Roger Arnaud

Diels–Alder reactions: An assessment of quantum chemical procedures

Kinetic and thermodynamic parameters for the Diels–Alder reactions of butadiene with ethylene, formaldehyde, and thioformaldehyde have been computed at a variety of levels of theory with the aim of identifying the most suitable approach for general application. Especially in the presence of heteroatoms, inclusion of electron correlation significantly modifies the geometries of stationary points obtained at the Hartree–Fock level. However, energetic quantities are not very sensitive to the employed geometries provided that the structures of all the species are optimized at the same computational level. On the other hand, both extended basis sets and inclusion of most of the dynamical correlation are needed to obtain reliable energetic quantities. In particular, converged results are obtained only using a triple zeta contraction for the valence space and double sets of polarization functions on nonhydrogen atoms, whereas f functions and hydrogen polarization functions play a minor role. Among cheaper approa...

Study of prototypical Diels-Alder reactions by a hybrid density functional/Hartree-Fock approach

Abstract Kinetic and thermodynamic parameters for the Diels-Alder reactions of butadiene with ethylene, formaldehyde and thioformaldehyde have been computed by a density functional method (B3LYP) which incorporates gradient corrections and some Hartree-Fock exchange. Post Hartree-Fock computations have also been performed for comparison. The activation energies computed for the addition of C 2 H 4 at the B3LYP and QCISD[T] levels approach the experimental value, whereas HF and MP2 approaches are not sufficiently reliable. The computed activation energy for H 2 CO is close to that for C 2 H 4 by the B3LYP approach, whereas it significantly decreases at MP2. QCISD[T] computations give an intermediate result. A low activation energy is computed for H 2 CS, in agreement with observations that thioaldehydes and activated thioketones rapidly undergo hetero-Diels-Alder reactions at or below room temperature. The B3LYP result is closer to the QCISD[T] reference than the MP2 value. These trends have been interpreted by interactions between frontier orbitals or by energy partitioning based on natural bond orbitals.

Ab-initio mechanistic studies of radical reactions. Directive effects in the addition of methyl radical to unsymmetrical fluoroethenes

Addition of methyl radical to unsymmetrical fluoroethenes has been studied by ab-initio molecular orbital calculations. In agreement with experimental data, we find that the reaction rate decreases in going from ethene to mono- and 1,1 di-fluoroethene, but sharply increases in the case of trifluoroethene. Additions to the more fluorinated carbon atoms are always thermodynamically favoured, but addition to the CH2 end of mono- and 1,1 di-fluoroethene is kinetically favoured. The general trends of the potential energy barriers have been rationalized by means of the energy decomposition scheme proposed by Morokuma. Non potential energy effects have also been considered, but their role is negligible.

Ab initio mechanistic studies of radical reactions. Addition of methyl radical to acetylene and ethylene

Abstract Addition of methyl radicals to ethylene and acetylene has been studied by ab initio molecular-orbital calculations. In agreement with experimental data, we find that, although addition to ethylene is characterized by a lower activation energy, addition to acetylene is faster due to the opposite and larger trend of pre-exponential factors. The reaction barriers have been analyzed by the energy decomposition scheme proposed by Morokuma.

Etude théorique de la conformation des complexes moléculaires de type donneur accepteur d'électrons

The stabilisation energy of donor acceptor molecular complexes given by tetracyanoethylene with thiazole and phenylthiazoles have been calculated by several semiempirical approaches. The results obtained with the Extended Hückel theory, the CNDO/2 method and the PCILO method, the latter a perturbative one, are compared and this comparison shows that the PCILO method is well adapted to the study of molecular complexes. The different contributions to the stabilisation energy and the origin of the intermolecular forces have been discussed in relation to the geometry of the complex. For all the studied complexes, the calculated results obtained by the PCILO method agree very well with experiments.

Regioselectivity of methyl radical addition to fluoroethenes: a quantum mechanical study

Abstract The addition of the methyl radical to mono-, di- and tri-substituted alkenes has been studied by both post-Hartree–Fock and hybrid Hartree–Fock/density functional (B3LYP) methods. The B3LYP model confirms its reliability concerning activation energies and reaction enthalpies, but fails to reproduce the inversion of regioselectivity along the fluoroethene series. Only coupled cluster computations are completely reliable, but single-point computations at B3LYP geometries provide results close to those issuing from much more expensive coupled cluster geometry optimizations. The origin of regioselectivity trends is analysed in terms of spin densities and deformation energies.

Thioacrolein S-oxide

Abstract Thioacrolein S -oxide was synthesized for the first time and characterized by low temperature NMR spectroscopy. Theoretical calculations about this unsaturated sulfine and two of its possible cyclic isomers were performed.

The rearrangement of α-imino-thioaldehydes into dihydro-1,3-thiazoles

Abstract α-Imino-thioaldehydes were generated by retro-Diels-Alder reaction under flash vacuum thermolysis conditions. They were found to be unstable and cyclized to 2,3-dihydro-1,3-thiazoles. This cyclization was investigated by ab initio calculations.

Hydrogen abstraction from ethylbenzene by imide-N-oxyl radicals with and without O2: a DFT theoretical study

In this paper, we report a theoretical study of the hydrogen abstraction reactions from ethylbenzene by a series of imide-N-oxyl radicals. Geometry optimizations and vibrational frequencies were performed using density functional theory at the B3LYP/6-31G(d,p) level. Single-point energy calculations were carried out at the PMP2/6-31G(d,p) and B3LYP/6-311+G(2df,2p) levels. Calculations reproduce experimental trends. In the absence of dioxygen, calculated barriers are not too high to prevent H-abstraction but the process is endothermic. The factors governing the reactivity of nitroxide radicals have been discussed in the scope of the state correlation diagram approach. Moreover, the influence of dioxygen on the mechanism of these reactions has also been studied. Thus, the addition of dioxygen occurs after the H-abstraction by nitroxide radicals and no clear evidence for an energetic barrier to O2 addition was found. However, in the presence of dioxygen the whole process is exothermic and thus H-abstraction becomes irreversible.

Quantum mechanical study of regioselectivity of radical additions to substituted olefins

Regiochemical trends in the addition of free radicals to substituted olefins are investigated by different quantum chemical approaches with special reference to oxygen centered radicals. From a methodological point of view, density functional methods provide correct general trends but they do not reach quantitative accuracy, especially for intermediate complexes. More reliable results are obtained by single point post‐Hartree–Fock computations at density functional geometries. A number of test computations show that reoptimization of the geometry and computation of vibrational frequencies by correlated methods can be safely avoided. As a consequence, the overall computational approach has very reasonable computer costs. From a more chemical point of view, a careful analysis of computational results points out the significant role of anomeric and polar effects in tuning the common filicity of carbon centered radicals.