Vibrational Spectra of the Cyclic SN-Compounds S4N4, S4N4H4 and S3N3Cl3

The IR and Raman spectra of solid and dissolved S₄N₄, S₄N₄H₄, S₄N₄D₄ and S₃N₃Cl₃ have been recorded and are assigned according to D_2d, C_4v and C_3v symmetry respectively. In the solid state, many forbidden bands and splittings of degenerate vibrations are observed because of the symmetry lowering in the crystals. Due to the different size and shape of the rings and to strong coupling of the normal modes there is no clear correlation between the SN ring stretching vibrations and the strength of the SN bonds, except for the one of the E modes. However, the stretching force constant show the trend expected from changes in interatomic distances.     

Placement of medium-sized molecular fragments into active sites of proteins

Summary We present an algorithm for placing molecular fragments into the active site of a receptor. A molecular fragment is defined as a connected part of a molecule containing only complete ring systems. The algorithm is part of a docking tool, called FlexX, which is currently under development at GMD. The overall goal is to provide means of automatically computing low-energy conformations of the ligand within the active site, with an accuracy approaching the limitations of experimental methods for resolving molecular structures and within a run time that allows for docking large sets of ligands. The methods by which we plan to achieve this goal are the explicit exploitation of molecular flexibility of the ligand and the incorporation of physicochemical properties of the molecules. The algorithm for fragment placement, which is the topic of this paper, is based on pattern recognition techniques and is able to predict a small set of possible positions of a molecular fragment with low flexibility within seconds on a workstation. In most cases, a placement with rms deviation below 1.0 Å with respect to the X-ray structure is found among the 10 highest ranking solutions, assuming that the receptor is given in the bound conformation.     

Structure of a potent neuromuscular blocking agent: Caracurine-II dimethochloride octahydrate, [C40H44N4O2]2+·2CI−·8H2O

Crystals of this calabash curare alkaloid, space groupP2₁; havea=12.695(4), Å,b=7.424(2) Å,c=21.762(6) Å,=98.03(5)°,Z=2,F(000)=884. The structure was solved by direct methods and refined by least squares toR=0.10. The alkaloid cation, with two-fold molecular symmetry, has a highly fused ring system and is structurally rigid. This determination provides accurate stereochemical parameters for those atoms and groups (N⁺ centers, aromatic rings, and hydrogen bond acceptors) postulated by various theories as involved in binding to the acetylcholine receptor. In the crystal, layers of alkaloid cations parallel to thebc plane alternate with layers containing two Cl^– ions and eight water molecules distributed almost randomly over ten sites. Electrostatic attractions between N⁺ and Cl^– bind the alternating layers together. Each one of the ten sites is, on average, 4.60 Å from one or two N⁺, allowing the two Cl_– ions to be disordered. Binding interactions within the alkaloid layers are solely van der Waals attractions. Within each H²O/C1^– layer there is a complex hydrogen bond system, including four infinite spirals parallel to theb axis, with an average bonding distance of 2.94 Å. There are no hydrogen bonds between layers. The possible relevance to the activity of the alkaloid of its ability to organize large amounts of water is discussed.     

PHASE SHIFT INTERFEROMETRY FOR ACCURATE TEMPERATURE MEASUREMENT AROUND A VAPOR BUBBLE

An interferometric measurement technique for determining accurately the temperature field around an axial symmetric vapor bubble that might be used for boiling investigations was developed. The specific setup of optical components and the fluid test cell for the generation of a single vapor bubble are described. The procedure and main steps to derive an accurate temperature field from the recorded interferograms are explained in detail. These are: wrapped phase calculation based on a four frames algorithm, phase unwrapping using Itoh algorithm, and Abe inversion for the calculation of the local temperatures. The results from interferometry are verified by point wise measurements using micro-thermocouples. Compared to formerly applied measurement techniques the presented phase shift interferometry leads to higher accuracy and higher spatial resolution.     

