- Gas-phase Protonation and Methyl Cation Transfer from Methyl Halide Ions
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The ion-molecule reactions between +., +, + ions (X = F, Cl, Br, I) and a number of nucleophiles have been studied by ion cyclotron resonance techniques.Protonation of the nucleophiles is observed to occur from both the molecular ions X+. and protonated species + whereas dimethylalonium ions + react principally by methyl cation transfer.A notable exception occurs in methyl iodide where the molecular ions +. act both as proton and methyl cation donors, whereas dimethyliodonium ions are found unreactive.The results are discussed with reference to the use of alkyl halides as reagent gases in chemical ionization experiments.
- Houriet, R.,Rolli, E.,Flammang, R.,Maquestiau, A.,Bouchoux, G.
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- Kinetics and mechanism of the reaction of F atoms with CH3Br
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The reaction of F atoms with CH3Br at 296 K was studied using a pulse radiolysis/transient UV absorption spectroscopy absolute technique and a FTIR relative rate technique. The rate constant for this reaction was determined to be (4.46 ± 0.22) × 10-11. The reaction proceeds via two channels, 69 ± 5%, via hydrogen abstraction giving CH2Br radicals and HF, and 31 ± 5%, to give the adduct CH3Br-F. In the FTIR system the observed rate constant was 69 ± 8% of that measured using the pulse radiolysis system because the CH3Br-F adduct falls apart to re-form the reactants. The CH3Br-F adduct reacts with NO, with a rate constant of (2.25 ± 0.14) × 10-11 cm3 molecule-1 s-1, giving FNO as a product. There was no discernible reaction of the CH3Br-F adduct with O2 and an upper limit of 6 × 10-15 cm3 molecule-1 s-1 was derived for this reaction. The CH3Br-F adduct absorbs strongly at 260-340 nm. The absorption cross section at 280 nm of the CH3Br-F adduct was (2.06 ± 0.31) × 10-17 cm2 molecule-1. A lower limit for the equilibrium constant was [CH3Br-F]/([CH3Br][F]) > 5 × 10-16 cm3 molecule-1 at 296 K. A lower limit of 12 kcal mol-1 is estimated for the binding energy of the F atom in the CH3Br-F adduct. The UV absorption spectrum of the CH2BrO2 radical was determined; at 250 nm σ = (3.4 ± 0.9) × 10-18 cm2 molecule-1.
- Sehested, Jens,Bilde, Merete,Mogelberg, Trine,Wellington, Timothy J.,Nielsen, Ole John
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- First observation of the rotational spectrum of the bromomethyl radical, CH2Br
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The first high resolution rotational spectrum of the bromomethyl radical in the gas phase has been observed. The radical was produced in a fast flow system by hydrogen abstraction of CH3Br by Cl or F atoms obtained from a 2450 MHz microwave discharge in Cl2 or in CF4, respectively. The rotational, quartic centrifugal distortion constants have been obtained as well as the electron spin-rotation interaction constants. The small positive inertial defect, Δ0 = 0.032 amu A2 deduced from the rotational constants of the CH279Br species clearly indicates that the radical is planar in its ground vibronic state.
- Bailleux, Stephane,Drean, Pascal,Godon, Marc,Zelinger, Zdenek,Duan, Chuanxi
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- Halogen abstraction reaction between aminoalkyl radicals and alkyl halides: Unusual high rate constants
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The very high reactivity of aminoalkyl radicals toward the halogen abstraction reaction is reported for the first time. Reaction rate constants with CCl4 and CBr4 are close to the diffusion limit: they are about 4-5 orders of magnitude higher than those previously determined for typical alkyl radicals. A better understanding of this unusual behavior is obtained using molecular orbitals (MO) calculations. The participation of polar effects is directly evidenced. This approach can be useful for the design of new reducing agents.
- Lalevée,Allonas,Fouassier
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- Rates and Mechanisms of Reduction of Tris(phenanthroline)iron(III) by Various Radicals. Effect of Solvent
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Tris(phenthroline)iron(III) (FeIII(phen)3) is reduced to FeII complex by a wide variety of radicals in aqueous solutions.Reduction by α-hydroxyalkyl radicals takes place with nearly diffusion-controlled rate constants (3-5*1E9 M-1 s-1) and involves an outer-sphere electron transfer.Reduction by other substituted alkyl radicals, as well as methyl and ethyl, is less rapid to varying degrees (k ca. 1E6-1E9 M-1 s-1) and may involve an inner-sphere mechanism, whereby the radical attaches to the iron center in an intermediate stage.Addition of alkyl radicals to a phenanthroline carbon, which occurs in acetonitrile solutions, becomes less likely in water, because it is slow relative to the other processes.In the case of the OH radical reaction, however, addition to carbon followed by an intramolecular electron transfer may be the only operative mechanism for reduction of FeIII(phen)3).
