Glauciete S. Maciel

Experimental benchmarks and phenomenology of interatomic forces: open-shell and electronic anisotropy effects

This article gives a perspective view of some representative experimental information available on interatomic forces. They play a role in gaseous properties, but modern quantitative information comes from spectroscopy and molecular beam scattering. This latter technique is emphasized here: recent experimental results and consideration of physical properties of interacting species is complementary to progress of modelling based on ab initio or other quantum chemical calculations. Interactions involved in closed-shell–closed-shell species are considered to be typical of the so-called ‘non-covalent’ forces, although additional effects of a ‘chemical’ nature are demonstrated to be non-negligible in some cases. The partition of the interaction into van der Waals (repulsion + dispersion) and possibly electrostatic and/or induction components is analysed. Interactions involving open-shell species offer a most interesting phenomenology, because electronic anisotropy often provides further strength to the bonds, which are usually weaker than ordinary chemical bonds. Again, the focus is on experimental information (especially on scattering of magnetically analysed open-shell atoms) and on the understanding that comes from the analysis of the ample phenomenology accumulated. Additional terms such as those of specific ‘covalent’ nature appear in the partition of the interaction, besides those already mentioned. The extension of this approach for describing molecular anisotropies is also outlined. Contents PAGE 1.  Introduction  1.1. Motivation and dedication 166  1.2. Scope and outline of the paper 167 2.  Isotropic interactions and van der Waals forces 168  2.1. 2 S +1S atom– 168  2.2. Combination rules and correlation formulas 170  2.3. 2 S +1S Ion– 172 3.  Anisotropic interactions and open-shell effects 175  3.1. General 175  3.2. 2 S +1P atom– 176  3.3. Charge transfer and bond stabilization 178  3.4. 2 S +1P ion – 180  3.5. Dications 181 4.  Final remarks 182  4.1.  Towards atom–molecule and molecule–molecule interactions 182  4.2.  Prospects for future work 184  4.3.  From van der Waals interactions to chemical bonds 185 Acknowledgements 185 Appendix A– Interatomic forces by molecular beam scattering 185 Appendix B–Basic contributions to the interatomic interactions and their dependence on physical properties f involved species 188 Appendix C–Electronic anisotropy and orbital alignment 192 References

Quantum Chemistry of C3H6O Molecules: Structure and Stability, Isomerization Pathways, and Chirality Changing Mechanisms

Electronic structure calculations were carried out to study the various isomers of formula C(3)H(6)O, as a part of our current quantum chemical and dynamical approaches to intra- and intermolecular kinetics for the C(n)H(2n)O (n = 1, 2, 3) molecules. The usefulness of the GRRM (global reaction route mapping) program developed by Ohno and Maeda in predicting the structure of all isomers and of the transition states connecting them is fully exploited. All the isomers are identified as local minima on the MP2/CC-PVDZ potential energy surface. Acetone is the most stable isomer. In increasing order of stability the others are propanal, 2-propenol, 1-propenol, allyl alcohol, methyl vinyl ether, cyclopropanol, propylene oxide, and oxetane. Various isomerization paths connecting them are identified. All the transition states are fully characterized using intrinsic reaction coordinate calculations. The isomerization reactions may proceed through a single step or involve an intermediate species which is either a carbene or a diradical. Special attention is devoted to propylene oxide, a favorite molecule in current photochemical and stereodynamical studies because of its chiral nature. It is a rigid molecule, and chirality switching is found to be supported by its isomers. Two different chirality switching mechanisms which are assisted by propanal and allyl alcohol are presented.

A quantum chemical study of H2S2: Intramolecular torsional mode and intermolecular interactions with rare gases

The structural and energetic properties of the H(2)S(2) molecule have been studied using density functional theory, second-order Moller-Plesset method, and coupled cluster theory with several basis sets. In order to extend previous work on intra- and intermolecular dynamics of the chirality changing modes for H(2)O(2) and its derivatives, our focus has been on the torsion around the S-S bond, along with an extensive characterization of the intermolecular potentials of H(2)S(2) with the rare gases (He, Ne, Ar, and Kr). Use is made of previously defined coordinates and expansion formulas for the potentials which allow for a faithful representation of geometrical and symmetry properties of these systems that involve the interaction of an atom with a floppy molecule. The potential energy surfaces obtained in this work are useful for classical and quantum mechanical simulations of molecular collisions responsible for chirality changing processes of possible interest in the modeling of prebiotic phenomena.

The origin of chiral discrimination: supersonic molecular beam experiments and molecular dynamics simulations of collisional mechanisms

The target of the present paper is the study of chirality effects in molecular dynamics from both a theoretical and an experimental point of view under the hypothesis of a molecular dynamics mechanism as the origin of chiral discrimination. This is a fundamental problem per se, and of possible relevance for the problem of the intriguing homochirality in Nature, so far lacking satisfactory explanations. We outline the steps that have been taken so far toward this direction, motivated by various experimental studies of supersonic molecular beams carried out in this laboratory, such as the detection of aligned oxygen in gaseous streams and further evidence on nitrogen, benzene and various hydrocarbons, showing the insurgence of molecular orientation in the dynamics of molecules in flows and in molecular collisions. Chiral effects are theoretically demonstrated to show up in the differential scattering of oriented molecules, also when impinging on surfaces. Focus on possible mechanisms for chiral bio-stereochemistry of oriented reactants may be of pre-biotical interest, for example when flowing in atmospheres of rotating bodies, specifically the planet Earth, as well as in vortex motions of celestial objects. Molecular dynamics simulations and experimental verifications of the hypothesis are reviewed and objectives of future research activity proposed.

