Recent contributions of high resolution gas chromatography to the analysis of environmental hydrocarbons

Analyses of hydrocarbon fractions from different areas of the marine environment are described to illustrate the possibilities and limitations of high resolution gas chromatography (HRGC) in the analysis of environmental samples. Examples are given of dissolved, particulate, and sedimentary hydrocarbons and organochlorine compounds; the importance of an adequate sampling of the marine environment is stressed. HR chromatographic profiles obtained in two columns of low and high polarity (SE-52 and PEG 20M) permit the sources and transport pathways of both natural and anthropogenic hydrocarbons to be traced. Analysis of tissues of marine mammals, which metabolize or excrete many of the biogenic and petrogenic hydrocarbons present in other areas of the marine environment, may provide an opportunity to obtain relatively clean profiles of many anthropogenic compounds of interest. The plotting of mass chromatograms from data compiled by COM-GC-MS remains the most appropriate method for the conclusive indentification of these compounds.     

Decomposition channels for multiply charged ammonia clusters

Multiply charged ammonia cluster ions are produced by adiabatic nozzle expansion and subsequent ionization by electron impact. They are analyzed in a double focussing sector field mass spectrometer (reversed geometry). Doubly charged clusters are only detected above a critical size of 51 and triply charged clusters above 121. Some of these multiply charged ions decay via metastable dissociation processes in the experimental time window accessible. Doubly charged ammonia clusters with sizes ofn≧51 lose one neutral monomer or, roughly ten times less probable, two neutral monomers. Conversely, triply charged ammonia clusters with sizes 110≦n≦120 show an extremely asymmetric Coulomb dissociation resulting in doubly charged cluster ions of about 90% of the initial mass     

Water effect on the excited-state decay paths of singlet excited cytosine

Two low-energy deactivation paths for singlet excited cytosine, one through a S₁/S₀ conical intersection of the ethylene type, and one through a conical intersection that involves the (n_N, π^*) state, are calculated in the presence of water. Water is included explicitly for several cytosine monohydrates, and as a bulk solvent, and the calculations are carried out at the complete active space self-consistent field (CASSCF) and complete active space second order perturbation (CASPT2) levels of theory. The effect of water on the lowest-energy path through the ethylenic conical intersection is a lowering of the energy barrier. This is explained by stabilization of the excited state, which has zwitterionic character in the vicinity of the conical intersection due to its similarity with the conical intersection of ethylene. In contrast to this, the path that involves the (n_N, π^*) state is destabilized by hydrogen bonding, although the bulk solvent effect reduces the destabilization. Overall, this path should remain energetically accessible.     

Thermal and mechanical properties of PE/organoclay nanocomposites

Polyethylene/montmorillonite clay nanocomposites were obtained via direct melt intercalation. The clay was organically modified with four different types of quaternary ammonium salts. The objective of this work is to study the use of montmorillonite clay in the production of nanocomposites by means on rheological, mechanical and crystallization properties of nanocomposites and to compare to the properties of the matrix and PE/unmodified clay nanocomposites. In general, the tensile test showed that the yield strength and modulus of the nanocomposites are close to the pure PE. Apparently, the mixture with Dodigen salt seems to be more stable than the pure PE and PE/unmodified clay.     

Density Functional Theory and Low-Temperature Matrix Investigations of CO-Loss Photochemistry from [(C<Subscript>5</Subscript>R<Subscript>5</Subscript>)Ru(CO)<Subscript>2</Subscript>]<Subscript>2</Subscript> (R = H,Me) Complexes

The photochemical CO-loss products of the diruthenium complexes [CpRu(CO) ₂]₂ (5; Cp = ⁵-C₅H₅), [Cp*Ru(CO)₂]₂ (5*; Cp* = ⁵-C₅(CH₃)₅) and CpCp*[Ru(CO)₂]₂ (5) have been studied experimentally in low-temperature (96 K) matrices in 3-methylpentane by using IR spectroscopy. It is proposed that all three complexes undergo single-CO-loss chemistry but that the products have different structures. The single-CO-loss product from 5 is proposed to have one bridging and two terminal carbonyl ligands, whereas 5* and 5 generate triply bridged CO-loss products similar to that observed from [CpFe(CO)₂]₂ and [Cp*Fe(CO)₂]₂. Double-CO-loss from 5* and 5* 9 is also apparently observed. Relativistic DFT calculations have been carried out on various isomers of the starting materials and on potential CO-loss products from 5. The calculations suggest that the triply bridged product Cp₂Ru₂(-CO)₃ (6) might have a singlet ground state in contrast to the corresponding diiron complex Cp₂Fe₂(-CO)₃ (3), which has a triplet ground state.     

