Synthesis of silanized polyether urethane hybrid systems. Study of the curing process through hydrogen bonding interactions

Different types of silanized polyurethanes (SPUR) were prepared in two ways: the first type (a), by reacting a polyether diol with an isocyanatesilane and the second type (b) by reacting it with diisocyanate (IPDI) and afterwards with aminosilane. These systems are able to cure with atmospheric moisture and, as a consequence, a tridimensional hybrid structure is formed where the inorganic and organic phases are bonded with covalent bonds. The evolution of the curing process in both systems has been monitored by means of Fourier transform infrared spectroscopy, through the changes observed in the carbonyl stretching vibration region. The results obtained show that the SPURs of kind (b) present stronger hydrogen bonding interactions as a consequence of urea groups present in the final structure. Moreover, given the proximity of both urethane and urea groups to alkoxysilane end groups, during the alkoxysilane curing process these groups are forced to approximate themselves even further and therefore the reticulation process leads to an increase of the self association of urethane and urea groups.Finally, DSC has been used to measure T_g values of the systems studied before and after the curing process. The obtained results have confirmed the main conclusions obtained in FTIR analysis.     

Pd-catalyzed arylation/ring-closing metathesis approach to azabicycles

Palladium-catalyzed arylation followed by Grignard addition to imines and ring-closing metathesis, using Grubbs’ catalysts, provides a route to six-, seven-, and eight-membered azabicycles.     

First principles study of photostability within hydrogen-bonded amino acids

The photochemistry and photophysics of a two-glycine minimal model is studied at the CASPT2//CASSCF level of theory. Different photoinduced processes are discussed, on the basis of the calculated minimum energy paths and the characterization of the electronic state crossings. Two main processes could provide UV-photostability to the hydrogen-bonded peptide system: (i) forward-backward photoinduced electron/proton transfer involving the H in the hydrogen bond, (ii) singlet-singlet energy transfer between two amino acids, providing ultrafast population of the low-energy n,π* state.     

Adaptative and developmental responses to stress in Aspergillus nidulan

Development in the ascomycete A. nidulans is principally determined by environmental signals. Adaptability to oxidative stimuli can derive in changes of growth patterns and /or the activation of sexual or asexual reproductive cycles but this model fungus might also respond to high osmotic or salt concentrations, the redox state, the availability and quantity of carbon or nitrogen sources and the degree or quality of illumination. Since each cell within the colony follows a single morphogenetic program at a time, all these environmental cues might be sensed and integrated into a limited number of intracellular signals which, finally, would activate the required morphogenetic program and repress the others. This signaling mainly occurs through stress response pathways. The present review aims to summarize the available knowledge on how these pathways transduce environmental stimuli to mediate morphological changes in Aspergillus nidulans.     

Intramolecular triplet-triplet energy transfer in oxa- and aza-di-pi-methane photosensitized systems

Two different mechanisms are proposed for the intramolecular triplet-triplet energy transfer in oxa- and aza-di-pi-methane systems, one thermally induced and the other photoinduced. These mechanisms involve a key structure that corresponds to an avoided crossing. The triplet sensitized photochemistry of these compounds, focusing attention in the di-pi-methane rearrangement, is discussed in light of these proposed mechanisms.     

Triplet versus singlet photoreaction mechanism in the barrelene di-pi-methane rearrangement

A detailed study of the potential energy surfaces involved in the di-pi-methane rearrangement (singlet and triplet states) reveals the factors that modulate the mechanisms (pathways) and reactivity in strained di-pi-methane systems such as bicyclo[2.2.2]octa-2,5,7-triene (barrelene). [structure: see text]     

The role of the intersection space in the photochemistry of tricyclo[3.3.0.0(2,6)]octa-3,7-diene

CASSCF and CASPT2 methods were used to study the photochemistry of tricyclo[3.3.0.0(2,6)]octa-3,7-diene (TOD). The analysis of different S1 reaction paths as well as the topology of the S1/S0 intersection space allows us to establish two novel properties associated with the photochemical behavior of this compound: (i) simple low-lying intersection space domains can mediate different photoproducts, and (ii) TOD photochemistry is probably mediated by two disconnected intersection space domains, related to the formation of cyclooctatetraene and semibulvalene in different time-scales. It is shown that these domains are chemically distinct since the first, leading to COT, mediates barrierless pericyclic reactions while the second, leading to SBV, is accessed through the formation of an excited-state biradical intermediate. To the best of our knowledge, in the domain of single molecule photochemistry, TOD represents the first example where a different chemical role of distinct low-lying intersection spaces has been computationally documented. The observed photoproducts can be rationalized in terms of branching space diagrams, constructed by determining the branching space (derivative coupling and gradient difference vectors) for each conical intersection involved in the photochemical process.     

Does employee participation matter? An empirical study on the effects of participation on well-being and organizational performance

Central European Journal of Operations Research - Employability, talent and/or motivation of people can be a source of sustainable competitive advantage; difficult for competitors to imitate. The...     

Photooxidation and stabilization of silanised poly(ether-urethane) hybrid systems

The photooxidative behaviour of a new kind of hybrid system based on silanised poly(ether-urethane)s (SPURs) has been analyzed by Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). The influence of polyether molecular weight and SPUR chemical nature as well as the addition of HALS and UVA stabilizers on the system photodegradation has been studied.FTIR results and AFM images have shown that the light stabilizers considerably enhance the photodegradation resistance, specially the HALS type stabilizer.