Observation of a reversible isomorphous phase transition and an interplay of "sigma-holes'' and "pi-holes'' in Fmoc-Leu-psi[CH2-NCS]

Fmoc-Leu-psi[CH2NCS] undergoes a reversible isomorphous phase transition upon cooling. The crystal structure at 100 K displays a short N=C=S center dot center dot center dot N=C=S intermolecular interaction, which has been characterized based on experimental charge density analysis, as a stabilizing interaction with both sigma-holes and pi-holes acting cooperatively

The Reaction of a Nitro-Capped Cobalt(III) Cage Complex With Base: the Crystal Structure of a Contracted Cage Complex, and the Mechanism of Its Formation

The synthesis, properties and crystal structure of the cage complex (1-hydroxy-8-methyl-3,6,10,13,15,18-hexaazabicyclo[6.6.5]nonadecane)cobalt(III) chloride hydrate ([Co(Me,OH-absar)] C13.H2O) are reported. The mechanism of the formation of this contracted cavity cage from a nitro-capped hexaazabicycloicosane type cage has been investigated. Treatment of (1-methyl-8-nitro-3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane)cobalt(III) chloride ([Co(Me,NO2-sar)] 3+) with excess base in aqueous solution leads initially to rapid (t1/2 < 1 ms) and reversible deprotonation of one coordinated secondary amine. This species undergoes a retro-Mannich type reaction and imine hydrolysis (t1/2 almost-equal-to 90 s). Quenching the reaction with acid gives rise to a pair of isomeric intermediate species which have been isolated and characterized. They have a pendant arm macrocyclic structure, resulting from the loss of a methylene unit from one of the arms of the cap. Heating either isomer in aqueous solution gives the new cage compound with the contracted cap. It is postulated that this occurs through a Nef reaction, resulting in the formation of a ketone which then condenses with the coordinated primary amine. A comparison with the corresponding bicycloicosane analogue indicates a reduced chromophoric cavity size for the contracted cage. The reduction potential of the cobalt(III)/cobalt(II) couple is 170 mV more negative for the smaller cage, and, in the electronic spectrum of the cobalt(III) complex, the d-d transitions are both shifted to higher energy, corresponding to a stronger ligand field

Mechanistics of stabilisation via doping in bismuthsesquioxide (Bi2-2xHo2xO3; x=0.2) - A combined approach of Rietveld and microstructural analysis

delta-Bi1.6Ho0.4O3, is cubic, Fm3m, with a=5.5291(5) Angstrom, V=169.03(3) Angstrom(3), D-calc=8.838 g/cm(3), D-measured (powder technique)=8.607 g/cm(3), with Z=2. Rietveld profile refinements on high resolution X-ray diffraction data give R-p=0.143, R-wp=0.188, R-(I,R-HKL)=0.090, R-(expected)=0.078 confirming the earlier results on the structure of this phase to have a fluorite type lattice with highly disordered oxygen sublattice. Strain and crystal size analysis of all the reflections in the range (2 theta=5-100.94 degrees) from holmium doped bismuth sesquioxide were carried out and compared with the results of the simulated whole powder pattern of Bi2O3 for alpha and delta phases. These results provide pointers to the possible mechanism of stabilization of delta phase on holmium substitution

Influence of inherent strain on the curie temperature of rare earth ion-doped bismuth vanadate

X-ray line broadening is found to be an effective parameter to estimate the strain associated with rare earth ion (Gd3+)-doped polycrystalline bismuth vanadate(Bi2VO5.5). The strain increases with increasing Gd3+ concentration. It is anisotropic and found to be maximum in (111) plane. The Curie temperature which is known to decrease with increase in the rare earth ion concentration in these compounds is correlated with increase in strain