Frank De Proft

Vrije Universiteit Brussel, Bruxelles, Brussels Capital, Belgium

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Publications (321)1116.1 Total impact

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    ABSTRACT: Several expanded porphyrins switch between Hückel, Möbius and twisted-Hückel topologies, encoding different aromaticity and NLO properties. Since the topological switch can be induced by different external stimuli, expanded porphyrins represent a promising platform to develop molecular switches for molecular electronic devices. In order to determine the optimum conditions for efficient molecular switches from octaphyrins, we have carried out a comprehensive quantum chemical study focused on the conformational preferences and aromaticity of [36]octaphyrins. Different external stimuli for triggering the topological switch have been considered in our work, such as protonation and redox reactions. Importantly, the structure-property relationships between the molecular conformation, number of π-electrons and aromaticity in octaphyrins have been established by using energetic, magnetic, structural and reactivity descriptors. Importantly, we found that the aromaticity of octaphyrins is highly dependent on the π-conjugation topology and the number of π-electrons and it can be modulated by protonation and redox reactions. A non-aromatic figure-eight conformation is strongly preferred by neutral [36]octaphyrins, that switches to a Möbius aromatic conformation upon protonation. Such a change of topology involves an aromaticity switch in a single molecule and is accompanied by a drastic change in the NLO properties. By contrast, the twisted-Hückel topology remains the most stable one in the oxidized and reduced species, but the aromaticity is totally reverse upon redox reactions. Aromaticity is shown to be a key concept in expanded porphyrins, determining the electronic, magnetic and NLO properties of these macrocycles.
    No preview · Article · Feb 2016 · Physical Chemistry Chemical Physics
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    ABSTRACT: The role of the solvent and the influence of dynamics on the kinetics and mechanism of the SNAr reaction of several halonitrobenzenes in liquid ammonia, using both static calculations and dynamic ab initio molecular dynamics simulations, are investigated. A combination of metadynamics and committor analysis methods reveals how this reaction can change from a concerted, one-step mechanism in gas phase to a stepwise pathway, involving a metastable Meisenheimer complex, in liquid ammonia. This clearly establishes, among others, the important role of the solvent and highlights the fact that accurately treating solvation is of crucial importance to correctly unravel the reaction mechanism. It is indeed shown that H-bond formation of the reacting NH3 with the solvent drastically reduces the barrier of NH3 addition. The halide elimination step, on the other hand, is greatly facilitated by proton transfer from the reacting NH3 to the solvent. Furthermore, the free energy surface strongly depends on the halide substituent and the number of electron-withdrawing nitro substituents.
    No preview · Article · Jan 2016 · The Journal of Organic Chemistry
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    ABSTRACT: In recent years, expanded porphyrins have emerged as a promising class of π-conjugated molecules that display unique electronic, optical and conformational properties. Several expanded porphyrins can switch between planar and twisted conformations, which have different photophysical properties. Such a change of topology involves a Hückel-Möbius aromaticity switch in a single molecule and it can be induced by solvent, pH and metallation. These features make expanded porphyrins suitable for the development of a novel type of molecular switches for molecular electronic devices. Octaphyrins consisting of eight pyrrole rings, exhibit twisted-Hückel, Möbius and Hückel -conjugation topologies depending on the oxidation and protonation state, with distinct electronic structures and aromaticity. Our working hypothesis is that a significant change in the conductance of expanded porphyrins will be observed after the topology switching. Despite the potential of Hückel-Möbius systems as conductance switches, the relathionship between the conductance and the molecular topology is not yet understood. We have explored the performance of local descriptors of conductivity in simple molecules, as well as the relationship with conductance. Since these indexes provide a qualitiative measure of delocalization and conductance in the probe molecules, we have carried out a local analysis of electrical conductance changes as a function of the π-conjugation in two examples. In one of them, the locality of the electronic changes ensures the ability of these indexes to describe the conductance as local. However, we also show that this is not always the case, and in those molecules where a deep rearrangement occurs far from the structural perturbation, local measures show a limited efficiency. This is a first step for the description of the connection between the molecular structure and conductance in molecular switches.
