P. Vansteenkiste

Experimental and Computational Study of the Conrotatory Ring Opening of Various 3-Chloro-2-azetines

S. Mangelinckx, V. Van Speybroeck, P. Vansteenkiste, M. Waroquier, N. De Kimpe
Journal of Organic Chemistry
73 (14) 5481-5488
2008
A1

Abstract 

A combined experimental and theoretical study is presented on 2-azetines, a class of azaheterocyclic compounds, which are difficult to access but have shown a unique reactivity as strained cyclic enamines. New highly substituted 2-azetines bearing aryl substituents at the 2- and 4-position were synthesized from 3,3-dichloroazetidines. Whereas 2-aryl-3,3-dichloroazetidines gave stable 2-aryl-3-chloro-2-azetines upon treatment with sodium hydride in DMSO, 2,4-diaryl-3,3-dichloroazetidines showed a remarkably different reactivity in that they afforded benzimidoyl-substituted alkynes under similar mild treatment with base. The formation of the alkynes involves electrocyclic ring opening of intermediate 2,4-diaryl-3-chloro-2-azetines and elimination of hydrogen chloride. Ab initio theoretical calculations confirmed the experimental findings and demonstrated that the 4-aryl substituent is responsible for this remarkably enhanced reactivity of 2-azetines toward electrocyclic conrotatory ring opening by a significant decrease in reaction barrier of about 30 kJ/mol. This activation effect by an aryl group in the allylic position toward electrocyclic ring opening of unsaturated four-membered rings is of general importance since a similar increased reactivity of 4-aryloxetes, 4-arylthiete-1,1-dioxides, and 3-arylcyclobutenes has been reported in literature as well.

MFI Fingerprint: How Pentasil-Induced IR Bands Shift during Zeolite Nanogrowth

D. Lesthaeghe, P. Vansteenkiste, T. Verstraelen, A. Ghysels, C. Kirschhock, J.A. Martens, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry C
112 (25), 9186-9191
2008
A1

Abstract 

Silicalite-1 zeolite exhibits a characteristic pentasil framework vibration around 540−550 cm−1. In the initial stages of zeolite synthesis, however, this band is observed at much higher wavenumbers: literature shows this vibration to depend on particle size and to shift over 100 cm−1 with increasing condensation. In this work, the pentasil vibration frequency was derived from theoretical molecular dynamics simulations to obtain the correct IR band assignments for important nanoparticles. The IR spectroscopic fingerprint of oligomeric five-ring containing precursors proposed in the literature was computed and compared with experimental data. Our theoretical results show that, while isolated five-membered rings show characteristic vibrational bands around 650 cm−1, the combination of five-membered rings in the full MFI-type structure readily generates the bathochromic shift to the typical pentasil vibration around 550 cm−1. As opposed to what was previously believed, the IR band does not shift gradually as nanoparticle size increases, but it is highly dependent on the specific way structural units are added. The most important feature is the appearance of an additional band when double five-membered rings are included, which allows for a clear distinction between the key stages of early zeolite nucleation. Furthermore, the combination of the simulated spectra with the experimental observation of this spectral feature in nanoparticles extracted from silicalite-1 clear solutions supports their structured nature. The theoretical insights on the dependency of pentasil vibrations with the degree of condensation offer valuable support toward future investigations on the genesis of a zeolite crystal.

Four-Membered Heterocycles with a Carbon−Heteroatom Exocyclic Double Bond at the 3-Position:  Puckering Potential and Thermodynamic Properties

P. Vansteenkiste, V. Van Speybroeck, G. Verniest, N. De Kimpe, M. Waroquier
Journal of Physical Chemistry A
111 (14), 2797-2803
2007
A1

Abstract 

Despite the specific importance of four-membered heterocycles with a carbon−heteroatom double bond at the 3-position in organic and medicinal chemistry, little attention has been given up to now to theoretical computational studies on these molecules. However, the overall geometry, and degree of ring puckering especially, could significantly influence the reactivity and biological properties of these four-membered ring compounds. In this paper, focus is made on the influence of different substituents on the equilibrium geometry, ring puckering potential, and thermodynamic quantities. It was found that these properties are mainly affected by the heteroatom (oxygen, nitrogen, sulfur, phosphorus) contained in the ring skeleton. Moreover, the correct description of the puckering potential with the hindered rotor treatment leads to substantial corrections on the thermodynamic properties in the harmonic oscillator approximation.

