Yannick Carissan

Huckel-Lewis Projection Method: A "Weights Watcher" for Mesomeric Structures †

A new approach to extract the coefficients and weights of Lewis structures from the Hückel wave function is designed: Hückel-Lewis projection (HL-P). The weights are obtained by projection on overlapping Lewis structures. This straightforward alternative to ab initio approaches is detailed and used on typical cases, including acrolein, allyl radical, pyrrole-like systems, and imidazolylidene. A trust parameter is defined and shown as a guide to retrieve the most important Lewis structures. The emblematic examples of butadiene and benzene are chosen to illustrate the use of this parameter.

On the Catalysis of the Cycloisomerization of 1,6-Dienes with Tin(IV) Salts: The Important Role of a Water Molecule

A diene cycloisomerisation reaction catalysed by tin(IV) triflimidate is studied by using DFT computations. It is proposed that the mechanism does not involve the direct addition of the tin(IV) cation to a double bond because the catalyst regeneration step would be energetically unfeasible. We show that a water molecule may play a decisive role to enable the smooth completion of the catalytic cycle. The proposed active catalyst is thus a hydrated triflimidate salt. The hydrolysis and hydration energies are computed for three ligands of SnL4, L=triflate (OTf), triflimidate (NTf2) and a chlorosulfonate model (OSO2Cl). The diastereoselectivity observed in the cycloisomerisation is discussed in light of the transition‐state geometries. The hypothesis of hidden Brønsted acid catalysis is discussed and ruled out on the basis of new experimental results.

trans-1,2-Disiloxybenzocyclobutene, an adequate partner for the auto-oxidation: EPR/spin trapping and theoretical studies

The auto-oxidation of trans-1,2-disiloxybenzocyclobutene was found to be very efficient, giving endo-peroxide in quantitative yield. Each step of the mechanism of spin-forbidden addition of triplet oxygen O2((3)Σg) was studied by both EPR/spin trapping and theoretical studies.

Labile ligands on some Lewis super acids: a computational study

Lewis super-acids, like Sn(OTf)(4) or In(OTf)(3), OTf = CF(3)SO(3)(-), are efficient catalysts for cycloisomerization reactions. We report here a study on the coordination of ligands (or of solvent molecules) around such catalysts at density functional theory level. Using the Sn-based compound as an example, we consider the interaction with several molecules, usually nearby in reaction conditions. These include nitromethane, esters and diesters, dimethylsulfoxide, and so on. We establish a ladder of interaction strengths and show that dimethylsulfoxyde molecules can displace triflate ligands from the metallic center. This leads to the main conclusions that solvent molecules directly coordinate Sn centers and that non-coordinating triflates are potentially free in solution despite their anionic characteristic.

The interaction of beryllium with benzene and graphene: a comparative investigation based on DFT, MP2, CCSD(T), CAS-SCF and CAS-PT2.

The interaction of beryllium with benzene, graphene and graphitic compounds involves multi-reference electronic states, Jahn-Teller distortion, charge transfer and van der Waals interactions. This is investigated herein using periodic and molecular models at different levels of theory: (i) the second-order Møller-Plesset (MP2) perturbation theory, (ii) the coupled cluster method with inclusion of single double and perturbative triple excitations (CCSD(T)), (iii) the complete active space self-consistent field (CAS-SCF) and (iv) the complete active space with perturbation theory truncated at the 2nd order (CAS-PT2). Molecular and periodic Density Functional Theory (DFT) methods are also used. The two major failures of DFT are addressed with regard to the beryllium benzene and graphene interaction: the degeneracy problem is the source of no specific problem while the delocalization error causes DFT with the Perdew Burke, Ernzerhof functional plus the Grimme correction (DFT/PBE-D2) to be over-binding by about 0.4 eV at a short-range. The agreement between DFT/PBE-D2 and wave-function based methods is nevertheless good; DFT/PBE-D2 provides an accurate description of the electronic structure of the system. By the end of this work, we shall get a better insight into the mechanisms leading beryllium to physisorb on graphene and to chemisorb into the bilayer of graphite.

Recasting wave functions into valence bond structures: A simple projection method to describe excited states

A method is proposed to obtain coefficients and weights of valence bond (VB) determinants from multi configurational wave functions. This reading of the wave functions can apply to ground states as well as excited states. The method is based on projection operators. Both energetic and overlap‐based criteria are used to assess the quality of the resulting VB wave function. The approach gives a simple access to a VB rewriting for low‐lying states, and it is applied to the allyl cation, to the allyl radical and to the ethene (notably to the V‐state). For these states, large overlap between VB and multi reference wave functions are easily obtained. The approach proves to be useful to propose an interpretation of the nature of the V‐state of ethene.

Pseudopotentials for hybridized carbon atoms

Pseudopotentials for hybridized atoms are extracted and combined. The study focuses on sp2 carbon atoms and their combination to give rise to a π electronic system. As electrons of interest are treated explicitly, any ab initio method can be used, thus, configuration interaction methods and time‐dependent density functional theory are used and compared. All electron and pseudopotential calculations are in good agreement for electronic transition spectroscopy (0.2 eV difference), geometrical parameters (error of 0.8 pm), and reaction energies.

Theoretical Studies of Autoxidation of 2-Alkylidene-1,3-cyclohexadione Leading to Bicyclic-Hemiketal Endoperoxides

Mechanism of the addition of molecular oxygen on the dienolic form of the 2-alkylidene-1,3-cyclohexadione was investigated by quantum chemical calculations using the approximate projection method developed by Yamaguchi. The complete reaction pathway of the formation of the endoperoxide is described. The crossing between triplet and singlet potential energy surfaces has been located. A multireference complete active space self-consistent field calculation has been performed to strengthen the results.

HuLiS, a program to teach mesomerism and more

The HuLiS program is presented as a tool to decompose a delocalized wave function as a linear combination of localized electronic structures. The principles of the energy based HL-CI and the overlap based schemes HL-P are developed. The results obtained for a set of 10 relevant organic molecules are shown to compare very well with high level quantum chemistry calculations. A trust factort is introduced and its use is shown on the allyl radical case where symmetry must be taken into account.