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871.
We investigate the local structures of oxynitride La-Si-(Al)-O-N glasses by 29Si and 27Al magic-angle spinning (MAS) solid state Nuclear Magnetic Resonance (NMR). The glasses studied span an unprecedented range of compositions, up to >50 at.% lanthanum and nitrogen out of the cations and anions, respectively, and achievable through a recently introduced glass preparation route. Transmission as well as scanning electron microscopy verified homogeneous samples over length-scales down to 20 nm. As the nitrogen content of the glasses increased, 29Si NMR evidenced a progressive formation of Si-N bonds, with SiO2N2 tetrahedra dominating in the nitrogen-rich glass networks. In the oxygen-rich end of the series, aluminum is predominantly present in tetrahedral coordination as AlO4, whereas the glasses with highest nitrogen contents have a major fraction of AlO3N structural units. Trends in isotropic 29Si and 27Al chemical shifts and 27Al quadrupolar couplings are compared with results of La-Si-Al-O glasses and are discussed in relation to the glass compositions and their proposed structures. 相似文献
872.
Alexei R. Khokhlov Felix F. Ternovsky Ekaterina A. Zheligovskaya 《Macromolecular theory and simulations》1993,2(2):151-168
The exact solution of the problem of adsorption of a long ideal polymer chain with variable degree of stiffness on a plane surface is presented. It is shown that the adsorption of stiff polymer chains is a second-order phase transition; in the adsorbed state “train” (i.e. adsorbed) sections are relatively longer and loop sections relatively shorter than for flexible chains. This effect is very pronounced: already for moderately stiff chains the number of Kuhn segment lengths in one “train” section at the temperature T = Tcr/2 (Tcr is the critical temperature for adsorption transition) can reach several thousands, and deviation from the surface occurs only in the form of small “hairpins”. The maximum length of the chain, which at the given conditions would flatten completely on the surface, is estimated. 相似文献
873.
Dr. Vyacheslav Sushev Dr. Yulia Panova Dr. Alexandra Khristolyubova Dr. Natalia Zolotareva Matvey Grishin Dr. Roman Rumyantcev Ekaterina Kozlova Prof. Dr. Georgy Fukin Prof. Dr. Alexander Kornev 《欧洲无机化学杂志》2023,26(23):e202300252
A number of two-deck dicationic 3a,6a-diaza-1,4-diphosphapentalenes (DDP)2X2 (X=halogen or complex ion) have been characterized. Interdeck distances P(1)⋅⋅⋅P(3) and P(2)⋅⋅⋅P(4) in crystal structures, as a rule, significantly exceed the sum of covalent radii, however they are still within the sum of van der Waals radii. The 31P NMR data indicate that phosphorus atoms are equivalent in solution even at low temperatures (233 K). Based on DFT calculations, the formation of two equivalent P−P covalent bonds (<2.4 Å) is energetically unfavorable, despite the absence of steric barriers. The total energy of the model dication as a function of the P⋅⋅⋅P separation has a minimum at the P−P distance of 2.85 Å. This distance corresponds to the highest aromaticity index in the interdeck space (NICS(1.0)=−20.72). The energy of interdeck interactions is estimated at 10–15 kcal/mol. 相似文献