed as secondary reactions from the H + CCl3Br, CF2ClBr, and CH2Cl–CH2I chemical systems. The conditions in the flow reactor were controlled so that the nascent vibrational distributions of HCl and HF were recorded. The pattern of vibrational energy disposal to HCl was used to differentiate between Cl atom abstraction and recombination–elimination mechanisms. The H atom reactions with CCl3 and CF2Cl radicals occur only via a recombination–elimination mechanism and give HCl(υ) or HF(υ) in a unimolecular step. Thus, the Cl atom abstraction reactions must have ≥3.0 kcal mol−1 higher activation energy than the recombination reaction. From observation of the ratio of the HCl and HF products from CHF2Cl*, the difference in threshold energies for HF and HCl elimination was determ...
The Journal of Physical Chemistry A, 2015
Following photodissociation of gaseous acryloyl chloride, CH 2 CHC(O)-Cl, at 193 nm, temporally resolved vibration−rotational emission spectra of HCl (v ≤ 7, J ≤ 35) in region 2350−3250 cm −1 and of CO (v ≤ 4, J ≤ 67) in region 1865−2300 cm −1 were recorded with a step-scan Fourier-transform spectrometer. The HCl emission shows a minor low-J component for v ≤ 4 with average rotational energy E rot = 9 ± 3 kJ mol −1 and vibrational energy E vib = 28 ± 7 kJ mol −1 and a major high-J component for v ≤ 7 with average rotational energy E rot = 36 ± 6 kJ mol −1 and vibrational energy E vib = 49 ± 9 kJ mol −1 ; the branching ratio of these two channels is ∼0.2:0.8. Using electronic structure calculations to characterize the transition states and each intrinsic reaction coordinate, we find that the minor pathway corresponds to the four-center HClelimination of CH 2 ClCHCO following a 1,3-Cl-shift of CH 2 CHC(O)Cl, whereas the major pathway corresponds to the direct four-center HCl-elimination of CH 2 CHC-(O)Cl. Although several channels are expected for CO produced from the secondary dissociation of C 2 H 3 CO and H 2 CCCO, each produced from two possible dissociation channels of CH 2 CHC(O)Cl, the CO emission shows a near-Boltzmann rotational distribution with average rotational energy E rot = 21 ± 4 kJ mol −1 and average vibrational energy E vib = 10 ± 4 kJ mol −1. Consideration of the branching fractions suggests that the CO observed with greater vibrational excitation might result from secondary decomposition of H 2 C CCO that was produced via the minor low-J HCl-elimination channel, while the internal state distributions of CO produced from the other three channels are indistinguishable. We also introduce a method for choosing the correct point along the intrinsic reaction coordinate for a roaming HCl elimination channel to generate a Franck−Condon prediction for the HCl vibrational energy.
The Journal of Physical Chemistry A, 2007
The reaction kinetics of chlorine atoms with a series of partially fluorinated straight-chain alcohols, CF 3 -CH 2 CH 2 OH (1), CF 3 CF 2 CH 2 OH (2), CHF 2 CF 2 CH 2 OH (3), and CF 3 CHFCF 2 CH 2 OH (4), were studied in the gas phase over the temperature range of 273-363 K by using very low-pressure reactor mass spectrometry. The absolute rate coefficients were given by the expressions (in cm 3 molecule -1 s -1 ): k 1 ) (4.42 ( 0.48) × 10 -11 exp(-255 ( 20/T); k 1 (303) ) (1.90 ( 0.17) × 10 -11 , k 2 ) (2.23 ( 0.31) × 10 -11 exp(-1065 ( 106/ T); k 2 (303) ) (6.78 ( 0.63) × 10 -13 , k 3 ) (8.51 ( 0.62) × 10 -12 exp(-681 ( 72/T); k 3 (303) ) (9.00 ( 0.82) × 10 -13 and k 4 ) (6.18 ( 0.84) × 10 -12 exp(-736 ( 42/T); k 4 (303) ) (5.36 ( 0.51) × 10 -13 . The quoted 2σ uncertainties include the systematic errors. All title reactions proceed via a hydrogen atom metathesis mechanism leading to HCl. Moreover, the oxidation of the primarily produced radicals was investigated, and the end products were the corresponding aldehydes (R F -CHO; R F ) -CH 2 CF 3 , -CF 2 CF 3 , -CF 2 CHF 2 , and -CF 2 CHFCF 3 ), providing a strong experimental indication that the primary reactions proceed mainly via the abstraction of a methylenic hydrogen adjacent to a hydroxyl group. Finally, the bond strengths and ionization potentials for the title compounds were determined by density functional theory calculations, which also suggest that the R-methylenic hydrogen is mainly under abstraction by Cl atoms. The correlation of room-temperature rate coefficients with ionization potentials for a set of 27 molecules, comprising fluorinated C2-C5 ethers and C2-C4 alcohols, is good with an average deviation of a factor of 2, and is given by the expression log(k) (in cm 3 molecule -1 s -1 ) ) (5.8 ( 1.4) -(1.56 ( 0.13) × (ionization potential (in eV)).
