Luciano A. Xavier

Bio: Luciano A. Xavier is an academic researcher from University of São Paulo. The author has contributed to research in topics: Ion & Molecule. The author has an hindex of 6, co-authored 14 publications receiving 159 citations.

Recent advances in the energetics and mechanisms of gas-phase ionic reactions

Abstract: Three important aspects are still at the forefront of gas-phase ion chemistry: structure and energetics, mechanisms of gas-phase reactions relevant to condensed phases, and bridging the gap between gas-phase and solution. In this paper, we review some recent results of our laboratories: 1) The use of incoherent infrared radiation to promote multiphoton dissociation of ions is shown to be an efficient and convenient method to determine the energetics of ions and provide structural information of ions. 2) New experimental and theoretical data provide some interesting comparison between nucleophilic reactions at carbon, silicon and germanium centers. In the latter cases, the mechanism involves primarily an attachment-detachment process. 3) The ability to make gas-phase anions attached to neutral molecules provides an interesting approach towards the study of the stability, reactivity, structure and spectroscopy of gas-phase solvated ions.

A SN2 Reaction That Avoids Its Deep Potential Energy Minimum

Abstract: Chemical dynamics trajectory simulations were used to study the atomic-level mechanisms of the OH- + CH3F --> CH3OH + F- SN2 nucleophilic substitution reaction. The reaction dynamics, from the [OH...CH3...F]- central barrier to the reaction products, are simulated by ab initio direct dynamics. The reaction's potential energy surface has a deep minimum in the product exit channel arising from the CH3OH...F- hydrogen-bonded complex. Statistical theories of unimolecular reaction rates assume that the reactive system becomes trapped in this minimum and forms an intermediate, with random redistribution of its vibrational energy, but the majority of the trajectories (90%) avoided this potential energy minimum and instead dissociated directly to products. This finding is discussed in terms of intramolecular vibrational energy redistribution (IVR) and the relation between IVR and molecular structure. The finding of this study may be applicable to other reactive systems where there is a hierarchy of time scales for intramolecular motions and thus inefficient IVR.

Concerted nucleophilic aromatic substitutions

Abstract: Nucleophilic aromatic substitution (SNAr) is one of the most widely applied reaction classes in pharmaceutical and chemical research, providing a broadly useful platform for the modification of aromatic ring scaffolds. The generally accepted mechanism for SNAr reactions involves a two-step addition–elimination sequence via a discrete, non-aromatic Meisenheimer complex. Here we use 12C/13C kinetic isotope effect (KIE) studies and computational analyses to provide evidence that prototypical SNAr reactions in fact proceed through concerted mechanisms. The KIE measurements were made possible by a new technique that leverages the high sensitivity of 19F as an NMR nucleus to quantitate the degree of isotopic fractionation. This sensitive technique permits the measurement of KIEs on 10 mg of natural abundance material in one overnight acquisition. As a result, it provides a practical tool for performing detailed mechanistic analyses of reactions that form or break C–F bonds. Nucleophilic aromatic substitution reactions have long been thought to occur primarily via stepwise mechanisms. New and sensitive methodology for measuring carbon kinetic isotope effects now shows that most such substitutions actually occur through concerted mechanisms.

Rate-Determining Factors in Nucleophilic Aromatic Substitution Reactions

Abstract: Quantum chemical calculations (OPBE/6-311++G(d,p)) have been performed to uncover the electronic factors that govern reactivity in the prototypical S(N)Ar reaction. It was found that intrinsic nucleophilicity--expressed as the critical energy (the energy required for forming the Meisenheimer structure Ph(X)(2)(-)) in the identity substitution reaction X(-) + PhX --> X(-) + PhX (Ph = phenyl)--shows the following approximate trend: NH(2)(-) approximately OH(-) approximately F(-) >> PH(2)(-) approximately SH(-) approximately Cl(-) > AsH(2)(-) approximately SeH(-) approximately Br(-). The periodic trends are discussed in terms of molecular properties (proton affinity of X(-) expressing Lewis basicity of the nucleophile and C(1s) orbital energy expressing Lewis acidity of the substrate) based on a dative bonding model. Furthermore, the stepwise progress of the reactions and the critical structures are analyzed applying energy decomposition analysis. Increased stability, and thereby increased intrinsic nucleophilicity, correlates with decreasing aromatic character of the Meisenheimer structure. This apparent contradiction is explained in consistency with the other observations using the same model.