1,721,032 research outputs found
Host/guest interactions and femtosecond scale proton exchange in a zeolitic cage
No full textWe present a first-principles computer simulation study of the sodium hydroxosodalite dihydrate Na-8[Al6Si6O24](OH)(2) . 2H(2)O. We have found that a fast proton exchange occurs, on the femtosecond scale, between two OH- encapsulated in the cavities (beta-cages) of the sodalite. The hosted species, formally two O2H3- groups not directly linked to the framework, show a correlation with the motion of the sodalite framework, indicating a nonthermal coupling between the zeolitic host and its molecular guests. The presented results are in agreement with experimental data
On the adiabatic dynamics of paramagnetic ion pairs
No full textA model describing the adiabatic dynamics of a paramagnetic ion pair in a simple fluid is presented. A mixed classical-quantum mechanical approach is adopted. The quantum degrees of freedom, i.e. the unpaired electron in the authors case, are treated via the time dependent Schrodinger equation, while the other particle are dealt with by classical molecular dynamics. The time evolution of the excess electron density as a function of environmental changes is discusse
Dynamical host/guest interactions in zeolites: Framework isotope effects on proton transfer studied by Car-Parrinello molecular dynamics
No full textTS-1 from First Principles
No full textFirst principles Studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO2 are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi95O192] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Ti's per unit cell [Ti2Si94O192] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO4 tetrahedra, is discussed. By using Cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Ti's occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from Occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites
Structure and dynamics of nitrite sodalite. An ab initio study
No full textWe present a first-principles study of nitrite sodalite, a synthetic zeolite. The NO2- anions are hosted in octahedral cavities typical of such crystals, and each anion is surrounded by four Na cations in a tetrahedral geometry. We found that NO2- is in a rotational state inside the zeolite's cages. This rotational state explains the orientational disorder experimentally detected in nitrite sodalite
The hydrophobic hydration of oxygen: A Monte Carlo investigation
No full textA simulation of a dilute solution of O-2 in water under standard conditions was performed with the Monte Carlo technique. In line with previous simulations of apolar solutes in water, our results predict a clathrate like structure of the water molecules around the solute. In the first solvation shell, formed by 17 water units, the molecules are tangentially arranged around the solute, causing a local distortion of the three-dimensional hydrogen bond network typical of water. This distortion is compensated by stronger interactions among the water molecules within the hydration shell
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