    No preview · Article · Jan 2016 · Physical Chemistry Chemical Physics
  • Yannick Geboes · Frank De Proft · Wouter A. Herrebout
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    ABSTRACT: The formation of complexes between hexafluorobenzene and the Lewis bases dimethyl ether, dimethyl ether-d6, trimethylamine and trimethylamine-d9 was investigated experimentally using FTIR spectroscopy on solutions of liquid krypton. Additional bands found in the spectra of mixtures were assigned to lone pair···π complexes using ab initio calculations at the MP2/aug-cc-pVDZ level. By constructing Van ‘t Hoff plots, experimental complexation enthalpies were determined of -6.0(6) kJ mol−1 for the complex with dimethyl ether(-d6) and -6.7(9) kJ mol−1 for the complex with trimethylamine(-d9). These values are in good agreement with calculated enthalpy values.
    No preview · Article · Jan 2016 · Chemical Physics Letters
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    ABSTRACT: An integrated experimental and computational study was carried out on molybdenum loaded BEA zeolite in order to scrutinize the most stable state of the molybdenum site at various oxidation and hydration states. After a two-step post-synthetic process, the incorporation of Mo ions into the vacant T-atom sites of the SiBEA framework zeolite as mononuclear MoVI centers has been evidenced by combined use of XRD, FTIR and diffuse reflectance UV-vis spectroscopy. The consumption of OH groups upon Mo loading has been monitored by FTIR and adsorbed pyridine is used as probe molecules for characterization of Brønsted and Lewis acidity in SiBEA and Mo3.0SiBEA. Periodic DFT calculations in sodalite (SOD) framework indicate a trigonal-bipyramidal conformer at the MoVI site with molybdenum being incorporated into the zeolite framework by four SiO– bonds and a Mo=O functionality under anhydrous conditions. Water splitting formally to O2- and 2H+ is computed to be feasible in SOD cage, whereas it is strongly favored in MoBEA framework at the T1 position. Depending on the hydration state, it is shown that the Mo sites formed can exhibit two Mo=O functionalities in a trigonal-bipyramidal and a pseudo-octahedral arrangement.
    No preview · Article · Jan 2016 · Microporous and Mesoporous Materials
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    ABSTRACT: The metal-free reaction of terminal arylacetylenes with α,α-dichloroaldimines in 1,1,1,3,3,3-hexafluoro-2-propanol as the sole solvent results in the rapid and selective formation of γ,γ-dichloro-β-amino ketones. In this solvent the expected dichlorinated propargylamines and/or allylic amines are not formed. The dichloromethylene moiety of the aldimine acts as an activating group and is essential to accomplish this transformation. Electron-rich acetylenes lead to the best results and work well with all imines (with or without α′-H at the nitrogen substituent), while electron-deficient acetylenes only reacted with N-tert-butylaldimines (no α′-H). The mechanistic pathway showed 1,1,1,3,3,3-hexafluoro-2-propanol to protonate the aldimine, which in the rate-determining step will react with the arylacetylene to form a resonance-stabilized allene cation, which is trapped by a HFIP molecule giving rise to an enol ether, which promptly hydrolyzes to furnish exclusively the β-amino ketones. Using DFT techniques we found that the first CC bond forming step is the rate-determining step and is associated with a barrier of about 21kcalmol-1.
    Full-text · Article · Dec 2015 · Advanced Synthesis & Catalysis
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    Mercedes Alonso · Balazs Pinter · Paul Geerlings · Frank De Proft

    Full-text · Article · Dec 2015 · Chemistry - A European Journal
  • Thijs Stuyver · Stijn Fias · Frank De Proft · Paul Geerlings
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    ABSTRACT: A new selection rule for transmission under small voltage for molecular electronic devices consisting of alternant hydrocarbons is established, based on curly arrow drawings of the displacement of electrons through the conjugated system of the molecule. If the displacement of electrons from one contact atom to the other can be drawn, transmission is possible. If not, then no transmission will take place. This simple, back of the envelope ansatz allows determination of quantum interferences for a wide range of experimentally studied molecules (linear polyenes, benzenoids, cross-conjugated molecules,...) avoiding any calculation. This new selection rule bridges the conceptual gap between traditional reactivity theory and molecular electronics.
    No preview · Article · Nov 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: This chapter focuses on the importance of aziridinium ions as intermediates in organic chemistry. The principal aim is to gain insight into the factors to take into account for the selective synthesis of a variety of functionalized amines via aziridinium salts, such as the nature of the aziridinium ion (ring strain and N- and C-substituents of the aziridine ring), the nucleophile, and the solvent environment. Molecular modeling is used to investigate kinetics, electrostatics, and frontier molecular orbitals of reactions involving intermediate aziridinium ions, such as the nucleophilic ring opening of aziridines, the ring expansion of nitrogen heterocycles, and the ene reactions with triazolinedione.
