Polyisocyanides as a new alignment medium to measure residual dipolar couplings for small organic molecules

Polyisocycanides were found to give anisotropic molecular alignment in the magnetic field and are useful to measure residual dipolar couplings (RDCs) from analytes, e.g. strychnine. They show less quadrupolar splitting of the deuterated solvent signal compared with other liquid crystal systems such as Poly-γ-benzyl-L-glutamate (PBLG) and hence less undesired line broadening.     

Configuration assignment in small organic molecules via residual dipolar couplings

Here we propose a new method to assign relative configurations of stereocenters in small organic molecules by using residual dipolar couplings; the main advantage of this method is that spatial proximity of the stereocenters is not required.     

Unusual assembly of small organic building molecules in common solvent

During the process of self-association, reaching a thermodynamic equilibrium state in dilute solution is usually very fast, taking at most seconds for small organic (such as surfactants) solutions and hours for polymer solutions. It is very rare that days are necessary for soluble small organic molecules to reach thermodynamic stability in dilute solutions. This work reports such an unusually slow association of two polymerizable organic molecules, HOOC(CH2)3CCCC(CH2)3COOH and (EtO)3Si(CH2)3NH2, in their common solvent. The self-organization process of above complexes spanned several minutes to several days, depending on their concentrations. The morphologies of resultant aggregates, ranging from vesicles to solid spheres and to hollow spheres, were also tunable by varying the molar ratios of two precursors. Enriched functional COOH/NH2 groups on the aggregate surface can attach various antibodies, which endow the nanaoparticles with great potential applications as targeted drug-delivery vehicles. In addition, as-synthesized hybrid aggregates could be further stabilized by either addition reaction of diacetylenic acid or hydrolysis and condensation reactions of 3-aminopropyltriethoxysilane. In particular, the derived polydiacetylenic aggregates demonstrate a thermochromatic property and may be applied as sensing materials. Those novel phenomena, along with the simplicity in the preparation of aggregates, make the system promising in addressing related theoretical problems and practical applications.     

Achiral deuterated derivatizing agent for enantiomeric analysis of carboxylic acids by NMR in a chiral liquid crystalline solvent

The use of a deuterated ‘probe’ for the enantiomeric analysis of chiral carboxylic acids is proposed. The probe is the perdeuterobenzyl fragment which can be easily attached to the acid and removed from the corresponding ester. The analysis is performed through the measurement of the proton-decoupled deuterium NMR spectrum of the chiral benzyl ester dissolved in poly-γ-benzyl-l-glutamate/dimethylformamide liquid crystal. Enantiomeric discrimination was observed for all the studied compounds on the para and/or α deuterons. There is no need for the deuterons to be located close to the stereogenic centre. Thus enantiomers were correctly distinguished from the signal of a deuterium located 12 bonds away from the asymmetric carbon. The major interest of this general technique is that no kinetic resolution should occur during the derivatization process.     

The static electric permittivity of solutions of organic molecules in dipolar solvents

The permittivity of solutions of pyrazine and of 1,4-diazabicyclo-[2,2,2]-octane (TED) in various dipolar solvents has been measured at 10 MHz by means of a sensitive bridge method. The solvents used were water, methanol, formamide, N-methylformamide, N,N-dimethylformamide, and dimethylsulfoxide. The results of the measurements are compared with those of theoretical mixture formulas. It is shown that — depending on structural properties — the permittivity attributed to the solvation shells around the organic solute molecules is changed with respect to the pure solvent value.     

Direct prediction of residual dipolar couplings of small molecules in a stretched gel by stochastic molecular dynamics simulations

Residual dipolar couplings are highly useful NMR parameters for calculating and refining molecular structures, dynamics, and interactions. For some applications, however, it is inevitable that the preferred orientation of a molecule in an alignment medium is calculated a priori. Several methods have been developed to predict molecular orientations and residual dipolar couplings. Being beneficial for macromolecules and selected small‐molecule applications, such approaches lack sufficient accuracy for a large number of organic compounds for which the fine structure and eventually the flexibility of all involved molecules have to be considered or are limited to specific, well‐studied liquid crystals. We introduce a simplified model for detailed all‐atom molecular dynamics calculations with a polymer strand lined up along the principal axis as a new approach to simulate the preferred orientation of small to medium‐sized solutes in polymer‐based, gel‐type alignment media. As is shown by a first example of strychnine in a polystyrene/CDCl₃ gel, the simulations potentially enable the accurate prediction of residual dipolar couplings taking into account structural details and dynamic averaging effects of both the polymer and the solute. Copyright © 2015 John Wiley & Sons, Ltd.     

Dielectric relaxation processes of some nearly spherical dipolar organic molecules in a few glassy media

Summary Dielectric absorption studies have been carried out on some dipolar, almost spherical organic molecules (camphane derivatives) in a polystyrene matrix in the frequency range 10–10⁵ Hz between 80 and 300 K. Some molecules have also been studied in other glass forming media such as carbon tetrachloride, glassy-o-terphenyl and polyphenyl ether (Santovac). All of these molecules show dielectric relaxation due to molecular reorientation in each dispersion medium with characteristic activation parameters. Alcohol molecules do not show dispersion due to hydrogen-bonded species below five percent concentration, a range usually used in dielectric studies. Relaxation due to the hydrogen-bonded species is observed at higher concentrations. High activation parameters for molecular reorientation of camphane derivatives are accounted for by the strong intermolecular interactions at low temperatures.

