Stairway to the conical intersection: a computational study of the retinal isomerization

The potential-energy surface of the first excited state of the 11-cis-retinal protonated Schiff base (PSB11) chromophore has been studied at the density functional theory (DFT) level using the time-dependent perturbation theory approach (TDDFT) in combination with Becke's three-parameter hybrid functional (B3LYP). The potential-energy curves for torsion motions around single and double bonds of the first excited state have also been studied at the coupled-cluster approximate singles and doubles (CC2) level. The corresponding potential-energy curves for the ground state have been calculated at the B3LYP DFT and second-order M?ller-Plesset (MP2) levels. The TDDFT study suggests that the electronic excitation initiates a turn of the beta-ionone ring around the C6-C7 bond. The torsion is propagating along the retinyl chain toward the cis to trans isomerization center at the C11=C12 double bond. The torsion twist of the C10-C11 single bond leads to a significant reduction in the deexcitation energy indicating that a conical intersection is being reached by an almost barrierless rotation around the C10-C11 single bond. The energy released when passing the conical intersection can assist the subsequent cis to trans isomerization of the C11=C12 double bond. The CC2 calculations also show that the torsion barrier for the twist of the retinyl C10-C11 single bond adjacent to the isomerization center almost vanishes for the excited state. Because of the reduced torsion barriers of the single bonds, the retinyl chain can easily deform in the excited state. Thus, the CC2 and TDDFT calculations suggest similar reaction pathways on the potential-energy surface of the excited state leading toward the conical intersection and resulting in a cis to trans isomerization of the retinal chromophore. According to the CC2 calculations the cis to trans isomerization mechanism does not involve any significant torsion motion of the beta-ionone ring.     

Sorption and diffusion of methanol and water in PVDF‐g‐PSSA and Nafion® 117 polymer electrolyte membranes

Sorption and diffusion properties of poly(vinylidene fluoride)‐graft‐poly(styrene sulfonic acid) (PVDF‐g‐PSSA) and Nafion® 117 polymer electrolyte membranes were studied in water/methanol mixtures. The two types of membranes were found to have different sorption properties. The Nafion 117 membrane was found to have a maximum in‐solvent uptake around 0.4 to 0.6 mole fraction of methanol, while the PVDF‐g‐PSSA membranes took up less solvent with increasing methanol concentration. The proton NMR spectra were recorded for membranes immersed in deuterated water/methanol mixtures. The spectra showed that the hydroxyl protons inside the membrane exhibit resonance lines different from the resonance lines of hydroxyl protons in the external solvent. The spectral features of the lines of these internal hydroxyl groups in the membranes were different in the Nafion membrane compared with the PVDF‐g‐PSSA membranes. Diffusion measurements with the pulsed field gradient NMR (PFG‐NMR) method showed that the diffusion coefficient of the internal hydroxyl groups in the solvent immersed Nafion membrane mirrors the changes in the diffusion coefficients of hydroxyl and methyl protons in the external solvent. For the PVDF‐g‐PSSA membranes, a decrease in the diffusion coefficient of the internal hydroxyl protons was seen with increasing methanol concentration. These results indicate that the morphology and chemical structure of the membranes have an effect on their solvent sorption and diffusion characteristics. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3277–3284, 2000     

Synthesis and properties of sulfonated and crosslinked poly[(vinylidene fluoride)-graft-styrene] membranes

Crosslinked proton exchange membranes were prepared by electron beam grafting of styrene with divinylbenzene, DVB, or bis(vinyl phenyl)ethane, BVPE, on to poly(vinylidene fluoride), PVDF, followed by sulfonation. The area and thickness increase, crystallinity and mechanical properties of the membranes were investigated. All these properties were influenced by the crosslinker structure and its concentration. The crosslinkers increase the final degree of grafting. For noncrosslinked membranes it was about 80% and with 10% of BVPE crosslinker about 180% after 8 hr. The area increase with grafting is very similar for the noncrosslinked membranes and the differently crosslinked membranes, but the crosslinkers reduce swelling for sulfonated membranes swelled in boiling water. DVB reduces swelling more than BVPE. With grafting, the crystallinity of the PVDF phase decreases; with 10% BVPE the decrease is smallest and with 10% DVB largest. The mechanical properties of the sulfonated swelled membranes decrease with the degree of grafting, for a 10% DVB crosslinked membrane the strength decreases to zero at degrees of grafting higher than 75%. © 1998 John Wiley & Sons, Ltd.     

Molecular Dynamics Simulations of Polymers of Unsubstituted and Substituted Poly(p‐phenylene terephthalate)s in the Bulk State

A molecular dynamics simulation has been carried out on polymers of p‐phenylene terephthalate (PPT) and 1,4‐phenylene‐2‐octyloxy terephthalate (PTA8) at six different temperatures: 300 K, 375 K, 450 K, 575 K, 600 K, and 625 K. The periodic simulation boxes consisted of 18 (PPT) or 16 (PTA8) chains with 9 monomer units. The Compass force field was used in the simulations. The crystallographic data and density obtained from the NPT simulation at room temperature agreed well with the experimental data of PPT. Our simulations showed that, in the case of PTA8, the crystal → layered nematics phase transition temperature (T_m) is close to the experimental value for the polymer similar to PTA8. The average initial structure was obtained from X‐ray scattering data. The simulated X‐ray data indicated the existence of a biaxial nematic phase, in accordance with experimental findings.     

