Soledad Gutiérrez-Oliva

A new perspective on chemical and physical processes: the reaction force

The reaction force F(R) of a chemical or physical process is the negative derivative of the systems potential energy V(R) along the reaction coordinate. The features of F(R) – its maxima, minima and zeroes – divide the process into well-defined stages which can, in general, be characterized in terms of changes in structural and/or electronic properties. This has been demonstrated for bond dissociation/formation and for reactions that have activation barriers in both forward and reverse directions. An important aspect of the reaction force is that it naturally and unambiguously divides activation energies into two components, one corresponding to the preparative structural stage of the process and the other to the first phase of the transition to products. It is shown how this can help to elucidate the effect of a solvent or a catalyst upon an activation barrier.

The reaction force and the transition region of a reaction

The reaction force F(R) and the position-dependent reaction force constant κF(R) are defined by F(R)=-∂V(R)/∂R and κ(R)=∂2V(R)/∂R2, where V(R) is the potential energy of a reacting system along a coordinate R. The minima and maxima of F(R) provide a natural division of the process into several regions. Those in which F(R) is increasing are where the most dramatic changes in electronic properties take place, and where the system goes from activated reactants (at the force minimum) to activated products (at the force maximum). κ(R) is negative throughout such a region. We summarize evidence supporting the idea that a reaction should be viewed as going through a transition region rather than through a single point transition state. A similar conclusion has come out of transition state spectroscopy. We describe this region as a chemically-active, or electronically-intensive, stage of the reaction, while the ones that precede and follow it are structurally-intensive. Finally, we briefly address the time dependence of the reaction force and the reaction force constant.

The reaction force : Three key points along an intrinsic reaction coordinate

The concept of the reaction force is presented and discussed in detail. For typical processes with energy barriers, it has a universal form which defines three key points along an intrinsic reaction coordinate: the force minimum, zero and maximum. We suggest that the resulting four zones be interpreted as involving preparation of reactants in the first, transition to products in the second and third, and relaxation in the fourth. This general picture is supported by the distinctive patterns of the variations in relevant electronic properties. Two important points that are brought out by the reaction force are that (a) the traditional activation energy comprises two separate contributions, and (b) the transition state corresponds to a balance between the driving and the retarding forces.

Insights into the chemical meanings of the reaction electronic flux

Abstract The negative derivative of the chemical potential with respect to the reaction coordinate is called reaction electronic flux and has recently focused a wide interest to better understand chemical reactions at molecular level. After much consideration, it is now well accepted that positive REF values are associated with spontaneous processes, while negative REF ones translate unspontaneous phenomena. These characteristics of the REF are based on a thermodynamic analogy and have been shown right through computational results. In this paper, we develop two analytical expressions of the REF in both the canonical and the grand canonical ensembles. The connection between both equations is established. They are then analyzed, and some arguments are put forward to support the alleged characteristic of the REF and its ability to properly discriminate spontaneous from unspontaneous phenomena.

Analysis of two intramolecular proton transfer processes in terms of the reaction force

The negative derivative of the potential energy along an intrinsic reaction coordinate defines a force that has qualitatively a universal form for any process having an energy barrier: it passes through a negative minimum before the transition state, at which it is zero, followed by a positive maximum. We have analyzed two intramolecular proton transfer reactions in terms of several computed properties: internal charge separation, the electrostatic potentials of the atoms involved, their Fukui functions, and the local ionization energies. The variation of each of these properties along the intrinsic reaction coordinate shows a marked correlation with the characteristic features of the reaction force. We present a description of the proton transfer processes in terms of this force.

