Reactivity enhancement of ultracold O(3P)+H2 collisions by van der Waals interactions

The role of van der Waals forces in O((3)P)+H(2)(upsilon=1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial (3)A(") potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandao et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures.     

A quantum wave packet dynamical study of the electronic and spin-orbit coupling effects on the resonances in Cl(2P) + H2 scattering

Dynamical resonances in Cl(2P) + H2 scattering are investigated with the aid of a time-dependent wave packet approach using the Capecchi-Werner coupled ab initio potential energy surfaces [Phys. Chem. Chem. Phys. 2004, 6, 4975]. The resonances arising from the prereactive van der Waals well (approximately 0.5 kcal/mol) and the transition-state (TS) region of the 2Sigma(1/2) ground spin-orbit (SO) state of the Cl(2P) + H2 system are calculated and assigned by computing their eigenfunctions and lifetimes. The excitation of even quanta along the bending coordinate of the resonances is observed. The resonances exhibit an extended van der Waals progression, which can be attributed to the dissociative states of ClH2. Excitation of H2 vibration is also identified in the high-energy resonances. The effect of the excited 2P(1/2) SO state of Cl on these resonances is examined by considering the electronic and SO coupling in the dynamical simulations. While the electronic coupling has only a minor impact on the resonance structures, the SO coupling has significant effect on them. The nonadiabatic effect due to the SO coupling is stronger, and as a result, the spectrum becomes broad and diffuse particularly at high energies. We also report the photodetachment spectrum of ClD2- and compare the theoretical findings with the available experimental results.     

Collisional stabilization of van der Waals states of ozone

The mixed quantum-classical theory developed earlier [M. Ivanov and D. Babikov, J. Chem. Phys. 134, 144107 (2011)] is employed to treat the collisional energy transfer and the ro-vibrational energy flow in a recombination reaction that forms ozone. Assumption is that the van der Waals states of ozone are formed in the O + O(2) collisions, and then stabilized into the states of covalent well by collisions with bath gas. Cross sections for collision induced dissociation of van der Waals states of ozone, for their stabilization into the covalent well, and for their survival in the van der Waals well are computed. The role these states may play in the kinetics of ozone formation is discussed.     

Long-range-corrected hybrids using a range-separated Perdew-Burke-Ernzerhof functional and random phase approximation correlation

We build on methods combining a short-range density functional approximation with a long-range random phase approximation [B. G. Janesko, T. M. Henderson, and G. E. Scuseria, J. Chem. Phys. 130, 081105 (2009)] or second-order screened exchange [J. Paier et al., J. Chem. Phys. 132, 094103 (2010)] by replacing the range-separated local density approximation functional with a range-separated generalized gradient approximation functional in the short range. We present benchmark results that show a marked improvement in the thermodynamic tests over the previous local density approximation-based methods while retaining those methods' excellent performance in van der Waals interactions.     

Ab initio potential energy surfaces, bound states, and electronic spectrum of the Ar-SH complex

New ab initio potential energy surfaces for the (2)Pi ground electronic state of the Ar-SH complex are presented, calculated at the RCCSD(T)/aug-cc-pV5Z level. Weakly bound rotation-vibration levels are calculated using coupled-channel methods that properly account for the coupling between the two electronic states. The resulting wave functions are analyzed and a new adiabatic approximation including spin-orbit coupling is proposed. The ground-state wave functions are combined with those obtained for the excited (2)Sigma(+) state [D. M. Hirst, R. J. Doyle, and S. R. Mackenzie, Phys. Chem. Chem. Phys. 6, 5463 (2004)] to produce transition dipole moments. Modeling the transition intensities as a combination of these dipole moments and calculated lifetime values [A. B. McCoy, J. Chem. Phys. 109, 170 (1998)] leads to a good representation of the experimental fluorescence excitation spectrum [M.-C. Yang, A. P. Salzberg, B.-C. Chang, C. C. Carter, and T. A. Miller, J. Chem. Phys. 98, 4301 (1993)].     

