K. Hemelsoet

An assessment of theoretical procedures for predicting the thermochemistry and kinetics of hydrogen abstraction by methyl radical from benzene

K. Hemelsoet, D. Moran, V. Van Speybroeck, M. Waroquier, L. Radom
Journal of Physical Chemistry A
110 (28), 8942-8951
2006
A1

Abstract 

The reaction enthalpy (298 K), barrier (0 K), and activation energy and preexponential factor (600−800 K) have been examined computationally for the abstraction of hydrogen from benzene by the methyl radical, to assess their sensitivity to the applied level of theory. The computational methods considered include high-level composite procedures, including W1, G3-RAD, G3(MP2)-RAD, and CBS-QB3, as well as conventional ab initio and density functional theory (DFT) methods, with the latter two classes employing the 6-31G(d), 6-31+G(d,p) and/or 6-311+G(3df,2p) basis sets, and including ZPVE/thermal corrections obtained from 6-31G(d) or 6-31+G(d,p) calculations. Virtually all the theoretical procedures except UMP2 are found to give geometries that are suitable for subsequent calculation of the reaction enthalpy and barrier. For the reaction enthalpy, W1, G3-RAD, and URCCSD(T) give best agreement with experiment, while the large-basis-set DFT procedures slightly underestimate the endothermicity. The reaction barrier is slightly more sensitive to the choice of basis set and/or correlation level, with URCCSD(T) and the low-cost BMK method providing values in close agreement with the benchmark G3-RAD value. Inspection of the theoretically calculated rate parameters reveals a minor dependence on the level of theory for the preexponential factor. There is more sensitivity for the activation energy, with a reasonable agreement with experiment being obtained for the G3 methods and the hybrid functionals BMK, BB1K, and MPW1K, especially in combination with the 6-311+G(3df,2p) basis set. Overall, the high-level G3-RAD composite procedure, URCCSD(T), and the cost-effective DFT methods BMK, BB1K, and MPW1K give the best results among the methods assessed for calculating the thermochemistry and kinetics of hydrogen abstraction by the methyl radical from benzene.

Unexpected Four-Membered over Six-Membered Ring Formation during the Synthesis of Azaheterocyclic Phosphonates: Experimental and Theoretical Evaluation

V. Van Speybroeck, K. Moonen, K. Hemelsoet, C.V. Stevens, M. Waroquier
JACS (Journal of the American Chemical Society)
128 (26), 8468-8478
2006
A1

Abstract 

The cyclization of functionalized aminophosphonates is studied on both experimental and theoretical grounds. In a recently described route to phosphono-β-lactams [Stevens C. V.; Vekemans, W.; Moonen, K.; Rammeloo, T. Tetrahedron Lett. 2003, 44, 1619], it was found that starting from an ambident allylic anion only four-membered rings were formed without any trace of six-membered lactams. New anion trapping experiments revealed that the γ-anion is highly reactive in intermolecular reactions. Ab initio calculations predict higher reaction barriers for the γ-anion due to restricted rotation about the C−N bond and due to highly strained transition states during ring closure. The sodium or lithium counterion, explicit dimethyl ether solvent molecules, and bulk solvent effects were properly taken into account at various levels of theory.

Bifunctional acid-base catalyzed reactions in zeolites from the HSAB viewpoint

K. Hemelsoet, D. Lesthaeghe, V. Van Speybroeck, M. Waroquier
Chemical Physics Letters
419 (1-3), 10-15
2006
A1

Abstract 

The applicability of the hard and soft acids and bases principle is investigated for the interaction of 5T zeolite clusters with probe molecules such as chloromethane, methanol and olefins. The reactions are intermediately hard–hard and, therefore, mainly charge-controlled. This is confirmed by the success of the atomic charges and the electrostatic interaction energy at the acid site as correct descriptors of regio-selectivity and reactivity sequences. Both acid and basic reactive sites can be clearly indicated using frontier orbitals. Moreover, an excellent correlation is found between the activation hardnesses and the energy barriers at the absolute zero.

