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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., 13 figures, 5 tables, special issue in honor of Walther Caminati

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.

Investigations of the stereodynamics of elementary processes provide the background for perspective demonstration of the manifestation of chiral effects in molecular collisions. Advances in experimental vacuum technology allow the generation of molecular beams containing oriented molecules. Here, we focus on collisional alignment, a molecular polarization phenomenon occurring in supersonic expansions of gaseous mixtures. The underlying physical mechanisms, relevant gas-phase experiments and molecular dynamics simulations are illustrated, with reference to applications of these tools to the study of elementary processes occurring both in homogeneous and in heterogeneous phases, and are discussed in view of their fundamental relevance and also of possible interest in an astrochemical, protobiological context.

This study is intended as a continuation of previous experimental and theoretical works on the systems H2O-Rg, H2S-Rg, H2O2-Rg, and H2S2-Rg, where Rg = He, Ne, Ar, Kr, Xe. For the H2O-Rg and H2S-Rg systems, molecular and atomic polarizabilities have been calculated and from them, using phenomenological correlation formulas modeling the dispersion-repulsion (van der Waals) forces, the isotropic interaction parameters have been determined and compared with experimental data from this laboratory. For the H2O2-Rg and H2S2-Rg systems, the molecular polarizabilities have been calculated and used in correlation formulas to predict well depths and positions of van der Waals forces and a comparison made with the corresponding potential energy surfaces calculated in previous works. The approach correctly predicts the interaction parameters, except for H2O and H2O2 with the heavier rare gases. The correlation formulas have been then extended to include an attractive induction contribution accounting for the interaction between the permanent molecular dipole moment and the instantaneous induced atomic dipole moment, to improve the predicted parameters for H2O and H2O2-Ar, Kr and Xe. The agreement with experimental and theoretical data is improved but the predicted data still underestimate the interaction. This is probably due to the presence of a significant non van der Waals contribution to the interaction for the heavier gases, as suggested by analogy with the previously studied water-Rg case. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

The equilibrium and cis and trans geometric structures of a series of disulfides RS-SR' (HSSH, HSSF, HSSCl, HSSCH3, HSSC2H5, FSSF, ClSSCl, ClSSF, CH3SSCH3, C2H5SSC2H5) have been studied using both density functional theory (B3LYP/6-311++G(3df, 3pd)) and second-order Møller-Plesset perturbation theory (MP2(FU)/aug-cc-pVTZ). The effects of internal rotation on the structural parameters have been analyzed and the torsional potentials around the S-S bond have been compared with experimental and theoretical information, when available, as well as with the analogues of the peroxydic family. Torsional levels were calculated, and their distribution as a function of temperature was determined, obtaining partition functions. This information is of interest for statistical approaches to equilibrium properties and to rates of processes in which torsional anharmonicity is relevant. It is also required for recent atmospheric modeling studies and also for prototypical chiral separation experiments, in view of a possible dynamic mechanism for chirality exchange by molecular collisions. In general, barriers for such processes are higher than for the corresponding peroxides, and accordingly, rates for chirality change are estimated to be consistently smaller.

Este trabalho é calculada o potencial de interação entre o H2 O2 e H2 S2 com os gases nobres, He, Ne, Ar, Kr, e Xe. O peróxido e persulfeto de hidrogênio foram modelados como moléculas rígidas, exceto pelo modo torsional ao londo da ligação X-X, com X = O ou S. A maior preocupação foi a definição do sistema de coordenadas e a expansão nos harmônicos hiperesféricos para o potencial, para que o mesmo reproduzisse as propriedades geométricas e de simetria dos sistemas, tornando-o um protótipo para a interação entre um átomo e uma molécula fracamente ligada.

We present here a systematic study by quantum mechanical methods of a series of molecules (HOOF, HOOCl, HOONO, HOOCN, FOOF, ClOOF, ClOOCl, and FOONO), corresponding to substitutions of one or both hydrogens in hydrogen peroxide. The emphasis is on the structural and energetic properties and on the features of the internal modes, in particular, the torsion around the O-O bond, which leads to the chirality changing isomerization. The cis and trans barriers appear to vary remarkably upon substitution by halogen groups. They are compared with experimental and theoretical information, when available, and analyzed by reference to a previous systematic analysis of the effects of alkyl substitutions. Torsional levels were calculated, and their distribution as a function of temperature was determined. This information is of interest for statistical approaches to equilibrium properties and to rates of processes where torsional anharmonicity is relevant, as required for recent atmospheric modeling studies and also for prototypical chiral separation experiments, in view of a possible dynamic mechanism for chirality exchange by molecular collisions. Dipole moments are also presented.