Elementary Processes in Atmospheric Chemistry: Quantum Studies of Intermolecular Dimer Formation and Intramolecular Dynamics

Abstract The present article provides an account of recent progress in the use of quantum mechanical tools for understanding structure and processes for systems of relevance in atmospheric chemistry. The focus is on problems triggered by experimental activity in this laboratory on investigations of intermolecular interactions by molecular beam scattering. Regarding the major components of the atmosphere, results are summarized on dimers (N 2 –N 2 , O 2 –O 2 , N 2 –O 2 ) where experimental and phenomenogically derived potential energy surfaces have been used to compute quantum mechanically the intermolecular clusters dynamics. Rovibrational levels and wave functions are obtained, for perspective use in atmospheric modelling, specifically of radiative absorption of weakly bound complexes. Further work has involved interactions of paramount importance, those of water, for which state-of-the-art quantum chemical calculations for its complexes with rare gases yield complementary information on the interaction (specifically the anisotropies) with respect to molecular beam scattering experiments that measure essentially the isotropic forces. Similar approaches and results have been pursued and obtained for H 2 S. Stimulated in part by the interesting problem of large amplitude vibrations, such as the chirality change transitions associated with the torsional motions around O O and S S bonds, a systematic series of quantum chemical studies has been undertaken on systems that play also roles in the photochemistry of the minor components of the atmosphere. They are H 2 O 2 , H 2 S 2 and several molecules obtained by substitutions of the hydrogens by alkyl groups or halogens. Quantum chemistry is shown to have reached the stage of resolving many previously controversial features regarding these series of molecules (dipole moment, equilibrium geometries, heights of barriers for torsion), which are crucial for intramolecular dynamics. Quantum dynamics calculations are also performed to compute torsional levels and the temperature dependence of their distributions.

Screens for Displaying Chirality Changing Mechanisms of a Series of Peroxides and Persulfides from Conformational Structures Computed by Quantum Chemistry

A great variety of data on molecular structure and changes, accumulated both experimentally and theoretically, need be compacted and classified to extract the information arguably relevant to understand the basic mechanisms of chemical transformations. Here a screen for displaying four-center processes is developed and as an illustration applied to conformations involving torsions around O – O and S – S bonds, extending the structural properties previously calculated in this laboratory. The construction of the screen follows from connections recently established between the classical kinematic mechanism – the four-bar linkage – and the basic ingredient of quantum angular momentum theory – the 6j symbol.

Structural Properties and Torsional Dynamics of Peroxides and Persulfides

For the study of molecules containing O—O and S—S bonds, an analysis on the effect of level of theory and basis sets on electronic properties and geometrical parameters for H2O2 and H2S2 was done. Substitutions of one or both hydrogens in these molecules either by halogen atoms or alkyl groups were investigated for properties like geometries, dipole moments, cis and trans barriers. Attention has also been dedicated to the study of energy levels in the very anharmonic torsional potentials, obtaining their distributions as a function of temperature and partition functions for the torsional motion, of relevance for the isomerization rate leading to exchange between chiral enantiomers. Estimated rates both for underbarrier tunnelling and overbarrier transitions are consistently smaller for the S—S cases with respect to the corresponding O—O ones, due to the generally higher barriers. Regarding intermolecular interactions, of specific importance for collisional chirality exchange, an exploration was done for both H2O2– and H2S2–rare gases systems, extending the joint experimental and theoretical approach already tackled in this laboratory for interactions of H2O and H2S with the rare gases.

The spherical-harmonics representation for the interaction between diatomic molecules: The general case and applications to COCO and COHF

Abstract The spherical-harmonics expansion is a mathematically rigorous procedure and a powerful tool for the representation of potential energy surfaces of interacting molecular systems, determining their spectroscopic and dynamical properties, specifically in van der Waals clusters, with applications also to classical and quantum molecular dynamics simulations. The technique consists in the construction (by ab initio or semiempirical methods) of the expanded potential interaction up to terms that provide the generation of a number of leading configurations sufficient to account for faithful geometrical representations. This paper reports the full general description of the method of the spherical-harmonics expansion as applied to diatomic-molecule – diatomic-molecule systems of increasing complexity: the presentation of the mathematical background is given for providing both the application to the prototypical cases considered previously (O2 O2, N2 N2, and N2 O2 systems) and the generalization to: (i) the CO CO system, where a characteristic feature is the lower symmetry order with respect to the cases studied before, requiring a larger number of expansion terms necessary to adequately represent the potential energy surface; and (ii) the CO HF system, which exhibits the lowest order of symmetry among this class of aggregates and therefore the highest number of leading configurations.