Tomás L. Sordo

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.

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,… ).

Rearrangement of azirine intermediates to nitriles: Theoretical study of cleavage of 3,4‐dihydro‐1aH‐azirine[2,3‐c]pyrrol‐2‐one to cyanoketene–formaldimine complex

A detailed investigation of the reaction path for the thermal rearrangement of 3,4‐dihydro‐1aH‐azirine[2,3‐c]pyrrol‐2‐one to yield a cyanoketene–formaldimine complex is carried out at the MP2/6‐31G* and B3LYP/6‐31G* levels of theory. The ring opening of the five‐membered pyrrolinone ring and the formation of the nitrile group takes place in a concerted manner, presenting a significant strain energy release and allowing for an electronic stabilization by coarctate conjugation of the transition structure (TS). These two factors make possible a moderate energy barrier. Although the structural features B3LYP/6‐31G* theoretical levels, it is found that the MP2 energy barrier (28.8) CCSD(T)/6‐31G*//MP2/6‐31G* value (17.1 kcal/mol). The complex electronic rearrangement can be rationalized using the theory of coarctate transition structures developed by Herges as the evolution of an azirine structure without referring to a hypothetical vinyl nitrene intermediate. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 912–922, 1998

DOES A COOPERATIVE MECHANISM EXPLAIN THE UNUSUALLY LARGE GAS-SOLID STRUCTURE DIFFERENCES FOUND FOR HCN...BF3 ?

Abstract Ab initio calculations, at the MP2/6-31G ∗∗ level, suggest that a cooperative mechanism seems to play an important, though not decisive, role in explaining the short B-N distance and the large out-of-plane distortion of the BF 3 in the HCN…BF 3 crystal.

THEORETICAL STUDY OF THE GAS-PHASE ADDITION OF HF AND HCL TO ETHYLENE: ANALYSIS OF THE CATALYTIC ACTION OF DIMERIC HALIDES

Ab initio MP2/6‐31G** calculations render the same mechanism for the gas‐phase addition of HF and HCl to ethylene in contrast with previous HF/3‐21G calculations. The leading interaction is in both cases the electrophilic attachment of the hydrogen atom in the hydrogen halide to a carbon atom in ethylene. The presence of a second molecule of hydrogen halide causes a catalytic effect by allowing an alternative mechanism for electron density rearrangement through ethylene polarization.

Water‐Assisted Alkaline Hydrolysis of Monobactams: A Theoretical Study

A theoretical study of the water-assisted alkaline hydrolysis of 2-azetidinone, 3-formylamino-2-azetidinone and 3-formylamino-2-azetidine-1-sulfonate ion is carried out at the B3LYP/6-31+G* level. The effect of bulk solvent is taken into account using the PCM solvation model while specific solvent effects are represented by the inclusion of an ancillary water molecule along the reaction profile. The calculated free energy barriers in solution are in reasonable agreement with experimental values. The observed substituent effects due to the presence of the 3-formylamino and the SO(3) groups attached to the beta-lactam ring are crucial factors determining the hydrolysis of monobactam antibiotics.

Ammonolysis and Aminolysis ofβ-Lactams: A Theoretical Study

A comparative quantum chemical study of the uncatalyzed ammonolysis of 2-azetidinone and N-methylacetamide reveals some interesting details of the ring-opening of the four-membered cycle and the role played by the strain energy in the process. The most favorable route is a stepwise mechanism through tetrahedral intermediates to give the product of an anti 1,2 addition (see diagram).

Ab initio study of the formation of C3H3+ from the reaction of CH3+ with acetylene

Ab initio molecular orbital theory has been used to study the mechanism of the formation of C3H3+ from the reaction of CH3+ with acetylene. The highest level geometry optimizations and frequencies were computed at MP2‐FC/6‐31G**; single point energies of all the critical structures were computed to the MP4‐FC/6‐31G**//MP2‐FC/6‐31G** theory level. One of the three alternative transition structures leading to the formation of C3H3+ gives the cyclopropenyl cation and the other two the propargyl cation. The proportions of C3H2D+ and C3HD2+ obtained when CD3+ reacts with acetylene, and the composite nature of the metastable peak observed for the [C3H5]+→[C3H3]+ + H2 fragmentation are explained by assuming a different degree of deuterium scrambling depending on the energy of the system.

An alternative method to assess the quality of hartree-fock-roothaan wave functions

Abstract The different contributions of the Breit Hamiltonian as well as those corresponding to the electric quadrupole and magnetic octupole interactions have been computed using several Slater-Type basis sets. These calculations allow one to assess the quality of the basis sets in regions of configuration space other than those emphasized by the operators appearing in the classical (non-relativistic) Hamiltonian. The results confirm the usefulness of the δ test (L.A.G. Bernardo and J.A. Sordo, J. Chem. Phys. 85 (1986) 1475) for measuring the quality of a given basis set.

Zn2+ catalysed hydrolysis of β-lactams: experimental and theoretical studies on the influence of the β-lactam structure

We present both experimental and theoretical results on simple model systems of zinc-β-lactamases. Kinetic studies show that the rate of degradation of β-lactam antibiotics in the presence of zinc ions and tris(hydroxymethyl)aminomethane buffers depends markedly on the structure of the β-lactam. Carbapenems are highly reactive whereas monobactam antibiotics like aztreonam, which are known to be non-susceptible to the catalytic action of the metallo-β-lactamases, are less reactive by three orders of magnitude. To complement the experimental studies, density functional calculations were carried out on model systems. These calculations allowed us to characterise the reactive mode of binding between the β-lactam nucleus and Zn2+ ions as well as to rationalise the kinetic trends observed experimentally. Docking analyses are reported for the complex formed between aztreonam and the mononuclear metallo-β-lactamase from Bacillus cereus. On the basis of all the results, we hypothesise that the aztreonam–metallo-β-lactamase complex might be poorly reactive due to a potential interaction of the N-sulfonate group of aztreonam with the essential Zn ion at the active site.