Synthesis, electropolymerization and electrochemical characterization of some new acrylate substituted thiophene derivatives

In this work, three new acrylate substituted thiophene monomers, (3-thienyl) methylacrylate, 6-(3-thienyl)methoxy-hexylacrylate and 11-(3-thienyl)methoxy-undecylacrylate were synthesized and electropolymerized. Electrochemical polymerisation by a potential step technique leads to the formation of an electroactive film on a Pt electrode in each case. The polymerization involves evidently only the thiophene ring, as no sign of polyacrylate formation could be detected. The oxidation of each polymer was studied by in situ external reflectance FTIR spectroscopy, which showed several bands assigned to (bi)polaron type charge carriers in the region 1500–1000 cm⁻¹. The evolution of the conductivity was studied in situ by the contact electric resistance technique. The conductivity of poly(3-thienyl methacrylate) was found to be higher in aqueous solutions than in acetonitrile. The surface of poly((3-thienyl) methacrylate) was found to have a granular structure observed for many polythiophenes, as studied by atomic force microscopy (AFM).     

Aromatic pathways of porphins, chlorins, and bacteriochlorins

Magnetically induced current densities have been calculated for free-base porphynoids using the gauge including magnetically induce current (GIMIC) method. Numerical integration of the current density passing selected chemical bonds yields current pathways and the degree of aromaticity according to the magnetic criterion. The ring-current strengths of the porphins, chlorins, and bacteriochlorins are 1.5-2.5 times stronger than for benzene. The calculations show that the 18π [16]annulene inner cross is not the correct picture of the aromatic pathway for porphyrins. All conjugated chemical bonds participate in the current transport independently of the formal number of π electrons. The ring current branches at the pyrrolic rings taking both the outer and the inner route. The NH unit of the pyrrolic rings has a larger resistance and a weaker current strength than the pyrroles without inner hydrogens. The traditional 18π [18]annulene with inactive NH bridges is not how the ring-current flows around the macroring. The porphins have the strongest ring current of ca. 27 nA/T among the investigated porphynoids. The current strengths of the chlorins and bacteriochlorins are 19-24 nA/T depending on whether the ring current is forced to pass an NH unit or not. The current strengths of the 3-fold and 4-fold β-saturated porphynoids are 13-17 nA/T, showing that the inner-cross 18π [16]annulene pathway is not a preferred current route.     

Divergent Carbocatalytic Routes in Oxidative Coupling of Benzofused Heteroaryl Dimers: A Mechanistic Update

Mildly thermal air or HNO₃ oxidized activated carbons catalyse oxidative dehydrogenative couplings of benzo[b]fused heteroaryl 2,2’-dimers, e.g., 2-(benzofuran-2-yl)-1H-indole, to chiral 3,3’-coupled cyclooctatetraenes or carbazole-type migrative products under O₂ atmosphere. DFT calculations show that the radical cation and the Scholl-type arenium cation mechanisms lead to different products with 2-(benzofuran-2-yl)-1H-indole, being in accord with experimental product distributions.     

Tuning the Protein‐Induced Absorption Shifts of Retinal in Engineered Rhodopsin Mimics

Rational design of light‐capturing properties requires understanding the molecular and electronic structure of chromophores in their native chemical or biological environment. We employ here large‐scale quantum chemical calculations to study the light‐capturing properties of retinal in recently designed human cellular retinol binding protein II (hCRBPII) variants (Wang et al. Science, 2012 , 338, 1340–1343). Our calculations show that these proteins absorb across a large part of the visible spectrum by combined polarization and electrostatic effects. These effects stabilize the ground or excited state energy levels of the retinal by perturbing the Schiff‐base or β‐ionone moieties of the chromophore, which in turn modulates the amount of charge transfer within the molecule. Based on the predicted tuning principles, we design putative in silico mutations that further shift the absorption properties of retinal in hCRBPII towards the ultraviolet and infrared regions of the spectrum.     

Two-dimensional fully numerical solutions of molecular Hartree-Fock equations: LiH and BH

A fully numerical Hartree-Fock approach is developed for diatomic molecules. The exchange potential is solved relaxing a local, Poisson-like equation. Improved Hartree-Fock limits are reported for LiH and BH.     

More economical use of high-performance thin-layer plates for chromatographic screening of illicit drug samples

A rapid thin-layer chromatographic method for the separation of illicit drugs and similar compounds by two simultaneous but different developments on the same chromatographic plate is described. One half of the plate was impregnated with an aqueous solution of KHSO4 and the whole plate developed in a Camag linear developing chamber from two opposite sides using methanol with 0.01 M KBr as eluent.