Ramón Hernández-Lamoneda

On the Unusual Properties of Halogen Bonds: A Detailed ab Initio Study of X2-(H2O)1-5 clusters (X ) Cl and Br)

Halogen bonds have received a great deal of attention in recent years. Their properties, sometimes paralleled with those of hydrogen bonds, have not yet been fully understood. In this work, we investigate the nature of the intermolecular interactions between Cl(2) and Br(2) with water. Our analysis of several features of MP2/aug-cc-pVDZ-optimized stable clusters with different number and arrangement of water molecules shows that two different kinds of halogen-water coordination patterns are involved in the stability and properties found for these systems: halogen bonds (X-X...O) and halogen-hydrogen interactions, (X-X...H-O-H). Both types of interactions result in a large polarization of the halogen molecule, which leads to important cooperative effects on these structures. Although the general structural aspects of these clusters can be understood in terms of dipole-quadrupole forces at long range, where it is the dominant term, the SAPT analysis shows that factors such as polarization of pi densities and dispersion become increasingly important close to equilibrium. In particular, we show that the halogen-hydrogen interactions are weaker than halogen-oxygen interactions mainly due to the electrostatic and dispersion forces. We also calculate vibrational and electronic shifts that should be helpful for the interpretation of experimental results and for investigating the microsolvation phenomena for halogens in an aqueous environment.

Long‐range interaction for dimers of atmospheric interest: dispersion, induction and electrostatic contributions for O2O2, N2N2 and O2N2

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 O2O2, N2N2, and O2N2 dimers. Our data is a comprehensive and consistent set that for N2N2 shows a very good agreement with previous accurate calculations, whereas for quantities involving open‐shell O2 represents a considerable improvement over previous estimations. Moreover, the long‐range interaction is analyzed and compared for the different interacting partners. It is found that the C8 dispersion interaction plays a nonnegligible role and that the induction component is only important for a detailed description of the highest order anisotropy terms in the spherical harmonics expansion of the long‐range potential. It is also found that the total long‐range interaction is quite similar in O2O2 and O2N2, and that differences with N2N2 are mainly because of the important role of the electrostatic interaction in that dimer. Comparison with high level supermolecular calculations indicates that the present long‐range potentials are accurate for intermolecular distances larger than about 15 bohr.

Does ozone have a barrier to dissociation and recombination

Abstract The barrier associated with the dissociation and recombination of ozone has been calculated using highly correlated ab initio methods. Our calculations show that, for fixed equilibrium values of the bending angle and one bond distance, there is a very small barrier, 100 cm −1 , as opposed to much larger values previously reported. When the saddle point geometry is optimized, the reaction path still contains a barrier but the top of the barrier lies below the dissociation limit.

Global ab initio potential energy surfaces for the O2(Σ3g−)+O2(Σ3g−) 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. Chem. Phys. 128, 214304 (2008)] with complete active space second order perturbation theory (CASPT2) or, alternatively, multireference configuration interaction (MRCI) calculations of the singlet-quintet and triplet-quintet splittings. Spherical harmonic expansions, containing a large number of terms due to the high anisotropy of the interaction, have been built from the ab initio data. The radial coefficients of these expansions are matched at long range distances with analytical functions based on recent ab initio calculations of the electric properties of the monomers [M. Bartolomei, E. Carmona-Novillo, M. I. Hernández, J. Campos-Martínez, and R. Hernández-Lamoneda, J. Comput. Chem. (2010) (in press)]. The singlet and triplet PESs obtained from either RCCSD(T)-CASPT2 or RCCSD(T)-MRCI calculations are quite similar, although quantitative differences appear in specific terms of the expansion. CASPT2 calculations are the ones giving rise to larger splittings and more attractive interactions, particularly in the region of the absolute minima (in the rectangular D(2h) geometry). The new singlet, triplet, and quintet PESs are tested against second virial coefficient B(T) data and, their spherically averaged components, against integral cross sections measured with rotationally hot effusive beams. Both types of multiconfigurational approaches provide quite similar results, which, in turn, are in good agreement with the measurements. It is found that discrepancies with the experiments could be removed if the PESs were slightly more attractive. In this regard, the most attractive RCCSD(T)-CASPT2 PESs perform slightly better than the RCCSD(T)-MRCI counterpart.

Jump in depletion rates of highly excited O2: reaction or enhanced vibrational relaxation?

Abstract A simple model is used to calculate vibrational self-relaxation rates of highly excited oxygen. In contrast to previous theoretical treatments, the model reproduces the experimental observation of a sharp increase in depletion rates above a critical value of v . It is proposed that the observed jump in O ‡ 2 (v) depletion rates is partially due to inelastic collisions, rather than to ozone formation. It is also shown that the presence of the reactive transition state region in the potential energy surface is the key for the enhancement of the relaxation rates.

