Isolation of equatorial conformers of chloro- and bromocyclohexane in a pure state as inclusion complexes with a host compound

The equatorial conformers of chloro- and bromocyclohexane were isolated in a pure state as inclusion complexes with a host compound, and their structures were studied by IR spectra and X-ray analysis.     

N-substituent effect on the cis-trans geometry of nine-membered lactams

The cis-trans geometry of a nine-membered lactam significantly depends on the N-substituents; N-acyl-1-aza-2-cyclononanones (1a-c) exist as cis form; in contrast, N-Z- 1-aza-2-cyclononanone (1d) exists as trans form both in the crystal and in solution.     

Global analysis of periodic orbit bifurcations in coupled Morse oscillator systems: time-reversal symmetry,permutational representations and codimension-2 collisions

In this paper we study periodic orbit bifurcation sequences in a system of two coupled Morse oscillators. Time-reversal symmetry is exploited to determine periodic orbits by iteration of symmetry lines. The permutational representation of Tsuchiya and Jaffe is employed to analyze periodic orbit configurations on the symmetry lines. Local pruning rules are formulated, and a global analysis of possible bifurcation sequences of symmetric periodic orbits is made. Analysis of periodic orbit bifurcations on symmetry lines determines bifurcation sequences, together with periodic orbit periodicities and stabilities. The correlation between certain bifurcations is explained. The passage from an integrable limit to nointegrability is marked by the appearance of tangent bifurcations; our global analysis reveals the origin of these ubiquitous tangencies. For period-1 orbits, tangencies appear by a simple disconnection mechanism. For higher period orbits, a different mechanism involving 2-parameter collisions of bifurcations is found. (c) 1999 American Institute of Physics.     

High-quality sub-100-fs optical pulse generation by fiber-optic soliton compression of gain-switched distributed-feedback laser-diode pulses in conjunction with nonlinear optical fiber loops

We report our investigations of a new and simpler femtosecond fiber-optic soliton compressor for chirp-compensated gain-switched diode-laser pulses and of pedestal suppression by a compound fiber loop mirror (CFLM) and nonlinear-optical loop mirrors (NOLM's), both of which are designed for sub-100-fs pulse generation of high quality. The soliton compressor, composed of a 21.1-m-long steplike dispersion profiled fiber, exhibited 44-fs pulses and a compression ratio of approximately 60. Pedestals of 100-fs soliton pulses were suppressed successfully (>20 dB) by the CFLM and the NOLM's. In particular, 63-fs pedestal-free sech(2) pulses were obtained by NOLM propagation.     

Studies on pyrazines. 7. The synthesis of 5-chloropyrazinecarboxylic acid

The titled carboxylic acid ( 1 ) was prepared by condensation of 2-furylglyoxal with aminoacetamide followed by chlorination of the resulting 2-hydroxy-5-(2′-furyl)pyrazine ( 2 ) and permanganate oxidation. The acid was further converted into methyl ester and 5-hydroxypyrazinecarboxylic acid.     

High-performance liquid chromatographic determination of alpha-keto acids in plasma with fluorometric detection

This paper describes a sensitive high-performance liquid chromatographic method for the quantitative determination of alpha-keto acids in plasma using a fluorescence detector. This method is about ten times more sensitive than that reported in a previous paper. Only 50 microliters of plasma are needed for the determination of alpha-keto acids. However, p-hydroxyphenylpyruvic acid could not be analysed because the quinoxalinol derived from it does not exhibit fluorescence.     

Coefficients of molecular packing and intrinsic birefringence of aromatic polyimides estimated using refractive indices and molecular polarizabilities

Molecular structure dependences of the coefficients of molecular packing (K_p) and the intrinsic birefringence (Δn⁰) were investigated for aromatic polyimides (PIs) using a method based on the modified Lorentz–Lorenz equation and the Vuks equation. This method needs only average refractive indices and polarizability tensors ( ) for PI repeating units. αˆ was calculated using the density functional theory at the B3LYP/6-31+G(d) theory level that was confirmed as optimal for PI repeating units. Because bent or kinked structures in main chain hinder the ordering of polymer chains, K_p decreases in cases that ether linkages are introduced in the main chain and that kinked-structured dianhydrides are used. Introduction of side groups, trifluoromethyl ( CF₃) groups in particular, also causes decreases in K_p. On the other hand, the estimated values of Δn⁰ are proportional to the calculated anisotropy in αˆ (Δα/V_vdw), thus Δn⁰ can be well predicted by computation. The values of Δα/V_vdw are decreased not only by bent structures in the main chain (e.g., ether linkage), presence of side groups, but also by large fractions of imide rings in the polymer structures. The estimated values of Δn⁰ range from 0.338 to 0.470, which are much larger than the reported ones for optically transparent conventional polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2354–2366, 2004