Unimolecular Reactions of CF2ClCFClCH2F and CF2ClCF2CH2Cl: Observation of ClF Interchange
The Journal of Physical Chemistry A, 2008
The unimolecular reactions of CF 2 ClCFClCH 2 F and CF 2 ClCF 2 CH 2 Cl molecules formed with 87 and 91 kcal mol-1 , respectively, of vibrational energy from the recombination of CF 2 ClCFCl with CH 2 F and CF 2 ClCF 2 with CH 2 Cl at room temperature have been studied by the chemical activation technique. The 2,3-and 1,2-ClF interchange reactions compete with 2,3-ClH and 2,3-FH elimination reactions. The total unimolecular rate constant for CF 2 ClCF 2 CH 2 Cl is 0.54 (0.15 × 10 4 s-1 with branching fractions for 1,2-ClF interchange of 0.03 and 0.97 for 2,3-FH elimination. The total rate constant for CF 2 ClCFClCH 2 F is 1.35 (0.39 × 10 4 s-1 with branching fractions of 0.20 for 2,3-ClF interchange, 0.71 for 2,3-ClH elimination and 0.09 for 2,3-FH elimination; the products from 1,2-ClF interchange could be observed, but the rate constant was too small to be measured. The D(CH 2 F-CFClCF 2 Cl) and D(CH 2 Cl-CF 2 CF 2 Cl) were evaluated by calculations for some isodesmic reactions and isomerization energies of CF 3 CFClCH 2 Cl as 84 and 88 kcal mol-1 , respectively; these values give the average energies of formed molecules at 298 K as noted above. Density functional theory was used to assign vibrational frequencies and moments of inertia for the molecules and their transition states. These results were combined with statistical unimolecular reaction theory to assign threshold energies from the experimental rate constants for ClF interchange, ClH elimination and FH elimination. These assignments are compared with results from previous chemical activation experiments with CF 3 CFClCH 2 Cl, CF 3 CF 2 CH 3, CF 3 CFClCH 3 and CF 2 ClCF 2 CH 3 .
Physical Chemistry Chemical Physics, 2001
Infrared chemiluminescence from a flow reactor has been used to study the H þ CH 2 XI and N þ CH 2 X (X ¼ Cl, F, I, H) reactions at 300 K. Both the HI þ CH 2 Cl and HCl þ CH 2 I channels were identified for the H þ CH 2 ClI reaction. The HCl channel involves adduct, HICH 2 Cl, formation as confirmed by the D þ CH 2 ClI reaction, which gave both HCl and DCl products. The nascent HCl(v) distribution from the H þ CH 2 ClI reaction was P 1-P 5 ¼ 25 : 29 : 26 : 13 : 7. The rate constant for the HCl(v) formation channel is estimated to be 4 times smaller than that for the H þ Cl 2 reaction. The highest HCl(v) level observed from the H þ CH 2 ClI reaction implies that the C-Cl bond energy is 50.2 kJ mol À1 lower than that of the Cl-CH 3 bond, which is in modest agreement with recent theoretical estimates. The H þ CH 2 FI reaction gave a HF(v) distribution of P 1-P 3 ¼ 77 : 15 : 8. The C-F bond energy in CH 2 FI is estimated to be 4 460.2 kJ mol À1 , based on the highest HF(v) level observed, the upper bound being the same as that of F-CH 3. When N atoms are added to the flow reactor, the HCl(v) emission intensities from H þ CH 2 ClI increased by up to 2-fold, which is attributed to the N þ CH 2 Cl ! HCl þ HCN reaction. Concomitant weak emission from HCN and HNC could also be observed; however, the main product channel is thought to be NCH 2 þ Cl. Strong visible CN(A-X) emission was also observed when H=N=CH 2 XI were present in the reactor. If the CH 2 X radicals were produced by the F þ CH 3 X reaction in the presence of N atoms, similar results were obtained. The N þ CH 2 N reaction is proposed as the first step that leads to CN(A) formation with NCN as an intermediate.