In Situ Monitoring of Polymer Processing Properties

Frequency-dependent dielectric measurements, often called frequency-dependent electromagnetic sensing (FDEMS), made over many frequency ranges, Hz–MHz, provide a sensitive, convenient automated means for monitoring in situ the processing properties of thermosets and thermoplastics. Using a planar wafer-thin sensor, measurements can be made in situ in almost any environment. Through the frequency dependence of the impedance, this sensing technique is able to monitor chemical and physical changes throughout the entire cure process. In this presentation we discuss how to use the frequency dependence in the Hz–MHz range to separate and determine parameters governing ionic and dipolar mobility is discussed. In turn, the relationship between these two molecular probes, ionic translational mobility and dipolar rotational mobility, and macroscopic processing properties is also discussed. The cure of a high-temperature polyimide (PMR-15) and the cure of several epoxy resins were monitored using dielectric sensing. The ionic and dipolar mobilities are quantitatively related to the viscosity, degree of cure and the build up in glass transition temperature. The ability of the change in ionic mobility to detect the gel point through the determination of an inflection point and through the use of a power law (1−t/t_gel)^x is discussed. Finally the application of in situ sensing to monitor processing properties and to control the cure process intelligently in a complex part in an autoclave is described. © 1997 John Wiley & Sons, Ltd.     

Adaptive Coupling and Fast Solution of FEM–BEM Equations for Parabolic–Elliptic Interface Problems

In this paper we prove an a posteriori error estimate for the symmetric coupling of finite elements and boundary elements applied to linear parabolic–elliptic interface problems. The discontinuous Galerkin method is used for the discretization in time. We present an adaptive algorithm for choosing the mesh size in space and time and we analyse the Hybrid Modified Conjugate Residual (HMCR) method as a solution method for the linear systems which arise. Computational results show that the number of HMCR-iterations grows slowly with the problem size. © 1997 by B. G. Teubner Stuttgart–John Wiley & Sons Ltd.     

Dynamic rheo-optical characterization of uniaxially oriented polyamide 11

Dynamic mechanical analysis, coupled with polarized step-scan FTIR transmission spectroscopy, has been used to monitor the submolecular motional behavior of uniaxially oriented polyamide 11. The dynamic in-phase spectra depend upon the morphology of the samples as well as on the polarization direction of the infrared radiation. The lineshape features of the dynamic in-phase spectra and their relationship to sample deformation are analyzed on the basis of changes of the internal coordinates, the reorientation movement of several functional groups, and the thickness change of the film during the stretching cycle. Dynamic infrared spectra are helpful for deconvolution of overlapping bands on the basis of their different responses to the external perturbation, which sometimes cannot be resolved well by derivative spectroscopy or curve-fitting analysis. The lineshape features have been used to follow microstructural changes after isothermal heat treatment. Near the N H stretching frequency, two bands at 3270 cm⁻¹ and 3200 cm⁻¹ are resolved and analyzed in terms of Fermi resonance between the N H stretching fundamental mode and the overtone and combination modes of the amide I and II vibrations. The dynamic response of the N H stretching mode correlates with the modulation of hydrogen bond strength in uniaxially oriented PA-11. After thermal treatment at the highest temperature (190°C), the dynamic response in this region is mainly caused by the modulation of crystals. In amide I region, three bands at 1680 cm⁻¹, 1648 cm⁻¹, and 1638 cm⁻¹ are separated and assigned to hydrogen bond-free, hydrogen-bonded amorphous, and hydrogen-bonded crystalline regions, respectively. The dynamic responses of the hydrogen-bonded regions are more sensitive to external perturbation. Two components are found in the amide II region, and the band at 3080 cm⁻¹ is assigned to the overtone resonance of the component with perpendicular polarization. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2895–2904, 1998     

Efficient,Reversible Redox‐Switching of Molecular First Hyperpolarizabilities in Ruthenium(II) Complexes Possessing Large Quadratic Optical Nonlinearities

Low‐energy metal‐to‐ligand charge‐transfer (MLCT) excitations are associated with the very large molecular nonlinear optical (NLO) coefficients, β, of the complexes [Ru^II(NH₃)₅(N‐R‐4,4′‐bipyridinium)]³⁺ (R=methyl, phenyl, or 4‐acetylphenyl). Chemical oxidation to the Ru^III forms causes bleaching of the MLCT absorptions and marked attenuation of the NLO responses. Both effects are completely reversed upon re‐reduction, and the extent of the β switching is about 10‐ to 20‐fold.