Ab initio calculation of entropy and heat capacity of gas-phase n-alkanes with hetero-elements O and S: Ethers/alcohols and sulfides/thiols

P. Vansteenkiste, T. Verstraelen, V. Van Speybroeck, M. Waroquier
Chemical Physics
328 (1-3), 251-258
2006
A1

Abstract 

In this paper, the performance of the one-dimensional hindered rotor approach (1D-HR) is evaluated for n-alkanes with hetero-elements O or S. The internal rotations in these molecules show a behavior distinct from those in n-alkanes, for which 1D-HR is a cost-efficient method to describe the thermochemical features (entropy and heat capacity). It turns out that also for ethers, alcohols, sulfides and thiols this approach gives a satisfactory experimental agreement. This work confirms earlier results, and consolidates the assumption that the 1D-HR model is highly suitable for reproducing thermodynamic properties of single chain molecules, and that multi-dimensional coupled hindered rotor approaches (nD-HR) are not necessarily required for attaining high accuracy. Moreover, it seems that the 1D-HR results are almost independent of the details of the level of theory.

Applicability of the Hindered Rotor Scheme to the Puckering Mode in Four-Membered Rings

P. Vansteenkiste, V. Van Speybroeck, G. Verniest, N. De Kimpe, M. Waroquier
Journal of Physical Chemistry A
110 (10), 3838-3844
2006
A1

Abstract 

The hindered rotor scheme, originally developed for internal rotors in flexible chains (Van Speybroeck, V.; Van Neck, D.; Waroquier, M.; Wauters, S.; Saeys, M.; Marin, G. B. J. Phys. Chem. A 2000, 104, 10939), is extended to puckering motions in four-membered rings. The applicability of the approach is tested in a variety of heterocyclic compounds for which the partition function, entropy, and heat capacity are calculated. The entropy may be substantially altered by a correct description of the puckering mode. The equilibrium puckering angle ranges between 0° and 30° depending on the heterosubstitution X (CH2, O, S, NH, PH, CO, CS, CNH, CPH) in the four-membered ring.

An extended hindered-rotor model with incorporation of Coriolis and vibrational-rotational coupling for calculating partition functions and derived quantities

P. Vansteenkiste, V. Van Speybroeck, D. Van Neck, M. Waroquier
Journal of Chemical Physics
124 (4), 044314
2006
A1

Abstract 

Large-amplitude motions, particularly internal rotations, are known to affect substantially thermodynamic functions and rate constants of reactions in which flexible molecules are involved. Up to now all methods for computing the partition functions of these motions rely on the Pitzer approximation of more than 50 years ago, in which the large-amplitude motion is treated in complete independence of the other (vibrational) degrees of freedom. In this paper an extended hindered-rotor model (EHR) is developed in which the vibrational modes, treated harmonically, are correctly separated from the large-amplitude motion and in which relaxation effects (the changes in the kinetic-energy matrix and potential curvature) are taken into account as one moves along the large-amplitude path. The model also relies on a specific coordinate system in which the Coriolis terms vanish at all times in the Hamiltonian. In this way an increased level of consistency between the various internal modes is achieved, as compared with the more usual hindered-rotor (HR) description. The method is illustrated by calculating the entropies and heat capacities on 1,3-butadiene and 1-butene (with, respectively, one and two internal rotors) and the rate constant for the addition reaction of a vinyl radical to ethene. We also discuss various variants of the one-dimensional hindered-rotor scheme existing in the literature and its relation with the EHR model. It is argued why in most cases the HR approach is already quite successful.