Electronic structure and spectrum of UO2 2+ and UO2Cl4 2-

A theoretical study is presented of the electronic spectra of the UO(2) (2+) and UO(2)Cl(4) (2-) ions, based on multiconfigurational perturbation theory (CASSCF/CASPT2), combined with a recently developed method to treat spin-orbit coupling [P.-A. Malmqvist et al., Chem. Phys. Lett. 357, 230 (2002); B. O. Roos and P.-A. Malmqvist, Phys. Chem. Chem. Phys. 6, 2919 (2004)]. The results are compared to the experimental spectroscopic data obtained for uranyl ions in Cs(2)UO(2)Cl(4) crystals from Denning [Struct. Bonding (Berlin) 79, 215 (1992)] and to previous theoretical calculations performed using a combined configuration-interaction spin-orbit treatment [Z. Zhang and R. M. Pitzer, J. Phys. Chem. A 103, 6880 (1999); S. Matsika and R. M. Pitzer, J. Phys. Chem. A. 105, 637 (2001)]. As opposed to the latter results, the calculations performed in this work point to a significant effect of the weakly bound equatorial chlorine ligands on the excitation energies.     

Through-space spin-spin coupling in van der Waals dimers and CH/pi interacting systems. An ab initio and DFT study

The through-space J(HH) and J(CH) spin-spin coupling constants of model van der Waals dimers (involving methane, ethylene, and benzene), and of selected compounds showing the CH/pi interaction, have been investigated by means of DFT and ab initio calculations. In the range of intermolecular separations for which the interaction is stabilizing, weak couplings (0.1-0.3 Hz) are predicted for J(CH), while the corresponding J(HH) couplings are much smaller. The relative contributions (Fermi-contact, spin-orbit, and spin-dipole) are strongly dependent on the geometry of the dimers and on the distance; the non-negligible values of J(CH) for pi systems stem largely from an incomplete cancellation of spin-orbit terms. The results obtained for the larger molecules, that is, acetonitrile@calix[4]arene 5, the imine 6, and the aryl ester 7 are consistent with those on the model dimers. For 7, the occurrence of a through-space mechanism for the transmission of coupling is established by examining trends in the magnitude of couplings as a function of the number of intervening covalent bonds.     

A post-Hartree-Fock model of intermolecular interactions: inclusion of higher-order corrections

We have previously demonstrated that the dipole moment of the exchange hole can be used to derive intermolecular C(6) dispersion coefficients [J. Chem. Phys. 122, 154104 (2005)]. This was subsequently the basis for a novel post-Hartree-Fock model of intermolecular interactions [J. Chem. Phys. 123, 024101 (2005)]. In the present work, the model is extended to include higher-order dispersion coefficients C(8) and C(10). The extended model performs very well for prediction of intermonomer separations and binding energies of 45 van der Waals complexes. In particular, it performs twice as well as basis-set extrapolated MP2 theory for dispersion-bound complexes, with minimal computational cost.     

Importance of spin-orbit coupling for the assignment of the photodetachment spectra of AuX2- (X=Cl, Br, and I).

Recently, photodetachment spectra of AuX2-, X=Cl, Br, and I, have been reported [D. Schr?der et al., Angew. Chem. Int. Ed. 2003, 42, 311] followed by a scalar-relativistic theoretical study of the assignment of these spectra [B. Dai, J. Yang, Chem. Phys. Lett. 2003, 379, 512]. Herein, the photodetachment spectra of the title molecules are reassigned, taking spin-orbit coupling into account and employing relativistic-effective core potentials for gold and the halogen atoms. The composition of the AuX2 electronic states are further analyzed in terms of scalar-relativistic electronic states. The relevance of spin-orbit coupling in the spectroscopy of heavy elements is emphasized by comparing the results of this work with experiment and with other scalar-relativistic theoretical studies.     

