Molecular Oxygen Trimer: Multiplet Structures and Stability.

We present a detailed theoretical study of the molecular oxygen trimer where the potential energy surfaces of the seven multiplet states have been calculated by means of a pair approximation with very accurate dimer ab initio potentials. In order to obtain all the states a matrix representation of the potential using the uncoupled spin representation has been applied. The and states are nearly degenerate and low-lying isomers appear for most multiplicities.

Halogen bonding and rotational disorder in chlorine clathrate hydrate cages.

We present a detailed theoretical characterization of the structure and interactions in dichlorine clathrate hydrate cages. In the case of the dodecahedral cage, there is clear evidence of the presence of halogen bonding, whereas in the tetrakaidecahedral cage, the expected signatures are there but in a weaker form. Comparison is made with the available structural data from x-ray experiments, where the rotational motion of dichlorine has been taken into account through Monte Carlo simulations illustrating delocalization effects associated with sampling multiple minima, specifically for the larger cage.

An unrestricted approach for the accurate calculation of the intermolecular potential of (O): Implications for the solid epsilon phase.

Oxygen in its elemental form shows a variety of magnetic properties in its condensed phases; in particular, the epsilon solid phase loses its magnetism. These phenomena reflect the nature of the intermolecular forces present in the solid and the changes that arise with variations in pressure and temperature. In this study, we use intermolecular potentials obtained with unrestricted ab initio methods to model the singlet state of the oxygen tetramer [(O)], which is the unit cell, consistent with the non-magnetic character of this phase.

Energy exchange rate coefficients from vibrational inelastic O(Σg-3) + O(Σg-3) collisions on a new spin-averaged potential energy surface.

A new spin-averaged potential energy surface (PES) for non-reactive O(Σg-3) + O(Σg-3) collisions is presented. The potential is formulated analytically according to the nature of the principal interaction components, with the main van der Waals contribution described through the improved Lennard-Jones model. All the parameters involved in the formulation, having a physical meaning, have been modulated in restricted variation ranges, exploiting a combined analysis of experimental and ab initio reference data.

Density Functional Study on the Fundamental and Valence Excited States of Dibromine in , , and Clathrate Cages.

This work evaluates the performance of different DFT models in the accurate prediction of the guest-host intermolecular potentials for the ground and excited states of Br in the tetrakaidecahedral (), pentakaidecahedral (), and hexakaidecahedral () clathrate cages. Of a set of density functionals, we found that PBE0-D3 and wb97XD provide a physically sound and quantitatively correct description of the interaction and transition energies of low-lying valence excited states of Br inside these clathrate cages. The importance of correctly modeling dispersive interactions is also analyzed.

An unrestricted approach for the accurate calculation of the interaction potentials of open-shell monomers: The case of O-O.

The properties of molecular oxygen including its condensed phases continue to be of great relevance for the scientific community. The richness and complexity of its associated properties stem from the fact that it is a very stable diradical. Its open-shell nature leads to low-lying multiplets with total electronic spin S = 0, 1, 2 in the case of the dimer, (O), and the accurate calculation of the intermolecular potentials represents a challenge to ab initio electronic structure methods.

Halogen Bonds in Clathrate Cages: A Real Space Perspective.

In this paper we present real space analyses of the nature of the dihalogen-water cage interactions in the 5 and 5 6 clathrate cages containing chlorine and bromine, respectively. Our Quantum Theory of Atoms in Molecules and Interacting Quantum Atoms results provide strong indications that halogen bonding is present even though the lone pairs of water molecules are already engaged in hydrogen bonding interactions.

Nature of the guest-host interactions for dibromine in the T, P, and H clathrate cages.

The guest-host intermolecular potentials for the ground states of Br in the tetrakaidecahedral (T), pentakaidecahedral (P), and hexakaidecahedral clathrate (H) cages have been calculated using ab initio local correlation methods. Applying the local correlation energy partitioning analysis together with first-order symmetry adapted perturbation theory, we obtain a detailed understanding of the nature of the interactions. In particular, the debated question concerning the possible presence of halogen bonding (XB) is carefully analyzed.

