Bond Fission Pathways Research Articles

A Quantum Chemical Study of the C−C Bond Fission Pathways of Alkoxy Radicals Formed following OH Addition to Isoprene

Isoprene is one of the most important non-methane organic compounds in tropospheric chemistry, yet its atmospheric degradation pathways remain incompletely understood. The uncertain fates of alkoxy radicals formed in the first stages of the OH-initiated degradation of isoprene contribute significantly to our ignorance. This paper examines the C−C bond fission pathways of these radicals at the B3LYP/6-311G(2df,2p) level of theory. For the four β-hydroxyalkoxy radicals that are expected to be formed, C−C bond fission (decomposition) pathways exist with very low barriers (0.7−2.1 kcal/mol) that are likely to dominate the chemistry. These radicals appear to possess intramolecular hydrogen bonds which typically persist in the transition states. For the two δ-hydroxyalkoxy radicals formed, C−C bond fission is endothermic (16−20 kcal/mol) and this pathway is unlikely to be important.

Dissociation of acetyl bromide: an experimental and theoretical study

We examine the ground state dissociation pathways of acetyl bromide, CH3C(O)Br, using infrared multiphoton dissociation and ab initio molecular orbital theory. Experiments reveal stable products C2H6, CH3Br, and CO which are consistent with the CBr bond fission pathway. However, production of C2H4 is consistent with ketene (CH2CO) formation, which has been a suggested intermediate in the dissociation of CH3C(O)Br. Calculations indicate that the activation energy for the molecular dissociation of CH3C(O)Br into ketene and HBr exceeds the CBr bond energy of 64 kcal mol−1 by about 30 kcal mol−1, implying that ketene may be formed by secondary chemistry rather than by molecular dissociation of acetyl bromide.

Competing C–Br and C–C bond fission following 1[n(O),π*(C=O)] excitation in bromoacetone: Conformation dependence of nonadiabaticity at a conical intersection

These experiments investigate the competition between C–C and C–Br bond fission in bromoacetone excited in the 1[n(O),π*(C=O)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the C–C fission reaction coordinate. In the first part of the paper, measurement of the photofragment velocity and angular distributions with a crossed laser-molecular beam time-of-flight technique identifies the primary photofragmentation channels at 308 nm. The time-of-flight spectra evidence two dissociation channels, C–Br fission and fission of one of the two C–C bonds, BrH2C–COCH3. The distribution of relative kinetic energies imparted to the C–Br fission and C–C fission fragments show dissociation is not occurring via internal conversion to the ground electronic state and allow us to identify these channels in the closely related systems of bromoacetyl- and bromopropionyl chloride. In the second part of the work we focus on the marked conformation dependence to the branching between C–C fission and C–Br fission. Photofragment angular distribution measurements show that C–Br fission occurs primarily from the minor, anti, conformer, giving a β of 0.8, so C–C fission must dominate the competition in the gauche conformer. Noting that the dynamics of these two bond fission pathways are expected to be strongly influenced by nonadiabatic recrossing of the reaction barriers, we investigate the possible mechanisms for the conformation dependence of the nonadiabatic recrossing with low-level ab initio electronic structure calculations on the C–Br reaction coordinate and qualitative consideration of the conical intersection along the C–C reaction coordinate. The resulting model proposes that C–C bond fission cannot compete with C–Br fission in the anti conformer because the dissociation samples regions of the phase space near the conical intersection along the C–C fission reaction coordinate, where nonadiabaticity inhibits C–C fission, while from the gauche conformer C–C fission can proceed more adiabatically and dominate C–Br fission. A final experiment confirms that the branching ratio changes with the relative conformer populations in accord with this model.