    No preview · Article · Nov 2015 · ChemInform
  • Mercedes Alonso · Balazs Pinter · Paul Geerlings · Frank De Proft
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    ABSTRACT: The heretofore unpredictable behavior of [26] and [28]hexaphyrins upon metalation has been elucidated through quantum chemical calculations. It is demonstrated that the molecular topology of Group 10 and Group 11 metal complexes of hexaphyrins depends on sensitive interplay between the intrinsic ligand strain and the metal-ligand interaction strength. As such, the aromaticity of the ligand and effective charge of the metal are revealed as key factors determining the binding mode and the preference for Möbius or Hückel structures. These findings offer a new perspective to rationalize experimental observations for metalated hexaphyrins. More importantly, the proposed guidelines could be useful for designing novel complexes of hexaphyrins, such as a hitherto unknown Möbius [26]hexaphyrin complex.
    No preview · Article · Oct 2015 · Chemistry - A European Journal
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    ABSTRACT: The reduction of N,C,N-chelated bismuth chlorides [C6 H3 -2,6-(CHNR)2 ]BiCl2 [where R=tBu (1), 2',6'-Me2 C6 H3 (2), or 4'-Me2 NC6 H4 (3)] or N,C-chelated analogues [C6 H2 -2-(CHN-2',6'-iPr2 C6 H3 )-4,6-(tBu)2 ]BiCl2 (4) and [C6 H2 -2-(CH2 NEt2 )-4,6-(tBu)2 ]BiCl2 (5) is reported. Reduction of compounds 1-3 gave monomeric N,C,N-chelated bismuthinidenes [C6 H3 -2,6-(CHNR)2 ]Bi [where R=tBu (6), 2',6'-Me2 C6 H3 (7) or 4'-Me2 NC6 H4 (8)]. Similarly, the reduction of 4 led to the isolation of the compound [C6 H2 -2-(CHN-2',6'-iPr2 C6 H3 )-4,6-(tBu)2 ]Bi (9) as an unprecedented two-coordinated bismuthinidene that has been structurally characterized. In contrast, the dibismuthene {[C6 H2 -2-(CH2 NEt2 )-4,6-(tBu)2 ]Bi}2 (10) was obtained by the reduction of 5. Compounds 6-10 were characterized by using (1) H and (13) C NMR spectroscopy and their structures, except for 7, were determined with the help of single-crystal X-ray diffraction analysis. It is clear that the structure of the reduced products (bismuthinidene versus dibismuthene) is ligand-dependent and particularly influenced by the strength of the N→Bi intramolecular interaction(s). Therefore, a theoretical survey describing the bonding situation in the studied compounds and related bismuth(I) systems is included. Importantly, we found that the C3 NBi chelating ring in the two-coordinated bismuthinidene 9 exhibits significant aromatic character by delocalization of the bismuth lone pair.
    Full-text · Article · Oct 2015 · Chemistry - A European Journal
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    ABSTRACT: N-Methyl-D-Aspartate (NMDA), which is a member of ionotropic glutamate receptors (iGluR) becomes active via binding of glycine and glutamate to the subunits of the receptor. Thus, the binding sites of these subunits are important targets for pharmaceutical research. NMDA receptor has two distinctive behaviours which makes it more prominent for medical researches 1. The first one is the inactivation of NMDA during its recessing period, and the second one is the "exitotoxic cell death" due to the prolonged stimulation of the receptor. Regulation of receptor activity by introducing partial agonist-antagonist ligands to the glycine and glutamate sites became a promising strategy for the treatment of neuropsychiatric disorders, which are related with dysfunction of NMDA. In this study, quantum chemical calculations are implemented to the ligands for the major subfamily of glutamate receptors, N-methyl-D-aspartate (NMDA). In this manner molecular properties playing a role in ligand binding to NMDA are investigated. Various quantum chemical descriptors are calculated to understand the link between chemical traits of molecules and their activities. These descriptors are further used to discriminate the partial agonism-antagonism nature of the ligands. A number of ligands which bind to the glycine binding site NR1 subunit with different activities 2-5 are selected and calculations are carried out using B3LYP with the PCM solvent model in water by utilizing the Gaussian09 program package. The next step of this project will be to connect these molecular descriptors to their activities in order to design a robust QSAR model for the ligands of the NMDA receptor. This will open new opportunities for the drug design in virtual screening and synthesis of novel classes of ligands for glutamate receptors.