A quantum chemical exploration is reported on the interaction potentials of H2O2 with the rare gases, He, Ne, Ar, Kr, and Xe. Hydrogen peroxide (the simplest example of chiral molecule in its equilibrium geometry) is modeled as rigid except for the torsional mode around the O-O bond. However, on the basis of previous work (Maciel, G. S.; et al. Chem. Phys. Lett. 2006 432, 383), the internal mode description is based, rather than on the vectors of the usual valence picture, on the orthogonal local representation, which was demonstrated useful for molecular dynamics simulations, because the torsion around the vector joining the center-of-mass of the two OH radicals mimics accurately the adiabatic reaction path for chirality changing isomerization, following the torsional potential energy profile from equilibrium through the barriers for the trans and cis geometries. The basic motivation of this work is the determination of potential energy surfaces for the interactions to be used in classical and quantum simulations of molecular collisions, specifically those leading to chirality changes of possible relevance in the modeling of prebiotic phenomena. Particular attention is devoted to the definition of coordinates and expansion formulas for the potentials, allowing for a faithful representation of geometrical and symmetry properties of these systems, prototypical of the interaction of an atom with a floppy molecule.

We present here a systematic study by quantum mechanical methods of structural and energetic properties for a series of substitutions by alkyl groups of the hydrogens in hydrogen peroxide. The emphasis is on the torsion around the peroxidic bond, which leads to the chirality changing stereomutation. The dihedral angle dependence of the geometrical features and of the dipole moment is discussed with reference to previous experimental and theoretical information, and with respect to the preceding paper on hydrogen peroxide (Maciel et al., Chem Phys Lett, 2006, 432, 383). This information is of interest for chiral separation experiments as well as in view of a possible dynamical mechanism for chirality exchange by molecular collisions. The cis and trans barriers appear to vary remarkably upon substitution by alkyl groups (methyl, ethyl, n- and iso-propyl, sec- and tert-butyl hydroperoxides), the most important property being their geometrical dimensions. As the latter increase, tendency for the equilibrium configuration towards the trans structure increases, so that the trans barrier becomes negligible for dimethyl and diethyl peroxides and for n- and iso-butyl hydroperoxides, giving essentially achiral molecules. For the chiral ones (HOOH, CH3OOH, and C2H5OOH) torsional level energies and eigenfunctions are calculated and their distribution as a function of temperature determined. Their use is exemplified by a calculation of the dipole moment of hydrogen peroxide at room temperature, reconciling previous disagreement between theory and experiment. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Structural and energetic properties are studied by quantum mechanical methods for hydrogen peroxide and its isotopomers, with the purpose of elucidating the features of the internal modes, and in particular the torsion around the O–O bond which leads to the chirality changing isomerization. The cis and trans barriers are found in good agreement with published data and the dihedral angle dependence of the dipole moment and of the reaction path is discussed with reference to previous experimental and theoretical information. The pictures in terms of stretching, bending and torsion modes based both on the usual valence type description of the molecule and on orthogonal local vectors are compared and the greater accuracy of the latter is demonstrated.

This Letter examines the topological neighborhood effects on atomic partial charges and their transferability. To determine neighborhood effects, we used Breneman and Wiberg’s CHELPG charges calculated at B3LYP/6-31G* theory level and AM1 geometries for a test set of 324 molecules of insecticides, ferroelectric liquid crystals (FLC) and antiinflammatories. The Qcode atomic descriptor was applied to represent topological neighborhood. Calculated molecular dipole moments using averaged charges with various neighbor numbers indicate that partial charges have a strong dependence with the molecular topology. Five or more neighbors are demonstrated to be typically needed for accurate partial charges transferability.

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, ...

Three procedures based on the charges derived by fitting the ESP were examined, the charge from electrostatic potential (CHELP), charge from ESP grid based (CHELPG) by Breneman and Wiberg and the method of Merz and Kollman (MK). The ability of these charges to reproduce SCF and experimental dipole moments was evaluated for 89 molecules among hydrocarbons, alkyl halides, aromatics, oxygen and nitrogen compounds. It is shown that accuracy for dipole moments depends more on the theory level and geometry optimization than on the charge models used. From a computational viewpoint, the DFT methods with AM1 geometry optimization provide the best accuracy/cost ratio for the calculation of molecular dipole moments with respect to experimental data, with mean absolute deviation smaller than 0.19 D for total database. All three methods reproduce well SCF dipole moments, but charges derived from CHELP fit have deviations as much as twice than CHELPG and MK methods.

