Stefano Crocchianti

A detailed three‐dimensional quantum study of the Li+FH reaction

Accurate quantum reactive scattering calculations in the full three‐dimensional physical space have been carried out for the Li+FH reaction at zero total angular momentum using the adiabatically adjusting principal axis of inertia hyperspherical coordinate formalism. The procedures for fitting the potential energy surface, calculating the surface functions, and propagating the solutions in a coupled channel treatment are given and discussed. Features of the resulting reactive probability plots are analyzed, and simple explanations of a number of the quantum resonance and oscillatory features are found.

Rate coefficients for the N+O2 reaction computed on an ab initio potential energy surface

Accurate calculations of the potential energy surface of the N+O2 reaction have been performed at complete active space self‐consistent field (CASSCF) and multireference single–double configuration interaction (MR‐SDCI) levels. Features of the calculated potential energy values are analyzed and compared with those of previous ab initio calculations and experimental data. The comparison has been extended to kinetic properties of the reaction.

Anion‐Dependent Tendency of Di‐Long‐Chain Quaternary Ammonium Salts to Form Ion Quadruples and Higher Aggregates in Benzene

The self-aggregation tendency of [N(CH(3))(2)(C(18)H(37))(2)]X [1X; X(-)=BF(4) (-), PF(6) (-), OTf(-), NTf(2) (-), BPh(4) (-), BTol(4) (-), BAr(F-), and B(C(6)F(5))(4) (-)] salts to form ion quadruples (IQs) and higher aggregates (HAggs) in [D(6)]benzene is investigated by means of diffusion NMR spectroscopy. The experimental results indicate that salts containing small anions (1BF(4), 1PF(6), and 1OTf) are present in solution as IQs even at the lowest investigated concentration of C=5×10(-5) M and show a limited tendency to further self-aggregate, reaching a maximum average aggregation number (N=V(H)/V(H)(0IP), where V(H)=measured hydrodynamic volume and V{H}{0IP}=hydrodynamic volume of the ion pair) of about 6-8 (C=0.050-0.100 M). Salts with larger counterions [1BPh(4), 1BTol(4), 1BAr(F), and 1B(C(6)F(5))(4)] form instead ion pairs at low concentration but steadily self-aggregate (especially the non-fluorinated ones) on increasing their concentration up to N values exceeding 50 (C=0.030-0.050 M). 1NTf(2) behaves in an intermediate fashion. The self-aggregation tendency of salts is quantified by formulating the dependence of V(H) on C by means of the equations of indefinitive aggregation models. The following rankings for the formation of IQs and HAggs are obtained: IQs: 1BF(4)≈1PF(6)≈1OTf> 1NTf(2)>1B(C(6)F(5))(4)≥1BPh(4)≥1BTol(4)≥1BAr(F); HAggs: 1BTol(4)>1BPh(4)> 1NTf(2)>1B(C(6)F(5))(4)> 1BAr(F)>1BF(4)≈1PF(6)≈1OTf. Interionic NOE NMR studies and DFT calculations were conducted in order to determine the relative anion-cation orientation in the self-aggregating units.

On the effect of increasing the total angular momentum on Li+HF reactivity

The effect of increasing the total angular momentum J on the value of the reaction probability has been investigated theoretically using exact quantum calculations. The contribution of higher J probabilities to the calculated value of the cross-section has been treated as a correction to its J-shifting model formulation. Two interesting features of the calculated cross-section have been evidenced in this way: (a) as more exact quantum contributions are added to the calculation, the agreement between theory and experiment improves; (b) the resonance structure characterizing low-energy probabilities survives the averaging over partial waves.

The quantum threshold behavior of the Na+HF reaction

Full three dimensional quantum calculations of reactive properties of the Na+HF system have been performed at zero total angular momentum (J=0) to investigate the energy dependence of the reactive probability of this reaction. The effect of increasing the vibrational excitation of reactants is also discussed.

Parallel Time Independent Quantum Calculations of Atom Diatom Reactivity

Some models for the parallel organization of quantum reactive computer codes are discussed. The need for articulating the parallelism at different levels is examined and possible solutions are worked out by analyzing the structure of related theoretical and computational approaches. For the reduced dimensionality program, for which the solution of the bound state problem needs little cpu time, a multilevel task farm parallelization over the angle at the upper level and over the propagation at the lower level was found to be appropriate. On the contrary, for the full dimensional method the solution of the bound state problem is time consuming and has to be parallelized. In this case, a different model has been adopted: the calculations of the surface functions relative to a given subset of sectors have been grouped together and the subsets have been distributed for parallel calculation using a single program multiple data model.

An extension of the molecular simulator GEMS to calculate the signal of crossed beam experiments

By exploiting the potentialities of collaborative work and of high throughput computing on the grid platform recently deployed within the European Grid Initiative and made available to the virtual organization COMPCHEM, it has been possible to extend GEMS, a simulator of molecular systems, to reproduce in an ab initio fashion the signal measured in molecular beam experiments. As a case study the crossed beam experiment measuring the differential cross section of the OH(vOH = 0, jOH = 0) + CO(vCO = 0, jCO = 0) → H + CO2 reaction has been considered. The results of the calculations provide a univocal evaluation of the accuracy of the ab initio potential energy surfaces proposed in the literature.

Quantum isotopic effects and reaction mechanisms: the Li+HF reaction

Zero total angular momentum exact quantum probabilities for Li atoms reacting with HF and its isotopic variants were calculated at total energy values ranging from threshold up to 0.6 eV. In this way it has been possible to estimate product distributions as well as the dependence of the reactive probability upon orientation and rotational excitation of reactants. Computed values are compared with quantities obtained from the experiment. To estimate the reactive cross section from zero total angular momentum probabilities the J-shifting model was adopted.

Computational granularity and parallel models to scale up reactive scattering calculations

The computational granularity is a key factor in determining the suitability of a parallel model for an efficient implementation of complex codes on concurrent processor architectures. The paper discusses these aspects with reference to various theoretical approaches to reactive scattering and to the scale up of related calculations.

Vertical Profiles and Chemical Properties of Aerosol Particles upon Ny-Ålesund (Svalbard Islands)

Size-segregated particle samples were collected in the Arctic (Ny-Alesund, Svalbard) in April 2011 both at ground level and in the free atmosphere exploiting a tethered balloon equipped also with an optical particle counter (OPC) and meteorological sensors. Individual particle properties were investigated by scanning electron microscopy coupled with energy dispersive microanalysis (SEM-EDS). Results of the SEM-EDS were integrated with particle size and optical measurements of the aerosols properties at ground level and along the vertical profiles. Detailed analysis of two case studies reveals significant differences in composition despite the similar structure (layering) and the comparable texture (grain size distribution) of particles in the air column. Differences in the mineral chemistry of samples point at both local (plutonic/metamorphic complexes in Svalbard) and remote (basic/ultrabasic magmatic complexes in Greenland and/or Iceland) geological source regions for dust. Differences in the particle size and shape are put into relationship with the mechanism of particle formation, that is, primary (well sorted, small) or secondary (idiomorphic, fine to coarse grained) origin for chloride and sulfate crystals and transport/settling for soil (silicate, carbonate and metal oxide) particles. The influence of size, shape, and mixing state of particles on ice nucleation and radiative properties is also discussed.