Influences of local polymer-solvent π-π-interaction on dynamics of phenyl ring rotation and its role on photophysics of conjugated polymer

In this report, the role of local polymer-solvent π-π-interaction on rotational dynamics of phenyl rings of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is examined by utilizing nuclear magnetic resonance (NMR) spectroscopy. We have found that an aromatic solvent can couple strongly with the phenyl rings via favorable local π-π-interaction, causing a restriction of the ring rotation. The dynamic process takes place at much faster rate in a non-aromatic solvent. NMR line shape analysis indicates the existence of two side chain configurations with relatively long life-time in toluene and pyridine while a single time-averaged configuration is detected in chloroform and tetrahydrofuran. Addition of chloroform or cyclohexane to a solution of MEH-PPV in toluene is accompanied by an increase of the rotational dynamics. This indicates that the expansion or collapse of main chain upon varying solvent quality play a minor role on the rotational dynamics. The relationship between the dynamics of ring rotation and photophysics of MEH-PPV in solution is discussed.     

Rotamer decomposition and protein dynamics: Efficiently analyzing dihedral populations from molecular dynamics

Molecular mechanics methods have matured into powerful methods to understand the dynamics and flexibility of macromolecules and especially proteins. As multinanosecond to microsecond length molecular dynamics (MD) simulations become commonplace, advanced analysis tools are required to generate scientifically useful information from large amounts of data. Some of the key degrees of freedom to understand protein flexibility and dynamics are the amino acid residue side chain dihedral angles. In this work, we present an easily automated way to summarize and understand the relevant dihedral populations. A tremendous reduction in complexity is achieved by describing dihedral timeseries in terms of histograms decomposed into Gaussians. Using the familiar and widely studied protein lysozyme, it is demonstrated that our approach captures essential properties of protein structure and dynamics. A simple classification scheme is proposed that indicates the rotational state population for each dihedral angle of interest and allows a decision if a given side chain or peptide backbone fragment remains rigid during the course of an MD simulation, adopts a converged distribution between conformational substates or has not reached convergence yet. © 2012 Wiley Periodicals, Inc.     

New acetylated flavone C-glycosides from Iris lactea

Chemical investigation of the aerial parts of Iris lactea afforded three new flavone C-glycosides including 4?-O-acetyl-embinin (1), 2?,4?-O-diacetyl-embinin (2) and 6″,4?-O-diacetyl-embinin (3) along with the known analogue embinin (4). Their structures were elucidated by 1D and 2D NMR spectroscopic analysis as well as by HRESIMS data. The sugars were characterized following acid hydrolysis of the respective glycosides and TLC analysis compared to known standards. Duplicated signals can be observed in the NMR spectra, indicating the presence of rotamers caused by rotational hindrance around the glycosyl-flavone CC linkage. All isolated compounds were tested for their antimicrobial and cytotoxic activities but found to be inactive.     

Progress toward the total synthesis of 9β-hydroxyvertine: Construction of an advanced quinolizidine intermediate

Progress toward the total synthesis of 9β-hydroxyvertine 2 is described. Our approach involves a Petasis borono-Mannich reaction to assemble a key intermediate A, securing the correct configuration at H-9 and H-10 in the final targeted molecule. Subsequent formation of the quinolizidine framework B allowed the synthesis of an advanced intermediate containing all but the lactone moiety of 2. Our unsuccessful attempts at introducing this lactone feature are also described.     

Variable temperature H NMR studies on Grubbs catalysts

Variable temperature ¹H NMR studies were conducted to investigate whether steric congestion is influencing the structural rigidity of (IMesH₂)(PCy₃)(Cl)₂RuCHPh (IMesH₂ = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) in solution. It was shown that both mesityl ligands rotate at about the same rate around the N-Mesityl bonds in the IMesH₂ ligand and that changing the solvent does not significantly alter this rotation. It was found that the increased steric congestion in (IMesH₂)(PCy₃)(Cl)₂RuCHPh compared to (PCy₃)₂(Cl)₂RuCHPh does affect the rates of rotation around the C_alkylidene-Ph bonds. Unusual chemical shift positions were also observed in the low temperature ¹H NMR spectrum for the aromatic proton signals for (IMesH₂)(PCy₃)(Cl)₂RuCHPh and (PCy₃)₂(Cl)₂RuCHPh.     

What is the enthalpy of formation of acrylic acid?

Acrylic acid is a key industrial compound with numerous uses. Despite its importance, its enthalpy of formation is still contentious—even ignoring “ancient” determinations, there is a 12 kJ mol⁻¹ range of values reported for the gas phase quantity, −320 to −332 kJ mol⁻¹. Our quantum chemical calculations using current methodology suggest the value of −321 ± 3 kJ mol⁻¹.     

Quantum chemical analysis of the energetics of the anti and gauche conformers of ethanol

Ethanol displays two stable conformers, the classic anti (or trans) form and a gauche conformation in which the hydroxyl hydrogen points toward one of the methyl hydrogens. Surprisingly, the two forms have nearly equal energies, and in the vapor phase the gauche form predominates because of its twofold degeneracy. An analysis of the energetics of these conformers based on natural bond orbital analysis helps to explain the apparently anomalous near degeneracy of these conformers.     

Unusually High Barriers to Rotation Involving the Tetrahedral Carbon Atom

Rotation about CC single bonds in suitably substituted compounds gives rise to rotational isomers (also called rotamers) which can be isolated if the barrier to rotation is high enough. The present article considers derivatives of triptycene and fluorene whose barriers to rotation are of the order of 30 kcal/mol.     

Stereochemistry of alkylated isoquinoline Reissert compounds

A series of alkylated isoquinoline Reissert compounds (2-acyl-1-alkyl-1,2-dihydroisoquinoline-1-nitriles) with either 3-H (3) or 3-CH₃ (4) substituents was examined by ¹H, ¹³C, and ¹⁹F NMR spectroscopies and X-ray crystallography. In all cases the trans-amide conformation, with reference to the carbonyl oxygen and the 3-position of the isoquinoline ring, exists exclusively in the solid state, and predominates in solution. As expected the 1-alkyl groups (isopropyl, isobutyl, benzyl) are nearly pseudo-axial or axial in all cases. In N-aroyl compounds the larger ortho-aroyl substituent lies syn to the 1-alkyl moiety in nine of the 11 cases, as revealed by X-ray crystallographic studies. In solution NMR results show that atropisomerism about the Ar/CO bond is usually rapid at room temperature, but slow at −50 °C and usually favors the syn isomer. CH?π interactions of the 3-methyl protons of 4 with the N-aroyl moieties appear to influence the atropisomerism.     

Atropisomerism about a heptafluoroisopropyl to aryl bond in 5-amino-4-heptafluoroisopropyl indazole

The atropisomerism about a heptafluoroisopropyl to aryl bond in 5-amino-4-heptafluoroisopropyl indazole is reported. The through-space hydrogen fluorine coupling of the indazole H-3 proton with the tertiary F atom is shown to be useful to distinguish the two rotamers. The same atropisomerism is also observed in the analogous indole, benzofuran, and naphthaline derivatives.