Comparative study of kinetics and reactivity indices of free radical polymerization reactions

K. Van Cauter, K. Hemelsoet, V. Van Speybroeck, M. Waroquier
International Journal of Quantum Chemistry
102 (4), 454-460
2005
A1

Abstract 

Density functional theory calculations are used to determine the kinetics and reactivity indices of the first propagation steps of the polyethylene and poly(vinyl chloride) polymerization. Transition state theory is applied to evaluate the rate coefficient from the microscopically determined energies and partition functions. A comparison with the experimental Arrhenius plots validates the level of theory. The ability of reactivity indices to predict certain aspects of the studied propagation reactions is tested. Global softnesses of the reactants give an indication of the relative energy barriers of subsequent monomer additions. The correlation between energy and hardness profiles along the reaction path confirm the principle of maximum hardness. Local indices predict the regioselectivity of the attack of the growing radical to vinyl chloride. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Reactivity Indices for Radical Reactions Involving Polyaromatics

K. Hemelsoet, V. Van Speybroeck, G.B. Marin, F. De Proft, P. Geerlings, M. Waroquier
Journal of Physical Chemistry A
108 (35) , 7281-7290
2004
A1

Abstract 

The reactivity of polyaromatics involved in various radical reactions is studied. The reactions under study are hydrogen abstractions by a methyl radical and additions to double bonds both intra- and intermolecular. The chemical reactivity of the involved molecules is described through different properties, which are calculated within the density functional theory (DFT) framework. The softness reactivity index is tested on its usefulness and reliability to provide information about the reactivity of the global molecule or about chemical selectivity. The applicability of the hard and soft acids and bases (HSAB) principle for bimolecular radical reactions is illustrated by comparing the results of the softness-matching criterion with kinetic and thermodynamic data. For large polyaromatic molecules several magnetic indices, in particular, magnetic susceptibilities, chemical shifts, and nucleus independent chemical shifts (NICS), are computed to quantify the aromatic character of the involved species. The applicability of these magnetic indices in the case of radical reactions is validated by comparing with kinetic results obtained from transition state theory.

Reactivity and aromaticity of polyaromatics in radical cyclization reactions

V. Van Speybroeck, K. Hemelsoet, M. Waroquier, G.B. Marin
International Journal of Quantum Chemistry
96(6), 568-576
2004
A1

Abstract 

Theoretical ab initio calculations are presented on cyclization reactions of polyaromatics that occur by a radical mechanism. Such processes are one of the elementary steps for polyaromatic hydrocarbon formation in thermal cracking units and during soot formation. Ring closure can take place at various sites of the polyaromatic surface. It is the aim of this study to obtain insight into the influence of the local structure on the reactivity of the polyaromatics. Aromaticity is a determining factor for the reactivity and can be probed by various magnetic properties such as the diamagnetic susceptibilities, proton chemical shifts, and nucear-independent chemical shifts. A correlation is established between the magnetic properties and activation energy of the studied reactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Bond Dissociation Energies of Organophosphorus Compounds: an Assessment of Contemporary Ab Initio Procedures

K. Hemelsoet, F. Van Durme, V. Van Speybroeck, M-F. Reyniers, M. Waroquier
Journal of Physical Chemistry A
114 (8), 2864–2873
2010
A1

Abstract 

Thermodynamic properties of phosphorus-containing compounds were investigated using high-level ab initio computations. An extended set of contemporary density functional theory (DFT) procedures was assessed for their ability to accurately predict bond dissociation energies of a set of phosphoranyl radicals. The results of meta- and double-hybrids as well as more recent methods, in particular M05, M05-2X, M06, and M06-2X, were compared with benchmark G3(MP2)-RAD values. Standard heats of formation, entropies, and heat capacities of a set of ten organophosphorus compounds were determined and the low-cost BMK functional was found to provide results consistent with available experimental data. In addition, bond dissociation enthalpies (BDEs) were computed using the BMK, M05-2X, and SCS-ROMP2 procedure. The three methods give the same stability trend. The BDEs of the phosphorus(III) molecules were found to be lower than their phosphorus(V) counterparts. Overall, the following ordering is found: BDE(P−OPh)

Open Access version available at UGent repository

Opposite Regiospecific Ring Opening of 2-(Cyanomethyl)aziridines by Hydrogen Bromide and Benzyl Bromide: Experimental Study and Theoretical Rationalization

S. Catak, M. D'Hooghe, T. Verstraelen, K. Hemelsoet, A. Van Nieuwenhove, H-J. Ha, M. Waroquier, N. De Kimpe, V. Van Speybroeck
Journal of Organic Chemistry
75 (13), 4530–4541
2010
A1

