Mohammednoor Altarawneh

A mechanistic and kinetic study on the formation of PBDD/Fs from PBDEs.

This study presents a detailed mechanistic and kinetic investigation that explains the experimentally observed high yields of formation of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) from the polybrominated diphenyl ethers (PBDEs), commonly deployed in brominated flame retardants (BFRs). Theoretical calculations involved the accurate meta hybrid functional of M05-2X. The previously suggested pathways of debromination and generation of bromophenols/bromophenoxys/bromobenzenes were found to be unimportant corridors for the formation of PBDD/Fs. A loss of an ortho Br or H atom from PBDEs, followed by a ring-closure reaction, is the most accessible pathway for the production of PBDFs via modest reaction barriers. The initially formed peroxy-type adduct (RO₂) is found to evolve in a complex, nevertheless very exoergic, mechanism to produce PBDDs. Results indicate that, degree and pattern of bromination, in the vicinity of the ether oxygen bridge, has a minor influence on governing mechanisms and that even fully brominated isomers of BFRs are capable of forming PBDD/Fs. We thoroughly discuss bimolecular reactions of PBDEs with Br and H, as well as the Br-displacement reaction by triplet oxygen. The rate of the Br-displacement reaction significantly exceeds that of the unimolecular inititiation reactions due to loss of ortho Br or H. Results presented herein address conclusively the intriguing question of how PBDEs form PBDD/Fs, a matter that has been in the center of much debate among environmental chemists.

Theoretical study of unimolecular decomposition of catechol

This study develops the reaction pathway map for the unimolecular decomposition of catechol, a model compound for various structural entities present in biomass, coal, and wood. Reaction rate constants at the high-pressure limit are calculated for the various possible initiation channels. It is found that catechol decomposition is initiated dominantly via hydroxyl H migration to a neighboring ortho carbon bearing an H atom. We identify the direct formation of o-benzoquinone to be unimportant at all temperatures, consistent with the absence of this species from experimental measurements. At temperatures higher than 1000 K, water elimination through concerted expulsion of a hydroxyl OH together with an ortho H becomes the most significant channel. Rice-Ramsperger-Kassel-Marcus simulations are performed to establish the branching ratio between these two important channels as a function of temperature and pressure. All unimolecular routes to the reported major experimental products (CO, 1,3-C(4)H(6) and cyclo-C(5)H(6)) are shown to incur large activation barriers. The results presented herein should be instrumental in gaining a better understanding of the decomposition behavior of catechol-related compounds.

Quantum chemical and kinetic study of formation of 2-chlorophenoxy radical from 2-chlorophenol: Unimolecular decomposition and bimolecular reactions with H, OH, Cl, and O2

This study investigates the kinetic parameters of the formation of the chlorophenoxy radical from the 2-chlorophenol molecule, a key precursor to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCCD/F), in unimolecular and bimolecular reactions in the gas phase. The study develops the reaction potential energy surface for the unimolecular decomposition of 2-chlorophenol. The migration of the phenolic hydrogen to the ortho-C bearing the hydrogen atom produces 2-chlorocyclohexa-2,4-dienone through an activation barrier of 73.6 kcal/mol (0 K). This route holds more importance than the direct fission of Cl or the phenolic H. Reaction rate constants for the bimolecular reactions, 2-chlorophenol + X --> X-H + 2-chlorophenoxy (X = H, OH, Cl, O2) are calculated and compared with the available experimental kinetics for the analogous reactions of X with phenol. OH reaction with 2-chlorophenol produces 2-chlorophenoxy by direct abstraction rather than through addition and subsequent water elimination. The results of the present study will find applications in the construction of detailed kinetic models describing the formation of PCDD/F in the gas phase.

Formation and Chlorination of Carbazole, Phenoxazine, and Phenazine

This contribution presents pathways for the formation of the three nitrogenated dioxin-like species, carbazole, phenoxazine, and phenazine via unimolecular rearrangements of diphenylamine (DPA) and its nitro substituents (NDPA). The latter represent major structural entities appearing in formulations of explosives and propellants. Intramolecular H transfer from the amine group to one of the two O atoms in the nitro group denotes the most accessible route in the unimolecular decomposition of NDPA. Further unimolecular rearrangements afford phenazine and carbazole. A loss of an ortho substituent from DPA, followed by addition of an oxygen molecule, prompts the formation of carbazole and phenoxazine in a facile mechanism. The consistency between trends in Fukui-based electrophilic indices and the experimental profiles of chlorinated carbazole, phenoxazine, and phenazine suggests the formation of these species by electrophilic substitution.

Mechanism of thermal decomposition of tetrabromobisphenol A (TBBA).

This study presents a detailed investigation into the gas-phase thermal decomposition of tetrabromobisphenol A (TBBA), that is, the most widely used brominated flame retardant (BFR). Elimination of one of the methyl groups characterizes the sole dominant channel in the self-decomposition of the TBBA molecule at all temperatures. A high-pressure rate constant for this reaction is fitted to k(T) = 2.09 × 10(10)T(1.93) exp(-37000/T) s(-1). The high A factor and low activation energy for this reaction arise from the formation of a delocalized radical upon the loss of a methyl group. We calculate rate constants for the bimolecular reactions of TBBA with H, Br, and CH3 radicals. Kinetic and mechanistic data provided herein should be instrumental to gain further understanding of the fate of TBBA during thermal degradation of materials laden with this BFR.

