Deuterium NMR studies of guest motions in urea inclusion compounds of 1,6-dibromohexane with analytical evaluation of spectra in the fast motion limit

²H NMR (nuclear magnetic resonance) spectroscopy, in conjunction with X-ray diffraction experiments, was used to characterize the guest motions of 1,6-dibromohexane in its urea inclusion compound. These motions are characterized by alkyl chain jumps between two conformations, each approximately gauche to the terminal bromines, which remain stationary. In this distorted urea channel, one conformer is heavily preferred, but thermally activated population of the unfavorable conformer leads to reversible, temperature-dependent changes in the unit cell parameters. Although rapid motions of the guest chain give rise to ambiguities in the interpretation of the²H NMR spectra, fortuitous temperature-independent spectral features of guests containing deuterium at the α, β and γ positions indicate that the guest motion resembles a two-site jump with unequal probabilities. Analytical lineshape calculations on the three sets of²H NMR spectra indicate a single jump mechanism in which the range of jump angles is narrowly prescribed. This NMR model provided a starting point for successful solution and refinement of the crystal structures at 213 and 298 K, which had been complicated by motional disorder.     

A broad frequency range dielectric spectrometer for colloidal suspensions: cell design, calibration, and validation

Electrode polarization complicates low-frequency measurements of the dielectric response of electrolyte solutions and colloidal suspensions. To deal with this longstanding problem, a new dielectric cell was developed along with a model based on the standard electrokinetic theory. The parallel plate cell utilizes a thin chamber that is easily filled and emptied; different chamber thicknesses are readily accommodated. The analytical form of the theoretical impedance model makes data analysis straightforward. Using standard electrolytes, the device and the theoretical model were tested over a wide range of frequencies for several electrolyte concentrations. Excellent agreement was found between the theory and the experimental data. The methodology developed to account for polarization effects exhibits a significant improvement over the conventional approaches and points up a deficiency in often-used equivalent circuit models.     

An NMR study of the freezing of emulsion-containing drops

Various nuclear magnetic resonance (NMR) techniques were used to monitor the freezing behaviour of suspended 2-mm-diameter drops. The drops were composed of hydrocarbon oils emulsified in either water or water/sucrose mixtures. As such they were good model systems for the study of spray freezing, sharing structural similarities with potential products such as ice cream. In particular, simple 1H NMR spectroscopy was used to monitor and individually quantify the freezing or solidification behaviour of the various constituent species of the drops. In addition, the effect of freezing on the emulsion droplet size distribution (and hence emulsion stability) was also measured based on NMR self-diffusion measurements. The effect of freeze/thaw cycling was also similarly studied. The nucleation temperature of the emulsion droplets was found to depend on the emulsion droplet size distribution: the smaller the droplets, the lower the nucleation temperature. Emulsion droplet sizing indicated that oil-in-sucrose-solution emulsions were more stable, showing minimal coalescence, whereas oil-in-water emulsions showed significant coalescence during freezing and freeze/thaw cycling.     

Synthesis and characterization of Co/CdSe core/shell nanocomposites: bifunctional magnetic-optical nanocrystals

Nanocomposite materials provide the possibility for multifunctional properties in contrast with their more-limited single-component counterparts. Here, we report the synthesis and characterization of the first all-inorganic core/shell hybrid magnetic-optical nanoparticle, cobalt/cadmium selenide. The core/shell nanocrystals are prepared in a facile one-pot reaction, and their microstructure is analyzed using low- and high-resolution transmission electron microscopy. Using magnetic and optical characterization, we demonstrate bifunctional behavior, whereby the core retains the magnetic properties of the starting Co nanoparticle, and the shell emits similarly to a single-component CdSe nanoparticle. Interestingly, while the coercivity was found to be unchanged by shell formation, the blocking temperature for the composite structure was observed to be substantially lower (Co: >350 K; Co/CdSe: 240 K). In addition, we observed that at low temperatures (20 K) shell CdSe photoluminescence (PL) decay was very rapid (<1 ns). In contrast, nanocrystalline CdSe PL decay is typically much slower at such temperatures (>50 ns). Finally, we propose possible explanations for the unusual magnetic and optical behavior of the core/shell hybrid nanostructures.     

Structure of 72,74Se at high spin

Lifetimes of high spin states up to { }=22⁺ in the yrast positive parity bands have been measured to investigate the shape evolution with increasing spin in ^{72, 74}Se. The Q _t values derived from these measurements indicate that prolate shape stabilizes for ⁷²Se, while a triaxial shape develops for ⁷⁴Se at higher spins. Comparison of the observed trend in Q _t with spin for ^{72, 74}Se with that of the corresponding kryptones isotones emphasizes the stability provided by N=38 prolate shell gap even at high rotational frequency.     

Multicolored forests in complete bipartite graphs

If the edges of a graph G are colored using k colors, we consider the color distribution for this coloring a=(a₁,a₂,…,a_k), in which a_i denotes the number of edges of color i for i=1,2,…,k. We find inequalities and majorization conditions on color distributions of the complete bipartite graph K_n,n which guarantee the existence of multicolored subgraphs: in particular, multicolored forests and trees. We end with a conjecture on partitions of K_n,n into multicolored trees.