The Color and Stability of Maya Blue: TDDFT Calculations

Car-Parrinello structural optimizations of realistic models of the Maya Blue (MB) hybrid material are combined with TDDFT calculations of the electronic excitation spectra to identify the nature of the fundamental guest-host interactions leading to the unusual stability of this pigment. The comparison with the features of the experimental visible spectrum reveals that the main mode of interaction between the host solid (the palygorskite clay) and the guest molecule (the organic indigo dye) involves the coordination of the carbonyl group of the dye by Al3+ ions exposed at the edge of the palygorskite tunnels. Analogous Mg2+-dye interactions which do not strongly affect the MB visible spectrum can also be present. Thermal treatment used in the preparation of the pigment appears therefore essential to release some of the structural water molecules tightly bound to the Al3+ ions in the internal clay surface, thus leaving them available to coordinate the organic molecule. Moderate heating also favors the oxidation of indigo to dehydroindigo (DHI): the spectral features of the latter complex with Al3+ are in remarkable agreement with the experimental spectrum, thus confirming the substantial role of DHI in the properties of Maya Blue

Competition between water and hydrogen peroxide at Ti center in Titanium zeolites. An ab initio study

A combined Car 12Parrinello molecular dynamics blue moon sampling approach has been adopted to study the competitive attack of H2O and H2O2 at a tetracoordinated Titanium site in a Ti 12zeolite. The results indicate that, although the attack of water to form a trigonal bipyramidal center is thermodynamically more stable, the attack of hydrogen peroxide to form a similar adduct is kinetically favored. In both cases, solvent cooperation is effective in the formation of the adducts. The relevance of such a result in relation to the catalytic properties of Ti 12zeolites is discussed

Interactions, Behavior, And Stability of Fluorenone inside Zeolite Nanochannels

The development of functional materials based on the supramolecular organization of photoactive species in nanosized porous matrices requires a deep knowledge of host 12guest interactions and of their influence on material properties and stability. Extensive first-principles investigations on the fluorescent dye fluorenone inside zeolite L, both at dry conditions and in the presence of water, have unraveled the molecular origin of the peculiar stability of this composite in humid environments, a fundamental prerequisite for practical applications. Results of first-principles molecular dynamics simulations, structural optimizations, and TDDFT calculations, validated by comparison with experimental data, provide a comprehensive picture of the structure, energetics, electronic excitation properties, and room-temperature behavior of the fluorenone/zeolite L composite and predict a substantial optical anisotropy for this material also maintained upon contact with water. The interaction of the fluorenone carbonyl group with the zeolite extraframework potassium cations is responsible for the dye stabilization in zeolite L nanochannels and features itself as a general leitmotiv regarding important properties of carbonyl functionalized photoactive species in hydrophilic matrices

Water in acid boralites: Hydration effects on framework B sites

Properties and behavior of protonated boron-containing zeolites at different hydration degree have been investigated by means of periodic DFT approaches. Geometry optimization and room-temperature Car-Parrinello molecular dynamics results, in line with experimental findings, indicate that the BO3-bound silanolic acid site typical of dry boralites should convert to a solvated H3O+ hydrogen bonded to tetrahedral BO4 at moderate water content. By increasing the water loading, the tetrahedral structure of the B site is stabilized and the physicochemical properties of the water molecules solvating the acid proton gradually approach the liquid-phase ones. A relevant role of structural and vibrational properties of the zeolite framework in the water-induced trigonal-to-tetrahedral transition at the B site is highlighted by simulation results

TS-1 from First Principles

First 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

"Hot" Surface Activation of Molecular Complexes: Insight from Modeling Studies

Rock-and-roll over hot floors: Theoretical modeling of the first activation stages of a Cu complex on top of a heated surface (750 K) revealed two mobility regimes, a slow bump-and-rock diffusion over the surface and a fast roll-and-go motion accompanied by significant temperature-induced bond oscillations. This study enables a deeper insight into "hot" surface molecular activation processes.Tanz auf dem Vulkan: Das Modellieren der ersten Aktivierungsstufen eines Cu-Komplexes auf einer beheizten Oberfl\ue4che (750\u2005K) enth\ufcllte zwei Bewegungsarten: eine langsame Diffusion durch \u201eAnsto fen und Taumeln\u201c und eine schnelle Rollbewegung, die mit deutlichen temperaturinduzierten Bindungsoszillationen einhergeht. Diese Befunde geben einen Einblick in die Prozesse bei der Aktivierung durch \u201ehei fe\u201c Oberfl\ue4chen

