Practical method to estimate the standard deviation in absorption coefficients measured with THz time-domain spectroscopy

A model of standard deviation in the intensity spectrum of electric field observed with the terahertz time-domain spectroscopy (THz-TDS) is proposed to estimate the random error in the transmittance and absorption coefficients from a single or a few measurements of a sample. The proposed standard deviation which is derived on the basis of the statistical standard deviation and noise floor of intensity spectrum of reference fits well to the standard deviation of transmittance as well as absorption coefficient computed statistically. This study contributes the simple and computationally efficient method to demonstrate the accuracy in optical constants like imaginary part of refractive index and absorption coefficients measured using the THz-TDS.     

Three-dimensional numerical simulation of the reciprocating flow in a branching tube network

The paper deals with the three dimensional numerical simulation of the reciprocating flow in a branching network, a phenomenon that might be found for example in the human bronchi. As the first approximation, the working fluid was water with laminar flow regime. The main issue was to elucidate the mechanism of the air exchange in the human lungs. The model used in this simulation consists of two branching generations. Three different cases have been analyzed with various frequency, Re and Wo numbers.     

Switching of macromolecular helicity of optically active poly(phenylacetylene)s bearing cyclodextrin pendants induced by various external stimuli

A series of novel phenylacetylenes bearing optically active cyclodextrin (CyD) residues such as alpha-, beta-, and gamma-CyD and permethylated beta-CyD residues as the pendant groups was synthesized and polymerized with a rhodium catalyst to give highly cis-transoidal poly(phenylacetylene)s, poly-1alpha, poly-2beta, poly-3gamma, and poly-2beta-Me, respectively. The polymers exhibited an induced circular dichroism (CD) in the UV-visible region of the polymer backbones, resulting from the prevailing one-handed helical conformations. The Cotton effect signs were inverted in response to external chiral and achiral stimuli, such as temperature, solvent, and interactions with chiral or achiral guest molecules. The inversion of the Cotton effect signs was accompanied by a color change due to a conformational change, such as inversion of the helicity of the polymer backbones with a different twist angle of the conjugated double bonds, that was readily visible with the naked eye and could be quantified by absorption and CD spectroscopies. The dynamic helical conformations of poly-2beta showing opposite Cotton effect signs in different solvents could be further fixed by intramolecular cross-linking between the hydroxy groups of the neighboring beta-CyD units in each solvent. The cross-link between the pendant CyD units suppressed the inversion of the helicity; therefore, the cross-linked poly-2betas showed no Cotton effect inversion, although the polymer backbones were still flexible enough to alter their helical pitch with the same handedness, resulting in a color change depending on the degree of intramolecular cross-linking.     

Flexible and shape-selective guest binding at Cu(II) axial sites in 1-dimensional Cu(II)-1,2-bis(4-pyridyl)ethane coordination polymers

A series of guest-binding Cu(II) coordination polymers, {[Cu(bpetha)2(acetone)2].2PF6}n (bpetha = 1,2-bis(4-pyridyl)ethane) (1), {[Cu(bpetha)2(DMF)2].2PF6}n (2), {[Cu(bpetha)(2)(MeCN)(2)].2PF6.2MeCN}n (3), {[Cu(bpetha)2(H2O)2].2PF6.3THF.2H2O}n (4), {[Cu(bpetha)2(H2O)2].2PF6.3dioxane}n (5), and {[Cu(bpetha)2(H2O)2].2PF6.2-PrOH.2H2O}n (6), have been synthesized and crystallographically characterized. Their framework stabilities and guest-exchange properties have also been investigated. All compounds form a similar framework motif, a "double chain", in which the bpetha ligands bridge Cu(II) centers to form 1-D [Cu(bpetha)2]n double chains. A variety of Lewis base guest molecules, such as H2O, acetone, DMF, MeCN, THF, dioxane, and 2-PrOH, are incorporated into the assembly of the 1-D double chains. These chains flexibly change their forms of assembly in a guest-dependent manner. Interestingly, acetone, DMF, and MeCN guests with a carbonyl or cyanide group coordinate directly to the axial sites of the Cu(II) centers; in contrast, THF, dioxane, and 2-PrOH guests with an ether or alcohol group are incorporated into the frameworks not via coordination bonds but via weak interactions (hydrogen bonds and van der Waals forces). This selectivity is probably due to steric effects at coordinated oxygen or nitrogen atoms of the guests. Crystal-to-crystal transformations triggered by guests are observed, during which guests coordinated to the Cu(II) axial sites are readily removed and replaced by other guests.     

Simple protocol for NMR analysis of the enantiomeric purity of primary amines

[reaction: see text] A practically simple three-component chiral derivatizing protocol for determining the enantiopurity of 13 chiral primary amines by (1)H NMR spectroscopic analysis is described, including analysis of those that contain remote stereocenters.     

Highly selective encaging of carbon dioxide molecules in the mixed carbon dioxide and nitrogen hydrate at low temperatures

The structural identification and guest compositions of the mixed CO(2) and N(2) hydrates at low temperature conditions were investigated by both theoretical predictions and experimental measurements. From the model calculations, at very low temperatures, the highly CO(2)-concentrated hydrates over 95 mol % CO(2) on the basis of water-free concentration could coexist with the gas mixtures of low CO(2) concentrations in equilibrium. X-ray diffraction measurements of the hydrates formed with the gas mixture of 3.16 mol % CO(2) and balanced N(2) indicate that the formed hydrates at all conditions considered in this study were identified as structure I, whereas the model predicts a structural transition to structure II around 220 K. However, it was also found that the formed hydrate samples contain a considerable amount of hexagonal ice resulting from incomplete conversion of ice to the hydrates. The compositional analysis suggests that a favorable encaging of CO(2) in the mixed hydrate can be obtained by the hydrate formation at low temperatures and relative amount of CO(2) molecules in the mixed hydrates increases with a decrease of temperature.