The structure and binding energies of the van der Waals complexes of Ar and N2 with phenol and its cation, studied by high level ab initio and density functional theory calculations

We have investigated, using both ab initio and density functional theory methods, the minimum energy structures and corresponding binding energies of the van der Waals complexes between phenol and argon or the nitrogen molecule, and the corresponding complexes involving the phenol cation. Structures were obtained at the MP2 level using a large basis, and the corresponding energies were corrected for basis set superposition error (BSSE), higher order electron correlation effects, and for basis set size. The structures of the global minima were further refined for the effects of BSSE and the corresponding binding energies were evaluated. For each neutral species, we find only a single true minimum, pi bonded for argon and OH bonded for nitrogen. For both cationic species, we find that the OH-bonded complex is preferred over other minima which we have identified as having Ar or N(2) between exogeneous atoms. The ab initio calculations are generally in excellent agreement with experimental binding energies and rotational constants. We find that the B3LYP functional is particularly poor at describing these complexes, while a density functional theory (DFT) method with an empirical correction for dispersive interactions (DFT-D) is very successful, as are some of the new functionals proposed by Zhao and Truhlar [J. Phys. Chem. A 109, 5656 (2005); J. Chem. Theory Comput. 2, 1009 (2006); Phys. Chem. Chem. Phys. 7, 2701 (2005); J. Phys. Chem. A 108, 6908 (2004)]. Both the ab initio and DFT-D methods accurately predict the intermolecular vibrational modes.     

A density-functional study on pi-aromatic interaction: benzene dimer and naphthalene dimer

The long-range correction (LC) scheme of density-functional theory (DFT) was applied to the calculation of the pi-aromatic interaction of the benzene dimer and naphthalene dimer. In previous calculations, it was confirmed that the LC scheme [Iikura et al., J. Chem. Phys. 115, 3540 (2001)] gives very accurate potential- energy surfaces (PESs) of small van der Waals (vdW) complexes by combining with the Anderson-Langreth-Lundqvist (ALL) vdW correlation functional [Andersson et al., Phys. Rev. Lett. 76, 102 (1996)] (LC-DFT + ALL). In this study, LC-DFT+ALL method was examined by calculating a wide range of PES of the benzene dimer including parallel, T-shaped, and parallel-displaced configurations. As a result, we succeeded in reproducing very accurate PES within the energy deviance of less than 1 kcalmol in comparison with the results of high-level ab initio molecular-orbital methods at all reference points on the PES. It was also found that LC-DFT + ALL gave accurate results independent of exchange-correlation functional used, in contrast with the strong functional dependencies of conventional pure functionals. This indicates that both exchange repulsion and van der Waals attractive interactions should be correctly incorporated in conventional pure functionals in order to calculate accurate pi-aromatic interactions. We also found that LC-DFT + ALL method has a low basis-set dependency in the calculations of pi-aromatic interactions. The present scheme was also successfully applied to the pi,[ellipsis (horizontal)],pi stacking interactions of naphthalene dimer. This may suggest that LC-DFT + ALL method would be a powerful tool in the calculations of large molecules such as biomolecules.     

Mass selective gas phase study of ClO, OClO, ClOO and ClAr by anion-ZEKE-photoelectron spectroscopy

Anion-ZEKE-photoelectron spectra of ClO-, OClO-, ClOO- and the van der Waals cluster ArCl- have been measured. Refined or new values for the electron affinity of ClO, OClO and ClOO have been found. The peak positions in these spectra are in very good agreement with former ClO- and OClO- anion-photoelectron spectra (K. M. Gilles, M. L. Polak and W. C. Lineberger, J. Chem. Phys., 1992, 96, 8012) and a recent ArCl- anion-ZEKE spectrum (T. Lenzer, I. Yourshaw, M. Furlanetto, G. Reiser and D. Neumark, J. Chem. Phys., 1992, 110, 9578). The higher resolution of our anion-ZEKE-photoelectron spectrum of OClO- led to a refined assignment of the corresponding anion-photoelectron spectrum. In addition, a strong difference in the relative intensities of the vibrational peaks has been found in the anion-ZEKE-spectrum of OClO- in comparison with the anion-photoelectron spectrum. For the first time, mass selective spectroscopic information has been obtained for ClOO. The strong similarity to the ArCl- spectrum indicates a weakly bound van der Waals cluster Cl.O2. Binding energies of the anion, neutral ground and neutral excited state could be deduced. These are in good agreement with the electron affinities of Cl and ClOO, but differ from theoretical values (K. A. Peterson and H. J. Werner, J. Chem. Phys., 1992, 96, 8948) by a factor of 4.5 and from thermochemically determined values (J. M. Nicovich, K. D. Kreutter, C. J. Shackelford and P. H. Wine, Chem. Phys. Lett., 1991, 179, 367 and S. Baer, H. Hippler, R. Rahn, M. Siefke, N. Seitzinger and J. Troe, J. Chem. Phys., 1991, 95, 6463) by a factor of 9.     