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Dielektrische Relaxationsprozesse einiger nahezu sphärischer dipolarer organischer Moleküle in glasartigen Medien

Zusammenfassung Es wurden die dielektrischen Absorptionen einiger dipolarer, nahezu sphärischer organischer Moleküle (Camphan-Derivate) in einer Polystyrolmatrix im Frequenzbereich 10–10⁵ Hz zwischen 80 und 300 K untersucht. Für einige Moleküle wurden auch andere glasbildende Medien herangezogen, z.B. Tetrachlorkohlenstoff, glasigeso-Terphenyl und Polyphenylether (Santovac). Alle untersuchten Moleküle zeigten eine dielektrische Relaxation mit charakteristischen Aktivierungsparametern, die auf molekulare Reorientierung in den jeweiligen Dispersionsmedien zurückzuführen sind. Alkoholmoleküle zeigen wegen wasserstoffbrückengebundener Spezies bei einer Konzentration unter 5% (ein bei dielektrischen Untersuchungen üblicher Bereich) keine Dispersion. Bei höheren Konzentrationen wird wegen wasserstoffbrückengebundener Spezies Relaxation beobachtet. Die hohen Aktivierungsparameter für die molekulare Reorientierung von Camphan-Derivaten wird auf starke intermolekulare Wechselwirkungen bei tiefen Temperaturen zurückgeführt.

     

Photoconduction in rare gas fluids doped by small organic molecules

Impurity photoconduction was studied in fluid argon, krypton and xenon doped by one of the following impurities ethane, propane, butane, and benzene. For several impurity/host combinations it was possible to determine, for a wide density range, the energy needed to raise an electron from the ground state of the impurity to the conduction level of the host. The polarization energy P₊ⁱ of the medium by the positive impurity ion was calculated from these results and compared with the Born charging energy formula.     

From polymer to small organic molecules: a tight relationship between radical chemistry and solid-phase organic synthesis

Since Gomberg's discovery of radicals as chemical entities, the interest around them has increased through the years. Nowadays, radical chemistry is used in the synthesis of 75% of all polymers, inevitably establishing a close relationship with Solid-Phase Organic Synthesis. More recently, the interest of organic chemists has shifted towards the application of usual "in-solution" radical chemistry to the solid-phase, ranging from the use of supported reagents for radical reactions, to the development of methodologies for the synthesis of small molecules or potential libraries. The aim of this review is to put in perspective radical chemistry, moving it away from its origin as a synthetic means for solid supports, to becoming a useful tool for the synthesis of small molecules.     

Dynamic polarization of solvent nuclei in solutions of diphenylpicrylhydrazyl in organic liquids

Measurements have been made of the dynamic polarization of nuclei of solvent in concentrated solutions of diphenylpicrylhydrazyl (DPPG) in a weak magnetic field of approximately 21 Oe. In proton-containing solutions, a negative dynamic polarization was observed, corresponding to dipole-dipole interaction of electronic and nuclear spins. In a hexafluorobenzene solution a considerable contribution of scalar interaction to the relaxation of the fluorine nucleus was observed. The temperature dependence of the dynamic polarization of the fluorine nucleus has been investigated, and an activation energy of 3.3 kcal/mole has been found for the bond between the molecules of hexafluorobenzene and the solvate shell of DPPH molecules.     

Excitonic effects in molecular crystals built up by small organic molecules

The excitonic effects of biphenyl and 2,2′-bithiophene are investigated within an ab initio framework. For this purpose the Bethe–Salpeter equation for the two-particle Greens function is solved. Therefrom the imaginary part of the dielectric function is derived, which includes the electron–hole interaction in the absorption process. It turns out that these organic molecular crystals, which are built by small molecules, give rise to sizeable exciton binding-energies, which are between 0.7 and 0.8 eV. To study the influence of the intermolecular interaction, the exciton binding energy of crystalline biphenyl is calculated as a function of pressure. The decrease of both, the band gap and the exciton binding energy, results in a slight red-shift of the lowest optically active singlet exciton.     

Configurational statistics of macromolecules in solution from correlations of liquid molecules

We address the relevant quest for a simple formalism describing the microstructure of liquid solutions of polymer chains. On the basis of a recent relativistic-type picture of self-diffusion in (simple) liquids named Brownian relativity (BWR), a covariant van Hove's distribution function in a Vineyard-like convolution approximation is proposed to relate the statistical features of liquid and chain molecules forming a dilute polymer solution. It provides an extension of the Gaussian statistics of ideal chains to correlated systems, allowing an analysis of macromolecular configurations in solution by the only statistical properties of the liquid units (and vice versa). However, the mathematical solution to this issue is not straightforward because, when the liquid and polymer van Hove's functions are equated, an inverse problem takes place. It presents some conceptual analogies with a scattering experiment in which the correlation of the liquid molecules acts as the radiation source and the macromolecule as the scatterer. After inverting the equation by a theorem coming from the Tikhonov's approach, it turns out that the probability distribution function of a real polymer can be expressed from a static Ornstein-Uhlenbeck process, modified by correlations. This result is used to show that the probability distribution of a true self-avoiding walk polymer (TSWP) can be modeled as a universal Percus-Yevick hard-sphere solution for the total correlation function of the liquid units. This method suits in particular the configurational analysis of single macromolecules. The analytical study of arbitrary many-polymer systems may require further mathematical investigation.