170 research outputs found
Cagelike clusters as models for the isolated hydroxyls of silica: ab-initio B3-LYP calculationsof the interaction with ammonia
No full textCage-like clusters as models for the hydroxys of silica: ab-initio calculations of 1H and 29Si NMR chemical shifts
No full textVibrational modes of isolated hydroxyls of silica computed ab-initio in a cluster approach
No full textQuantum mechanical ab-initio characterization of a simple periodic model of the silica surface
No full textAb Initio Calculation of the Crystalline Structure and IR Spectrum of Polymers: Nylon 6 Polymorphs
No full textState-of-the-art computational methods in
solid-state chemistry were applied to predict the structural
and spectroscopic properties of the α and γ crystalline
polymorphs of nylon 6. Density functional theory calculations
augmented with an empirical dispersion correction (DFT-D)
were used for the optimization of the two different crystal
structures and of the isolated chains, characterized by a
different regular conformation and described as one-dimen-
sional infinite chains. The structural parameters of both
crystalline polymorphs were correctly predicted, and new
insight into the interplay of conformational effects, hydrogen
bonding, and van der Waals interactions in affecting the
properties of the crystal structures of polyamides was obtained.
The calculated infrared spectra were compared to experimental data; based on computed vibrational eigenvectors, assignment of
the infrared absorptions of the two nylon 6 polymorphs was carried out and critically analyzed in light of previous investigations.
On the basis of a comparison of the computed and experimental IR spectra, a set of marker bands was identified and proposed as
a tool for detecting and quantifying the presence of a given polymorph in a real sample: several marker bands employed in the
past were confirmed, whereas some of the previous assignments are criticized. In addition, some new marker bands are proposed.
The results obtained demonstrate that accurate computational techniques are now affordable for polymers characterization,
opening the way to several applications of ab initio modeling to the study of many families of polymeric materials
Spectroscopic and thermodynamic study of the H-bonding of olefins onto the isolated hydroxyl of amorphous silica
No full textAdducts of alkali-metal ions with the C-C triple bond: an experimental and ab-initio study
No full textIntermolecular Interaction Energies in Molecular Crystals: Comparison and Agreement of Localized Møller–Plesset 2, Dispersion-Corrected Density Functional, and Classical Empirical Two-Body Calculations
No full textA comparative analysis of the intermolecular energy for a data set including 60 molecular crystals with a large variety of functional groups has been carried out using three different computational approaches: (i) a method based on a physically meaningful empirical partition of the interaction energy (PIXEL), (ii) density functional methods with a posteriori empirical correction for the dispersion interactions (DFT-D), and (iii) a full periodic ab initio quantum mechanical method based on Møller-Plesset perturbation theory for the electron correlation using localized crystal orbitals (LMP2). Due to the large computational cost, LMP2 calculations have been restricted to a subset of seven molecular crystal comprising benzene, formic acid, formamide, succinic anhydride, urea, oxalic acid, and nitroguanidine, and the results compared with PIXEL and DFT-D data as well as with the experimental data show excellent agreement among all adopted methods. This shows that both DFT-D and PIXEL approaches are robust predictive tools for studying molecular crystals. A detailed analysis shows a very similar dispersion contribution of the two methods across the 60 considered molecular crystals. The study also confirms that pure DFT shows serious deficiencies in properly handling molecular crystals in which the dispersive contribution is large. Due to the negligible requested computational resources, PIXEL is the method of choice in screening of a large number of molecular crystals, an essential step to predict crystal polymorphism or to study crystal growth processes. DFT-D can then be used to refine the ranking emerged from PIXEL calculations due to its general applicability and robustness in properly handling short-range interactions
Experimental and quantum chemical studies on the adsorption of carbon dioxide on alkali-metal-exchanged ZSM-5 zeolites
No full textThe adsorption of carbon dioxide onto M-ZSM-5 zeolites (M = Li, Na, K, Cs) was studied by means of FTIR spectroscopy and adsorption microcalorimetry. Quantum chemical calculations, at the B3-LYP level, on the interaction of CO2 with the bare alkali-metal cations were performed to assist interpretation of the experimental results. With the likely exception of Li+, CO2 was found to undergo a two-step interaction with the metal ions. At a low equilibrium pressure linear 1:1 adducts of the type M+···OCO (M = Na+, K+, Cs+) are formed; upon increasing the CO2 equilibrium pressure, the metal cation coordinates a second CO2 molecule, forming a 2:1 adduct. Calculated (ab initio) bond lengths for the 1:1 adduct are given, as well as corresponding values of the binding energy and enthalpic term. Experimentally derived values of the main thermodynamic functions (ΔH°, ΔG°, and ΔS°) are discussed and correlated with detailed results from IR spectroscopy. The interaction cation/CO2 alone cannot account for the body of evidence, and the contribution of nearby O2- anions has to be invoked
Thermo-chemical and thermo-physical properties of the high-pressure phase anhydrous B (Mg\u3csub\u3e14\u3c/sub\u3e Si \u3csub\u3e5\u3c/sub\u3e O \u3csub\u3e24\u3c/sub\u3e ): An ab-initio all-electron investigation
No full textUsing the hybrid B3LYP density functional method, we computed the ab-initio thermo-chemical and -physical properties of the mineral anhydrous B (Anh-B), which has been recently suggested as a potential phase responsible for the X-discontinuity in the Earth\u27s mantle at ∼300 km depth through the reaction forsterite + periclase = Anh-B, and also to likely split the 410 km discontinuity within the interior of a cold slab through the reaction wadsleyite/ringwoodite = Anh-B + stishovite. We first conducted an investigation of the static properties through a symmetry-preserving relaxation procedure and then computed, on the equilibrium structure, harmonic vibrational modes at the long-wavelength limit corresponding to the center of the Brillouin zone (k → 0). While optic modes are the eigenvectors of the Hessian matrix at G point, acoustic modes were obtained by solving the non-zero components of the strain matrix. Following the Kieffer model, acoustic branches were assumed to follow sine wave dispersion when traveling within the Brillouin zone. All thermodynamic properties that depend on vibrational frequencies namely, heat capacities, thermal expansion, thermal derivative of the bulk modulus, thermal correction to internal energy, enthalpy, Gibbs free energy, thermal pres-sure and Debye temperature, were computed on the basis of quasi-harmonic mode-gamma analysis of the volume effects on vibrational frequencies. Moreover, the strain tensor was used to calculate several thermo-physical properties of geophysical interest (transverse and longitudinal wave veloci-ties, shear modulus, Young\u27s modulus, and Poisson\u27s ratio). The ab-initio results derived in this study and the available data on molar volumes were used to calculate the univariant equilibrium forsterite + periclase = Anh-B. The results are in satisfactory agreement with the reversed experimental data of Ganguly and Frost (2006)
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