The Journal of Physical Chemistry A, 2005
Vibrationally activated CF 3 CH 2 CH 2 Cl molecules were prepared with 94 kcal mol-1 of vibrational energy by the combination of CF 3 CH 2 and CH 2 Cl radicals and with 101 kcal mol-1 of energy by the combination of CF 3 and CH 2 CH 2 Cl radicals at room temperature. The unimolecular rate constants for elimination of HCl from CF 3 CH 2 CH 2 Cl were 1.2 × 10 7 and 0.24 × 10 7 s-1 with 101 and 94 kcal mol-1 , respectively. The product branching ratio, k HCl /k HF , was 80 (25. Activated CH 3 CH 2 CH 2 Cl and CD 3 CD 2 CH 2 Cl molecules with 90 kcal mol-1 of energy were prepared by recombination of C 2 H 5 (or C 2 D 5) radicals with CH 2 Cl radicals. The unimolecular rate constant for HCl elimination was 8.7 × 10 7 s-1 , and the kinetic isotope effect was 4.0. Unified transition-state models obtained from density-functional theory calculations, with treatment of torsions as hindered internal rotors for the molecules and the transition states, were employed in the calculation of the RRKM rate constants for CF 3 CH 2 CH 2 Cl and CH 3 CH 2 CH 2 Cl. Fitting the calculated rate constants from RRKM theory to the experimental values provided threshold energies, E 0 , of 58 and 71 kcal mol-1 for the elimination of HCl or HF, respectively, from CF 3 CH 2 CH 2 Cl and 54 kcal mol-1 for HCl elimination from CH 3 CH 2 CH 2 Cl. Using the hindered-rotor model, threshold energies for HF elimination also were reassigned from previously published chemical activation data for CF 3 CH 2 CH 3, CF 3 CH 2 CF 3 , CH 3 CH 2 CH 2 F, CH 3 CHFCH 3 , and CH 3 CF 2-CH 3. In an appendix, the method used to assign threshold energies was tested and verified using the combined thermal and chemical activation data for C 2 H 5 Cl, C 2 H 5 F, and CH 3 CF 3 .
The Journal of Chemical Physics, 2003
The dynamics of chlorine and hydrogen atom formation in the 193.3 nm gas-phase laser photolysis of room-temperature 1,1-dichloro-1-fluoroethane, CH 3 CFCl 2 ͑HCFC-141b͒, were studied by means of the pulsed-laser-photolysis and laser-induced fluorescence ͑LIF͒ ''pump-and-probe'' technique. Nascent ground-state Cl(2 P 3/2) and spin-orbit excited Cl*(2 P 1/2) as well as H(2 S) atom photofragments were detected under collision-free conditions by pulsed Doppler-resolved laser-induced fluorescence measurements employing narrow-band vacuum ultraviolet probe laser radiation, generated via resonant third-order sum-difference frequency conversion of dye laser radiation in krypton. Using HCl photolysis as a reference source of well-defined Cl(2 P 3/2), Cl*(2 P 1/2), and H atom concentrations, values for the chlorine-atom spin-orbit branching ratio ͓Cl*͔/͓Cl͔ϭ0.36Ϯ0.08, the total chlorine atom quantum yield (⌽ ClϩCl * ϭ1.01Ϯ0.14), and the H atom quantum yield (⌽ H ϭ0.04Ϯ0.01) were determined by means of a photolytic calibration method. From the measured Cl and Cl* atom Doppler profiles the mean relative translational energy of the chlorine fragments could be determined to be E T(Cl) ϭ157Ϯ12 kJ/mol and E T(Cl *) ϭ165 Ϯ12 kJ/mol. The corresponding average values 0.56 and 0.62 of the fraction of total available energy channeled into CH 3 CFClϩCl/Cl* product translational energy were found to lie between the limiting values 0.36 and 0.85 predicted by a soft impulsive and a rigid rotor model of the CH 3 CFCl 2 →CH 3 CFClϩCl/Cl* dissociation processes, respectively. The measured total chlorine atom quantum yield along with the rather small H atom quantum yield as well as the observed energy disposal indicates that direct C-Cl bond cleavage is the most important primary fragmentation mechanism for CH 3 CFCl 2 after photoexcitation in the first absorption band.
