Br Bond Dissociation Research Articles

Computationally Efficient Methodology To Calculate C−H and C−X (X = F, Cl, and Br) Bond Dissociation Energies in Haloalkanes.

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionORIGINAL ARTICLEThis notice is a correctionComputationally Efficient Methodology To Calculate C−H and C−X (X = F, Cl, and Br) Bond Dissociation Energies in Haloalkanes.W. Sean McGivern, Agnes Derecskei-Kovacs, Simon W. North, and Joseph S. FranciscoCite this: J. Phys. Chem. A 2000, 104, 33, 7916Publication Date (Web):July 27, 2000Publication History Published online27 July 2000Published inissue 1 August 2000https://doi.org/10.1021/jp002317rCopyright © 2000 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views256Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (9 KB) Get e-Alerts Get e-Alerts

Computationally Efficient Methodology to Calculate C−H and C−X (X = F, Cl, and Br) Bond Dissociation Energies in Haloalkanes

A computationally efficient method for calculating C−H and C−X (X = F, Cl, and Br) bond dissociation energies in haloalkanes has been developed by determining correction factors to MP2/cc-pVtz energies. Corrections for basis set effects were determined by the difference in bond dissociation energies calculated at the MP2/cc-pVtz and MP2/cc-pV5z levels, and correlation effects were corrected by calculating the difference in energies at the MP2/cc-pVtz and CCSD(T)/cc-pVtz levels. Subsequent corrections for the spin−orbit energy of the atomic fragment and zero-point energy were applied to give a final bond dissociation energy. The correction factors were determined using CH4, CH3F, CH3Cl, and CH3Br and are found to yield bond dissociation energies in excellent agreement with experimental results. This correction may also be broadly applied to multihalogen compounds, as shown in calculations of the C−H and C−X bond dissociation energies of CH2X2 and CHX3 (X = F, Cl, and Br) compounds, which accurately reprodu...

Photodissociation dynamics of CH2BrCl studied using resonance enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometry

The photodissociation dynamics of CH2BrCl have been studied using resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry. Polarization dependent time-of-flight profiles were collected for a range of wavelengths from 248 to 268 nm, corresponding to the red wing of the absorption spectrum. Forward convolution fits to the data have provided translational energy distributions and anisotropy parameters over the entire wavelength range for both Br(2P3/2) and Br*(2P1/2). The average translational energies for the Br and Br* channels are 20 and 23 kcal/mol, respectively. The measured anisotropy parameters indicate that both channels arise preferentially from a parallel transition and that the relative contribution of this transition increases with decreasing wavelength. Nonadiabatic transitions appear to play a smaller role in CH2BrCl dissociation than in its monohalogenated analogues, specifically CH3Br. We suggest that this difference is the result of the intrinsic Cs symmetry and lower radial velocity of CH2BrCl, and it is discussed in terms of a one-dimensional Landau–Zener model. A C–Br bond dissociation energy of 67.5 kcal/mol in CH2BrCl was also calculated using ab initio methods at the MP2/cc-pVtz//MP2/cc-pVdz level.

Theoretical Study of Carbon−Halogen Bond Dissociation Enthalpies of Substituted Benzyl Halides. How Important Are Polar Effects?1

It has been suggested (Clark, K. B.; Wayner, D. D. M. J. Am. Chem. Soc. 1991, 113, 9363−9365) that C−Br bond dissociation enthalpies (BDEs) in 4-YC6H4CH2-Br decrease as Y becomes more electron-withdrawing because of increasing destabilization of the Cδ+−Brδ- dipole and, furthermore, that the direction and magnitude of the effect of Y on 4-YC6H4Z-X BDEs could be correlated with the sign and magnitude, respectively, of the electronegativity difference between Z and X. The results of density functional theory (DFT) calculations using a locally dense basis set approach with the B3LYP functional and 6-311+G(2d,2p) as the primary basis set on 4-YC6H4CH2-X, for X = H, Br, Cl, and F with Y = NH2, HO, CH3O, CH3, H, CF3, CN, NO2 and BH2, show that the effects of Y on CH2-X BDEs are small (≤2.0 kcal/mol) for all four classes of compounds and are roughly equal for each Y for the three halides. Furthermore, almost all Y's reduce CH2-X BDEs relative to Y = H. Clark and Wayner's intriguing hypothesis that the magnitude ...