Ab Initio Study of Free-Radical Polymerization: Polyethylene Propagation Kinetics

K. Van Cauter, V. Van Speybroeck, P. Vansteenkiste, M-F. Reyniers, M. Waroquier
ChemPhysChem
7 (1), 131-140
2006
A1

Abstract 

The chain-length dependence of the propagation rate coefficient for the free-radical polymerization of ethylene was investigated on an ab initio basis. Polyethylene was chosen as a test system because of its structural simplicity. Ab initio density functional theory at the B3LYP/6-31g(d) level was applied to study the kinetics of a set of addition reactions of a systematically growing radical alkyl chain to ethylene. These reactions are propagation steps in the free-radical polymerization of ethylene. Special attention was paid to low normal modes corresponding to internal rotations (IR), since the latter are important for an accurate description of the partition functions. The effect of coupling of the IR modes is also discussed. A comparison is made with the propagation rate constant derived from experiment. The results indicate that the propagation rate coefficient has largely converged by the hexyl radical stage, though a weaker chain-length dependence of kpfor longer chains was detected.

Rules for Generating Conformers and Their Relative Energies in n-Alkanes with a Heteroelement O or S:  Ethers and Alcohols, or Sulfides and Thiols

P. Vansteenkiste, E. Pauwels, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry A
109 (42), 9617–9626
2005
A1

Abstract 

With the aid of density functional theory calculations, all conformers of several single-chain alcohols, thiols, ethers, and sulfides are investigated. Starting from earlier computational works on n-alkanes, we construct an extended set of general rules for predicting the number and occurrence of conformers in these oxygen- or sulfur-containing compounds. In alcohols and thiols, it is found that only the conformers generated by internal rotations in the HXCH2CH2CH2 (X = O or S) top are distinctive from those in n-alkanes. In ethers and sulfides, the primary influence of the heteroelement also extends up to three internal rotations, but many more conformers are possible. However, a number of double gauche sequences are forbidden, and therefore, several conformers can be eliminated. These exclusions in particular make up a set of rules for eventually deducing all possible conformers. Furthermore, on the basis of only an exact calculation of these gg conformations in addition to single gauche conformers, it is possible to make an accurate estimate of the relative energy. This two-dimensional approximation scheme constitutes an effective tool for adequately describing the relative energies of all possible conformers at a minimal computational cost.

How should we calculate multi-dimensional potential energy surfaces for an accurate reproduction of partition functions?

P. Vansteenkiste, V. Van Speybroeck, E. Pauwels, M. Waroquier
Chemical Physics
314 (1-3), 109-117
2005
A1

Abstract 

The potential energy of n-hexane is studied since it constitutes a typical example of a single chain molecule in which various internal rotations are present and a large number of conformations are existing, which cannot be reached by using one-dimensional rotational energy profiles. For an accurate reproduction of the global partition function and all derived thermodynamic properties an adequate description of all possible conformers is necessary. The full three-dimensional potential energy surface of the internal rotations in n-hexane (3D-PES) is calculated at an ab initio level and compared with one-dimensional schemes to reproduce the energy. Due to the higher dimensionality of the relevant potential energy surface, the computational cost is very high. A new approximate scheme based on two-dimensional cuts is proposed that gives good accuracy for the relative conformational energies and kinetic energies at a reasonable computational cost. This scheme is of general use for any long chain molecule.

Why does the uncoupled hindered rotor model work well for the thermodynamics of n-alkanes?

V. Van Speybroeck, P. Vansteenkiste, D. Van Neck, M. Waroquier
Chemical Physics Letters
402 (4-6), 479 - 484
2005
A1

Abstract 

In this Letter, we unravel the origin of the good-behavior of the one-dimensional hindered rotor model to describe the partition function and derived thermodynamic properties of n-alkanes. The simplified uncoupled model predicts entropies of n-alkanes up to decane with a standard deviation less than 1% (P. Vansteenkiste, V. Van Speybroeck, G.B. Marin, M. Waroquier, J. Phys. Chem. A 107 (2003) 3139). Application of a fully coupled scheme for the internal rotations present in pentane and hexane gives a justification of the success of the uncoupled hindered rotor model based on microscopic grounds. The success of the separable rotor model is due to fortuitous cancellation of errors and cannot be generalized.

Pagina's

Abonneren op RSS - P. Vansteenkiste