Potential-energy surface and van der Waals motions of p-difluorobenzene-argon cation

The structure and dynamics of the van der Waals complex of argon with the p-difluorobenzene cation are investigated using the ab initio theory. The restricted open-shell M?ller-Plesset second-order perturbation method combined with the augmented correlation-consistent polarized valence double-zeta basis set is employed to determine the electronic ground-state potential-energy surface of the cationic complex. This surface is extremely flat in a wide region of the configuration space of the Ar atom which moves almost freely over the monomer ring. However, it is bound to the monomer stronger in the cationic than in the neutral complex. Its binding energy is calculated to be 621 cm(-1) at a distance of 3.445 A from the monomer center. The calculated dissociation energy of 572 cm(-1) agrees perfectly with the experimental value of 572+/-6 cm(-1) [S. M. Belm, R. J. Moulds, and D. Lawrence, J. Chem. Phys. 115, 10709 (2001)]. The effect of a strong coupling of large-amplitude intermolecular motions on the character of van der Waals vibrational states is investigated. The vibrational structure of the spectrum of the complex is explained and its earlier assignment is partly corrected.     

Recombination of ozone via the chaperon mechanism

The recombination of ozone via the chaperon mechanism, i.e., ArO+O2 --> Ar+O3 and ArO2+O --> Ar+O3, is studied by means of classical trajectories and a pairwise additive Ar-O3 potential energy surface. The recombination rate coefficient has a strong temperature dependence, which approximately can be described by T(-n) with n approximately 3. It is negligible for temperatures above 700 K or so, but it becomes important for low temperatures. The calculations unambiguously affirm the conclusions of Hippler et al. [J. Chem. Phys. 93, 6560 (1990)] and Luther et al. [Phys. Chem. Chem. Phys. 7, 2764 (2005)] that the chaperon mechanism makes a sizable contribution to the recombination of O3 at room temperature and below. The dependence of the chaperon recombination rate coefficient on the isotopomer, studied for two different isotope combinations, is only in rough qualitative agreement with the experimental data. The oxygen atom isotope exchange reaction involving ArO and ArO2 van der Waals complexes is also investigated; the weak binding of O or O2 to Ar has only a small effect.     

Resonances of triatomic van der Waals molecules by the complex discrete variable representation

Summary The results of Light and co-workers [J. Chem. Phys. 85:4594 (1986); 86:3065 (1987); 92:2129 (1990)] for the Hamiltonian matrix of a triatomic van der Waals molecule in the discrete variable representation, DVR, is extended to complex-scaled Hamiltonians. As an illustrative numerical example theJ=1 resonances positions and widths of a van der Waals model system were obtained by the calculation of the complex-scaled Hamiltonian matrix in the DVR formalism.Supported in part by the Albert Einstein Research Fund, and the Fund for the Promotion of Research at the Technion     

On the role of scattering resonances in the F+HD reaction dynamics

We study scattering resonances in the F+HD-->HF+D reaction using a new method for direct evaluation of the lifetime Q-matrix [Aquilanti et al., J. Chem. Phys. 2005, 123, 054314]. We show that most of the resonances are due to van der Waals states in the entrance and exit reaction channels. The metastable states observed in the product reaction channel are assigned by calculating the energy levels and wave functions of the HF...D van der Waals complex. The behavior of resonance energies, widths, and decay branching ratios as functions of total angular momentum is analyzed. The effect of isotopic substitution on resonance energies and lifetimes is elucidated by comparison with previous results for the F+H2 reaction. It is demonstrated that HF(v'=3) products near threshold are formed by decay of the narrow resonances supported by van der Waals wells in the exit channel. State-to-state differential cross sections in the HF(v'=3) channel exhibit characteristic forward-backward peaks due to the formation of a long-lived metastable complex. The role of the exit-channel resonances in the interpretation of molecular beam experiments is discussed.