- Grodkowski, J.,Neta, P.,Schlesner, C. J.,Kochi, J. K.
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- Femtosecond photolysis of CH2Br2 in acetonitrile: Capturing the bromomethyl radical and bromine-atom charge transfer complex through deep-to-near UV probing
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Dibromomethane (CH2Br2) in acetonitrile is a suitable precursor to characterize the absorption signatures of the CH 2Br· radical and solvent ·Br charge-transfer complexes. Following irradiation of CH2Br2 at 255 nm, the iso-H2CBrBr isomer product rapidly converts back to the parent species, and transient absorption spectra reveal the bands of solvent-separated radical species, the CH2Br· radical peaking at 235 nm, as well as the CH3CN·Br complex at 272 nm. The absorption of CH2Br· exhibits minor solvatochromic shifts upon going from acetonitrile to cyclohexane, and the molecular decadic extinction coefficient of CH3CN·Br is estimated to be 1470 M-1 cm-1.
- Pal, Suman K.,Mereshchenko, Andrey S.,El-Khoury, Patrick Z.,Tarnovsky, Alexander N.
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- Adiabatic and diabatic dynamics in the photodissociation of CH2BrCl
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The photodissociation dynamics of chlorobromomethane (CBM) were investigated between 193 and 242 nm by resonance-enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry (TOFMS). Translational energy distributions, anisotropy parameters, and Br: Br(*) branching ratios were determined at 193 and 235 nm to explore the non-adiabatic dynamics near the avoided crossing. Additional measurements were made at intermediate wavelengths to characterize the wavelength dependence of the Br and Br(*) anisotropy parameters. The non-adiabatic crossing probabilities calculated by applying a one-dimensional Landau-Zener model were relatively insensitive to the excitation wavelength, indicating that the avoided crossing between 3A' and 4A' potentials lies in the exit channel. Additionally, we have determined the partial absorption cross sections for the excited states that contribute to the ultraviolet absorption spectrum of CBM.
- Zou,McGivern,North
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- Chemical Ionization Mass Spectrometry of Halomethanes with Tetramethylsilane as Reagent Gas
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Chemical ionization mass spectra of halomethanes measured using tetramethylsilane as reagent gas exhibit three major peaks corresponding to (1+), (1+) and (MeSi)2X(1+) ions (X = Cl, Br or I).Dihalomethanes CH2X2 form the most stable silylated molecular ions, whereas in the mass spectra of tetrahalomethanes (CX4) these ions have not been detected and the ions CX3(1+) are the most abundant.Production of bistrimethylsilylhalonium ions is the most pronounced process for haloforms (CHX3).
- Chizhov, Oleg S.,Kadentsev, Valentin I.,Stomakhin, Andrei A.
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- Ultraviolet Absorption Spectra and Kinetics of the Self-Reaction of CH2Br and CH2BrO2 Radicals in the Gas Phase at 298 K
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The ultraviolet absorption spectra of CH2Br and CH2BrO2 radicals and the kinetics of their self-reactions have been studied in the gas phase at 298 K by using the pulse radiolysis technique.Absorption cross sections were quantified over the wavelength range 220-350 nm.Measured cross sections near the absorption maxima were ?CH2Br(280 nm) = (6.26 +/- 1.15)*10-18 cm2 molecule-1 and ?CH2BrO2(250 nm) = 7.20 +/- 0.83)*10-18 cm2 molecule-1.Errors represent statistical errors (2?) together with our estimate of potential systematic errors (10percent).The absorption cross-sectional data were then used to derive the observed self-reaction rate constants for reactions 1 and 2 products (1), CH2BrO2 + CH2BrO2 -> products (2)>, defined as -d/dt = 2kobs2 (R = CH2Br or CH2BrO2) of k1= (2.93 +/- 0.60)*10-11 cm3 molecule-1 s-1 and kobs = (3.26 +/- 0.31)*10-11 cm3 molecule-1 s-1 (quoted errors represent 2?).These results are discussed with respect to previous studies of the absorption spectra and kinetics of peroxy radicals.
- Nielsen, Ole J.,Munk, Jette,Locke, Garrett,Wallington, Timothy J.