Nature of the valence excited states of bromine in the T and P clathrate cages.

The guest-host intermolecular potentials for the valence excited states of Br in the tetrakaidecahedral(T) and pentakaidecahedral(P) clathrate cages have been calculated using ab initio local correlation methods. We find that the excited states are more strongly bound than the corresponding ground states even in the small T cage where bromine has a tight fit. The angular dependence of the interaction energies is quite anisotropic; this reflects in the corresponding electronic shifts where regions of maxima for blue-shifts in the T cage indicate the presence of halogen bonding.

Communication: Evidence of halogen bonds in clathrate cages.

We present a theoretical characterization of the interaction of Cl and Br in the 5 and 56 clathrate cages, respectively, based on energy partitioning analysis and a study of the electronic shifts associated with transitions to the main valence bands. Our analysis clearly shows that while Br@56 does not show halogen bonding interactions in its equilibrium geometry, Cl@5 presents all the characteristics expected for halogen bonding. This is accomplished by the interaction of the usual sigma-hole with the lone pair of the closest oxygen atom involved in hydrogen bonding within the cage framework, though breaking of the hydrogen bond is not required.

Density-Difference-Driven Optimized Embedding Potential Method To Study the Spectroscopy of Br₂ in Water Clusters.

A variant of the density difference driven optimized embedding potential (DDD-OEP) method, proposed by Roncero et al. (J. Chem.

Performance of local correlation methods for halogen bonding: The case of Br2-(H2O)n,n = 4,5 clusters and Br2@5(12)6(2) clathrate cage.

The performance of local correlation methods is examined for the interactions present in clusters of bromine with water where the combined effect of hydrogen bonding (HB), halogen bonding (XB), and hydrogen-halogen (HX) interactions lead to many interesting properties. Local methods reproduce all the subtleties involved such as many-body effects and dispersion contributions provided that specific methodological steps are followed. Additionally, they predict optimized geometries that are nearly free of basis set superposition error that lead to improved estimates of spectroscopic properties.

Ab initio study of the O4H(+) novel species: spectroscopic fingerprints to aid its observation.

A detailed ab initio characterization of the structural, energetic and spectroscopic properties of the novel O4H(+) species is presented. The equilibrium structures and relative energies of all multiplet states have been determined systematically by analyzing static and dynamical correlation effects. The two and three body dissociation processes have been studied and indicate the presence of conical intersections in various states including the ground state.

Ground state analytical ab initio intermolecular potential for the Cl(2)-water system.

The chlorine/water interface is of crucial importance in the context of atmospheric chemistry. Modeling the structure and dynamics at this interface requires an accurate description of the interaction potential energy surfaces. We propose here an analytical intermolecular potential that reproduces the interaction between the Cl2 molecule and a water molecule.

Communication: Ab initio study of O4H+: a tracer molecule in the interstellar medium?

The structure and energetics of the protonated molecular oxygen dimer calculated via ab initio methods is reported. We find structures that share analogies with the eigen and zundel forms for the protonated water dimer although the symmetrical sharing of the proton is more prevalent. Analysis of different fragmentation channels show charge transfer processes which indicate the presence of conical intersections for various states including the ground state.

Ab initio rovibrational structure of the lowest singlet state of O2-O2.

Rovibrational bound states of the O(2)((3)Σ(g)(-), v = 0)-O(2)((3)Σ(g)(-), v = 0) dimer in its singlet electronic state have been obtained by solving the time-independent Schrödinger equation for the nuclear degrees of freedom. We have employed two different ab initio potential energy surfaces, based on high level multiconfigurational methods, which are expected to give upper and lower bounds for the real values of the interaction. Results are compared with spectroscopy experiments as well as with calculations using other semi ab initio and empirical interaction potentials.

A new ab initio potential energy surface for studying vibrational relaxation in NO(v) + NO collisions.

