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The optical absorption properties of (poly)aromatic hydrocarbons occluded in a nanoporous environment were investigated by theoretical and experimental methods. The carbonaceous species are an essential part of a working catalyst for the methanol-toolefins (MTO) process. In situ UV/Vis microscopy measurements on methanol conversion over the acidic solid catalysts H-SAPO-34 and H-SSZ-13 revealed the growth of various broad absorption bands around 400, 480, and 580 nm. The cationic nature of the involved species was determined by interaction of ammonia with the methanol- treated samples. To determine which organic species contribute to the various bands, a systematic series of aromatics was analyzed by means of time-dependent density functional theory (TDDFT) calculations. Static gas-phase simulations revealed the influence of structurally different hydrocarbons on the absorption spectra, whereas the influence of the zeolitic framework was examined by using supramolecular models within a quantum mechanics/molecular mechanics framework. To fully understand the origin of the main absorption peaks, a molecular dynamics (MD) study on the organic species trapped in the inorganic host was essential. During such simulation the flexibility is fully taken into account and the effect on the UV/ Vis spectra is determined by performing TDDFT calculations on various snapshots of the MD run. This procedure allows an energy absorption scale to be provided and the various absorption bands determined from in situ UV/Vis spectra to be assigned to structurally different species.

Ammonia-selective catalytic reduction (NH3-SCR) using Cu zeolites is a well-established strategy for the abatement of NOx gases. Recent studies have demonstrated that Cu is particularly active when exchanged into the SSZ-13 zeolite, and its location in either the 6r or 8r renders it an excellent model system for fundamental studies. In this work, we examine the interaction of NH3-SCR relevant gases (NO and NH3) with the Cu2+ centers within the SSZ-13 structure, coupling powder diffraction (PD), X-ray absorption spectroscopy (XAFS), and density functional theory (DFT). This combined approach revealed that, upon calcination, cooling and gas exposure Cu ions tend to locate in the 8r window. After NO introduction, Cu-ions are seen to coordinate to two framework oxygens and one NO molecule, resulting in a bent Cu-nitrosyl complex with a Cu-N-O bond angle of similar to 150 degrees. Whilst Cu seems to be partially reduced/changed in coordination state, NO is partially oxidized. On exposure to NH3 while the PD data suggest the Cu2+ ion occupies a similar position, simulation and XAFS pointed toward the formation of a Jahn-Teller distorted hexaamine complex [Cu(NH3)(6)](2+) in the center of the cha cage. These results have important implications in terms of uptake and storage of these reactive gases and potentially for the mechanisms involved in the NH3-SCR process.

Trans Effect and Trans Influence: importance of metal mediated ligand-ligand repulsion.

The formation and nature of active sites for methanol conversion over solid acid catalyst materials are studied by using a unique combined spectroscopic and theoretical approach. A working catalyst for the methanol-to-olefin conversion has a hybrid organic-inorganic nature in which a cocatalytic organic species is trapped in zeolite pores. As a case study, microporous materials with the chabazite topology, namely, H-SAPO-34 and H-SSZ-13, are considered with trapped (poly) aromatic species. First-principle rate calculations on methylation reactions and in situ UV/Vis spectroscopy measurements are performed. The theoretical results show that the structure of the organic compound and zeolite composition determine the methylation rates: 1) the rate increases by 6 orders of magnitude if more methyl groups are added on benzenic species, 2) transition state selectivity occurs for organic species with more than one aromatic core and bearing more than three methyl groups, 3) methylation rates for H-SSZ-13 are approximately 3 orders of magnitude higher than on H-SAPO-34 owing to its higher acidity. The formation of (poly) aromatic cationic compounds can be followed by using in situ UV/Vis spectroscopy because these species yield characteristic absorption bands in the visible region of the spectrum. We have monitored the growth of characteristic peaks and derived activation energies of formation for various sets of (poly) aromatic compounds trapped in the zeolite host. The formation-activation barriers deduced by using UV/Vis microspectroscopy correlate well with the activation energies for the methylation of the benzenic species and the lower methylated naphthalenic species. This study shows that a fundamental insight at the molecular level can be obtained by using a combined in situ spectroscopic and theoretical approach for a complex catalyst of industrial relevance.