Distance dependence of nonadiabaticity in the branching between C–Br and C–Cl bond fission following 1[n(O),π*(C=O)] excitation in bromopropionyl chloride

These experiments on bromopropionyl chloride investigate a system in which the barrier to C–Br fission on the lowest 1A″ potential energy surface is formed from a weakly avoided electronic configuration crossing, so that nonadiabatic recrossing of the barrier to C–Br fission dramatically reduces the branching to C–Br fission. The results, when compared with earlier branching ratio measurements on bromoacetyl chloride, show that the additional intervening CH2 spacer in bromopropionyl chloride reduces the splitting between the adiabatic potential energy surfaces at the barrier to C–Br fission, further suppressing C–Br fission by over an order of magnitude. The experiment measures the photofragment velocity and angular distributions from the 248 nm photodissociation of Br(CH2)2COCl, determining the branching ratio between the competing primary C–Br and C–Cl fission pathways and detecting a minor C–C bond fission pathway. While the primary C–Cl:C–Br fission branching ratio is 1:2, the distribution of relative kinetic energies imparted to the C–Br fission fragments show that essentially no C–Br fission results from promoting the molecule to the lowest 1A″ potential energy surface via the 1[n(O),π*(C=O)] transition; C–Br fission only results from an overlapping electronic transition. The results differ markedly from the predictions of statistical transition state theories which rely on the Born–Oppenheimer approximation. While such models predict that, given comparable preexponential factors, the reaction pathway with the lowest energetic barrier on the 1A″ surface, C–Br fission, should dominate, the experimental measurements show C–Cl bond fission dominates by a ratio of C–Cl:C–Br=1.0:<0.05 upon excitation of the 1[n(O),π*(C=O)] transition. We compare this result to earlier work on bromoacetyl chloride, which evidences a less dramatic reduction in the C–Br fission pathway (C–Cl:C–Br=1.0:0.4) upon excitation of the same transition. We discuss a model in which increasing the distance between the C–Br and C=O chromophores decreases the electronic configuration interaction matrix elements which mix and split the 1n(O)π*(C=O) and np(Br)σ*(C–Br) configurations at the barrier to C–Br bond fission in bromopropionyl chloride. The smaller splitting between the adiabats at the barrier to C–Br fission increases the probability of nonadiabatic recrossing of the barrier, nearly completely suppressing C–Br bond fission in bromopropionyl chloride. Preliminary ab initio calculations of the adiabatic barrier heights and the electronic configuration interaction matrix elements which split the adiabats at the barrier to C–Br and C–Cl fission in both bromopropionyl chloride and bromoacetyl chloride support the interpretation of the experimental results. We end by identifying a class of reactions, those allowed by overall electronic symmetry but Woodward–Hoffmann forbidden, in which nonadiabatic recrossing of the reaction barrier should markedly reduce the rate constant, both for ground state and excited state surfaces.

ChemInform Abstract: Very Low Pressure Pyrolysis (VLPP) of Methyl‐ and Ethynyl Cyclopentanes and Cyclohexanes

Studies of the kinetics of thermal unimolecular decomposition of methylcyclopentane, methylcyclohexane, ethynylcyclopentane, and ethynylcyclohexane have been carried out at temperatures in the range 861–1218 K using the technique of very low-pressure pyrolysis (VLPP). Multiple reaction pathways and secondary decomposition of primary products results in a complex array of reaction products. VLPP rate data (fall-off regime) were obtained for the overall decompositions and interpreted via the application of RRKM theory. The data for methylcyclopentane and methylcyclohexane were interpreted in terms of ring-opening bond fission pathways and bond fission to methyl and cycloalkyl radicals. By selecting Arrhenius parameters consistent with the analogous pathways in open-chain alkanes, a good fit to the overall decomposition is obtained. The data for ethynylcyclopentane and ethynylcyclohexane were interpreted in terms of ring-opening bond fission and alkyne to allene isomerization. The A factors for ring opening were based on known values for C-C fission in open-chain alkynes and the Arrhenius parameters for isomerization were chosen to be consistent with previously reported alkyne to allene isomerizations. The VLPP data are consistent with the following high-pressure rate expressions (at = 1100 K) for the dominant primary reaction channel of ring opening adjacent to the substitutent group: where θ = 2.303RT kJ mol−. Comparison of the activation energies for the ethynyl-cycloalkanes with those for the methyl-cycloalkanes shows that the effect of the ethynyl substituent is consistent with the propargyl resonance energy. This evidence supports the assumption of a biradical mechanism for ring opening in these cycloalkanes.