International Journal of Chemical Kinetics, 1986
The kinetics of the reactions of CH21, CH2Br, CH2CI, and CHCI2 with HI were studied in a tubular reactor coupled to a photoionization mass spectrometer. Rate constants were measured as a function of temperature (typically between 294 and 552 K) to determine Arrhenius parameters. For these and other R + HI reactions studied to date (Le,, those involving alkyl radicals), a linear free energy relationship was discovered which correlates the large differences in reactivity among all these R + HI reactions with the inductive effect of the substituent atoms or groups on the central carbon atom.
The Journal of Physical Chemistry, 1996
The absolute rate coefficients for the hydrogen abstraction reactions from CH 3 CCl 3 (k 1), CH 3 CCl 2 F (k 2), and CH 3 CClF 2 (k 3) by chlorine atoms in gas phase have been measured as a function of temperature using the discharge flow/mass spectrometric technique (DF/MS). The reactions were investigated under pseudo-firstorder conditions with Cl atoms in large excess with respect to the haloethanes. The temperature dependence of the rate coefficients is expressed in the Arrhenius form: k 1 (298-416K)) (2.8-1.7 +4.1) ×10-12 exp[-(1790 (320)/T], k 2 (299-429K)) (3.0-1.3 +2.4) × 10-12 exp[-(2220 (150)/T], k 3 (298-438K)) (1.5-1.0 +3.1) × 10-12 exp-[-(2420 (400)/T]. The units of the rate constants are cm 3 molecule-1 s-1 , and the quoted uncertainties are (2σ. For understanding the reaction path mechanism of the chlorination of the studied halogen-substituted ethanes, ab initio molecular orbital calculations were performed. Transition state structures were determined. These calculations lead to predictions of preexponential factors in the same order of magnitude of measured values. The ab initio energetics of the reactions were corrected using the ISO-M method, a mixing of isodesmic reactions for obtaining reaction enthalpies and concept of intrinsic energy of Marcus to deduce activation energies. A reasonably good agreement with the experimental values were found.
Theoretical Studies on the CH 3 CO + Cl Reaction: Hydrogen Abstraction versus CO Displacement
The Journal of Physical Chemistry A, 1998
The geometries, energies, and vibrational frequencies of the reactants, transition structures, intermediates, and products of the reaction of the acetyl radical with atomic chlorine have been determined by ab initio molecular orbital theory at the second-order Møller Plesset perturbation (MP2) level. Energies have been recalculated at the quadratic configuration interaction QCISD(T) level by using geometries obtained at MP2 level. The energy of the initial acetyl chloride adduct CH 3 COCl (1), formed by barrier-free combination, lies 78 kcal/mol below the reactants. Two major reaction routes are open to the chemically activated adduct 1: molecular dissociation to H 2 CdCdO + HCl (3), and the secondary formation of ketene via 1-chlorovinyl alcohol (2). Both these processes are energetically feasible to the thermal reactants and should hence lead to a spontaneous emission of a vibrationally hot HCl molecule as observed by Maricq et al. (Int. J. Chem. Kinet. 1997, 29, 421). The thermodynamically most stable products, CH 3 Cl + CO, should preferably be formed via direct displacement of CO from CH 3 CO by Cl; this reaction proceeds via a loose complex between Cl δand CH 3 CO δ+ , which explains the delayed emission of CO in the diode laser study of the Cl + CH 3 CO reaction. The energy barrier for decarbonylation of the adduct 1 is quite high and thereby is not accessible to the thermal reactants. The present potential energy surface reveals this reaction to be a capture-limited association-elimination reaction with a very high and pressure-independent rate coefficient.