J.A. Sordo

Lattice defects and magnetic ordering in plutonium oxides: A hybrid density-functional-theory study of strongly correlated materials

Experimental studies of actinide oxides are challenging, and conventional electronic structure calculations fail to qualitatively reproduce the scarce data. We employ a new generation of hybrid density functionals to model a defective plutonium dioxide lattice. The procedure is first tested against stoichiometric bulk PuO2 and Pu2O3, for which predictions agree well with experiment where known. The interstitial oxygen in PuO2.25 is found to be singly charged, consistent with experimental observations and contrary to the O2- previously proposed theoretically.

On the use of the Boys–Bernardi function counterpoise procedure to correct barrier heights for basis set superposition error

Abstract The application of Boys–Bernardis function counterpoise procedure to correct for the basis set superposition error when computing barrier heights associated with transition structures using well-balanced basis sets is analyzed in detail. It is shown that Emsley et al.’s formulation of the counterpoise method including the fragment relaxation terms may involve considerable errors when the relative orientation of the reference fragments is very different in the two chemical species defining the energy barrier.

Pseudorotation motion in tetrahydrofuran: An ab initio study

The use of different models based on experimental information about the observed level splitings, rotational constants, and far-infrared transition frequencies leads to different predictions on the equilibrium geometry for tetrahydrofuran. High-level ab initio calculations [coupled cluster singles, doubles (triples)/complete basis set (second order Moller-Plesset triple, quadrupole, quintuple)+zero-point energy(anharmonic)] suggest that the equilibrium conformation of tetrahydrofuran is an envelope C(s) structure. The theoretical geometrical parameters might be helpful to plan further microwave spectroscopic studies in order to get a physical interpretation of the measurements.

Performance of CCSDT for first row AB/AB− diatomics: Dissociation energies and electron affinities

CCSDT/aug-cc-pVXZ (X=D,T,Q) calculations were performed on the C2, C2−, CN, CN−, O2, and O2− first row diatomic molecules. The inclusion of diffuse functions improves the dissociation energies of the anionic systems by 2.0–3.4 kcal/mol, which is relevant bearing in mind the goal of achieving chemical accuracy. The contribution of the diffuse functions in the case of neutral O2 (0.6 kcal/mol) is by no means negligible in this context. A serious discrepancy between the theoretical prediction and the experimental values available for the dissociation energy of C2− was found. Since the theoretical deficiences commonly ascribed to the CCSDT method (single-reference and spin contamination when using UHF zeroth-order wave functions) cannot be invoked in this case, further experimental work is required to throw some light on the origin of such a discrepancy. The performance of CCSDT for adiabatic electron affinities is excellent in the case of O2/O2− and CN/CN−. For C2/C2−, the observed discrepancies can be expla...

Ruthenium(IV)‐Catalyzed Isomerization of the CC Bond of O‐Allylic Substrates: A Theoretical and Experimental Study

A general mechanism to rationalize Ru(IV) -catalyzed isomerization of the C=C bond in O-allylic substrates is proposed. Calculations supporting the proposed mechanism were performed at the MPWB1K/6-311+G(d,p)+SDD level of theory. All experimental observations in different solvents (water and THF) and under different pH conditions (neutral and basic) can be interpreted in terms of the new mechanism. Theoretical analysis of the transformation from precatalyst to catalyst led to structural identification of the active species in different media. The experimentally observed induction period is related to the magnitudes of the energy barriers computed for that process. The theoretical energy profile for the catalytic cycle requires application of relatively high temperatures, as is experimentally observed. Participation of a water molecule in the reaction coordinate is mechanistically essential when the reaction is carried out in aqueous medium. The new mechanistic proposal helped to develop a new experimental procedure for isomerization of allyl ethers to 1-propenyl ethers under neutral aqueous conditions. This process is an unique example of efficient and selective catalytic isomerization of allyl ethers in aqueous medium.

Theoretical analysis of the role of the solvent on the reaction mechanisms: one-step versus two-step ketene-imine cycloaddition

The effect of correlation energy, basis set size, zero‐point energy (ZPE) correction, and solvation on the reaction mechanism of the ketene–imine cycloaddition reaction has been investigated. The electrostatic solvent effect was studied with a self‐consistent reaction field method in which the solvation energy is obtained using a multipole expansion of the molecular charge distribution. The ab initio results have been analyzed by means of a theoretical method based on the expansion of the MOs of the supermolecule in terms of those of the reactants and the performance of the configuration analysis. In gas phase, due to the correlation energy and/or the ZPE corrections, the reaction is predicted to be a one‐step process. In solution, the stabilization of the charge‐transferred configurations results in the occurrence of a very stable, Zwitterionic intermediate giving a two‐step mechanism.

