Visualization of a Discrete Solid-State Process with Steady-State X-ray Diffraction: Observation of Hopping of Sulfur Atoms in Single Crystals of Realgar

Relationship between spatially heterogeneous reaction dynamics and photochemical kinetics in single crystals of anthracene derivatives

Understanding physicochemical property changes based on reaction kinetics is required to design materials exhibiting desired functions at arbitrary timings. In this work, we investigated the photodimerization of anthracene derivatives in single crystals. Single crystals of 9-cyanoanthracene (9CA) and 9-anthraldehyde (9AA) exhibited reaction front propagation on the optical length scale, while 9-methylanthracene and 9-acetylanthracene crystals underwent spatially homogeneous conversion. Moreover, the sigmoidal behavior in the absorbance change associated with the reaction was much pronounced in the case of 9CA and 9AA and correlated with the observation of heterogeneous reaction progress. A kinetic analysis based on the Finke-Watzky model showed that the effective quantum yield of the photochemical reaction changes by more than an order of magnitude during the course of the reaction in 9CA and 9AA. Both the reaction front propagation and nonlinear kinetic behavior could be rationalized in terms of the difference in the cooperativity of the reactions. We propose a plausible mechanism for the heterogeneous reaction progress in single crystals that depends on the magnitude of the conformational change required for reaction. Our results provide useful information to understand the connection between photochemical reaction progress in the crystalline phase and the dynamic changes in the physicochemical properties.

Parchments and coloring materials in two IXth century manuscripts: On-site non-invasive multi-techniques investigation

One among the most famous ancient parchments in the Islamic world, dating back to the IXth century, along with another contemporary one have been investigated by means of a completely non-invasive multi-techniques analysis combining all of elemental XRF and structural/molecular Raman, ATR-FTIR and FOR spectroscopies besides pHs and colorimetric measurements. The materials initially used in the preparation of the writing supports were identified; in addition to calcite at different extents, the parchments seem to be condensed tannins-pretreated. Furthermore, the exploration of amide I and II vibrational bands and sub-bands illustrated collagen gelatinization and molecular helix disorders phenomena. Parchments and inks degradation products, gypsum and calcium oxalates, have been also identified. In both manuscripts the writing black inks have been characterized as iron gall type. The corrosive effect of these black inks appears deeply damaging the acidic parchment supports. In this case, the high frequency side of the infrared spectrum disappears completely, illustrating thus irreversible parchment degradations. Gum Arabic, when added sufficiently in the black ink recipe, seems preventing the corrosive damaging effect induced by Fe²⁺ ions spreading from the black ink into the parchment body. Besides, all coloring materials have been identified; gold in gilded scriptures and natural pigments for vocalizations and basic decorations: cinnabar for reds, lapis-lazuli for blue and orpiment/realgar/pararealgar arsenic sulfides for yellow/orange shades. The anthraquinone-based kermesic acid extracted from Kermes female insects had been used for dark pink decorations, while the copper-based pigment verdigris had been synthetized to achieve green motifs. These results enrich the knowledge of ancient writing supports and materials, and highlight technologies and practices developed by middle-ages craftsmen.

The Jacopo Tintoretto "Wedding Feast at Cana": A non-invasive and multi-technique analytical approach for studying painting materials

This paper presents the results of a fully non-invasive photographic and spectroscopic study on a masterpiece by the famous Venetian painter Jacopo Tintoretto (1518-1594): "The Wedding feast at Cana" (1561), located inside the sacristy of the Santa Maria della Salute Basilica in Venice. A restoration campaign was carried out on the painting between 2016 and 2017. Various techniques were chosen to characterize in detail the materials used by the painter, especially pigments: Near Infrared Reflectography (NIR) to identify preparatory sketch and also allowing formulating hypothesis on some pigments; Raman and Fiber Optics Reflectance Spectroscopy (FORS) applied directly to the surface of the object, avoiding sample collection, to identify the pigments palette applied by Tintoretto. Information about binders was also obtained in a totally non-invasive way by using NIR spectroscopy. All analysis points were precisely (±2 mm) registered on a painting virtual map in order to recover them in a future possible conservation project.

Study on electronic structures and properties of neutral and charged arsenic sulfides [As n S3 ((-1,0,+1)), n =1-6] with the Gaussian-3 scheme

The structures and energies of neutral and charged arsenic sulfides As n S3 ((-1,0,+1)) (n = 1-6) were studied systematically with the G3 method. The ground-state structures of these species are reported. The ground-state structures of As n S3 with n ≥ 4 can be considered as resulting from the replacement of an As atom of the ground-state structure of neutral As n+1S2 by an S atom. In neutral As n S3, the character of sulfur bonding is edge-bridging. The ground-state structures of anion As n S3 (-) sometimes differ from their corresponding neutral structures. In such case, they exhibit a terminal sulfur atom. The ground-state structures of cationic As n S3 (+) are also sometimes different from the corresponding neutral ones. There, sulfur bonding can exhibit face-capping and arsenic can be four-fold coordinated. The potential energy surfaces of As4S3 (+) and As5S3 (+) are very flat and co-existence of various isomers of As4S3 (+) and As5S3 (+) is possible. Reliable adiabatic electron affinities (AEAs) and adiabatic ionization potentials (AIPs) of As n S3 are predicted. There are odd-even alternations in both AEAs and AIPs as a function of size. In addition, the reliable vertical detachment energies (VDEs) and vertical ionization potentials (VIPs) are presented. The dissociation energies (DEs) of S (and/or its ion S((-/+))) from As n S3 species and their ions were calculated to examine relative stabilities. The hardnesses and HOMO-LUMO gaps of As n S3 (n = 1-6) were evaluated and used to discuss relative chemical reactivity.

Chemical x-ray photodiffraction: principles, examples, and perspectives

X-ray photodiffraction (in the chemical literature also referred to as photocrystallography), which is based on the combination of X-ray diffraction methods with samples excited by UV or visible light to solve fundamental photochemical or photophysical issues, has developed in the last couple of decades into a very promising technique for direct observation of photoinduced chemical species in the solid state. The capability of providing direct information on very small perturbations in atomic positions and thus on the minute changes in molecular geometry during (or as a consequence of) photoexcitation appears to be the most important asset of this emerging analytical technique. When combined with other physicochemical methods, X-ray photodiffraction can be a powerful tool for analysis of steady-state photoinduced structures as well as slow or very fast time-dependent phenomena. Despite being a very useful approach, however, due to a number of practical requirements that it places with regard to the system to be studied, at the present stage of developments the technique is not widely and indiscriminately applicable to any photoinduced process. In some particular chemical systems the inherent pitfalls could be practically overcome by practical or theoretical means. In this short chapter, the basic principles of X-ray photodiffraction are briefly summarized, and the prospects of its application to "physical" and "chemical" problems is illustrated with selected examples from recent literature. Some possible future developments and alternative approaches with this and related methods are also presented.