K. Hemelsoet

Levofloxacin ozonation in water: Rate determining process parameters and reaction pathway elucidation

B. De Witte, H. Van Langenhove, K. Hemelsoet, K. Demeestere, P. De Wispelaere, V. Van Speybroeck, J. Dewulf
Chemosphere
76 (5), 683-689
2009
A1

Abstract 

Ozonation of the quinolone antibiotic levofloxacin was investigated with focus on both the levofloxacin degradation rate and degradation product formation. Degradation was about 2 times faster at pH 10 compared to pH 3 and 7 explained by direct ozonation at the unprotonated , one of the tertiary amines of the piperazinyl substituent. H2O2 concentration (2–100 μM) had only limited effect. Liquid chromatography – high resolution mass spectrometry revealed degradation at the piperazinyl substituent and the quinolone moiety, with the relative importance of both pathways being strongly affected by changes in pH. Levofloxacin N-oxide concentrations reached up to 40% of the initial levofloxacin concentration during ozonation at pH 10. Degradation at the quinolone moiety resulted in isatin and anthranilic acid type metabolites, probably formed through reaction with hydroxyl radicals. Ab initio molecular orbital calculations predicted radical attack mainly at C2 of the quinolone moiety. This is the carbon atom with the largest Fukui function. Reaction with ozone is expected to mainly occur at , characterized by the largest negative charge.

Bond Dissociation Enthalpies of Large Aromatic Carbon-Centered Radicals

K. Hemelsoet, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry A
112 (51), 13566-13573
2008
A1

Abstract 

Carbon−hydrogen bond dissociation enthalpy (BDE) values are computed for the class of benzylic radicals. An extended and representative set of large methylated polyaromatics has been submitted to an accurate computational study using various levels of theory. The hybrid B3P86 as well as two contemporary functionals (BMK and M05-2X) are applied. For a selection of species, the suitability of the DFT methods is validated through comparison with high-level G3(MP2)-RAD and SCS-ROMP2 results. The influence of the polyaromatic environment on the BDE results is thoroughly discussed. The results are compared with other hydrocarbon radical types in order to obtain a generalized radical stability scale. In order to complete this investigation, also carbon−carbon BDE values have been calculated, giving information about the influence of the local environment on removing the methyl group from the polyaromatic.

A DFT-Based Investigation of Hydrogen Abstraction Reactions from Methylated Polycyclic Aromatic Hydrocarbons

K. Hemelsoet, V. Van Speybroeck, M. Waroquier
ChemPhysChem
9 (16), 2349-2358
2008
A1

Abstract 

The growth of polycyclic aromatic hydrocarbons (PAHs) is in many areas of combustion and pyrolysis of hydrocarbons an inconvenient side effect that warrants an extensive investigation of the underlying reaction mechanism, which is known to be a cascade of radical reactions. Herein, the focus lies on one of the key reaction classes within the coke formation process: hydrogen abstraction reactions induced by a methyl radical from methylated benzenoid species. It has been shown previously that hydrogen abstractions determine the global PAH formation rate. In particular, the influence of the polyaromatic environment on the thermodynamic and kinetic properties is the subject of a thorough exploration. Reaction enthalpies at 298 K, reaction barriers at 0 K, rate constants, and kinetic parameters (within the temperature interval 700–1100 K) are calculated by using B3LYP/6-31+G(d,p) geometries and BMK/6-311+G(3df,2p) single-point energies. This level of theory has been validated with available experimental data for the abstraction at toluene. The enhanced stability of the product benzylic radicals and its influence on the reaction enthalpies is highlighted. Corrections for tunneling effects and hindered (or free) rotations of the methyl group are taken into account. The largest spreading in thermochemical and kinetic data is observed in the series of linear acenes, and a normal reactivity–enthalpy relationship is obtained. The abstraction of a methyl hydrogen atom at one of the center rings of large methylated acenes is largely preferred. Geometrical and electronic aspects lie at the basis of this striking feature. Comparison with hydrogen abstractions leading to arylic radicals is also made.

Using elementary reactions to model growth processes of polyaromatic hydrocarbons under pyrolysis conditions of light feedstocks

K. Hemelsoet, V. Van Speybroeck, K.M. Van Geem, G.B. Marin, M. Waroquier
Molecular Simulation
34 (2) ,193-199
2008
A1

Abstract 

Density functional theory results are presented for elementary steps leading to coke growth within a steam cracking unit. The discussed pathway starts from toluene and ultimately, 1-methylnaphthalene is formed. In order to find the rate determining step for coke formation, the pseudo first-order rate coefficients of the various steps are compared taking into account the concentrations of diverse coke precursors. The influence of the polyaromatic environment is studied for a large set of methylated polycyclic aromatic molecules, by means of carbon–hydrogen bond dissociation enthalpy values. Subsequent hydrogen abstraction reactions at the methylated polyaromatics, by a methyl radical, are also examined. The abstraction is found to preferentially occur at the larger systems and is in general faster compared to abstractions at the analogous non-methylated species.