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- Absolute rate constants for the reactions of Cl atoms with CH3Br, CH2Br2, and CHBr3
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The rate constants for the reactions of chlorine atoms with the complete series of the three bromomethanes CH3Br (1), CH2Br2 (2), and CHBr3 (3) were measured in a very low pressure reactor, employing a microwave discharge for the generation of Cl atoms with mass spectrometric detection of reactants and products. The experiments were performed in the temperature range 273-363 K and at total pressures approximately 1 m Torr. The reactions proceed via hydrogen atom transfer leading to HCl product and the corresponding bromomethyl radicals. Their rate constant expressions are (in cm3 molecule-1 s-1): k1 = (1.66±0.14)×10-11 exp(-1072±46/T), k2 = (0.84±0.15)×10-11 exp(-911±101/T), and k3 = (0.43±0.11)×10-11 exp(-809±142/T). The activation energy of the reaction decreases with additional bromine substitution, which is attributed to the gradual weakening of the corresponding C-H bond strength. Ab initio theoretical calculations performed at the MP2/6-31++G(2d,2p) level of theory suggest C-H bond strengths for CH3Br, CH2Br2, and CHBr3 of 416.58, 407.03, and 396.60 kJ mol-1, respectively.
- Kambanis,Lazarou,Papagiannakopoulos
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p. 8496 - 8502
(2007/10/03)
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- Atmospheric chemistry of CH2Br2: Rate coefficients for its reaction with Cl atoms and OH and the chemistry of the CHBr2O radical
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Rate coefficients for the reaction of OH radicals and Cl atoms with dibromomethane, OH + CH2Br2 → CHBr2 + H2O (1) and Cl + CH2Br2 → CHBr2 + HCl (3), and for the reaction of Cl atoms with methyl bromide, Cl + CH3Br → HCl + CH2Br (4), have been obtained using relative rate techniques. At 298 K, the value of k1, was determined to be (1.2 ± 0.3) × 10-13 cm3 molec-1 s-1. The rate coefficients k3 and k4 were determined at a series of temperatures ranging from 228 K to 296 K and combined with the measurements of Gierczak et al., yielding the following Arrhenius expressions for k3 and k4; k3 = (6.35 ± 0.6) × 10-12 exp(-807 ± 50/T) cm3 molec-1 s-1 and k4 = ( 1.49 ± 0.2) × 10-11 exp(- 1056 ± 50/T) cm3 molec-1 s-1. In addition, the Cl atom initiated oxidation mechanism of CH2Br2 in air has been studied at 298 K. The products observed were HC(O)Br and small amounts of CO. The dominant atmospheric fate of the alkoxy radical, CHBr2O, is elimination of a Br atom, which occurs at a rate estimated to be greater than 4 × 106 s-1 in 700 torr of O2 at 298 K.
- Orlando, John J.,Tyndall, Geoffrey S.,Wallington, Timothy J.,Dill, Marchoe
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p. 433 - 442
(2007/10/03)
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- Formation and homolysis of organonickel(III) complexes
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The reactions of Ni(1,4,8,11-tetraazacyclotetradecane)2+ with alkyl radicals were studied. Data were collected for the R,R,S,S (or β) and R,R,R,R (or α) isomers. The reactions form σ -bonded organometalllc cations, as in the equation NIL2+
- Kelley, Dougles G.,Marchaj, Adam,Bakac, Andreja,Espenson, James H.
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p. 7583 - 7587
(2007/10/02)
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- Reactions of cobalt(II) macrocycles with alkyl and alkylperoxy radicals
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The cobalt-carbon bond formation in the reactions of cobalt(II) macrocycles, LCoII (L = C-meso-Me6[14]aneN4, Me6[14]-4,11-dieneN4, corrin, tim), with primary alkyl and substituted primary alkyl radicals takes place with rate constants in the range 107-109 M-1 s-1. The kinetics are influenced by the nature of the macrocycle to a much greater extent than by the nature of the radicals, such that all the rate constants, kCo, for a given cobalt complex fall within a factor of ~2. For example, the values of 10-8kCo/M-1 s-1 for B12r are 4.2 (R = CH3), 5.1 (C2H5), 5.1 (CH3OCH2), 5.8 (ClCH2), 6.1 (C3H7), and 9.1 (BrCH2). The alkylperoxy radicals RO2? (R = CH3, C2H5, BrCH2) react with Co(tim)(H2O)22+ with a rate constant k = 1.4 × 108 M-1 s-1.