Desulfinylation of prop-2-enesulfinic acid: experimental results and mechanistic theoretical analysis.

The potential energy surfaces of the desulfinylation of prop-2-enesulfinic acid (13) in CH(2)Cl(2) solution at -15 degrees C have been explored by quantum calculations and analyzed with kinetic data obtained for the reaction in absence or presence of additives. Monomeric 13 adopts a preferred conformation with gauche S=O/sigma(C(1)-C(2) bond pairs and the O-H bond pointing toward C(3). It equilibrates with the more stable dimer (13)(2) (at -15 degrees C) formed by two O-H...O=S hydrogen bonds and in which the S=O/sigmaC(1)-C(2) are gauche also, but the SOH moieties are antiperiplanar with respect to sigma(C(1)-C(2)). Dimer (13)(2) undergoes desulfinylation into propene + SO(2) + 13 following a one-step, concerted mechanism. The preferred transition state is a six-membered, chairlike transition structure (C...S elongation and S-O...H...C(3) hydrogen transfer occur in concert) in which the S=O/sigma(C(1)-C(2)) bonds are gauche (S=O adopt pseudoaxial positions). There are at least 48 transition states, each one defining a different pathway, all with similar calculated free energies (DeltaG(double dagger) = 25.3-28.6 kcal/mol), which makes the bimolecular (autocatalyzed) retro-ene elimination of SO(2) competing (entropy factor) with a monomolecular process for which the transition state (calculated DeltaG(double dagger) = 24.3 kcal/mol) implies only one molecule of sulfinic acid. This agrees with the experimental rate law of the reaction which is first order in the concentration of dimer (13)(2). SO(2), CF(3)COOH, and BF(3) x Me(2)O do not catalyze the reaction. In the presence of an excess of BF(3) x Me(2)O the desulfinylation is completely inhibited due to the formation of a stable tetramolecular complex of type (CH(2)=CHCH(2)SO(2)H x BF(3))(2) (18), for which quantum calculations show that the S=O/sigma(C(1)-C(2)) bonds are antiperiplanar whereas the S-OH/sigma(C(1)-C(2)) bonds are gauche. Independently of the additive, the retro-ene eliminations of SO(2) are calculated to be concerted and have transition states adopting six-membered cyclic structures in which S=O and sigma(C(1)-C(2)) are gauche, the S=O interacting with the additive. Preliminary experiments suggested that the thermodynamically unfavored ene reaction of SO(2) with propene can occur at low temperature using 1 equiv of BF(3).

Ab initio and DFT studies on van der Waals trimers: The OCS · (CO2)2 complexes

Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6‐31G(d,p), cc‐pVXZ (X = D, T, Q), and aug‐cc‐pVDZ] and density functional theory [B3LYP/6‐31G(d,p)] calculations were carried out to study the OCS · (CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel‐like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm−1, respectively, at the CBS‐MP2/cc‐pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many‐body symmetry‐adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three‐body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer.

ANACAL: a program to carry out a configurational analysis of the wave function of reactive systems

Abstract A method to analyze the wave function of a closed-shell composed system in terms of the electronic configuration of its closed-shell components is presented. The molecular orbitals of a supersystem A-B (or A-B-C) are presented as linear combinations of the molecular orbitals of the fragments A, B (or A, B, and C), thus allowing the wave function of a two-fragment (A-B) or a three-fragment (A-B-C) composed system to be interpreted in terms of the electronic configurations of the reactants ( ABC, A + B - C,…,A ∗ BC,… ).

Theoretical approach to the mechanism of reactions between halogen atoms and unsaturated hydrocarbons: The Cl + propene reaction

The potential energy surface for the Cl + propene reaction was analyzed at the MP2 level using Poples 6‐31G(d,p) and 6‐311+G(d,p), and Dunnings cc‐pVDZ and aug‐cc‐pVDZ basis sets. Two different channels for the addition reaction leading to chloroalkyl radicals and five alternative channels for the abstraction reaction leading to C3H  5· + HCl were explored. The corresponding energy profiles were computed at the QCISD(T)/aug‐cc‐pVDZ//MP2/aug‐cc‐pVDZ level of theory. Theoretical results suggest that the previously established mechanism consisting of (1) direct abstraction and (2) addition–elimination steps is instead made up of (1) addition through an intermediate and (2) two‐step abstraction processes. No direct abstraction mechanism exists on the potential energy surface. The kinetic equations derived for the new mechanism are consistent with the pressure dependence experimentally observed for this reaction.