Mechanism and control of molecular energy flow: a modeling perspective

 Vibrational energy flow in organic molecules occurs by a multiple-time-scale mechanism that can be modeled by a single exponential only in its initial stages. The mechanism is a consequence of the hierarchical structure of the vibrational Hamiltonian, which leads to diffusion of vibrational wavepackets on a manifold with far fewer than the 3N−6 dimensions of the full vibrational state space. The dynamics are controlled by a local density of states, which does not keep increasing with molecular size. In addition, the number of vibrational coordinates severely perturbed during chemical reaction is small, leading to preservation of the hierarchical structure at chemically interesting energies. This regularity opens up the possibility of controlling chemical reactions by controlling the vibrational energy flow. Computationally, laser control of intramolecular vibrational energy redistribution can be modeled by quantum-classical, or by purely quantum-mechanical models of the molecule and control field. Received: 26 July 2002 / Accepted: 30 September 2002 / Published online: 2 December 2002 Electronic Supplementary Material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00214-002-0394-2. Acknowledgements. This work was supported by NSF grant CHE 9986670. Correspondence to: M. Gruebele e-mail: gruebele@scs.uiuc.edu     

Functional dynamics of ion channels: modulation of proton movement by conformational switches

Detailed comparative studies of proton relay in native and chemically modified gramicidin channels provide a unique opportunity to uncover the structural basis of biological proton transport. The function of ion channels hinges on their ability to provide surrogate solvation in narrow pore filters so as to overcome the dielectric barrier presented by biological membranes. In the potassium channel KcsA and in the cation channel gramicidin, permeant selectivity and mobility are determined by the proteinaceous matrix via hydrogen bonding, charge-dipole, and dipole-dipole interactions. In particular, main-chain carbonyl groups in these pore interiors play an essential role in the solvation of alkali ions and of protons. In this study, molecular dynamics simulations reveal how the translocation of H(+) is controlled by nanosecond conformational transitions exchanging distorted states of the peptidic backbone in the single-file region of a dioxolane-linked analogue of the gramicidin dimer. These results underline the functional role of channel dynamics and provide a mechanism for the modulation of proton currents by fluctuating dipoles.     

Evaluation of molecular structure in Langmuir monolayers of zinc stearate and zinc 12-hydroxystearate by IR external reflection spectroscopy

 In situ polarized Fourier transform IR external reflection spectra of Langmuir monolayers of zinc stearate and zinc 12-hydroxystearate on a water surface were recorded for various surface areas, and their molecular structures were estimated. In the zinc stearate monolayer, the wavenumbers and the absorbances of the antisymmetric and symmetric methylene stretching bands did not change during monolayer compression, which means that orientational and conformational changes of the hydrocarbon chain did not occur. However, wavenumber changes of the antisymmetric and symmetric carboxylate stretching bands were observed during surface compression. The change in the binding nature of the zinc cation to the carboxylate group was speculated. Moreover, it was elucidated that the structure of the hydrocarbon chain in the zinc 12-hydroxystearate monolayer was different from that in the zinc stearate monolayer. Received: 21 March 2001 Accepted: 6 July 2001     

Hyperfine structure of Mn II by fast ion beam-laser spectroscopy

We report the first measurements of the hyperfine splitting of the fine structure levels 3d ⁵'4s a ⁵ S ₂ and 3d ⁵ 4p z ⁵ P _1,2,3 ⁰ in singly ionised manganese in a collinear fast ion beam-laser experiment. The ions are excited by the intracavity frequency doubled output of a ring dye laser. The observed linewidth is sufficiently narrow to identify all hyperfine components of each fine structure level.     

Improvement of the HPLC analysis of anthocyanins in red wines by use of recently developed columns

Comparison of the separation performance of five newly developed pH-stable HPLC columns is described for the analysis of anthocyanins in red wines. Separation of twenty anthocyanins in a single run is described using the most appropriate method.