11 research outputs found
Kinetics and thermodynamics of the hydroxylation products in the photodegradation of the herbicide Metolachlor
Electronic structure calculations have been performed to determine the thermochemistry and kinetics of the reaction between OH and the radicals of the S enantiomer of the herbicide Metolachlor, 2-chloro-N-(2-methyl-6-ethylphenyl)-N(2-methoxy-1-methylethyl) acetamide (MC), produced by photoinduced breaking of the C–Cl bond. Both density functional and ab initio composite methods were employed to calculate the structure of reactants, intermediates, transition states and products, in gas phase and in aqueous solution. The expected relative abundance of each product was calculated and compared to the experimentally observed concentrations. It is shown that a combination of thermodynamic and kinetic characteristics interplay to produce the expected theoretical abundances, which turn out to be in agreement with the experimentally observed distribution of products
A reinvestigation of the deceptively simple reaction of toluene with OH, and the fate of the benzyl radical : a combined thermodynamic and kinetic study on the competition between OH-addition and H-abstraction reactions
This work reports density functional and composite model chemistry calculations performed on the reactions of toluene with the hydroxyl radical. Both the experimentally observed H-abstraction from the methyl group and possible OH additions to the phenyl ring were investigated. Reaction enthalpies and barrier heights suggest that H-abstraction is more favorable than OH-addition to the ring. The calculated reaction rates at room temperature and the radical intermediate product fractions support this view. At first sight, this might seem to disagree with the fact that, under most experimental conditions, cresols are observed in a larger concentration than benzaldehyde. Since the accepted mechanism for benzaldehyde formation involves H-abstraction, a contradiction arises that calls for a more elaborate explanation. In this first exploratory study, we provide evidence that support the preference of H-abstraction over OH addition and present an alternative mechanism which shows that cresols can be actually produced also through H-abstraction and not only from OH-addition, thus justifying the larger proportion of cresols than benzaldehyde among the products
Kinetics and Thermodynamics of the Hydroxylation Products in the Photodegradation of the Herbicide Metolachlor
Electronic
structure calculations have been performed to determine the thermochemistry and
kinetics of the reaction between OH and the radicals of the S enantiomer of the
herbicide Metolachlor,
2-chloro-N-(2-methyl-6-ethylphenyl)-N(2-methoxy-1-methylethyl) acetamide (MC),
produced by photoinduced breaking of the C-Cl bond. Both density functional and
ab initio composite methods were employed to calculate the structure of
reactants, intermediates, transition states and products. The expected relative
abundance of each product was calculated. and compared to the
experimentally observed concentrations. It is shown that a combination of
thermodynamic and kinetic characteristics interplay to produce the expected
theoretical abundances, which turn out to be in agreement with the experimentally
observed distribution of products.</p
Quantum Mechanical and Kinetic Studies of the Reaction of Methyl Radicals with Chlorine Molecules
Theoretical Study of the Microhydration of 1-Chloro and 2-Chloro Ethanol as a Clue for Their Relative Propensity Toward Dehalogenation
This work reports a computational
analysis of hydrogen bonded clusters of mono-, di-, tri- and tetra hydrates of
the chlorohydrins CH3CHClOH (1ClEtOH) and CH2ClCH2OH
(2ClEtOH). The goal of the study is to assess the role of the water solvent into
the facilitation of the initial step for dehalogenation of these compounds, a
process of interest in several contexts. Molecular orbital methods (MP2),
density functional methods (B3LYP, M06 and wB97X-D)
and composite model chemistries (CBS-QB3, G4) were employed to investigate the
structure, electronic distribution and hydrogen-bonded structure of 7 monohydrates,
6 dihydrates, 5 trihydrates and 5 tetrahydrates of both species. Standard
reaction enthalpy and standard Gibbs free reaction energy were computed for all aggregates with
respect to n independent water molecules and with respect to the dimer,
trimer and tetramer of water, respectively, in order to evaluate stability and
hydrogen bonding network. The influence of the water chains on the length and
vibrational frequencies, especially of the C-Cl and O-H bonds, was evaluated.</p
Computational Evidence Suggests That 1-chloroethanol May Be an Intermediate in the Thermal Decomposition of 2-chloroethanol into Acetaldehyde and HCl
The dehalogenation of
2-chloroethanol (2ClEtOH) in gas phase with and without participation of
catalytic water molecules has been investigated using methods rooted into the density
functional theory. The well-known HCl elimination leading to vinyl alcohol (VA)
was compared to the alternative elimination route towards oxirane and shown to
be kinetically and thermodynamically more favorable. However, the isomerization
of VA to acetaldehyde in the gas phase, in the absence of water, was shown to
be kinetically and thermodynamically less favorable than the recombination of
VA and HCl to form the isomeric 1-chloroethanol (1ClEtOH) species. This species
is more stable than 2ClEtOH by about 6 kcal mol-1, and the reaction barrier
is 22 kcal mol-1 vs 55 kcal mol-1 for the direct
transformation of VA to acetaldehyde. In a successive step, 1ClEtOH can
decompose directly to acetaldehyde and HCl with a lower barrier (29 kcal mol-1)
than that of VA to the same products (55 kcal mol-1). The
calculations were repeated using a single ancillary water molecule (W) in the
complexes 2ClEtOH_W and 1ClEtOH_W. The latter adduct is now more stable than
2ClEtOH_W by about 8 kcal mol-1, implying that the water molecule
increased the already higher stability of 1ClEtOH in the gas phase. However,
this catalytic water molecule lowers dramatically the barrier for the
interconversion of VA to acetaldehyde (from 55 to 6 kcal mol-1).
This barrier is now smaller than the one for the conversion to 1ClEtOH (which
also decreases, but not so much, from 22 to 12 kcal mol-1). Thus, it
is concluded that while 1ClEtOH may be a plausible intermediate in the gas
phase dehalogenation of 2ClEtOH, it is unlikely that it plays a major role in
water complexes (or, by inference, aqueous solution). It is also shown that
neither in the gas phase nor in the cluster with one water molecule, the
oxirane path is competitive with the VA alcohol path.</p
A Reinvestigation of the Deceptively Simple Reaction of Toluene with ●OH, and the Fate of the Benzyl Radical. I. a Combined Thermodynamic and Kinetic Study on the Competition Between ●OH Addition and Hydrogen Abstraction Reactions.
This work reports density functional and composite model chemistry calculations performed on the reactions of toluene with the hydroxyl radical. Both experimentally observed H-abstraction from the methyl group and possible additions to the phenyl ring were investigated. Reaction enthalpies and heights of the barriers suggest that H-abstraction is more favorable than ●OH addition to the ring. The calculated reaction rates at room temperature and the radical-intermediate product fractions support this view. This is somehow contradictory with the fact that, under most experimental conditions, cresols are observed in a larger concentration than benzaldehyde. Since the accepted mechanism for benzaldehyde formation involves H-abstraction, a contradiction arises that begs for an explanation. In this first part of our work we give the evidences that support the preference of hydrogen abstraction over ●OH addition and suggest an alternative mechanism which shows that cresols can actually arise also from the former reaction and not only from the latter