Theoretical and experimental IR data are combined to gain insight into the methanol and ethanol conversion over an acidic H-SAPO-34 catalyst. The theoretical simulations use a large finite cluster and the initial physisorption energy of both alcohols is calculated. Dispersive contributions turn out to be vital and ethanol adsorbs stronger than methanol with approximately 14 kJ mol(-1). Calculated IR spectra of the alcohols and of formed aromatic cations upon conversion are also analyzed and support the peak assignment of the experimental in situ DRIFT spectra, in particular for the C-H and C=C regions. Theoretical IR spectra of the gas phase compounds are compared with those of the molecules loaded in a SAPO cluster and the observed shifts of the peak positions are discussed. To get a better understanding of these framework-guest interactions, a new theoretical procedure is proposed based on a normal mode analysis. A cumulative overlap function is defined and enables the characterization of individual peaks as well as induced frequency shifts upon adsorption. (C) 2010 Elsevier B. V. All rights reserved.

The kinetics of the free-radical propagation of methyl acrylate (MA), methyl methacrylate (MMA), ethyl alpha-fluoroacrylate (EFA), ethyl alpha-chloroacrylate (ECA), ethyl alpha-cyanoacrylate (ECNA), and methyl alpha-hydroxymethacrylate (MHMA) have been calculated using quantum chemical tools. Various DFT functionals such as BMK, BBIK, MPW1B95, MPW1K, and MPWB1K were used to model the relative propagation kinetics of the monomers. Among the methodologies used, MPWB1 K/6-311+G(3df,2p)//B3LYP/6-31+G(d) was found to yield the best qualitative agreement with experiment. We explored chain length effects by examining addition reactions of monomeric, dimeric, trimeric, and tetrameric radicals to the monomers. We have also modeled the tacticity of the widely used monomers MA and MMA by considering all of the alternatives of attack of the radical in the 3D space around the monomer. This study has qualitatively confirmed the experimental syndiotactic/isotactic ratio of 66/3 for MMA. Finally, the kinetics of the initiation to polymerization for MA and MMA is also successfully reproduced.

Microscopic calculations of Compton scattering on the free proton and light nuclei are presented. For the description of Compton scattering on the proton the conventional K-matrix approach and the "Dressed K-Matrix" model are introduced. The latter approach can be used to calculate polarizabilities as well as Compton scattering for photon energies upto 1 GeV since it obeys the symmetry properties which are appropriate in the different energy regions. In particular, crossing symmetry, gauge invariance and unitarity are satisfied. The extent of violation of analyticity (causality) is used as an expansion parameter. Coherent Compton scattering on light nuclei at 200-300 MeV is studied in the impulse approximation and is shown to be a sensitive probe of the in-medium properties of the Delta-resonance. Modifications of the properties of the Delta-resonance due to the nuclear medium are accounted for through the self-energy operator of the Delta. The dominant medium effects such as the Pauli blocking effects in the decay width, effective nucleon mass and particle-hole excitations in the pion propagator axe consistently included in nuclear matter.

Coherent Compton scattering on light nuclei in the Δ-resonance region is studied in the impulse approximation and is shown to be a sensitive probe of the in-medium properties of the Δ resonance. The elementary amplitude on a single nucleon is calculated from the unitary K-matrix approach developed previously. Modifications of the properties of the Δ resonance due to the nuclear medium are accounted for through the self-energy operator of the Δ, calculated from the one-pion loop. The dominant medium effects such as the Pauli blocking, mean-field modification of the nucleon and Δ masses, and particle-hole excitations in the pion propagator are consistently included in nuclear matter.

The process p + d He-3 + gamma * at intermediate energies is described using a covariant and gauge-invariant model, and a realistic pd(3)He vertex, Both photodisintegration of He-3 and proton-deuteron capture with production of e(+)e(-) pairs are studied, and results for cross sections and response functions are presented. The effect of time-like form factors on the dilepton cross sections is investigated as well

The spectroscopic factors for the low-lying quasihole states observed in the O-16(e,e'p)N-15 reaction are reinvestigated with a variational Monte Carlo calculation for the structure of the initial and final nucleus. A computational error in a previous report is rectified. It is shown that a proper treatment of center-of-mass motion does not lead to a reduction of the spectroscopic factor for p-shell quasihole states, but rather to a 7% enhancement. This is in agreement with analytical results obtained in the harmonic oscillator model. The center-of-mass effect worsens the discrepancy between present theoretical models and the experimentally observed single-particle strength. We discuss the present status of this problem, including some other mechanisms that may be relevant in this respect. [S0556-2813(98)03005-2].