Thermochemical properties and decomposition pathways of three isomeric semiquinone radicals

Semiquinones are persistent free environmental radicals formed as important initial products from the decomposition of dihydroxylated benzene isomers. This study develops detailed decomposition pathways for the thermal decomposition of the three isomeric semiquinone radicals. Branching ratios based on the calculated high-pressure limit reaction rate constants predict that p-benzoquinone is a major product from the unimolecular decomposition of the p-semiquinone radical, while the formation of o-benzoquinone from the o-semiquinone radical corresponds to a minor channel. This finding is consistent with the absence of o-benzoquinone from the thermal degradation of the 1,2-dihydroxybenzene isomer and the abundance of p-benzoquinone from the thermal decomposition of 1,4-dihydroxybenzene. Ring contraction/CO elimination is shown to be the dominant sink pathway for the o-semiquinone and m-semiquinone radicals. Thermochemical properties, in terms of enthalpies of formation, entropies, and heat capacities for dihydroxylated benzene isomers, semiquinone radicals, and benzoquinones, are evaluated by quantum chemical calculations. Values of the enthalpies of formation calculated by the B3LYP/GTLarge method show good agreement with those obtained at the G3B3 level of theory.

Computational Study of the Oxidation and Decomposition of Dibenzofuran under Atmospheric Conditions

The atmospheric degradation of dibenzofuran (DF) initiated by OH addition has been studied by using density functional theory (B3LYP method). Site C1 in DF is predicted to be the favored site for OH addition, with a branching ratio of 0.61 to produce a DF-OH(1) adduct. The calculated reaction rate constant for OH addition to DF has been used to predict the atmospheric lifetime of DF to be 0.45 day. Three different modes of attack of O2 ((3)Sigma(g)) on DF-OH(1) have been examined. Abstraction of hydrogen gem to OH in DF-OH(1) by O2 ((3)Sigma(g)) (producing 1-dibenzofuranol I) and dioxygen addition in the three radical sites in cis and trans orientation (relative to the ispo-added OH) of the pi-delocalized electron system of DF-OH(1) are feasible under atmospheric conditions. The free energy of activation (at 298.15 K) for the formation of 1-dibenzofuranol is 15.1 kcal/mol with a free energy change of -36.3 kcal/mol, while the formation of DF-OH(1)-O2 adducts are endergonic by 9.2-21.8 kcal/mol with a 16.3-23.6 kcal/mol free energy of activation. On the basis of the calculated reaction rate constants, the formation of 1-dibenzofuranol is more important than the formation of DF-OH-O2 adducts. The results presented here are a first attempt to gain a better understanding of the atmospheric oxidation of dioxin-like compounds on a precise molecular basis.

A first-principles density functional study of chlorophenol adsorption on Cu2O(110):CuO

First-principles density functional theory and a periodic-slab model have been employed to explore the adsorption of a two-chlorophenol molecule on a Cu(2)O(110) surface containing surface Cu-O bonds, namely, the Cu(2)O(110):CuO surface. The two-chlorophenol molecule is found to interact very weakly with the Cu(2)O(110):CuO surface, forming several vertical and flat orientations. These weakly bound states tend to result from interaction between the phenolic hydrogen and an oxygen surface atom. The formation of a two-chlorophenoxy moiety and an isolated hydrogen on the Cu(2)O(110):CuO surface from a vacuum two-chlorophenol molecule is determined to have an endothermicity of 8.2 kcal/mol (0.37 eV). The energy required to form a two-chlorophenoxy radical in the gas phase is also found to be much smaller when assisted by the Cu(2)O(110):CuO surface than direct breaking of the hydroxyl bond of a free two-chlorophenol molecule. The calculated binding energy of a two-chlorophenoxy radical adsorbed directly onto the Cu(2)O(110):CuO surface is -12.5 kcal/mol (0.54 eV). The Cu(2)O(110):CuO and Cu(100) surfaces are found to have similar energy barriers for forming a surface-bound two-chlorophenoxy moiety from the adsorption of a two-chlorophenol molecule.

Rate constants for hydrogen abstraction reactions by the hydroperoxyl radical from methanol, ethenol, acetaldehyde, toluene, and phenol

An important step in the initial oxidation of hydrocarbons at low to intermediate temperatures is the abstraction of H by hydroperoxyl radical (HO2). In this study, we calculate energy profiles for the sequence: reactant + HO2 → [complex of reactants] → transition state → [complex of products] → product + H2O2 for methanol, ethenol (i.e., C2H3OH), acetaldehyde, toluene, and phenol. Rate constants are provided in the simple Arrhenius form. Reasonable agreement was obtained with the limited literature data available for acetaldehyde and toluene. Addition of HO2 to the various distinct sites in phenol is investigated. Direct abstraction of the hydroxyl H was found to dominate over HO2 addition to the ring. The results presented herein should be useful in modeling the lower temperature oxidation of the five compounds considered, especially at low temperature where the HO2 is expected to exist at reactive levels.

Theoretical Study of the Ammonia−Hypochlorous Acid Reaction Mechanism

A mechanism for the oxidation of ammonia by hypochlorous acid to form nitrogen gas has been developed at the B3LYP/6-31G(d,p) level of theory using the Gaussian 03 software package. The formation of NH(2)Cl, NHCl(2), and NCl(3) was studied in the gas phase, with explicit water molecules included to examine the transition state energy in aqueous solution. The inclusion of explicit water molecules in the transition state dramatically reduced the reaction barrier in reactions involving transfer of a hydrogen atom between molecules, effects that were not taken into account through use of a solvation model alone. Three mechanisms were identified for the decomposition of chloramine species to form N(2), involving the combination of two chloramine species to form hydrazine, dichlorohydrazine and tetrachlorohydrazine intermediates. The highest barrier in each pathway was found to be the formation of the hydrazine derivative.