André G. H. Barbosa

On the electronic structure of the diazomethane molecule

The electronic structure and chemical bonding in the diazomethane molecule are investigated using full-valence generalized valence bond (GVB) methods. We point out that an ab initio-based bonding model must correspond directly to a wave function that yields at least qualitatively corrects values for the structural parameters of a molecule, that is, molecular geometry, vibrational frequencies, and dipole moment. However, in the case of diazomethane, when trying to emulate the bonding models proposed in the literature through full-valence GVB wave functions, we found out that all of them are directly associated with optimized molecular geometries that are saddle points in the molecular potential energy surface. This spurious behavior is corrected by a multiconfiguration–self-consistent field (MCSCF) wave function that incorporates an enlarged “pi-like” active space enabling a complete active space self-consistent field (CASSCF) block, with more active orbitals than electrons, together with a “sigma-like” generalized valence bond with restricted configuration interaction (GVB-RCI) block. With this wave function, we are able to generate the best calculated set of harmonic frequencies to date for the diazomethane molecule. The physical effects that are important for the correct description of its electronic and vibrational structure are then discussed using a series of MCSCF wave functions. This result leads to a decomposition of the electronic wave function into diabatic GVB-RCI chemical structures along the CH2 wagging mode illustrating the necessity to understand the chemical bonding in this molecule as a superposition of bonding patterns. Some structural properties of diazomethane and diazocompounds are then successfully analyzed using our model.

Multi-configuration spin-coupled description of organometallic reactions: a comparative study of the addition of RMBr (M = Mg and Zn) to acetone

The large differences in the reactivity between methyl and allyl Grignard reagents and allylzinc bromide in their addition reactions to acetone are theoretically studied. The chemical nature of the transition states is revealed through multi-configuration spin-coupled (MC-SC) calculations. The additional spin-coupled configuration proves to be instrumental in the description of the electronic rearrangements that lead to the reaction. The resulting singly occupied overlapping orbitals indicate clearly that in all cases the transition states present a pronounced carbanion character. The greater reactivity of Grignard reagents is associated with a product-like nature of the transition state, while the zinc reagent presents a reactant-like transition state. Through the analysis of the wave functions around the transition state geometries, we are able to provide unique insights into the electronic inner workings of the alkylation process.

An Alternative Mechanism for the 1,4-Asymmetric Induction in the Stereoselective Addition of (R)-Pantolactone to 2-Phenylpropylketene

An alternative mechanism for the 1,4-assymmetric induction in the stereoselective addition of (R)-pantolactone to 2-phenylpropylketene was theoretically investigated. A mechanism involving an intermolecular proton transfer assisted by two amine molecules was proposed, which considered the active participation of the dimethylethylamine and its ion as proton transfer agents. In the first step, a neutral dimethylethylamine interacts with the seven-membered ring of the enol intermediate. A specific acid-base interaction is established between the hydroxyl group of the enol and the nitrogen atom of the dimethylethylamine. The neutral dimethylethylamine is basic enough to remove the proton. Another protonated dimethylethylamine is considered to donate its proton to the C=C double bond to give the desired products. The stereochemical outcome was defined by the way that the neutral and protonated dimethylethylamine approached to the enol. The diastereoisomeric ratio found is in good agreement with experiments [for (S, R) and (R, R) it is 99:1].

Quantum Mechanics of Many-Electrons Systems and the Theories of Chemical Bond

In this paper we briefly review the basic requirements that must be satisfied by any wave function representing many-electron systems. Following that, we examine the conditions under which the classical concepts of molecular structure, chemical structure and chemical bond can be translated into a quantum-mechanical language. Essential to this aim is the utilization of an independent particle model (IPM) for a many-electron system. In spite of the great popularity of the HF model only Valence-Bond (VB) type wave functions with optimized, singly occupied and non-orthogonal (except when symmetry imposes) localized orbitals, can provide a quantum-mechanical translation of the classical concepts of chemical structure and chemical bond. Moreover, the use of the HF model as the reference IPM gives rise to unphysical effects such as non-dynamic correlation energy and delocalization energy. Finally, the concept of resonance is redefined in a physically meaningful way, being related to point-group or “accidental” degeneracy. For the latter case, a set of selection rules to determine the possible symmetries of the resonant structures is discussed and applied to representative cases, for which Generalized Multistructural (GMS) wave functions are used to recover the full symmetry of the molecule.

The multiconfiguration Spin-Coupled approach for the description of the three-center two-electron chemical bond of some carbenium and nonclassical ions

The intrinsically elusive concepts of electronic “delocalization” and “chemical resonance” are briefly reviewed emphasizing their connection with Spin-Coupled (SC) descriptions of electronic structure. Multiconfiguration Spin-Coupled (MC-SC) calculations are performed to describe the three-center two-electron (3c-2e) bonding in some representative carbenium and nonclassical carbonium ions. Within the MC-SC approach, it is found that these cations present significant electronic energy stabilization when described by more than one valence SC spatial orbital configuration. It is shown that it is necessary to have a superposition of two chemical structures to completely span the orbital valence space of these cations. Two characteristic bonding themes are clearly distinguished. One specific to allyl-type carbenium ions and another specific to the nonclassical carbonium ions. In both situations, the 3c-2e bond is described by two chemical structures. The 3c-2e bond present in these carbocations is described clearly within this conceptual framework. The results point out for the robustness of the Spin-Coupled description in yielding a general picture of bonding, even when considering valence-bond type multiconfiguration effects.

Assessing the Molecular Basis of the Fuel Octane Scale: A Detailed Investigation on the Rate Controlling Steps of the Autoignition of Heptane and Isooctane

N-Heptane and 2,2,4-trimethylpentane (isooctane) are the key species in the modeling of ignition of hydrocarbon-based fuel formulations. Isooctane is knock-resistant whereas n-heptane is a very knock-prone hydrocarbon. It has been suggested that interconversion of their associated alkylperoxy and hydroperoxyalkyl species via hydrogen-transfer isomerization reaction is the key step to understand their different knocking behavior. In this work, the kinetics of unimolecular hydrogen-transfer reactions of n-heptylperoxy and isooctylperoxy are determined using canonical variational transition-state theory and multidimensional small curvature tunneling. Internal rotation of involved molecules is taken explicitly into account in the molecular partition function. The rate coefficients are calculated in the temperature range 300-900 K, relevant to low-temperature autoignition. The concerted HO2 elimination is an important reaction that competes with some H-transfer and is associated with chain termination. Thus, the branching ratio between these reaction channels is analyzed. We show that variational and multidimensional tunneling effects cannot be neglected for the H-transfer reaction. In particular, the pre-exponential Arrhenius fitting parameter derived from our rate constants shows a strong dependence on the temperature, because tunneling increases quickly at temperatures below 500 K. On the basis of our results, the existing qualitative model for the reasons for different knock behavior observed for n-heptane and isooctane is quantitatively validated at the molecular level.

Theoretical studies on structure and dynamics of carbonium ions

Modem valence bond (VB) theories such as Spin-Coupled theory, together with DFT and Molller-Plesset MO methods, and ab initio molecular dynamics, were employed to study structure/dynamics in representative carbonium ions.