Selection and alignment of rotamers and, more in general, of conformers in the gas phase is a challenge that we tackle experimentally by supersonic expansion of seeded molecular beams and hexapolar electrostatic fields with quadrupole mass detection. The studied system involves the nine conformers of the asymmetric-top molecule 2-butanol, which coexist because of nearly free rotations around a CC and a CO bond. From the measured time-of-flight of a 2-butanol supersonic molecular beam seeded in either He or Ar, the corresponding velocity distributions are obtained. The different nature and masses of the seeding gas decrease selectively the vibrational temperature and determine the population of the conformers, which is assessed on the basis of their statistical distribution, derived from high level accompanying quantum mechanical calculations. The use of a hexapolar electrostatic field permits us to induce a variation of the population distribution as a function of the applied voltage and of the selective focusing and alignment of the conformers. A technique, recently developed for treating asymmetric tops and involving extensive trajectory simulations, is applied to obtain the link between the focusing curves, i.e. the dependence of the beam intensity on the hexapole voltage, and the conformers' populations and alignment. Perspectives are provided for photo- and stereo-dynamics experiments, particularly appealing also on account that 2-butanol is the simplest chiral alcohol. © the Partner Organisations 2014.

Astrochemistry is a discipline consolidated recently, although its roots extend back to the dawn of early civilization with the observation and mapping of the sky. The way to the understanding of the common natural laws on earth and in space paved by Galilei’s observations by the telescope, has been extended in the last decades, by new technologies such as radioastronomy and space missions. Plenty of new chemistry was surprisingly discovered. Extreme rich information on the chemical “composition” of the universe is being obtained, either from the other planets and satellites in the Solar System, from meteorites and comets, or from the interstellar space. In this article we will present selected topics regarding the chemical structures and reactions being discovered. Particular attention will be devoted to aspects considered as relevant for the prebiotic processes on earth, such as those involving chirality and its role played in the origin and evolution of life.

An account is given of isomerization and decomposition paths in the title molecules, as obtained by characterizing stable isomers and transition states using quantum chemistry and reaction rate theories. For n = 1 (formaldehyde) intrinsic reaction paths are calculated to provide rates for decomposition mechanisms dominated by quantum mechanical tunneling. The n = 2 closed shell isomers are acetaldehyde, ethylene oxide, and vinyl alcohol, which interconvert and decompose through alternative paths. For n = 3, the very large number of isomers are characterized using an efficient automatic search algorithm recently made available by Ohno and Maeda. Interconversion paths are also found, including those involving chiral change mechanisms for propylene oxide. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

With the aim of illustrating computationally the mechanism of chiral discrimination induced by molecular collisions, we present results of extensive classical dynamics simulations of oriented collisions of rare gas atoms with prototypical chiral molecules, such as hydrogen peroxide and hydrogen persulfide. Models for the intermolecular and intramolecular interactions are based on previous quantum chemical calculations. The phenomenon of right-left asymmetry in scattering directions is documented both by exemplary single trajectories and by angular distributions obtained by statistical averagings. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 00: 000-000, 2010

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.

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 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.

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.

In view of the particular attention recently devoted to hindered rotations, we have tested reduced kinetic energy operators to study the torsional mode around the O-O bond for H(2)O(2) and for a series of its derivatives (HOOCl, HOOCN, HOOF, HOONO, HOOMe, HOOEt, MeOOMe, ClOOCl, FOOCl, FOOF, and FOONO), for which we had previously determined potential energy profiles along the dihedral ROOR(') angle [R,R(')=H,F,Cl,CN,NO,Me (=CH(3)), Et (=C(2)H(5))]. We have calculated level distributions as a function of temperature and partition functions for all systems. Specifically, for the H(2)O(2) system we have used two procedures for the reduction in the kinetic energy operator to that of a rigid-rotor-like one and the calculated partition functions are compared with previous work. Quantum partition functions are evaluated both by quantum level state sums and by simple classical approximations. A semiclassical approach, using a linear approximation of the classical path and a quadratic Feynman-Hibbs approximation of Feynman path integral, introduced in previous work and here applied to the torsional mode, is shown to greatly improve the classical approximations. Further improvement is obtained by the explicit introduction of the dependence of the moment of inertia from the torsional angle. These results permit one to discuss the characteristic time for chirality changes for the investigated molecules either by quantum mechanical tunneling (dominating at low temperatures) or by transition state theory (expected to provide an estimate of racemization rates in the high energy limit).

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.