Towards understanding the molecular internal rotations and vibrations and chemical reactions through the profiles of reactivity and selectivity indices: an ab initio SCF and DFT study

Ab initio SCF and DFT(B3LYP) calculations are performed with 6–311G** basis sets for obtaining insights into molecular internal rotations in HXNX (X = O,S), different vibrational modes in water and double proton transfer reaction in (HONO)2. While chemical reactivity is analyzed in terms of the profile of the global reactivity parameters, such as energy, chemical potential, hardness, polarizability, molecular valency and electrophilicity indices, the site selectivity is understood through the variations in local descriptors, such as the Fukui function and atomic valency. Principles of maximum hardness and molecular valency and the minimum polarizability principle are found to be valid in almost all cases. Rotational isomerization reactions can be better characterized by making use of the maximum hardness principle along with Hammonds postulate. Extremum points in electrophilicity during internal rotations, vibrations and chemical reaction can be located from those of chemical potential and hardness. The Fukui function and atomic valency show inverse behaviour in most cases.

Energy, chemical potential and hardness profiles for the rotational isomerization of HOOH, HSOH and HSSH

A theoretical study is reported of the mechanisms for internal rotation of hydrogen peroxide (HOOH), hydrogen thioperoxide (HSOH) and hydrogen persulphide (HSSH). Calculations at the ab initio HF//6-311G∗∗ and MP2//6-311G∗∗ levels show that these are gauche molecules presenting double-barrier torsional potentials. Important results have been obtained: two different isomerization mechanisms (trans and cis) have been characterized in terms of specific local interactions; the corresponding energy barriers have been classified according to through bond and through space interactions; and the principle of maximum hardness is qualitatively verified in all three molecules.

Reaction force analysis of solvent effects in the addition of HCl to propene.

We have investigated computationally, via reaction force analyses, the addition of HCl to propene, both Markovnikov and anti-Markovnikov, in the gas phase and in chloroform solution. The calculations were carried out at the CCSD(T)/aug-pVTZ//B3LYP/aug-cc-pVDZ level. A particular interest was in the magnitudes of the two components of the activation energies that are defined by the minimum of the reaction force for each process. The total activation energies for Markovnikov and anti-Markovnikov addition are found to be, respectively, 39.7 and 45.9 kcal/mol in the gas phase and 27.1 and 34.9 kcal/mol in chloroform solution. In solution, the first portion of the reaction (prior to the reaction force minimum) involves substantial stretching of the H-Cl bond, which makes that contribution to the total activation energy greater than in the gas phase. However the second part of the activation is much less energy demanding in solution for both the Markovnikov and anti-Markovnikov additions. The overall preference for Markovnikov addition is due to the electrostatic potential of propene favoring the initial approach of the HCl hydrogen to the terminal carbon.

Perspectives on the Reaction Force

Abstract The reaction force F ( R ) and the reaction force constant κ ( R ) are intrinsic and universal properties of any process that can be represented by a potential energy profile V ( R ), where R is a reaction coordinate. F ( R ) is the negative gradient of V ( R ) along R ; κ ( R ) is the second derivative of V ( R ). The minima and maxima of F ( R ), which correspond to the zeroes of κ ( R ), define stages of a process. Some of these are dominated by structural effects within the reacting system, while others are transitions to new species. In a transition stage, the focus is upon that entire region of the reaction coordinate, not upon a single point, a transition state . It is shown that activation energies have two components. Awareness of these can help in understanding reaction mechanisms as well as the roles played by solvents or catalysts. F ( R ) and κ ( R ) depend solely upon the V ( R ) of a process and provide insight into its nature and energetics.

The mechanism of double proton transfer in dimers of uracil and 2-thiouracil - The reaction force perspective

The intermolecular double proton transfer in dimers of uracil and 2‐thiouracil is studied through density functional theory calculations. The reaction force framework provides the basis for characterizing the mechanism that in all cases has been associated to a dynamic balance between polarization and charge transfer effects. It has been found that the barriers for proton transfer depend upon the nature of the acceptor atoms and its position within the seminal monomer. Actually, the change in the nature of the hydrogen bonds connecting the two monomers along the reaction coordinate may favor or disfavor the double‐proton transfer.