Modeling elementary reactions in coke formation from first principles

V. Van Speybroeck, K. Hemelsoet, B. Minner, G.B. Marin, M. Waroquier
Molecular Simulation
33 (9), 879-887
2007
A1

Abstract 

Theoretical calculations are presented on elementary reactions which are important during coke formation in a thermal cracking unit. This process is known to proceed through a free radical chain mechanism. The elementary reaction steps that lead to the growth of the coke surface can be divided into five classes of reversible reactions: hydrogen abstraction, substitution, gas phase olefin addition to radical surface species, gas phase radical addition to olefinic bonds and cyclization. To identify the elementary reaction classes that determine the coking rate, all microscopic routes that start from benzene and lead to naphthalene have been investigated. It is found that initial creation of surface radicals, either by hydrogen abstraction or substitution and subsequent hydrogen abstractions, determines the global coking rate. The influence of the local polyaromatic structure on the kinetics of the hydrogen abstraction reactions is determined by performing calculations on a large set of polyaromatic hydrocarbons (PAHs). On basis of the BDE values six types of possible reactive sites at the coke surface can be distinguished. For the initial hydrogen abstraction the local polyaromatic structure strongly influences the reaction kinetics and abstraction is preferred from less congested sites of the polyaromatic.

How useful are reactivity indicators for the description of hydrogen abstraction reactions on polycyclic aromatic hydrocarbons?

K. Hemelsoet, V. Van Speybroeck, M. Waroquier
Chemical Physics Letters
444 (1-3), 17-22
2007
A1

Abstract 

Hydrogen abstraction reactions at polyaromatic hydrocarbons by a methyl radical are investigated from the viewpoint of DFT-based reactivity descriptors. The BMK functional succeeds in accurately reproducing experimental data for the global indicators. All species are found to be soft. The local HSAB principle shows an overall good qualitative agreement with kinetic barriers, and the local softness is successful for describing the general reactivity trends. However, the indicators do not succeed in predicting the particularly high barriers encountered in some abstraction reactions, as these barriers are mainly caused by steric hindrance effects in the transition structures.

Global DFT-Based Reactivity Indicators:  An Assessment of Theoretical Procedures in Zeolite Catalysis

K. Hemelsoet, D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry C
111 (7), 3028-3037
2007
A1

Abstract 

The dependence of global reactivity descriptors on electronic structure method as well as basis set is investigated for typical reactions in zeolite catalysis. This research is especially focused on hard−hard interactions between small probe molecules (such as chloromethane, methanol, ethylene, and propene) and different zeolite clusters containing both oxygen and amine functionalities. The performance of novel hybrid metafunctionals (such as BMK and MPWB1K) on crucial reactivity predictors is assessed through comparison with both Hartree−Fock and B3-LYP results. For the complex bifunctional zeolite systems, we find accurate results using any of the DFT functionals, in conjunction with a basis set of at least double-ζ quality further augmented with both polarization and diffuse functions. Reactivity sequences, based on global softness differences as well as activation hardness values, are generally found to be independent of the level of theory whenever a DFT functional is used.

Spin-Polarized Conceptual Density Functional Theory Study of the Regioselectivity in Ring Closures of Radicals

B. Pinter, F. De Proft, V. Van Speybroeck, K. Hemelsoet, M. Waroquier, E. Chamorro, T. Veszpremi, P. Geerlings
Journal of Organic Chemistry
72 (2), 348-356
2007
A1

Abstract 

The regioselectivity of ring-forming radical reactions is investigated within the framework of the so-called spin-polarized conceptual density functional theory. Two different types of cyclizations were studied. First, a series of model reactions of alkyl- and acyl-substituted radicals were investigated. Next, attention was focused on the radical cascade cyclizations of N-alkenyl-2-aziridinylmethyl radicals (a three-step mechanism). In both of these reactions, the approaching radical (carbon or nitrogen centered) adds to a carbon−carbon double bond within the same molecule to form a radical ring compound. In this process, the number of electrons is changing from a local point of view (a charge transfer occurs from one part of the molecule to another one) at constant global spin number Ns (both the reactant and the product ring compound are in the doublet state). It is shown that the experimentally observed regioselectivities for these ring-closure steps can be predicted using the spin-polarized Fukui functions for radical attack, (r).

Ab initio thermochemistry and Kinetics of Hydrogen Abstraction by Methyl Radical from Polycyclic Aromatic Hydrocarbons

K. Hemelsoet, V. Van Speybroeck, D. Moran, G.B. Marin, L. Radom, M. Waroquier
Journal of Physical Chemistry A
110 (50), 13624-13631
2006
A1

Abstract 

Thermodynamic and kinetic properties relating to hydrogen abstraction by methyl radical from various sites in polycyclic aromatic hydrocarbons (PAHs) have been investigated. The reaction enthalpies (298 K), barriers (0 K), and activation energies and pre-exponential factors (700−1100 K), have been calculated by means of density functional theory, specifically with B3-LYP/6-311G(d,p) geometries, followed by BMK/6-311+G(3df,2p) single-point energy calculations. For uncongested sites in the PAHs, a reasonable correlation is obtained between reactivities (as characterized by the reaction barriers) and reaction enthalpies. This is reflected in a Bell−Evans−Polanyi (BEP) relationship. However, for congested sites, abstraction is accompanied both by lower reaction enthalpies (due to relief of steric strain) and also by reduced reactivities (due to significantly increased steric hindrance effects in the transition structures), so that the BEP relationship does not hold. In addition, the reaction enthalpies and kinetic parameters for the series of linear acenes indicate that abstraction is more difficult from the central rings.

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