The spectroscopic factors for the low-lying quasi-hole states observed in the 16O(e,e'p)15N reaction are reinvestigated with a variational Monte Carlo calculation for the structure of the initial and final nucleus. A computational error in a previous report is rectified. It is shown that a proper treatment of center-of-mass motion does not lead to a reduction of the spectroscopic factor for $p$-shell quasi-hole states, but rather to a 7% enhancement. This is in agreement with analytical results obtained in the harmonic oscillator model. The center-of-mass effect worsens the discrepancy between present theoretical models and the experimentally observed single-particle strength. We discuss the present status of this problem, including some other mechanisms that may be relevant in this respect., 14 pages, no figures, uses Revtex, to be published in Phys. Rev. C 58 (1998)

The process p+d \leftrightarrow 3He + \gamma* at intermediate energies is described using a covariant and gauge-invariant model, and a realistic pd3He vertex. Both photodisintegration of 3He and proton-deuteron capture with production of e+e- pairs are studied, and results for cross sections and response functions are presented. The effect of time-like formfactors on the dilepton cross sections is investigated as well., Comment: 10 pages, 4 figures, Revtex, to be published in Physics Letters B

A technique to deal with Coulomb electron distortions in the analysis of (e,e'p) reactions is presented. Thereby, no approximations are made. The suggested technique relies on a partial-wave expansion of the electron wave functions and a multipole decomposition of the electron and nuclear current in momentum space. In that way, we succeed in keeping the computational times within reasonable limits. This theoretical framework is used to calculate the quasielastic (e,e'p) reduced cross sections for proton knockout from the valence shells in $^{16}$O, $^{40}$Ca, $^{90}$Zr and $^{208}$Pb. The final-state interaction of the ejected proton with the residual nucleus is treated within an optical potential model. The role of electron distortion on the extracted spectroscopic factors is discussed., 45 pages, 10 encapsulated postscript figures, Revtex, uses epsfig.sty and fancybox.sty, to be published in Physical Review C

The reduced (e,e$'$p) cross section is calculated for kinematics that probe high missing momenta. The final-state interaction is handled within a non-relativistic many-body framework. One- and two-body nuclear currents are included. Electron distortion effects are treated in an exact distorted wave calculation. It is shown that at high missing momenta the calculated (e,e$'$p) cross sections exhibit a pronounced sensitivity to ground-state correlations of the RPA type and two-body currents. The role of these mechanisms is found to be relatively small at low missing momenta., Comment: 15 pages in REVtex with embedded psfigures

The contributions from $^{12}$C$(e,e'pn)$ and $^{12}$C$(e,e'pp)$ to the semi-exclusive $^{12}$C$(e,e'p)$ cross section have been calculated in an unfactorized model for two-nucleon emission. We assume direct two-nucleon knockout after virtual photon coupling with the two-body pion-exchange currents in the target nucleus. Results are presented at several kinematical conditions in the dip and $��$(1232) regions. The calculated two-nucleon knockout strength is observed to account for a large fraction of the measured $(e,e'p)$ strength above the two-nucleon emission threshold., 12 Revtex pages, 4 postscript figures (available upon request), University of Gent preprint SSF94-02-01

The similarities in the experimental indications for multinucleon mechanisms in $(\gamma,p)$ and $(e,e'p)$ processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive $(\gamma,N)$ reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behaviour of the $^{12}$C$(\gamma,p)$ and $^{12}$C$(\gamma,n)$ data at high excitation energies in the residual nucleus. In the calculations effects from non-resonant and resonant pion exchange currents are included. Photoabsorption on these currents are predicted to produce the major contributions to the exclusive $^{16}$O$(\gamma,n_0)^{15}$O process at photonenergies above the pion threshold. Double differential cross sections for photon induced $pp$ and $pn$ emission from $^{16}$O are calculated and compared with the data., Comment: 18 pages, Revtex 3.0, 13 figures (available upon request), SSF93-10-01

