Modelling the Interaction between Carboxylic Acids and Zinc Oxide: Insight into Degradation of ZnO Pigments

Computational modelling applied to cultural heritage can assist the characterization of painting materials and help to understand their intrinsic and external degradation processes. The degradation of the widely employed zinc oxide (ZnO)—a white pigment mostly used in oil paints—leads to the formation of metal soaps, complexes of Zn ions and long-chain fatty acids coming from the degradation of the oil binder. Being a serious problem affecting the appearance and the structural integrity of many oil paintings, it is relevant to characterize the structure of these complexes and to understand the reaction pathways associated with this degradation process. Density functional theory (DFT) calculations were performed to investigate the adsorption of the acetate and acetic acid on relatively large ZnO clusters and the formation of Zn–acetate complexes. Carboxylic acids with longer alkyl chains were then investigated as more realistic models of the fatty acids present in the oil medium. In addition, DFT calculations using a periodic ZnO slab were performed in order to compare the obtained results at different levels of theory. Optimization calculations as well as the formation energies of the ZnO@carboxylate coupled systems and the thermodynamics leading to possible degradation products were computed. Our results highlight the potential for DFT calculations to provide a better understanding of oil paint degradation, with the aim of contributing to the development of strengthening and conservation strategies of paintings.     

Toda flows and real Hessenberg manifolds

We relate the Toda flow on the “p-part” of a semi-simple Lie algebra to the topology of real Hessenberg manifolds, and we obtain their mod2 Betti numbers by reversing Morse inequalities using a theorem of Floyd and a result on the Weyl group.     

Cyclohexenylphenyldiazene: a simple surrogate of the azobenzene photochromic unit

We have carried out an experimental and computational study on the ground- and excited-state photochemical and photophysical properties of (1-cyclohexenyl)phenyldiazene (CPD), a species formally derived from azobenzene in which one of the phenyl rings is replaced by a 1-cyclohexene substituent. The results show that CPD does substantially behave like azobenzene, but with a higher (approximately 70%) Phi(Z-->E) (npi*) photoisomerization quantum yield, calling for CPD as an effective alternative of azobenzene itself with new functionalization possibilities. By use of state-of-the-art ab initio CASPT2//CASSCF minimum energy path computations, we have identified the most efficient decay and isomerization routes of the absorbing singlet (pipi*), S1 (npi*), T1, and S0 states of CPD. The resulting mechanistic scheme agrees with experimental findings and provides a rationale of the observed photoisomerization quantum yields. Furthermore, this study provides a deep insight on the photophysical and photochemical properties of compounds based on the -N=N- double bond which supplies a general model for the photoreactivity of azobenzene-type compounds in general. This is expected to be a useful guideline for the design of novel photoreactive azo compounds.     

Controlled sound field with a dual layer loudspeaker array

Controlled sound interference has been extensively investigated using a prototype dual layer loudspeaker array comprised of 16 loudspeakers. Results are presented for measures of array performance such as input signal power, directivity of sound radiation and accuracy of sound reproduction resulting from the application of conventional control methods such as minimization of error in mean squared pressure, maximization of energy difference and minimization of weighted pressure error and energy. Procedures for selecting the tuning parameters have also been introduced. With these conventional concepts aimed at the production of acoustically bright and dark zones, all the control methods used require a trade-off between radiation directivity and reproduction accuracy in the bright zone. An alternative solution is proposed which can achieve better performance based on the measures presented simultaneously by inserting a low priority zone named as the “gray” zone. This involves the weighted minimization of mean-squared errors in both bright and dark zones together with the gray zone in which the minimization error is given less importance. This results in the production of directional bright zone in which the accuracy of sound reproduction is maintained with less required input power. The results of simulations and experiments are shown to be in excellent agreement.     

Monotone weak Lindelöfness

The definition of monotone weak Lindelöfness is similar to monotone versions of other covering properties: X is monotonically weakly Lindelöf if there is an operator r that assigns to every open cover U a family of open sets r(U) so that (1) ∪r(U) is dense in X, (2) r(U) refines U, and (3) r(U) refines r(V) whenever U refines V. Some examples and counterexamples of monotonically weakly Lindelöf spaces are given and some basic properties such as the behavior with respect to products and subspaces are discussed.     