- Bakac, Andreja,Espenson, James H.
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p. 4319 - 4322
(2008/10/08)
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- Photochemical generation of alkyl radicals and their reactions with methyl viologen radical cation and with transition-metal complexes in aqueous solution
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The irradiation of aqueous solutions of RCo([14]aneN4)(H2O)2+ (R = CH3, C2H5, 1-C3H7, CH2Cl, CH2Br, CH2OCH3) with visible light, λ ≤ 500 nm, results in the homolysis of the cobalt-carbon bond. These complexes are quite unreactive toward a variety of dealkylating and redox reagents and thus represent a convenient source of small unstabilized carbon-centered radicals in the presence of potential substrates. The radicals react rapidly with the methyl viologen radical cation, kMV = (1-2) × 109 M-1 s-1. The kinetics of the formation of RCr2+ and RCo([14]aneN4)(H2O)2+ in the reactions of radicals with Cr2+ and Co([14]-aneN4)(H2O)22+, respectively, were studied by using MV+ as a probe. The respective rate constants have values kCr = (1.9-2.4) × 108 M-1 s-1 and kCo = (1-2) × 107 M-1 s-1 for all the radicals examined.
- Bakac, Andreja,Espenson, James H.
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p. 3901 - 3904
(2008/10/08)
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- Kinetics of Photochlorination of Halogen (F, Cl, Br) Substituted Methanes
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The reactions of photochemically generated Cl(2PJ) atoms with CH3Br, CH2Cl2, CH2Br2, CH2ClF, and CH2BrCl have been studied over the temperature range 0-95 deg C by using the competitive method with methane as a primary standard.Relative and absolute rate constants are reported.The preexponential factors for this group of compounds fall within the range (3.1-9.5)E-11 cm3s-1 and activation energies vary from 2.4 to 3.1 kcal mol-1.The results are compared with the results for other members of this series including CH3F, CH2F2, CHF3, CH3Cl, CH2Cl2, and CHCl3 from this and other laboratories and discussed in terms of the thermoch emistry involved.A sample calculation of the A factor using the thermochemical kinetics version of transition-state theory (TST) shows reasonable agreement with experiment.The sensitivity of the input parameters in the bond energy-bond order (BEBO) method prediction of activation energy and tunneling is discussed in some detail with CH2Cl2 as an example.
- Tschuikow-Roux, E.,Faraji, F.,Paddison, S.,Niedzielski, J.,Miyokawa, K.
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p. 1488 - 1495
(2007/10/02)
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- Hypovalent Radicals. 4. Gas-Phase Studies of the Ion-Molecule Reactions of Cyclopentadienylidene Anion Radical in a Flowing Afterglow
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The carbene anion radical, cyclopentadienylidene-.(c-C5H4-., 1), was generated by dissociative electron attachment with diazocyclopentadiene (2) in a flowing afterglow apparatus.The ion-molecule reaction of 1 with 2 produced c-C5H4N=N-c-C5H4-., c-C5H4=c-C5H4-., and c-C5H5- by coupling at Nβ and C1 of 2 and H. abstraction from 2, respectively.The PA(1) = 377 +/- 2 kcal mol-1 was determined from bracketing reactions of ROH + 1 -> RO- + c-C5H5., which gives ΔHf0(1) = 70.7 +/- 3.2 kcal mol-1.Although the H. abstraction process by 1 was observed in most of its ion-molecule reactions, 1 failed to react with CH4, C2H4, and c-C3H6 probably because of an activation barrier of (*) 3 kcal mol-1 in these cases.With dipolar CH3OH and 1, the only observed reaction was H. abstraction from the O-H bond (shown with CH3OD).This lower limit of the H. affinity of 1 gives ΔHf0(1) (*) 67.7 +/- 3 kcal mol-1, in excellent agreement with the value derived from protonation studies.The reactions of 1 with CH3X (Cl, Br) occur by H. abstraction and halide ion (SN2) displacement.Anion radical 1 adds to activated olefins H2C=CHX (CN, CO2CH3, Cl) by a nucleophilic Michael addition mechanism.The EA of the carbene c-C5H4 was bracketed by charge-transfer reactions between 1 and C6F6 and NO2.All of these and certain other results are consistent with the ?1?2 electronic configuration as the ground state of 1.The reactions of 1 with alcohols are postulated to proceed via a hydrogen-bonded complex.
- McDonald, Richard N.,Chowdhury, A.Kasem,Setser, D.W.
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p. 6491 - 6498
(2007/10/02)
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