Hexatic ordering in freely suspended liquid crystal films

In this paper, we report results from synchrotron X-ray scattering studies of thefluid/hexactic/solid phases and phase transitions in both very thick and very thin, freely suspended films of tilted hexatic liquid crystals. Contrary to the thick film case, the higher Fourier coefficients describing the bond orientational order are suppressed in very thin films. This suppression is consistent with a two-dimensional bond orientational order parameter, ₆, rather than the three-dimensional bond orientational order parameter found in very thick films. For a film containing twently-three (23) smectic layers we find that ₆ is two-dimensional whereas the positional order in the crystallineS _J phase is three-dimensional. We present an analysis of the thick film data in terms of the three-dimensionalXY-model and a new mean field theory model which incorporates explicitly the quasi two-dimensional nature of bulk smectic phases.     

Production of crosslinked,hollow nanoparticles by surface‐initiated living free‐radical polymerization

Surface‐initiated living free‐radical polymerization is employed in a multistep procedure to prepare hollow polymeric nanocapsules. Initially, trichlorosilyl‐substituted alkoxyamine initiating groups are attached to the surface silanol groups of silica nanoparticles. This surface layer of initiating groups is then used to grow functionalized linear chains leading to a core–shell morphology. The choice of functional groups is governed by their ability to undergo facile crosslinking reactions, with both active ester and benzocyclobutene groups being examined. Under either chemical or thermal conditions, the reaction of these functionalities gives a crosslinked polymeric shell that is covalently attached to, and surrounds, the central silica core. Removal of the silica core with HF then gives the hollow polymeric nanocapsules, which are stable under solvent dissolution and thermal treatment because of their crosslinked structure. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1309–1320, 2002     

Reduction pathway of end-on terminally coordinated dinitrogen. V. N-N bond cleavage in Mo/W hydrazidium complexes with diphosphine coligands. Comparison with triamidoamine systems

N-N cleavage of the dialkylhydrazido complex [W(dppe)2(NNC5H10)] (B(W)) upon treatment with acid, leading to the nitrido/imido complex and piperidine, is investigated experimentally and theoretically. In acetonitrile and at room temperature, B(W) reacts orders of magnitude more rapidly with HNEt3BPh4 than its Mo analogue, [Mo(dppe)2(NNC5H10)] (B(Mo)). A stopped-flow experiment performed for the reaction of B(W) with HNEt3BPh4 in propionitrile at -70 degrees C indicates that protonation of B(W) is completed within the dead time of the stopped-flow apparatus, leading to the primary protonated intermediate B(W)H+. Propionitrile coordination to this species proceeds with a rate constant k(obs(1)) of 1.5 +/- 0.4 s(-1), generating intermediate RCN-B(W)H+ (R = Et) that rapidly adds a further proton at Nbeta and then mediates N-N bond splitting in a slower reaction (k(obs(2)) = 0.35 +/- 0.08 s(-1), 6 equiv of acid). k(obs(1)) and k(obs(2)) are found to be independent of the acid concentration. The experimentally observed reactivities of B(Mo) or B(W) with acids in nitrile solvents are reproduced by DFT calculations. In particular, geometry optimization of models of solvent-coordinated, Nbeta-protonated intermediates is found to lead spontaneously to separation into the nitrido/imido complexes and piperidine/piperidinium, corresponding to activationless heterolytic N-N bond cleavage processes. Moreover, DFT indicates a spontaneous cleavage of nonsolvated B(W) protonated at Nbeta. In the second part of this article, a theoretical analysis of the N-N cleavage reaction in the Mo(III) triamidoamine complex [HIPTN3N]Mo(N2) is presented (HIPTN3N = hexaisopropylterphenyltriamidoamine). To this end, DFT calculations of the Mo(III)N2)triamidoamine complex and its protonated and reduced derivatives are performed. Calculated structural and spectroscopic parameters are compared to available experimental data. N-N cleavage most likely proceeds by one-electron reduction of the Mo(V) hydrazidium intermediate [HIPTN3N]Mo(NNH3)+, which is predicted to have an extremely elongated N-N bond. From an electronic-structure point of view, this reaction is analogous to that of Mo/W hydrazidium complexes with diphos coligands. The general implications of these results with respect to synthetic N2 fixation are discussed.     

The stereochemistry of some new chiral brominated compounds with a 2,4,8,10-tetraoxaspiro[5.5]undecanic skeleton

Summary New compounds containing the 2,4,8,10-tetraoxaspiro[5.5]undecanic skeleton, substituted with brominated groups, have been synthesized by a regioselective radicalic bromination reaction. The stereochemistry of the compounds was studied by high resolution NMR methods. The anancomericity or the flipping of the rings was inferred from the conformational analysis. The chirality of the spiranic skeleton was investigated by means of the diastereotopicity of hydrogen and carbon atoms.On leave from Organic Chemistry Department, Babes-Bolyai University, RO-3400 Cluj-Napoca, Romania     

A method for investigations of solid surfaces by nuclear spin relaxation

A method is presented which allows to investigate hot metal surfaces (1,000 K to 1,600 K) by use of nuclear spin polarized alkali atom beams (⁶Li,⁷Li,²³Na). At these temperatures the atoms are adsorbed for about 10^–4 s to 1 s. During the residence time on the surface the nuclear moments interact with the electromagnetic fields of their environment. The nuclear spin polarization of the ions desorbing from the surface is detected by beam foil spectroscopy. The depolarization gives information on electric field gradients and magnetic fields originating in the surface and on the electronic state of the absorbed alkali atoms.Supported partly by the Deutsche Forschungsgemeinschaft and the Bundesministerium für Forschung und Technologie, both Bonn, FRG     

Multipole electrostatic model for MNDO-like techniques with minimal valence spd-basis sets

We report an implementation of an atomic multipole model (up to quadrupole) for calculating the electrostatic properties of molecules based on electron densities derived from MNDO-like NDDO-based semiempirical MO calculations with minimal s,p,d valence basis sets. The results were validated by a comparison of the calculated values of the molecular electrostatic potential with those obtained from fine grain numerical integrations (both with AM1*), B3LYP/6–31G(d) and MP2/6–31G(d). The DFT and ab initio potentials can be reproduced remarkably well (mean unsigned error <2 kcal mol⁻¹ e⁻¹) using simple linear regression equations to correct the AM1* (multipole) results. Dedicated to Prof. Karl Jug on the occasion of his 65th birthday