The reaction between silylene and ammonia: some gas-phase kinetic and quantum chemical studies

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by 193 nm laser flash photolysis of silacyclopent-3-ene, have been carried out in the presence of ammonia, NH3. Second order kinetics were observed. The reaction was studied in the gas phase at 10 Torr total pressure in SF6 bath gas at each of the three temperatures, 299, 340 and 400 K. The second order rate constants (laser pulse energy of 60 mJ/pulse) fitted the Arrhenius equation: log(k/cm3 molecule-1 s-1) = (-10.37 ± 0.17) + (0.36 ± 1.12 kJ mol-1)/RTln10 Experiments at other pressures showed that these rate constants were unaffected by pressure in the range 10-100 Torr, but showed small decreases in value at 3 and 1 Torr. There was also a weak intensity dependence, with rate constants decreasing at laser pulse energies of 30 mJ/pulse. Ab initio calculations at the G3 level of theory, show that SiH2 + NH3 should form an initial adduct (donor-acceptor complex), but that energy barriers are too great for further reaction of the adduct. This implies that SiH2 + NH3 should be a pressure dependent association reaction. The experimental data are inconsistent with this and we conclude that SiH2 decays are better explained by reaction of SiH2 with the amino radical, NH2, formed by photodissociation of NH3 at 193 nm. The mechanism of this previously unstudied reaction is discussed

The parent?infant dyad and the construction of the subjective self

Developmental psychology and psychopathology has in the past been more concerned with the quality of self-representation than with the development of the subjective agency which underpins our experience of feeling, thought and action, a key function of mentalisation. This review begins by contrasting a Cartesian view of pre-wired introspective subjectivity with a constructionist model based on the assumption of an innate contingency detector which orients the infant towards aspects of the social world that react congruently and in a specifically cued informative manner that expresses and facilitates the assimilation of cultural knowledge. Research on the neural mechanisms associated with mentalisation and social influences on its development are reviewed. It is suggested that the infant focuses on the attachment figure as a source of reliable information about the world. The construction of the sense of a subjective self is then an aspect of acquiring knowledge about the world through the caregiver's pedagogical communicative displays which in this context focuses on the child's thoughts and feelings. We argue that a number of possible mechanisms, including complementary activation of attachment and mentalisation, the disruptive effect of maltreatment on parent-child communication, the biobehavioural overlap of cues for learning and cues for attachment, may have a role in ensuring that the quality of relationship with the caregiver influences the development of the child's experience of thoughts and feelings

Computational studies of Bronsted acid sites in zeolites

The authors have performed high-level ab initio calculations using both Hartree-Fock (HF) and Moller-Plesset perturbation theory (MP2) to study the geometry and energetics of the adsorption complex involving H{sub 2}O and the Bronsted acid site in the zeolite H-ZSM-5. In these calculations, which use aluminosilicate cluster models for the zeolite framework with as many a 28 T atoms (T = Si, Al), we included geometry optimization in the local vicinity of the acid site at the HF/6-31G(d) level of theory, and have calculated corrections for zero-point energies, extensions for zero-point energies, extensions to higher basis sets, and the influence of electron correlation. Results for the adsorption energy and geometry of this complex are reported and compared with previous theoretical and experimental values

Size-and composition optimized sub-nanometer and nm size catalysts for low-temperature jet-fuel activation

Adding catalysts to jet fuels has advantages such as accelerated combustion and increased heat sink capacity. The multitude of parameters determining catalyst performance forms a considerable challenge, such as optimal particle composition and size which primarily determine reaction rates and energetics, plus sintering resistance. Prerequisites for such studies are 1) catalysts with well defined size/composition and 2) characterization of the catalyst at work. In this study, cyclohexane is used as fuel surrogate and its dehydrogenation on Pt- and Co-based catalysts is monitored by temperature programmed reaction combined with in situ X-ray scattering and absorption. The results shows that efficient dehydrogenation of cyclohexane can be performed on subnanometer and nanometer size catalysts, moreover at low temperatures. Catalytic performance can be tuned by varying composition and functionalization of the support material, the size and doping of the nanocatalyst. DFT studies are used to help understand the structures and reaction pathways

Reaction Mechanism for Direct Propylene Epoxidation by Alumina Supported Silver Aggregates The Role of the Particle Support Interface

Subnanometer Ag aggregates on alumina supports have been found to be active toward direct propylene epoxidation to propylene oxide by molecular oxygen at low temperatures, with a negligible amount of carbon dioxide formation (Science 2010, 328, 224, ). In this work, we computationally and experimentally investigate the origin of the high reactivity of the subnanometer Ag aggregates. Computationally, we study O₂ dissociation and propylene epoxidation on unsupported Ag₁₉ and Ag₂₀ clusters, as well as alumina-supported Ag₁₉. The O₂ dissociation and propylene epoxidation apparent barriers at the interface between the Ag aggregate and the alumina support are calculated to be 0.2 and 0.2–0.4 eV, respectively. These barriers are somewhat lower than those on sites away from the interface. The mechanism at the interface is similar to what was previously found for the silver trimer on alumina and can account for the high activity observed for the direct oxidation of propylene on the Ag aggregates. The barriers for oxygen dissociation on these model systems both at the interface and on the surfaces are small compared to crystalline surfaces, indicating that availability of oxygen will not be a rate limiting step for the aggregates, as in the case of the crystalline surfaces. Experimentally, we investigate Ultrananocrystalline Diamond (UNCD)-supported silver aggregates under reactive conditions of propylene partial oxidation. The UNCD-supported Ag clusters are found to be not measurably active toward propylene oxidation, in contrast to the alumina supported Ag clusters. This suggests that the lack of metal-oxide interfacial sites of the Ag-UNCD catalyst limits the epoxidation catalytic activity. This combined computational and experimental study shows the importance of the metal-oxide interface as well as the noncrystalline nature of the alumina-supported subnanometer Ag aggregate catalysts for propylene epoxidation