A new ab initio potential energy surface for the ground state of the NO-NO system has been calculated within a reduced dimensionality model. We find an unusually large vibrational dependence of the interaction potential which explains previous spectroscopic observations. The potential can be used to model vibrational energy transfer, and here we perform quantum scattering calculations of the vibrational relaxation of NO(v).

Large shift and small broadening of Br2 valence band upon dimer formation with H2O: an ab initio study.

Valence electronic excitation spectra are calculated for the H(2)O···Br(2) complex using highly correlated ab initio potentials for both the ground and the valence electronic excited states and a 2-D approximation for vibrational motion. Due to the strong interaction between the O-Br and the Br-Br stretching motions, inclusion of these vibrations is the minimum necessary for the spectrum calculation. A basis set calculation is performed to determine the vibrational wave functions for the ground electronic state and a wave packet simulation is conducted for the nuclear dynamics on the excited state surfaces.

Global ab initio potential energy surfaces for the O2(3Σg-)+O2(3Σg-) interaction.

Completely ab initio global potential energy surfaces (PESs) for the singlet and triplet spin multiplicities of rigid O(2)((3)Σ(g)(-))+O(2)((3)Σ(g)(-)) are reported for the first time. They have been obtained by combining an accurate restricted coupled cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)] quintet potential [Bartolomei et al., J.

Long-range interaction for dimers of atmospheric interest: dispersion, induction and electrostatic contributions for O(2)-O(2), N(2)-N(2) and O(2)-N(2).

Electric multipole moments, static dipole polarizabilities, and dynamic dipole, quadrupole, and mixed dipole-octupole polarizabilities of molecular oxygen and nitrogen in their ground electronic states have been obtained by means of high level multiconfigurational ab initio calculations. From these properties, we have obtained electrostatic, dispersion, and induction coefficients for the long-range interactions of the O(2)-O(2) , N(2)-N(2) , and O(2)-N(2) dimers. Our data is a comprehensive and consistent set that for N(2)-N(2) shows a very good agreement with previous accurate calculations, whereas for quantities involving open-shell O(2) represents a considerable improvement over previous estimations.

Communications: A model study on the electronic predissociation of the NeBr(2) van der Waals complex.

Recently, the predissociation lifetimes of the NeBr(2)(B) complex for different initial vibrational excitation (10 View Article and Find Full Text PDF

Quantum-mechanical study of the collision dynamics of O2(3Sigma(g)-) + O2(3Sigma(g)-) on a new ab initio potential energy surface.

The quantum mechanical theory for the scattering of two identical rigid rotors is reviewed and applied to the collision of O2(3Sigma(g)-) molecules using a new accurate ab initio potential energy surface (PES) for the quintet state of the composite system. The PES is based on calculations using restricted coupled-cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)] [Bartolomei; et al. J.

On the local relaxation of solid neon upon Rydberg excitation of a NO impurity: the role of the NO(A)-Ne interaction potential and zero-point quantum delocalization.

The local relaxation of solid neon subsequent to the impulsive excitation of the NO chromophore to its A(3s sigma) Ryberg state is investigated using molecular dynamics simulations. This study makes use of empirical NO(X,A)-Ne isotropic pair potentials as well as a recently developed ab initio triatomic potential energy surface for the excited state. The role of these interaction potentials is analyzed, including many-body effects.

NeCl2 and ArCl2: transition from direct vibrational predissociation to intramolecular vibrational relaxation and electronic nonadiabatic effects.

Pump-probe results are reported for NeCl(2) excited to the Cl(2) B state, undergoing vibrational predissociation, and then probed via E <-- B transitions. Intensities, lifetimes and product vibrational branching ratios are reported for 16 < or = v' < or = 19 Cl(2) stretching quanta. The intensity of the signal rapidly decreases above v' = 17.

An ab initio calculation of the valence excitation spectrum of H2O...Cl2: comparison to condensed phase spectra.

Valence electronic excitation spectra are calculated for the H(2)O...