Very low‐pressure pyrolysis (VLPP) of methyl‐ and ethynyl‐cyclopentanes and cyclohexanes

AbstractStudies of the kinetics of thermal unimolecular decomposition of methylcyclopentane, methylcyclohexane, ethynylcyclopentane, and ethynylcyclohexane have been carried out at temperatures in the range 861–1218 K using the technique of very low‐pressure pyrolysis (VLPP). Multiple reaction pathways and secondary decomposition of primary products results in a complex array of reaction products. VLPP rate data (fall‐off regime) were obtained for the overall decompositions and interpreted via the application of RRKM theory. The data for methylcyclopentane and methylcyclohexane were interpreted in terms of ring‐opening bond fission pathways and bond fission to methyl and cycloalkyl radicals. By selecting Arrhenius parameters consistent with the analogous pathways in open‐chain alkanes, a good fit to the overall decomposition is obtained. The data for ethynylcyclopentane and ethynylcyclohexane were interpreted in terms of ring‐opening bond fission and alkyne to allene isomerization. The A factors for ring opening were based on known values for C‐C fission in open‐chain alkynes and the Arrhenius parameters for isomerization were chosen to be consistent with previously reported alkyne to allene isomerizations. The VLPP data are consistent with the following high‐pressure rate expressions (at < T > = 1100 K) for the dominant primary reaction channel of ring opening adjacent to the substitutent group: where θ = 2.303RT kJ mol−. Comparison of the activation energies for the ethynyl‐cycloalkanes with those for the methyl‐cycloalkanes shows that the effect of the ethynyl substituent is consistent with the propargyl resonance energy. This evidence supports the assumption of a biradical mechanism for ring opening in these cycloalkanes.

ChemInform Abstract: VERY LOW‐PRESSURE PYROLYSIS (VLPP) OF PENTYNES. I. KINETICS OF THE RETRO‐ENE DECOMPOSITION OF PENT‐1‐YNE

The thermal unimolecular decomposition of pent-1-yne has been investigated over the temperature range of 923–1154 K using the technique of very low-pressure pyrolysis (VLPP). Under the experimental conditions the reaction proceeds predominantly via a molecular retro-ene pathway to yield allene and ethylene. There was some evidence for the occurrence of the higher energy C3C4 bond fission pathway at the high end of the temperature range. Interpretation of the data with the aid of RRKM theory and taking into account a decrease in gas-wall collision efficiency with temperature yields the following high-pressure rate constant expression for the retro-ene pathway: at 1100 K where θ = 2.303 RT kcal/mol and the A factor was assigned from the results of shock-tube studies of similar molecules. These rate parameters are independent of the inclusion of the bond fission pathway in the RRKM calculations. The results are compared with previous data on the retro-ene decomposition of alkynes.

Very low‐pressure pyrolysis (VLPP) of pentynes. I. Kinetics of the retro‐ene decomposition of pent‐1‐yne

AbstractThe thermal unimolecular decomposition of pent‐1‐yne has been investigated over the temperature range of 923–1154 K using the technique of very low‐pressure pyrolysis (VLPP). Under the experimental conditions the reaction proceeds predominantly via a molecular retro‐ene pathway to yield allene and ethylene. There was some evidence for the occurrence of the higher energy C3C4 bond fission pathway at the high end of the temperature range. Interpretation of the data with the aid of RRKM theory and taking into account a decrease in gas‐wall collision efficiency with temperature yields the following high‐pressure rate constant expression for the retro‐ene pathway: at 1100 K where θ = 2.303 RT kcal/mol and the A factor was assigned from the results of shock‐tube studies of similar molecules. These rate parameters are independent of the inclusion of the bond fission pathway in the RRKM calculations. The results are compared with previous data on the retro‐ene decomposition of alkynes.