    Full-text · Conference Paper · Oct 2015
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    ABSTRACT: A set of tri- and diorganotin(IV) pseudohalides bearing the 2-(N,N-dimethylaminomethyl)phenyl-as a C,N-chelating ligand (LCN) has been prepared and structurally characterized. Triorganotin(IV) pseudohalides of the type LCNR2SnX [R = n-Bu, X = NCO (1a), NCS (1b), NCSe (1c), CN (1d); R = Ph, X = NCO (2a), NCS (2b), NCSe (2c), CN (2d)] and (LCN)2(n-Bu)SnX [X = NCO (3a), NCS (3b), NCSe (3c), CN (3d)] are monomeric both in solution and the solid state. The central tin atom in these species is five-coordinated in the solid state with distorted trigonal bipyramidal geometry. Monomeric diorganotin(IV) pseudohalides of the type (LCN)2SnX2 [X = NCO (4a), NCS (4b), NCSe (4c), CN (4d)] contain six-coordinated tin atoms with heavily distorted octahedral geometry both in solution and solid state due to the presence of two LCN units. Finally, we have found that LCN(n-Bu)Sn(NCS)2 (5) is presumably dimeric in CDCl3 and monomeric in THF-d8 or DMSO-d6 solution while it forms infinite linear polymer in the solid state via thiocyanate bridges. According to IR, multinuclear NMR spectroscopy and XRD analysis, it was found that the pseudohalide ligands of the N=C=Y type (Y = O, S, Se) are bound via nitrogen while the CN substituent act as a common cyanide ligand (i.e. binds via carbon). DFT calculations were performed for selected compounds (2a, 2b, 2d and 4d) to further support our conclusions about the nature of the Sn-pseudohalide bonds. Compound LCNSnPhI2 (6), which was prepared in the scope of this work as well has a trigonal bipyramidal structure with non-equivalent iodine atoms.
    No preview · Article · Oct 2015 · Journal of Organometallic Chemistry
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    ABSTRACT: Two ethylene-bridged methylaluminium amidinates and one aluminium amidinate containing three terminal trimethylstannyl-ethynyl groups interconnected by π-coordinated potassium ions were prepared in situ. The re-oxidation of the ethylene-bridged compound by iodine followed by further reduction using the same activation procedure demonstrated the versatility of the approach. The reactivity of an ethylene-bridged methylaluminum amidinate towards HCl was examined to demonstrate the building block concept. DFT calculations were performed to gain insight into the mechanism of the in situ activation of diphenylacetylene.
    No preview · Article · Sep 2015 · Dalton Transactions
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    ABSTRACT: Ionotropic glutamate receptors (iGluR) are located at the membranes of neuronal cells and play key roles in synaptic plasticity. Proper function of the receptors is crucial for the central nervous system. Members of iGluR, become active via binding of glycine and glutamate to the subunits of the receptors. Thus, the binding sites of these subunits are important targets for pharmaceutical research. Regulation of receptor activity by introducing partial agonist-antagonist ligands to the glycine and glutamate sites became a promising strategy for the treatment of neuropsychiatric disorders, which are related with dysfunction of glutamate receptors. An experimentally validated quantitative structure-activity relationship (QSAR) model would be enormously helpful for virtual screening of vast amount of molecules before synthesizing the ones with the highest expected activities. Intrinsic properties of molecules calculated by quantum chemical methods are one of the most common and effective approaches for QSAR modeling. In this study, various quantum chemical descriptors are calculated to understand the link between chemical traits of molecules with their activities. These descriptors are further used to discriminate the partial agonism-antagonism nature of the ligands. The next step is to connect these molecular descriptors to their activities in order to design a robust QSAR model for ligands of NMDA and AMPA receptors. The results emerging from this study will be presented.
