Recovering Invisible Signals by Two‐Field NMR Spectroscopy

Nuclear magnetic resonance (NMR) studies have benefited tremendously from the steady increase in the strength of magnetic fields. Spectacular improvements in both sensitivity and resolution have enabled the investigation of molecular systems of rising complexity. At very high fields, this progress may be jeopardized by line broadening, which is due to chemical exchange or relaxation by chemical shift anisotropy. In this work, we introduce a two‐field NMR spectrometer designed for both excitation and observation of nuclear spins in two distinct magnetic fields in a single experiment. NMR spectra of several small molecules as well as a protein were obtained, with two dimensions acquired at vastly different magnetic fields. Resonances of exchanging groups that are broadened beyond recognition at high field can be sharpened to narrow peaks in the low‐field dimension. Two‐field NMR spectroscopy enables the measurement of chemical shifts at optimal fields and the study of molecular systems that suffer from internal dynamics, and opens new avenues for NMR spectroscopy at very high magnetic fields.     

Total synthesis of two 12-nordrimanes and the pharmacological active sesquiterpene hydroquinone yahazunol

The synthesis of two 12-nordrimanes and yahazunol was achieved via 8-oxo-12-nordrimanic acid methyl ester. The cytotoxic activity of yahazunol and seven other sesquiterpene hydroquinones and sesquiterpene quinones has been determined.     

On the Validity of Stokes–Einstein and Stokes–Einstein–Debye Relations in Ionic Liquids and Ionic‐Liquid Mixtures

Stokes–Einstein (SE) and Stokes–Einstein–Debye (SED) relations in the neat ionic liquid (IL) [C₂mim][NTf₂] and IL/chloroform mixtures are studied by means of molecular dynamics (MD) simulations. For this purpose, we simulate the translational diffusion coefficients of the cations and anions, the rotational correlation times of the C(2)? H bond in the cation C₂mim⁺, and the viscosities of the whole system. We find that the SE and SED relations are not valid for the pure ionic liquid, nor for IL/chloroform mixtures down to the miscibility gap (at 50 wt % IL). The deviations from both relations could be related to dynamical heterogeneities described by the non‐Gaussian parameter α(t). If α(t) is close to zero, at a concentration of 1 wt % IL in chloroform, both relations become valid. Then, the effective radii and volumes calculated from the SE and SED equations can be related to the structures found in the MD simulations, such as aggregates of ion pairs. Overall, similarities are observed between the dynamical properties of supercooled water and those of ionic liquids.     

Formation of superlattices via blending of block copolymers

Block copolymers are well‐known for their large number of microphase morphologies on mesoscopic length scales. After a short review of the different morphologies observed in binary block copolymers and ternary triblock copolymers, the self‐assembling in blends of different block copolymers into common superlattices is discussed in detail. Besides similar morphologies known for pure triblock and diblock copolymers, the blends can also show new morphologies. Examples of such new morphologies are periodic non‐centrosymmetric lamellae and multiple gyroid interface structures. The discussion of the superlattices is primarily based on investigations by transmission electron microscopy (TEM), which are supplemented in a few cases by small angle X‐ray scattering (SAXS) or results from computer simulations.     

On‐Surface Synthesis of Cumulene‐Containing Polymers via Two‐Step Dehalogenative Homocoupling of Dibromomethylene‐Functionalized Tribenzoazulene

Cumulene compounds are notoriously difficult to prepare and study because their reactivity increases dramatically with the increasing number of consecutive double bonds. In this respect, the emerging field of on‐surface synthesis provides exceptional opportunities because it relies on reactions on clean metal substrates under well‐controlled ultrahigh‐vacuum conditions. Here we report the on‐surface synthesis of a polymer linked by cumulene‐like bonds on a Au(111) surface via sequential thermally activated dehalogenative C?C coupling of a tribenzoazulene precursor equipped with two dibromomethylene groups. The structure and electronic properties of the resulting polymer with cumulene‐like pentagon–pentagon and heptagon–heptagon connections have been investigated by means of scanning probe microscopy and spectroscopy methods and X‐ray photoelectron spectroscopy, complemented by density functional theory calculations. Our results provide perspectives for the on‐surface synthesis of cumulene‐containing compounds, as well as protocols relevant to the stepwise fabrication of carbon–carbon bonds on surfaces.     

Simulation of Ion Motion at Atmospheric Pressure: Particle Tracing Versus Electrokinetic Flow

Results obtained with two computational approaches for the simulation of ion motion at elevated pressure are compared with experimentally derived ion current data. The computational approaches used are charged particle tracings with the software package SIMION ver. 8 and finite element based calculations using the software package Comsol Multiphysics ver. 4.0/4.0a. The experimental setup consisted of a tubular corona discharge ion source coupled to a cylindrical measurement chamber held at atmospheric pressure. Generated ions are flown into the chamber at essentially subsonic laminar isothermal conditions. In the simulations, strictly stationary conditions were assumed. The results show very good agreement between the SIMION/SDS model and experimental data. For the Comsol model, only qualitative agreement is observed.     

Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

An analysis for the DIIS acceleration method used in quantum chemistry calculations

This work features an analysis for the acceleration technique DIIS that is standardly used in most of the important quantum chemistry codes, e.g. in DFT and Hartree–Fock calculations and in the Coupled Cluster method. Taking up results from Harrison (J Comput Chem 25:328, 2003), we show that for the general nonlinear case, DIIS corresponds to a projected quasi-Newton/secant method. For linear systems, we establish connections to the well-known GMRES solver and transfer according (positive as well as negative) convergence results to DIIS. In particular, we discuss the circumstances under which DIIS exhibits superlinear convergence behaviour. For the general nonlinear case, we then use these results to show that a DIIS step can be interpreted as step of a quasi-Newton method in which the Jacobian used in the Newton step is approximated by finite differences and in which the according linear system is solved by a GMRES procedure, and give according convergence estimates.