Photodissociation dynamics of the NO dimer. I. Theoretical overview of the ultraviolet singlet excited states

Molecular orbital theory and calculations are used to describe the ultraviolet singlet excited states of NO dimer. Qualitatively, we derive and catalog the dimer states by correlating them with monomer states, and provide illustrative complete active space self-consistent field calculations. Quantitatively, we provide computational estimates of vertical transition energies and absorption intensities with multireference configuration interaction and equations-of-motion coupled-cluster methods, and examine an important avoided crossing between a Rydberg and a valence state along the intermonomer and intramonomer stretching coordinates. The calculations are challenging, due to the high density of electronic states of various types (valence and Rydberg, excimer and charge transfer) in the 6-8 eV region, and the multiconfigurational nature of the ground state. We have identified a bright charge-transfer (charge-resonance) state as responsible for the broadband seen in UV absorption experiments. We also use our results to facilitate the interpretation of UV photodissociation experiments, including the time-resolved 6 eV photodissociation experiments to be presented in the next two papers of this series.     

Two-compartment radial diffusive exchange analysis of the NMR lineshape of (129)Xe dissolved in a perfluorooctyl bromide emulsion

Hyperpolarized (129)Xe (xenon) gas dissolved in a perfluorooctyl bromide (PFOB) emulsion stabilized with egg yolk phospholipid (EYP) is a possible contrast agent for quantitative blood flow measurements using magnetic resonance imaging. The NMR line shape of xenon dissolved in PFOB emulsion depends strongly on the exchange of spins between PFOB and water. The exchange in this system depends on three factors: the geometrical factors (i.e., droplet size and surrounding water volume), the permeability of the EYP monolayer surrounding the droplet, and the diffusion coefficients of xenon in the two media. A theoretical model which predicts the line shape of xenon in the emulsion based on the Bloch-Torrey equations is presented. Fitting the full width at half maximum (FWHM) of the theoretical line shapes with the FWHM of the experimental spectra obtained from emulsions with different water dilutions allows estimation of the volume-weighted average diameter of the PFOB droplets (3.5+/-0.8) microm and the permeability of the EYP membrane surrounding the droplet (58+/-14) microm / s.     

Soft core thermodynamics from self-consistent hard core fluids

In an effort to generalize the self-consistent Ornstein-Zernike approximation (SCOZA)-an accurate liquid state theory that has been restricted so far to hard core systems-to arbitrary soft core systems we study a combination of SCOZA with a recently developed perturbation theory. The latter was constructed by Ben-Amotz and Stell [J. Phys. Chem. B 108, 6877 (2004)] as a reformulation of the Weeks-Chandler-Andersen [J. Chem. Phys. 54, 5237 (1971)] perturbation theory directly in terms of an arbitrary hard sphere reference system. We investigate the accuracy of the combined approach for the Lennard-Jones fluid in comparison with simulation data and pure perturbation theory predictions and determine the dependence of the thermodynamic properties and the phase behavior on the choice of the effective hard core diameter of the reference system.     

Fumarate of (S,S)-alpha,alpha'-bis(trifluoromethyl)-1,8-anthracenedimethanol. A chiral macrocycle for the Diels-Alder reaction with cyclopentadiene

The synthesis and structural study of the fumarate of (S,S)-alpha,alpha'-bis(trifluoromethyl)-1,8-anthracenedimethanol by NMR spectroscopy are reported. The conformational study of the 13-membered macrocycle is presented. The cited alcohol is assayed as a chiral auxiliary in a Diels-Alder reaction with cyclopentadiene, and after methanolysis, provides the norbornene derivative with high enantioselectivity.     

Remarkable electron accepting properties of the simplest benzenoid cyanocarbons: hexacyanobenzene, octacyanonaphthalene and decacyanoanthracene

The optimised structures, electron affinities, and vibrational frequencies of the simplest benzenoid cyanocarbons, namely hexacyanobenzene C6(CN)6, octacyanonaphthalene C10(CN)8, and decacyanoanthracene C14(CN)10, have been studied using carefully calibrated density functional methods (Chem. Rev., 2002, 102, 231-282); the predicted adiabatic electron affinities are 3.53 eV for C6(CN)6, 4.35 eV for C10(CN)8 and 5.02 eV for C14(CN)10, which are significantly larger than those of the analogous benzenoid fluorocarbons as well as tetracyanoethane and tetracyanoquinodimethane.     

Unprecedented intermolecular C-H bond activation of a solvent toluene molecule leading to a seven-membered platinacycle

A novel platinum-mediated process involving intermolecular activation of a C(aryl)-H bond of toluene, intramolecular activation of an imine C(aryl)-Cl bond and formation of a C-C bond is reported.     