Accurate ab initio intermolecular potential energy surface for the quintet state of the O2(Σg−3)–O2(Σg−3) dimer

A new potential energy surface (PES) for the quintet state of rigid O(2)((3)Sigma(g)(-)) + O(2)((3)Sigma(g)(-)) has been obtained using restricted coupled-cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)]. A large number of relative orientations of the monomers (65) and intermolecular distances (17) have been considered. A spherical harmonic expansion of the interaction potential has been built from the ab initio data. It involves 29 terms, as a consequence of the large anisotropy of the interaction. The spherically averaged term agrees quite well with the one obtained from analysis of total integral cross sections. The absolute minimum of the PES corresponds to the crossed (D(2d)) structure (X shape) with an intermolecular distance of 6.224 bohrs and a well depth of 16.27 meV. Interestingly, the PES presents another (local) minimum close in energy (15.66 meV) at 6.50 bohrs and within a planar skewed geometry (S shape). We find that the origin of this second structure is due to the orientational dependence of the spin-exchange interactions which break the spin degeneracy and leads to three distinct intermolecular PESs with singlet, triplet, and quintet multiplicities. The lowest vibrational bound states of the O(2)-O(2) dimer have been obtained and it is found that they reflect the above mentioned topological features of the PES: The first allowed bound state for the (16)O isotope has an X structure but the next state is just 0.12 meV higher in energy and exhibits an S shape.

Molecular oxygen tetramer (O2)4: intermolecular interactions and implications for the epsilon solid phase

Recent data have determined that the structure of the high pressurephase of solid oxygen consists of clusters composed of four O2 molecules. This finding has opened the question about the nature of the intermolecular interactions within the molecular oxygen tetramer. We use multi- configurational ab initio calculations to obtain an adequate characterization of the ground singlet state of (O2)4 which is compatible with the non magnetic character of thephase. In contrast to previous suggestions implying chemical bonding, we show that (O2)4 is a van der Waals like cluster where exchange interactions preferentially stabilize the singlet state. However, as the clus- ter shrinks, there is an extra stabilization due to many-body interactions that yields a significant softening of the repulsive wall. We show that this short range behavior is a key issue for the understanding of the structure of �-oxygen.

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. Chem. Phys. 2008, 128, 214304.]. This PES is extended here for large intermolecular distances using the ab initio long-range coefficients of Hettema et al. [J. Chem. Phys. 1994, 100, 1297.]. Elastic and rotationally inelastic integral cross sections have been obtained by means of close coupling calculations in the subthermal energy range (center-of-mass velocities below 500 m/s). Results are compared with those obtained using a PES derived from molecular beam experiments [Aquilanti; et al. J. Am. Chem. Soc. 1999, 121, 10794.]. General agreement is found between both PESs, although the experimentally derived PES appears as somewhat more anisotropic at least for the studied energy range. There is, however, a significant difference in the absolute value of the elastic cross sections that is due to differences in the long-range dispersion interaction. The performance of the ab initio PES for higher velocities (relevant to experiments) is also explored by retaining just the isotropic component of the interaction. A satisfactory agreement is found for the shape of the glory pattern but shifted toward lower absolute values of the cross sections.

Theoretical evidence for the reaction O2(ν) + O2(ν = 0) → O3(X1A1) + O(3P)

The reaction O2(ν) + O2(ν = 0) → O3(X1A1) + O(3P) is studied by quantum time-dependent and time-independent methods, for high vibrational excitation of one of the reactants. State-selected reaction rate constants are computed showing evidence of reaction but their values are too small to explain previous experimental measurements.

Spin-orbit coupling in O2(υ)+O2 collisions: I. Electronic structure calculations on dimer states involving the XΣg−3, aΔg1, and bΣg+1 states of O2

The importance of vibrational-to-electronic (V-E) energy transfer mediated by spin-orbit coupling in the collisional removal of O2(X 3Sigmag-,upsilon>or=26) by O2 has been reported in a recent communication [F. Dayou, J. Campos-Martinez, M. I. Hernandez, and R. Hernandez-Lamoneda, J. Chem. Phys. 120, 10355 (2004)]. The present work provides details on the electronic properties of the dimer (O2)2 relevant to the self-relaxation of O2(X 3Sigmag-,upsilon>>0) where V-E energy transfer involving the O2(a 1Deltag) and O2(b 1Sigmag+) states is incorporated. Two-dimensional electronic structure calculations based on highly correlated ab initio methods have been carried out for the potential-energy and spin-orbit coupling surfaces associated with the ground singlet and two low-lying excited triplet states of the dimer dissociating into O2(X 3Sigmag-)+O2(X 3Sigmag-), O2(a 1Deltag)+O2(X 3Sigmag-), and O2(b 1Sigmag+)+O2(X 3Sigmag-). The resulting interaction potentials for the two excited triplet states display very similar features along the intermolecular separation, whereas differences arise with the ground singlet state for which the spin-exchange interaction produces a shorter equilibrium distance and higher binding energy. The vibrational dependence is qualitatively similar for the three studied interaction potentials. The spin-orbit coupling between the ground and second excited states is already nonzero in the O2+O2 dissociation limit and keeps its asymptotic value up to relatively short intermolecular separations, where the coupling increases for intramolecular distances close to the equilibrium of the isolated diatom. On the other hand, state mixing between the two excited triplet states leads to a noticeable collision-induced spin-orbit coupling between the ground and first excited states. The results are discussed in terms of specific features of the dimer electronic structure (including a simple four-electron model) and compared with existing theoretical and experimental data. This work gives theoretical insight into the origin of electronic energy-transfer mechanisms in O2+O2 collisions.