Abstract 

Ring opening of 1-arylmethyl-2-(cyanomethyl)aziridines with HBr afforded 3-(arylmethyl)amino-4-bromobutyronitriles via regiospecific ring opening at the unsubstituted aziridine carbon. Previous experimental and theoretical reports show treatment of the same compounds with benzyl bromide to furnish 4-amino-3-bromobutanenitriles through ring opening at the substituted aziridine carbon. To gain insights into the regioselective preference with HBr, reaction paths have been analyzed with computational methods. The effect of solvation was taken into account by the use of explicit solvent molecules. Geometries were determined at the B3LYP/6-31++G(d,p) level of theory, and a Grimme-type correction term was included for long-range dispersion interactions; relative energies were refined with the meta-hybrid MPW1B95 functional. Activation barriers confirm preference for ring opening at the unsubstituted ring carbon for HBr. HBr versus benzyl bromide ring opening was analyzed through comparison of the electronic structure of corresponding aziridinium intermediates. Although the electrostatic picture fails to explain the opposite regiospecific nature of the reaction, frontier molecular orbital analysis of LUMOs and nucleophilic Fukui functions show a clear preference of attack for the substituted aziridine carbon in the benzyl bromide case and for the unsubstituted aziridine carbon in the HBr case, successfully rationalizing the experimentally observed regioselectivity.

Modeling the Solvent Effect on the Tacticity in the Free Radical Polymerization of Methyl Methacrylate

I. Değirmenci, S. Eren, V. Aviyente, B. De Sterck, K. Hemelsoet, V. Van Speybroeck, M. Waroquier
Macromolecules
43 (13), 5602–5610
2010
A1

Abstract 

The control of stereochemistry in the free radical polymerization of methyl methacrylate (MMA) is important because the physical properties of PMMA are often significantly affected by the main-chain tacticity. In this study, the role of the solvent on the tacticity of MMA polymerization has been investigated by considering the propagation rate constants for the syndiotactic and isotactic free radical polymerization of MMA in vacuum, in methanol (CH3OH), and in 1,1,1,3,3,3-hexafluoro-2-(trifluoromethyl)propan-2-ol ((CF3)3COH). All geometry optimizations have been carried out with the B3LYP/6-31+G(d) methodology. The kinetics of the propagating dimer have been evaluated with the B3LYP/6-31+G(d), B3LYP/6-311+G(3df,2p), MPWB1K/6-311+G(3df,2p), and B2PLYP/6-31+G(d) methodologies. The role of the solvent has been investigated by using explicit solvent molecules and also by introducing a polarizable continuum model (IEF-PCM) with a dielectric constant specific to the solvent. Experimentally, the free radical polymerization of MMA in (CF3)3COH is found to be highly syndiotactic (rr = 75% at 20 °C): the stereoeffects of fluoroalcohols are claimed to be due to the hydrogen-bonding interaction of the alcohols with the monomers and growing species. This modeling study has revealed the fact that the solvents CH3OH and (CF3)3COH, which are H-bonded with the carbonyl oxygens located on the same side of the backbone hinder the formation of the isotactic PMMA to some extent. Methanol is less effective in reducing the isotacticity because of its small size and also because of the relatively loose hydrogen bonds (1.9 Å) with the carbonyl oxygens. The methodologies used in this study reproduce the solvent effect on the free radical polymerization kinetics of MMA in a satisfactory way.

Theoretical simulations elucidate the role of naphthalenic species during methanol conversion within H-SAPO-34

K. Hemelsoet, A. Nollet, V. Van Speybroeck, M. Waroquier
Chemistry - A European Journal
17(3) 9083–9093
2011
A1

Abstract 

The role of naphthalenic species during the methanol-to-olefins (MTO) process in a silicoaluminophosphate zeolitic material exhibiting the chabazite topology (H-SAPO-34) has been studied from first principles. These species could either act as active olefin-eliminating compounds or as precursors for deactivating species. Results incorporating van der Waals contributions for finite large clusters point out that successive methylation steps of naphthalenic compounds are feasible. The calculated intrinsic activation barrier is relatively independent of the number of methyl groups already attached on the aromatic compound and is approximately 140 kJ mol(-1) . The influence of the composition of the catalyst and hence the acidic strength on the intrinsic chemical kinetics was investigated in detail through comparison with the isostructural high-silicon material. Apparent chemical kinetics, starting from adsorbed methanol on the acid site, were also computed. The initiation steps of the side-chain route starting from a trimethylated naphthalenium ion were also examined. The actual side-chain methylation exhibits a high barrier and hence this mechanism involving methylated naphthalenes is not expected to be an active ethene-eliminating route in H-SAPO-34.

Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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