Beta-turn modified gramicidin S analogues containing arylated sugar amino acids display antimicrobial and hemolytic activity comparable to the natural product

This paper describes the design and synthesis of gramicidin S (GS) analogues 10a-c containing arylated sugar amino acids (SAAs) as a replacement of one of the two (D)Phe-Pro beta-turn regions. The cyclic, amphiphilic peptides adopt a beta-sheet conformation featuring an unusual reverse turn induced by the SAAs. The altered turn region induces a slight distortion of the antiparallel beta-sheet, as compared to GS; the overall geometry however closely resembles that of the nonarylated GS analogue 1. GS analogues 10a-c proved to be as active as the parent GS itself as antibacterial agents and are equally efficient in lysing red blood cells. These properties are in sharp contrast to the diminished biological activity displayed by 1. We conclude that the presence of aromaticity in the turn regions of GS derivatives is required for biological activity, whereas the native conformation of the beta-hairpin is not. Our findings may guide future research toward efficient and nonhemolytic GS analogues for combating bacterial infections.     

A new class of versatile chiral-bridged atropisomeric diphosphine ligands: remarkably efficient ligand syntheses and their applications in highly enantioselective hydrogenation reactions

A series of chiral diphosphine ligands denoted as PQ-Phos was prepared by atropdiastereoselective Ullmann coupling and ring-closure reactions. The Ullmann coupling reaction of the biaryl diphosphine dioxides is featured by highly efficient central-to-axial chirality transfer with diastereomeric excess >99%. This substrate-directed diastereomeric biaryl coupling reaction is unprecedented for the preparation of chiral diphosphine dioxides, and our method precludes the tedious resolution procedures usually required for preparing enantiomerically pure diphosphine ligands. The effect of chiral recognition was also revealed in a relevant asymmetric ring-closure reaction. The chiral tether bridging the two aryl units creates a conformationally rigid scaffold essential for enantiofacial differentiation; fine-tuning of the ligand scaffold (e.g., dihedral angles) can be achieved by varying the chain length of the chiral tether. The enantiomerically pure Ru- and Ir-PQ-Phos complexes have been prepared and applied to the catalytic enantioselective hydrogenations of alpha- and beta-ketoesters (C=O bond reduction), 2-(6'-methoxy-2'-naphthyl)propenoic acid, alkyl-substituted beta-dehydroamino acids (C=C bond reduction), and N-heteroaromatic compounds (C=N bond reduction). An excellent level of enantioselection (up to 99.9% ee) has been attained for the catalytic reactions. In addition, the significant ligand dihedral angle effects on the Ir-catalyzed asymmetric hydrogenation of N-heteroaromatic compounds were also revealed.     

Large area liquid crystal monodomain field-effect transistors

Butyl, hexyl, and decyl derivatives of the liquid-crystalline organic semiconductor 5,5' '-bis(5-alkyl-2-thienylethynyl)-2,2':5',2' '-terthiophene were synthesized and studied with respect to their structural, optical, and electrical properties. By means of an optimized thermal annealing scheme the hexyl and decyl compounds could be processed into self-assembled monodomain films of up to 150 mm in diameter. These were investigated with X-ray diffractometry, which revealed a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate. Within the lamellae the molecules were found to arrange with a tilt of about 50 degrees with the rubbing direction of the polyimide alignment layer. The resulting, close side-to-side packing was confirmed by measurements of the UV/vis absorption, which showed a dichroic ratio of 19 and indicated H-aggregation. AFM analyses revealed self-affinity in the surface roughness of the monodomain. The compounds showed bipolar charge transport in TOF measurements, with hole mobilities reaching up to 0.02 cm(2)/Vs and maximum electron mobilities around 0.002 cm(2)/Vs. The hexyl derivative was processed into large-area monodomain top-gate field-effect transistors, which were stable for months and showed anisotropic hole mobilities of up to 0.02 cm(2)/Vs. Compared to multidomain bottom-gate transistors the monodomain formation allowed for a mobility increase by 1 order of magnitude.     

High yield syntheses of stable, singly bonded Pd2(6+) compounds

A general method for the syntheses of dipalladium compounds having a singly bonded Pd26+ core and the formula R,S-cis-Pd2(C6H4PPh2)2(O2CR)2Cl2 is described. When the alkyl group in the carboxylate ligands is an electron donating group, the compounds are stable and the yields high. The Pd-Pd distances for the diamagnetic compounds with R = CF3 and CMe3 are 2.5434(4) and 2.5241(9) A, respectively. Calculations at the DFT level suggest that the electronic configuration is sigma2pi4delta2delta*2pi*4. These represent rare examples of palladium(III) compounds.