Structures, Vibrational Frequencies, Thermodynamic Properties, and Bond Dissociation Energies of the Bromomethanes and Bromomethyl Radicals: an Ab Initio Study

Reported here is a theoretical study of the entire series of bromomethanes (CH4-nBrn) and bromomethyl radicals (CH3-mBrm) establishing a self-consistent set of structural and thermodynamic information. Ab initio molecular orbital calculations were performed to compute equilibrium geometries for the molecules and radicals initially at the (U)HF/6-31G* and (R)HF/6-31G* levels, respectively, and then refined at the MP2/6-31G* level. Vibrational frequencies were determined for all species at the HF/6-31G* level and comparison with infrared measurements and matrix isolation studies is favorable. Electron correlation contributions were performed by single-point calculations using fourth-order Møller−Plesset perturbation theory for derived MP2/6-31G* geometries. Enthalpies of formation were obtained from a consideration of applicable isodesmic reactions using the derived MP4/6-31G**//MP2/6-31G* total energies in conjunction with experimentally established enthalpies of formation for CH3Br, CH4, and CH3•. The calculations predict the following standard enthalpies of formation in kilocalories per mole (at 298 K and 1 atm): CH2Br2, 1.07 ± 0.6; CHBr3, 12.16 ± 0.7; CBr4, 25.23 ± 0.8; CH2Br•, 41.63 ± 0.4; CHBr2•, 48.11 ± 0.6; and CBr3•, 55.36 ± 0.7. These data are then used to tabulate ΔH°f,T, ΔG°f,T, and Kf,T for all species over the temperature range 0−1500 K. Comparison is made to existing thermochemical data through calculation of C−H and C−Br bond dissociation energies.

Substituent effects on the benzylic bond dissociation enthalpy in benzyl bromides (C–Br) and tert-butylbenzenes (C–CH3): a gas phase thermolysis and liquid phase photoacoustic study

The bond dissociation enthalpies in a number of substituted benzyl bromides have been studied in the gas phase, using the toluene carrier technique, and in solution with photoacoustic calorimetry. Gas phase thermolysis with unsubstituted benzyl bromide gives an absolute C–Br bond dissociation enthalpy (Ed) at 298 K of 255 ± 4 kJ mol–1. Competition experiments in the gas phase reveal no substituent effect on the value of Ed(C–Br). Gas phase thermolysis with substituted tert-butylbenzenes also shows no effect on the C–CH3 bond dissociation enthalpy for p-CN, p-OH and p-But substituents, with a Ed(C–CH3) value at 298 K of 299 ± 2 kJ mol–1. In solution, photoacoustic experiments yield no detectable substituent effect for p-CN, p-But and m-CF3 substitution of the benzyl bromide, in contrast with other reports. With photoacoustic calorimetry a bond dissociation enthalpy of 254 ± 4 kJ mol–1 has been found for all benzyl bromides studied. A rationale for the absence of a substituent effect on the benzylic bond dissociation enthalpy is provided.

Bromination of fluoroalkanes. Part 5.—Kinetics of forward and reverse reactions in the system Br2+ i-C3F7H⇌HBr + i-C3F7Br

The kinetics of the forward and reverse reactions in the gas-phase system Br2+ i-C3F7H⇌HBr + i-C3F7Br have been studied. The slow steps for the forward and reverse reactions, respectively, are Br + i-C3F7H→HBr + i-C3F7(2), Br + i-C3F7Br→Br2+ i-C3F7. (–3)The Arrhenius parameters obtained, for the range 421–534 °C, are log A2/cm3 mol–1 s–1= 12.66 ± 0.34, E2= 81.2 ± 3.3, log A–3/cm3 mol–1 s–1= 13.80 ± 0.29, E–3= 86.3 ± 4.2 kJ mol–1The competitive brominations of mixtures of i-C3F7H + C2F5H and i-C3F7H + n-C3F7H have been studied over the ranges 249–430 and 220–402 °C, respectively. Each system yielded Arrhenius parameters for reaction (2) which are in excellent agreement with those given above. The results lead to the following bond dissociation energies at 298 K: D(i-C3F7—H)= 433.3 ± 2.4 kJ mol–1, D(i-C3F7—Br)= 273.8 ± 4.7 kJ mol–1.Attempts were made to measure D[(CF3)3C—H] using competitive photobromination and photochlorination. However, (CF3)3C—H is so ureactive that only the approximate results D[(CF3)3C—H]≈ 456 kJ mol–1 was obtained. The trends in C—H and C—Br bond dissociation energies are compared in alkanes, fluoroalkanes and the corresponding bromides.