On the substitution between saving and prevention

This work makes a joint analysis of prevention and saving decisions. First we determine the optimal levels of the two variables and we analyze substitution between them. Second we provide some results about the effects on optimal saving and prevention of changes in exogenous present and future wealth and in possible future loss. Finally we introduce insurance into the model and we extend the separation result, derived in the literature which studies the substitution between insurance and saving, to the case where prevention is considered too.     

Experimental study on hybrid control of multimodal cable vibrations

The effectiveness of a hybrid control policy in mitigating the linear and non-linear multimodal cable vibrations is investigated by means of a numerical analysis and a campaign of laboratory tests. The proposed control method is a hybrid solution combining wrapped shape memory alloy wires and an open-loop actuation. By operating in this way, a robust and economical control strategy is obtained which overcomes the damper/actuator localization and the state-tracking difficulties that significantly impair the overall control effectiveness of most of the control solutions already investigated in the literature. The proposed approach has shown, in a separate study, promising vibration mitigation capabilities when the motion was essentially dominated by the first in-plane mode. Here, the capability of the hybrid control policy in mitigating also the nonlinear vibrations of the second in-plane mode is under specific investigation.     

Expedient Route to the functionalized calyciphylline A-type skeleton via a Michael addition-RCM strategy

An efficient, robust, and scalable strategy to access the functionalized core of calyciphylline A-type alkaloids has been developed starting from commercially available 3-methylanisole. Key features of this approach are an intramolecular Michael addition/allylation sequence and a ring-closing metathesis step.     

Electron exchanges in nuclear spin conversion of hydrogen physisorbed on diamagnetic insulators

Models are provided and discussed to interpret new experiments on the ortho-paraconversion of hydrogen “physisorbed” on dielectric and diamagnetic surfaces.Electro-static and dynamical molecule-surface interactions complemented by hyperfinecontacts are shown to be generally more effective than the magnetic ones. Coulombrepulsion induces exchanges of molecular and surface electrons and excites triplet spinstates which are effective in the angular momenta transfers to the catalyst. Theconversion time is obtained as the square of a ratio of two energies: the exchange andexcitation ones. The main channel is found composed of triplet excitations of the order ofthe eV, induced by molecule-surface exchanges of about a hundred of meV. It explains thezinc and oxygen rates of about one minute observed on the MOF samples as well as the aboutten times slower ones on the ASW. The same mechanism is also shown to occur in thetransient regime, but faster. Finally it explains also the conversion of a few hoursobserved for interstitial hydrogen in silicium by transitions to the conduction bandinduced by about 10 meVelectron exchanges. The molecule-surface orbital geometries of the MOF and ASWconfigurations are displayed and the quantum path when unfolded exhibits the successivebroken symmetries.     

Mechanistic Investigations into the Enantioselective Conia‐Ene Reaction Catalyzed by Cinchona‐Derived Amino Urea Pre‐Catalysts and CuI

The enantioselective Conia‐ene cyclization of alkyne‐tethered β‐ketoesters is efficiently catalyzed by the combination of cinchona‐derived amino‐urea pre‐catalysts and copper(I) salts. The reaction scope is broad and a series of substrates can be efficiently cyclized with high yields and enantioselectivities. Herein, we present a detailed mechanistic study based on experimental considerations and quantum mechanical calculations. Several variables, such as the nature of the organic pre‐catalyst and the metal‐ion source, have been thoroughly investigated. Kinetic studies, as well as kinetic isotope effects and deuterium labeling experiments have been used to gain further insights into the mechanism and prove the cooperative nature of the catalytic system. Our studies suggest that the rate‐limiting step for the reaction involves the β‐ketoester deprotonation and that the active species responsible for the enantiodeterming step is monomeric in amino‐urea pre‐catalyst. Computational studies provide a quantitative understanding of the observed stereoinduction and identify hydrogen bonding from the urea group as a crucial factor in determining the observed enantioselectivity.