The response functions for the unpolarized $(e,e'p)$ and polarized $(\vec{e},e'p)$ reaction are calculated for medium-heavy nuclei under quasifree conditions. The formalism presented here incorporates two-body currents related to meson-exchange and the $\Delta (1232)$ excitation. The final-state interaction of the outgoing nucleon with the residual nucleus is handled in a HF-RPA formalism. The sensitivity of the results to the two-body currents is discussed for the five structure functions in quasielastic $(\vec{e},e'p)$ scattering off the target nuclei $^{16}O$ and $^{40}Ca$. A selective sensitivity to the two-body currents is obtained in the longitudinal-transverse interference term $W_{LT}$ where two-body currents can explain part of the discrepancy between the impulse-approximation calculations and the data., Comment: 12 pages in LaTex with 5 figures (available on request), INW0703

A model is presented to describe electromagnetically induced two-nucleon emission processes in a shell-model picture. Distortions in the outgoing nucleon waves are accounted for by performing a partial-wave expansion in a real mean-field potential. The antisymmetry condition for the A-body wavefunctions is shown to be naturally preserved. The model is used to calculate ($��$,pn) and ($��$,pp) cross sections off the target nuclei $^{16}$O and $^{12}$C for photon energies ranging from 50 MeV up to the $\bigtriangleup$(1232) isobar threshold. Effects due to the pionic currents and intermediate $\bigtriangleup$ creation are implemented. The impact of the distortions due to the interaction of the outgoing nucleon waves with the (A-2) core is examined. Hadronic form factors are introduced to regularize the $��$NN vertices and the sensitivity of the cross section to the pion cut-off mass is examined. The relative contribution of the ($��$,pp) and the ($��$,pn) channel to the total photoabsorption strength is discussed. Further, the photon energy dependence of the ($��$,pp)/($��$,pn) ratio is investigated., 30 pages, 11 figures (available from the authors upon request), INW0701

The similarities in the experimental indications for multinucleon mechanisms in $(��,p)$ and $(e,e'p)$ processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive $(��,N)$ reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behaviour of the $^{12}$C$(��,p)$ and $^{12}$C$(��,n)$ data at high excitation energies in the residual nucleus. In the calculations effects from non-resonant and resonant pion exchange currents are included. Photoabsorption on these currents are predicted to produce the major contributions to the exclusive $^{16}$O$(��,n_0)^{15}$O process at photonenergies above the pion threshold. Double differential cross sections for photon induced $pp$ and $pn$ emission from $^{16}$O are calculated and compared with the data., 18 pages, Revtex 3.0, 13 figures (available upon request), SSF93-10-01

Stretched spin configurations in 116 Sn have been studied via the reactions 116 Sn(~, p'), 116 Sn(e, e'), 115 In(3 He, d) and 115 In( a,t). The high-spin negative parity two-neutron quasiparticle states within the N = 51-82 major shell appear to be tittle fragmented. The most prominent examples are the P' = 9- state at E x = 3.522 MeV mad two 7- states at 2.909 and 3.120 MeV. It is found that in contrast the proton configurations (g9/2, - 1 h11/2) and (g9/2, g7/2) are strongly fragmented. Large-basis BCS shell model calculation, using a SkE-force have been made and DWBA analyses of the (e, e') data and of the (~, p') data are presented.

The role of core polarization and of the quenching of the leading shell model configuration in stretched spin states is investigated in a case study of the 9- state at 3.522 MeV in 116Sn. With data on five independent observables available, this is the best documented example of a natural parity stretched (yrast) state. The observables are the electron scattering longitudinal and transverse form factors, the proton-scattering cross section and asymmetry, and the proton-stripping cross section on 115In. By introducing a fully microscopic description of the core polarization, it is possible to account quantitatively for all five observables using an effective nucleon-nucleon interaction based on the free t matrix for the proton scattering and the free values for the coupling constants for electron scattering. A short discussion on the 5- Ex=9.70 MeV state in 28Si and the 12+ Ex=6.026 MeV state in 208Pb is given. Quenching factors for the leading shell model components are found to be similar in magnitude as for unnatural parity states

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Assessment of atomic chrge models for gas-phase computations on polypeptides

Spin Polarized Conceptual DFT Study of the Ring Closure in Radicals