Effects of nuclear dynamics in the low-kinetic-energy Auger spectra of CO and CO2

The CO and CO(2) carbon and oxygen Auger spectra have been measured by electron impact and compared with accurate theoretical calculations accounting for the effects of the dynamics of the nuclei on the energy and linewidth of the Auger bands. The calculations for CO were previously published [L. S. Cederbaum et al., J. Chem. Phys. 95, 6634 (1991)], while for CO(2) they are new and presented here for the first time. For both molecules, particular attention has been paid to the low-kinetic-energy region of the spectra, which corresponds to doubly charged ion states with the two holes mainly localized in the inner valence region. New bands have been observed. It is shown that a proper consideration of the vibrational broadening and shift of the bands due to the dynamics of the nuclei is needed to assign these features. For CO, very large energy shifts between corresponding features in the C 1s and O 1s spectra have been observed, confirming the theoretical predictions of 1991. The new computed spectra of CO(2) allow a very accurate analysis of the experiments over the whole energy range.     

Isomorphous replacement in nylon 6 by 4-aminomethylcyclohexanecarboxylic acid

It was desired to determine the effect of geometric configuration on the ability of 4-aminomethylcyclohexanecarboxylic acid (AMCC) to “isomorphously” replace ε-aminocaproic acid residues in nylon 6. However, we found that cis-AMCC isomerized to the more thermodynamically stable trans isomer during copolymerization with caprolactam (CL) and also during homopolymerization. A 20/80 cis/trans ratio of AMCC residues was found in the 50:50 copolyamides regardless of whether a high-cis (72/28) or high-trans (15/85) AMCC was used. Powder x-ray diagrams showed similar interplanar spacings for the copolyamides made from both high-cis and high-trans AMCC. The x-ray study also showed that the nylon 6 lattice can accommodate less than 30 mole-% AMCC residues before a new structure appears. DTA and TGA data of the CL:AMCC copolyamide showed that AMCC raised the melting point, T_g, T_c, and stability of nylon 6.     

Supramolecular control over donor-acceptor photoinduced charge separation

A novel donor-bridge-acceptor system has been synthesized by covalently linking a p-phenylene vinylene oligomer (OPV) and a perylene diimid (PERY) at opposite ends of a m-phenylene ethynylene oligomer (FOLD) of twelve phenyl rings, containing nonpolar (S)-3,7-dimethyl-1-octanoxy side chains. For comparison, model compounds have been prepared in which either the donor or acceptor is absent. In chloroform, the oligomeric bridge is in a random coil conformation. Upon addition of an apolar solvent (heptane) the oligomeric bridge first folds into a helical stack and subsequently intermolecular self-assembly of the stacks into columnar architectures occurs. Photoexcitation in the random coil conformation, where the interaction between the donor and acceptor chromophores is small, results only in long-range intramolecular energy transfer in which the OPV singlet-excited state is transformed into the PERY singlet-excited state. In the folded conformation of the bridge, donor and acceptor are closer and their enhanced interaction favors the formation the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state upon photoexcitation. As a result, the extent of photoinduced charge separation depends on the degree of folding of the bridge between donor and acceptor and therefore on the apolar nature of the medium. As a consequence, and contrary to conventional photoinduced charge separation processes, the formation of the OPV(*)(+)-FOLD-PERY(*)(-) charge-separated state is more favored in apolar media.     

Structure-Based Calculations of the Optical Spectra of the LH2 Bacteriochlorophyll-Protein Complex from Rhodopseudomonas acidophila

Abstract— The molecular structure of the light-harvesting complex 2 (LH2) bacteriochlorophyll-protein antenna complex from the purple non-sulfur photosynthetic bacterium Rhodopseudomonas acidophila , strain 10050 provides the positions and orientations of the 27 bacteriochlorophyll (BChl) molecules in the complex. Our structure-based model calculations of the distinctive optical properties (absorption, CD, polarization) of LH2 in the near-infrared region use a point-monopole approximation to represent the BChl Q_y transition moment. The results of the calculations support the assignment of the ring of 18 closely coupled BChl to B850 (BChl absorbing at 850 nm) and the larger diameter, parallel ring of 9 weakly coupled BChl to B800. All of the significantly allowed transitions in the near infrared are calculated to be perpendicular to the C9 symmetry axis, in agreement with polarization studies of this membrane-associated complex. To match the absorption maxima of the B800 and B850 components using a relative permittivity (dielectric constant) of 2.1, we assign different site energies (12 500 and 12260 cm⁻¹, respectively) for the Q_y transitions of the respective BChl in their protein binding sites. Excitonic coupling is particularly strong among the set of B850 chromophores, with pairwise interaction energies nearly 300 cm between nearest neighbors, comparable with the experimental absorption bandwidths at room temperature. These strong interactions, for the full set of 18 B850 chromophores, result in an excitonic manifold that is 1200 cm⁻¹ wide. Some of the upper excitonic states should result in weak absorption and perhaps stronger CD features. These predictions from the calculations await experimental verification.     

Electronic structure study by means of x-ray spectroscopy and theoretical calculations of the "ferric star" single molecule magnet

The electronic structure of the single molecule magnet system {M[Fe(L(1))(2)](3)}4CHCl(3) [M=Fe,Cr;L(1)=CH(3)N(CH(2)CH(2)O)(2) (2-)] has been studied using x-ray photoelectron spectroscopy, x-ray-absorption spectroscopy, soft-x-ray emission spectroscopy, as well as theoretical density-functional-based methods. There is a good agreement between theoretical calculations and experimental data. The valence band mainly consists of three bands between 2 and 30 eV. Both theory and experiments show that the top of the valence band is dominated by the hybridization between Fe 3d and O 2p bands. From the shape of the Fe 2p spectra it is argued that Fe in the molecule is most likely in the 2+ charge state. Its neighboring atoms (O,N) exhibit a magnetic polarization yielding effective spin S=52 per iron atom, giving a high-spin state molecule with a total S=5 effective spin for the case of M=Fe.     

Coexistence of chaotic and complexity dynamics of fluctuations with long-range temporal correlations under typical condition for formation of multiple anodic double layers in DC glow discharge plasma

Nonlinear Dynamics - Analysis of electrostatic floating potential fluctuations associated with multiple anodic double layer revealed a complexity dynamic coexisting with chaotic behavior. These...     

Weak Interactions in Silver Complexes of 15-Crown-5

Abstract

The X-ray crystal structures of two closely related Ag(I) complexes of 15-crown-5 and benzo-15-crown-5 are reported. In the case of [Ag(15-crown-5)₂][SbF₆] 1, pointing one of its oxygen atoms away from the Ag⁺ cation enables one of the crown ligands to take part in an intermolecular C?H…O hydrogen bond. The analogous benzo-15-crown-5 species, [Ag(benzo-15-crown-5)₂][SbF₆] 2, is too rigid to attain the necessary conformation. Crystal data for 1: P2_1/c, a = 8.4481(3), b = 25.5813(9), c = 13.2773(4) Å, β = 101.354(2)°. Z = 4, unique data: 5187 R ₁ [F ² > 2σ(F ²)] 0.0259. Compound 2: P1, a = 8.6511 (15) Å, b =10.2322(18) Å, c = 19.291(3) Å, α = 103.704 (2)°, β = 101.274(2)°, γ = 95.952(2)°, Z = 2, unique data: 5803 R ₁ [F ²>2σ(F ²)] 0.0931.