Determination of Some Carbon-Chlorine Bond Dissociation Energies

An investigation of the pyrolyses of benzyl chloride and benzoyl chloride has been carried out by means of the ``toluene carrier'' technique. It is shown that the course of these reactions is analogous to that of the previously investigated pyrolyses of benzyl bromide and benzoyl bromide, i.e., the rate determining steps are C6H5·CH2Cl→C6H5·CH2·+Clor C6H5·CO·Cl→C6H5·CO+Cl.From kinetic data we obtained the activation energies of the rate determining steps, and we identify these activation energies with the respective C–Cl bond dissociation energies. Thus we derive D(C6H5·CH2−Cl)=68 kcal/moleand D(C6H5·CO·−Cl)=73.2 kcal/mole.The consistency of these data with the respective C–Br bond dissociation energies is very satisfactory. For example, the heat of formation of the benzoyl radical, derived from the D(C6H5·CO–Cl), is 15.7 kcal/mole, and that calculated from the experimentally determined D(C6H5·CO–Br) is 15.6 kcal/mole.

Studies of the variations in bond dissociation energies of aromatic compounds - II. Substituted bromobenzenes

The paper reports the effects of substituents on C— Br bond dissociation energy in substituted bromobenzenes. The following substituents introduced in various positions of benzene ring were investigated: F, Cl, Br, CH 3 , C 6 H 5 , CN and OH. In addition, the studies were extended to bromopyridines and bromothiophene. Assuming that the frequency factor is constant for the series of decompositions, the following values were obtained for the difference ∆ D = D ( Ph — Br) - D ( Ph s — Br), where Ph s Br denotes a molecule of substituted bromobenzene: substituent ∆D (kcal/mole) substituent ∆D (kcal/mole) p -F 0-5 m -C 6 H 5 0-8 p -Cl 0-6 o -C 6 H 5 2-7 m -Cl 1-0 p -CN 0-3 o -Cl 1-2 m -CN 0-8 p -Br 0-3 o -CN 0-6 o -Br 1-8 p -OH 3-9 P -CH 3 0-2 o -OH 3-8 m -CH 3 0-2 3-bromopyridine -5-0 o -CH 3 0-8 2-bromopyridine -0-6 p -C 6 H 5 0-2 2-bromothiophene 2-4 The significance of these results is discussed, and a tentative explanation of the observed effects is proposed.

Studies of the variations in bond dissociation energies of aromatic compounds. I. Mono-bromo-aryles

Investigation of the pyrolyses of bromobenzene, β -bromonaphthalene, α -bromonaphthalene, 9-bromophenanthrene and 9-bromoanthraeene in the presence of an excess of toluene has shown that reaction (1) Ar .Br → Ar • + Br (1) is the primary and rate-determining step of the pyrolysis. The progress of reaction was measured by the rate of formation of hydrogen bromide, and it was shown that this rate obeys first-order kinetics. The following values were obtained for the activation energies and frequency factors of unimolecular decompositions represented by equation (1): compound E (kcal/mole) 10 -13 v (sec -1 ) bromobenzene 70.9 2 β -bromonaphthalene 700 1.5 α -bromonaphthalene 70.9 3.5 9-bromophenanthrene 67.7 1 9-bromoanthracene 65.6 1.5 Assuming that recombination of bromine atoms with aromatic radicals does not involve any activation energy we conclude tha t the determined activation energies correspond to the respective C—Br bond dissociation energies. The effect of molecular structure on the C—Br bond dissociation energy is discussed. The heat of formation of the phenyl radical is determined, and this result is used for calculating the various Ph — X bond dissociation energies.