Constraint methods for determining pathways and free energy of activated processes

Activated processes from chemical reactions up to conformational transitions of large biomolecules are hampered by barriers which are overcome only by the input of some free energy of activation. Hence, the characteristic and rate-determining barrier regions are not sufficiently sampled by usual simulation techniques. Constraints on a reaction coordinate r have turned out to be a suitable means to explore difficult pathways without changing potential function, energy or temperature. For a dense sequence of values of r, the corresponding sequence of simulations provides a pathway for the process. As only one coordinate among thousands is fixed during each simulation, the pathway essentially reflects the system's internal dynamics. From mean forces the free energy profile can be calculated to obtain reaction rates and insight in the reaction mechanism. In the last decade, theoretical tools and computing capacity have been developed to a degree where simulations give impressive qualitative insight in the processes at quantitative agreement with experiments. Here, we give an introduction to reaction pathways and coordinates, and develop the theory of free energy as the potential of mean force. We clarify the connection between mean force and constraint force which is the central quantity evaluated, and discuss the mass metric tensor correction. Well-behaved coordinates without tensor correction are considered. We discuss the theoretical background and practical implementation on the example of the reaction coordinate of targeted molecular dynamics simulation. Finally, we compare applications of constraint methods and other techniques developed for the same purpose, and discuss the limits of the approach

The Quantum-Mechanical Position Operator in Extended Systems

The position operator (defined within the Schroedinger representation in the standard way) becomes meaningless when periodic boundary conditions are adopted for the wavefunction, as usual in condensed matter physics. We show how to define the position expectation value by means of a simple many-body operator acting on the wavefunction of the extended system. The relationships of the present findings to the Berry-phase theory of polarization are discussed.Comment: Four pages in RevTe

Electron Localization in the Insulating State

The insulating state of matter is characterized by the excitation spectrum, but also by qualitative features of the electronic ground state. The insulating ground wavefunction in fact: (i) sustains macroscopic polarization, and (ii) is localized. We give a sharp definition of the latter concept, and we show how the two basic features stem from essentially the same formalism. Our approach to localization is exemplified by means of a two--band Hubbard model in one dimension. In the noninteracting limit the wavefunction localization is measured by the spread of the Wannier orbitals.Comment: 5 pages including 3 figures, submitted to PR

Direct Solvent-Free Amide Bond Formation Catalyzed by Anatase-TiO2 Surface: Insight from Modeling

Amide bond formation processes are of paramount relevance for a broad spectrum of applications. Conventional amidation protocols typically rely on drastic reaction conditions and the use/disposal of large amounts of chemicals. These limitations may be bypassed by heterogeneously catalyzed amidation at dry conditions. However, progress is hindered because the mechanisms of these processes are largely unexplored. By using ab initio metadynamics, a concerted one-step mechanism is proposed for the solvent-free condensation of methylamine and formic acid on TiO2(101)-anatase, leading to methylformamide with concomitant release of molecular water. The activation barrier-14.3 kcal mol-1-is in line with the mild conditions experimentally adopted in amide bond syntheses on TiO2 nanoparticles. The mechanism disclosed herein reveals the key role of Ti4+ sites located on stoichiometric (101) anatase surfaces in promoting amide-bond formation at the TiO2/vapor interface. The acid strength of the adsorbed HCOOH molecules may be tuned by the HCOOH surface coverage, thus influencing the outcome of the amidation reaction. These molecular-level insights may foster further endeavors to improve/upscale TiO2-catalyzed amide syntheses at dry conditions, while raising the interest toward amidation processes at the surface/vapor interface promoted by economically and environmentally sustainable metal oxide nanomaterials