    Full-text · Conference Paper · Aug 2015
  • T. Stuyver · S. Fias · F. De Proft · P. Geerlings
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    ABSTRACT: Pauling long bond orders have previously been used to obtain qualitative insight in the transmission through nanographenes. Here we show that this long bond order, the atom–atom polarizability (a mea- sure for delocalization) and the transmission probability are intimately linked and that their relationships are valid for all alternant hydrocarbons. These relationships allow a simple rationalization of the transport properties of a variety of molecules considered in molecular electronics. As an example, some molecular wires such as oligo(p-phenylene) are studied, leading to a simple explanation for the experimentally observed exponential decay of the transmission probability with the number of phenyl units.
    No preview · Article · Jun 2015 · Chemical Physics Letters
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    Gabriella Skara · Balazs Pinter · Paul Geerlings · Frank De Proft
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    ABSTRACT: Metal and ligand-based reductions have been modeled in octahedral ruthenium complexes revealing metal-ligand interactions as the profound driving force for the redox-active behaviour of orthoquinoid-type of ligands. Through an extensive investigation of redox-active ligands we revealed the most critical factors that facilitate or supress redox-activity of ligands in metal complexes, from which basic rules for designing non-innocent/redox-active ligands can be put forward. These rules also allow the rational redox-leveling, i.e. moderating redox potentials of ligand-centred electron transfer processes potentially leading to catalysts with low overpotential in multielectron activation processes.
    Full-text · Article · May 2015 · Chemical Science
  • Yannick Geboes · Frank De Proft · Wouter A Herrebout
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    ABSTRACT: The molecular electrostatic potential surface of unsaturated, locally electron deficient molecules shows a positive region perpendicular to (a part of) the molecular framework. In recent years it has been shown both theoretically and experimentally that molecules are able to form noncovalent interactions with Lewis bases through this π-hole. When studying unsaturated perfluorohalogenated molecules containing a higher halogen atom, a second electropositive region is also observed near the halogen atom. This region, often denoted as a σ-hole, allows the molecules to interact with Lewis bases and form a halogen bond. To experimentally characterize the competition between both these noncovalent interactions, Fourier transform infrared and Raman spectra of liquefied noble gas solutions containing perfluorohalogenated ethylene derivatives (C2F3X; X= F, Cl, Br or I) and trimethylamine(-d9) were investigated. Analysis of the spectra shows that in mixed solutions of trimethylamine(-d9) and C2F4 or C2F3Cl lone pair···π complex is present, while evidence for halogen bonded complex is found in solutions containing TMA(-d9) and C2F3Cl, C2F3Br or C2F3I. For all species observed, complexation enthalpies were determined, the values varying between -4.9(1) and -24.4 kJ mol(-1).
    No preview · Article · Apr 2015 · The Journal of Physical Chemistry A
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    ABSTRACT: We investigate the local polarisability or polarisability density using both a conceptual density functional theory approach based on the linear response function and time-dependent density functional theory. Using a zero frequency in the latter, we can immediately compare both approaches. Using an analytical expression for the linear response kernel, we are able to systematically analyse α(r) throughout the periodic table. An extension to molecules is also made with a study of the CO molecule retrieving the connection between local softness and local polarisability.
    No preview · Article · Mar 2015 · Molecular Physics
  • T Stuyver · S Fias · F De Proft · P W Fowler · P Geerlings
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    ABSTRACT: The atom-atom polarizability and the transmission probability at the Fermi level, as obtained through the source-and-sink-potential method for every possible configuration of contacts simultaneously, are compared for polycyclic aromatic compounds. This comparison leads to the conjecture that a positive atom-atom polarizability is a necessary condition for transmission to take place in alternant hydrocarbons without non-bonding orbitals and that the relative transmission probability for different configurations of the contacts can be predicted by analyzing the corresponding atom-atom polarizability. A theoretical link between the two considered properties is derived, leading to a mathematical explanation for the observed trends for transmission based on the atom-atom polarizability.
    No preview · Article · Mar 2015 · The Journal of Chemical Physics

Publication Stats

8k Citations
1,116.10 Total Impact Points

Institutions

  • 1993-2016
    • Vrije Universiteit Brussel
      • • Ultrastructure (ULTR)
      • • General Chemistry (ALGC)
      Bruxelles, Brussels Capital, Belgium
  • 2008-2015
    • University of Pardubice
      • Department of General and Inorganic Chemistry
      Pardubitz, Pardubický, Czech Republic
  • 2002
    • Universidad de Cartagena
      Cartagena de Indias, Bolívar, Colombia
  • 1999-2001
    • Ghent University
      Gand, Flanders, Belgium