The C—Br bond dissociation energy in substituted benzyl bromides

The ‘toluene-carrier’ technique has been used for the determination of the C—Br bond dissociation energies in the substituted benzyl bromides: p -, m - and o -xylyl bromides; p -, m - and o -chlorobenzyl bromides; p - and m -bromobenzyl bromides; p - and m -nitrobenzyl bromides; and p - and m -nitrilebenzyl bromides. The rate-determining step of the decompositions of all these compounds is represented by the unimolecular dissociation processes ( s ) Ph s . CH 2 . Br → Ph s . CH 2 • + Br, ( s ) where Ph s . CH 2 . Br refers to the substituted benzyl bromide. Assuming that the frequency factor of the decomposition of each benzyl bromide is equal to the frequency factor of reaction ( u ) Ph . CH 2 . Br → Ph . CH 2 • + Br, ( u ) the differences in activation energies between E u and E s were calculated using the relation E u ─ E s = RT In ( k s / k u ); (I) k s and k u denote the unimolecular rate constants of reactions ( s ) and ( u ) respectively. Since E s and E u are equal to the C—Br bond dissociation energies in the substituted benzyl bromides and benzyl bromide itself, equation (I) yields the differences, ∆ D’ s, between D ( Ph . CH 2 —Br) and the values for D ( Ph s . CH 2 —Br). The calculated differences in the C—Br bond dissociation energies are listed below: substituted ∆ D substituted ∆ D benzyl bromides (kcal. /mole) benzyl bromides (kcal. /mole) o -chloro 0·9 m -methyl 0-0 m -chloro 0·1 p -methyl 1·4 p -chloro 0·4 m -nitro 2·1 m -bromo 0·3 p -nitro 1·1 p -bromo 0·3 m -nitrile 1·4 o -methyl 2·0 p -nitrile 0·7 The significance of these findings is discussed, and the effect of substitution on a bond energy is contrasted with the effect of ionic reactions.

The C—Br bond dissociation energy in halogenated bromomethanes

The pyrolyses of methyl bromide and of the halogenated bromomethanes, CH 2 CI. Br, CH 2 Br 2 , CHCl 2 .Br, CHBr 3 , CF 3 Br, CCI 3 . Br and CBr 4 , have been investigated by the ‘toluene-carrier' technique. It has been shown that all these decompositions were initiated by the unimolecular process R Br → R + Br. (1) Since all these decompositions were carried out in the presence of an excess of toluene, the bromine atoms produced in process (1) were readily removed by the fast reaction C 6 H 5 .CH 3 + Br → C 6 H 5 . CH 2 • + HBr. Hence, the rate of the unimolecular process (1) has been measured by the rate of formation of HBr. The C—Br bond dissociation energies were assumed to be equal to the activation energies of the relevant unimolecular dissociation processes. These were calculated by using the expression k ═ 2 x 10 13 exp (- D/RT ). The reason for choosing this particular value of 2 x 10 13 sec. -1 for the frequency factor of these reactions is discussed. The values obtained for the C—Br bond dissociation energies in the investigated bromomethanes are: D (C—Br) D (C—Br) compound (kcal./mole) compound (kcal./mole) CH 3 Br (67.5) CHBr 3 55.5 CH 2 CIBr 61.0 CF 3 Br 64.5 CH 2 Br 2 62.5 CCI 3 Br 49.0 CHCl 2 Br 53.5 CBr 4 49.0 The possible factors responsible for the variation of the C—Br bond dissociation energy in these compounds have been pointed out.

The Effect of Halogenation on the C–Br Bond Dissociation Energy in Halogenated Bromomethanes

The Effect of Halogenation on the C–Br Bond Dissociation Energy in Halogenated Bromomethanes

The Influence of Aromatic Substitution on the C–Br Bond Dissociation Energy

The Influence of Aromatic Substitution on the C–Br Bond Dissociation Energy

The C–Br Bond Dissociation Energy in Trichlorobromomethane

First Page

The C–Br Bond Dissociation Energy in Benzyl Bromide and Allyl Bromide

The pyrolysis of organic bromides in a stream of toluene is described as a method for the determination of the C–Br bond dissociation energies. The technique used makes it possible to discriminate between two mechanisms of decomposition: R.Br→R·+BrR.Br→olefin+HBr.It was found that both benzyl and allyl bromide decomposed according to mechanism (a), the bromine atoms reacting readily with toluene to give hydrobromic acid. The rate of the primary dissociation process was measured by the rate of formation of HBr. It was proved that the thermal decompositions of benzyl and allyl bromide were homogeneous gas reactions obeying first order kinetics. The activation energies were calculated at 50.5±2 kcal./mole and 47.5±2 kcal./mole, and identified with D(C6H5.CH2–Br) and D(CH2:CH.CH2–Br), respectively. The fate of the allyl radical is discussed, and a rough estimate of the activation energy for the reaction CH2:CH. CH2·+C6H5. CH3→CH2:CH. CH3+C6H5CH2·leads to a value of 14–17 kcal./mole. The problem of ionic contribution in the C–Br bonds is discussed. The estimation of the dissociation energies of the C–Br bonds of various organic bromides is of great interest since the values for these dissociation energies, combined with the heats of formation of the relevant bromides in the gaseous state, make it possible to determine the heats of formation of various organic radicals. Furthermore, the latter data in conjunction with the heats of formation of hydrocarbons enable us to calculate the various C–H and C–C bond dissociation energies and the resonance energies of the relevant radicals.