Development of special optical fibers for evanescent-wave chemical sensing

The paper summarizes results achieved in the Institute of Radio Engineering and Electronics in the field of investigation and fabrication of novel fiber-optic structures for evanescent-wave chemical sensing. Several approaches for increasing the evanescent-wave sensitivity of multimode silica optical fibers to gaseous and liquid chemicals are shown. These approaches make use of the decrease of the core diameter in sectorial and capillary sectorial fibers, modification of trajectories of optical rays in the cores of inverted-graded-index fibers, tailoring of the refractive index of porous layers applied on silica cores, an output mode filter preventing the detection of rays propagating at low axial angles or fibers bent in single or multiple turns. Presented at the 1st Czech-Chinese Workshop “Advanced Materials for Optoelectronics”, Prague, Czech Republic, June 13–17, 1998. This work was supported by the Grant Agency of the Czech Republic (projects No. 102/96/0939 and No. 102/98/1358) and by joint project No. 4104 of AS CR and CNRS, France.     

On the semiclassical analysis of the ground state energy of the Dirichlet Pauli operator III: magnetic fields that change sign

We consider the semiclassical Dirichlet Pauli operator in bounded connected domains in the plane. Rather optimal results have been obtained in previous papers by Ekholm–Kova?ík–Portmann and Helffer–Sundqvist for the asymptotics of the ground state energy in the semiclassical limit when the magnetic field has constant sign. In this paper, we focus on the case when the magnetic field changes sign. We show, in particular, that the ground state energy of this Pauli operator will be exponentially small as the semiclassical parameter tends to zero and give lower bounds and upper bounds for this decay rate. Concrete examples of magnetic fields changing sign on the unit disk are discussed. Various natural conjectures are disproved, and this leaves the research of an optimal result in the general case still open.

     

Molecular Mobility in Amorphous Polylactic Acid Studied Using <Superscript>1</Superscript>H–<Superscript>13</Superscript>C Cross Polarization NMR

The ¹H–¹³C cross polarization nuclear magnetic resonance technique was used for the study of molecular mobility in amorphous polylactic acid. The cross polarization buildup curves for methine and methyl functional groups measured at magic angle spinning rates of 4 and 10 kHz, under Hartmann–Hahn conditions determined from corresponding Hartmann–Hahn matching profiles, confirmed the presence of ¹H–¹³C spin pairs isolated from lattice since transient oscillations were observed in the initial stages of buildup curves. The values of dipolar coupling constants derived from Fourier transforms of cross polarization buildup curves indicate the presence of rigid methine groups and two kinds of methyl functional groups which differ in mobility. This could be due to the presence of amorphous phase with chain aggregations and phase with less spatial restrictions in this material. Fitting of theoretical expressions to experimental data provided additional information on the cross polarization process in this material.     

Synthesis and dehydration reaction of 1-(4-benzyloxyphenyl)-6-methoxy-2-phenyl-1,2,3,4-tetrahydronaphth-2-ol: possible intermediate of Lasofoxifene

The paper deals with a simple and sufficient synthesis of key precursor of Lasofoxifene. The 1-(4-benzyloxyphenyl)-6-methoxy-2-phenyl-3,4-dihydronaphthalene was prepared by a sequence of five reactions steps: first 1-(4-benzyloxyphenyl)-6-methoxy-3,4-dihydronaphthalene was prepared (70%), and this was quantitatively epoxidized to 7b-[4-(benzyloxy)phenyl]-5-methoxy-1a,2,3,7b-tetrahydronaphtho[1,2-b]oxirene. Catalytic (ZnI₂) isomerization of the epoxide gave 1-(4-benzyloxyphenyl)-6-methoxy-1,2,3,4-tetrahydronaphthalen-2-one (75%). Its subsequent reaction with phenylmagnesium bromide gave 1-(4-benzyloxyphenyl)-6-methoxy-2-phenyl-1,2,3,4-tetrahydro-2-naphthol (87%). Acid-catalysed dehydration of this alcohol by polyphosphoric acid (25°C) provides 1-(4-benzyloxyphenyl)-6-methoxy-2-phenyl-1,4-dihydronaphthalene (80%). Dehydration in the system of acetic anhydride/polyphosphoric acid gives 1-(4-benzyloxyphenyl)-6-methoxy-2-phenyl-3,4-dihydronaphthalene (66%).     

High basicity of tris-(tetramethylguanidinyl)-phosphine imide in the gas phase and acetonitrile—a DFT study

It is shown by reliable density functional theory (DFT) calculations that compounds 4 and 6 are very powerful neutral organic superbases as evidenced by the calculated proton affinities in the gas phase 305.4 kcal mol⁻¹ (44.8) and 287.8 kcal mol⁻¹ (37.8), respectively, and the corresponding calculated pK_a values in acetonitrile given within parentheses.     

Laser induced mixing in multilayered Ti/Ta thin film structures

The possibility of interlayer mixing in a Ti/Ta multilayer system, induced by laser irradiation, was the main purpose of these experiments. Ti/Ta multilayer system, consisting of ten alternating Ti and Ta thin films and covered by slightly thicker Ti layer, was deposited on Si (100) wafers to a total thickness of 205 nm. Laser irradiation was performed in air by picoseconds Nd:YAG laser pulses in defocused regime with fluences of 0.057 and 0.11 J cm^?2. Laser beam was scanned over the 5?×?5 mm surface area with different steps along y-axes. Structural and compositional characterisation was done by auger electron spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy. Laser processing at lower fluence caused only oxidation of the top Ti layer, despite of the number of applied laser pulses. Interlayer mixing was not observed. Application of laser pulses at fluence of 0.11 J cm^?2 caused partial and/or complete ablation of deposited layers. In partially ablated regions considerable mixing between Ti and Ta films was registered.     

Nature of the water/aromatic parallel alignment interactions

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O? H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE_{CCSD(T)(limit)} = ?2.45 kcal mol^?1 at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry‐adapted perturbation theory, and extended transition state‐natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV‐based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O? H) between the monomers also exists. © 2014 Wiley Periodicals, Inc.     

Three Types of Induced Tryptophan Optical Activity Compared in Model Dipeptides: Theory and Experiment

The tryptophan (Trp) aromatic residue in chiral matrices often exhibits a large optical activity and thus provides valuable structural information. However, it can also obscure spectral contributions from other peptide parts. To better understand the induced chirality, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA) spectra of Trp‐containing cyclic dipeptides c‐(Trp‐X) (where X=Gly, Ala, Trp, Leu, nLeu, and Pro) are analyzed on the basis of experimental spectra and density functional theory (DFT) computations. The results provide valuable insight into the molecular conformational and spectroscopic behavior of Trp. Whereas the ECD is dominated by Trp π–π* transitions, VCD is dominated by the amide modes, well separated from minor Trp contributions. The ROA signal is the most complex. However, an ROA marker band at 1554 cm^?1 indicates the local χ₂ angle value in this residue, in accordance with previous theoretical predictions. The spectra and computations also indicate that the peptide ring is nonplanar, with a shallow potential so that the nonplanarity is primarily induced by the side chains. Dispersion‐corrected DFT calculations provide better results than plain DFT, but comparison with experiment suggests that they overestimate the stability of the folded conformers. Molecular dynamics simulations and NMR results also confirm a limited accuracy of the dispersion‐DFT model in nonaqueous solvents. Combination of chiral spectroscopies with theoretical analysis thus significantly enhances the information that can be obtained from the induced chirality of the Trp aromatic residue.