Fluorescence and photodissociation of rhodamine 575 cations in a quadrupole ion trap

The fluorescence and photodissociation of rhodamine 575 cations confined to a quadrupole ion trap are observed during laser irradiation at 488 nm. The kinetics of photodissociation is measured by time-dependent mass spectra and time-dependent fluorescence. The rhodamine ion signal and fluorescence decay are studied as functions of buffer gas pressure, laser fluence, and irradiation time. The decay rates of the ions in the mass spectra agree with decay rates of the fluorescence. Some of the fragment ions also fluoresce and further dissociate. The photodissociation rate is found to depend on the incident laser fluence and buffer gas pressure. The implications of rapid absorption/fluorescence cycling for photodissociation of dye-labeled biomolecular ions under continuous irradiation are discussed.     

Electrodiffusion of ions inside living cells

Electrodiffusion of ions, both inside and outside biologicalcells, are of utmost importance to proper cellular functions.Experiments indicate that both ion concentrations and electropotentialcan jump discontinuously across the cell membranes. We studya system of nonlinear partial differential equations modellingsuch phenomena. Jump conditions for species concentrations andelectropotential across cell membranes are imposed. Under zero-fluxboundary conditions for one-dimensional domains, the solutionsare proved to exist for all times. With further assumptions,these transient solutions will converge to the unique steady-statesolution. Numerical experiments in one- and two-dimensionaldomains are also performed in order to study some unresolvedtheoretical issues.     

Investigation of quark distributions in a family of pentaquarks using the Thomas-Fermi quark model

Using the Thomas-Fermi quark model,a collective,spherically symmetric density of states is created to represent a gas of interacting fermions with various degeneracies at zero temperature.Over a family of pentaquarks,uudcc,color interaction probabilities were obtained after averaging over all the possible configurations.Three different functions are developed for light,charm,and anti-charm quarks and are assumed to be linearly related by some proportionality constants.Interesting patterns of quark distributions are observed while analyzing the quark function consistency conditions for such constants.     

pH-enzyme di-dependent chronotherapeutic drug delivery system of theophylline for nocturnal asthma

The purpose of this research work was to develop and evaluate a chronotherapeutic based colon-targeted drug delivery system of theophylline (THEO) exploiting pH-enzyme sensitive property for the prevention of episodic attack of asthma in early morning. Guar gum microspheres of theophylline were prepared by emulsification technique. Coating of microspheres was performed using solvent evaporation method with pH sensitive Eudragit(?) polymers. The particle size and surface morphology, entrapment efficiency and degree of swelling of microspheres were examined. The in vitro drug release studies were performed in pH progression medium and also in the presence of 2% rat caecal content. Theophylline was efficiently microencapsulated in guar gum microspheres at different polymer concentrations (1-4%). Fourier transform infrared (FT-IR)-spectroscopy confirmed the intermolecular interactions between guar gum and glutaraldehyde. Coating of guar gum microspheres by Eudragit led to decelerate the in vitro drug release of THEO. Moreover in vitro drug release studies also performed with 2% rat caecal content showed marked increment in drug release. The controlled release of THEO after a lag time was achieved with developed formulation for chronotherapeutic delivery. The pH dependent solubility behavior of Eudragit and gelling properties of guar gum are found to be responsible for delaying the release.     

Gas‐Phase Synthesis of the Benzyl Radical (C6H5CH2)

Dicarbon (C₂), the simplest bare carbon molecule, is ubiquitous in the interstellar medium and in combustion flames. A gas‐phase synthesis is presented of the benzyl radical (C₆H₅CH₂) by the crossed molecular beam reaction of dicarbon, C₂(X¹Σ_g⁺, a³Π_u), with 2‐methyl‐1,3‐butadiene (isoprene; C₅H₈; X¹A′) accessing the triplet and singlet C₇H₈ potential energy surfaces (PESs) under single collision conditions. The experimental data combined with ab initio and statistical calculations reveal the underlying reaction mechanism and chemical dynamics. On the singlet and triplet surfaces, the reactions involve indirect scattering dynamics and are initiated by the barrierless addition of dicarbon to the carbon–carbon double bond of the 2‐methyl‐1,3‐butadiene molecule. These initial addition complexes rearrange via multiple isomerization steps, leading eventually to the formation of C₇H₇ radical species through atomic hydrogen elimination. The benzyl radical (C₆H₅CH₂), the thermodynamically most stable C₇H₇ isomer, is determined as the major product.     

Gas‐Phase Synthesis of 1‐Silacyclopenta‐2,4‐diene

Silole (1‐silacyclopenta‐2,4‐diene) was synthesized for the first time by the bimolecular reaction of the simplest silicon‐bearing radical, silylidyne (SiH), with 1,3‐butadiene (C₄H₆) in the gas phase under single‐collision conditions. The absence of consecutive collisions of the primary reaction product prevents successive reactions of the silole by Diels–Alder dimerization, thus enabling the clean gas‐phase synthesis of this hitherto elusive cyclic species from acyclic precursors in a single‐collision event. Our method opens up a versatile and unconventional path to access a previously rather obscure class of organosilicon molecules (substituted siloles), which have been difficult to access through classical synthetic methods.     

Optimization of a quadrupole ion storage trap as a source for time‐of‐flight mass spectrometry

Designs of a quadrupole ion trap (QIT) as a source for time‐of‐flight (TOF) mass spectrometry are evaluated for mass resolution, ion trapping, and laser activation of trapped ions. Comparisons are made with the standard hyperbolic electrode ion trap geometry for TOF mass analysis in both linear and reflectron modes. A parallel‐plate design for the QIT is found to give significantly improved TOF mass spectrometer performance. Effects of ion temperature, trapped ion cloud size, mass, and extraction field on mass resolution are investigated in detail by simulation of the TOF peak profiles. Mass resolution (m/Δm) values of several thousand are predicted even at room temperature with moderate extraction fields for the optimized design. The optimized design also allows larger radial ion collection size compared with the hyperbolic ion trap, without compromising the mass resolution. The proposed design of the QIT also improves the ion–laser interaction volume and photon collection efficiency for fluorescence measurements on trapped ions. Copyright © 2011 John Wiley & Sons, Ltd.     

Optimization of a quadrupole ion storage trap as a source for time-of-flight mass spectrometry

Designs of a quadrupole ion trap (QIT) as a source for time-of-flight (TOF) mass spectrometry are evaluated for mass resolution, ion trapping, and laser activation of trapped ions. Comparisons are made with the standard hyperbolic electrode ion trap geometry for TOF mass analysis in both linear and reflectron modes. A parallel-plate design for the QIT is found to give significantly improved TOF mass spectrometer performance. Effects of ion temperature, trapped ion cloud size, mass, and extraction field on mass resolution are investigated in detail by simulation of the TOF peak profiles. Mass resolution (m/Δm) values of several thousand are predicted even at room temperature with moderate extraction fields for the optimized design. The optimized design also allows larger radial ion collection size compared with the hyperbolic ion trap, without compromising the mass resolution. The proposed design of the QIT also improves the ion-laser interaction volume and photon collection efficiency for fluorescence measurements on trapped ions.     

Transferrable property relationships between magnetic exchange coupling and molecular conductance

Calculated conductance through Au_n–S–Bridge–S–Au_n (Bridge = organic σ/π-system) constructs are compared to experimentally-determined magnetic exchange coupling parameters in a series of Tp^Cum,MeZnSQ–Bridge–NN complexes, where Tp^Cum,Me = hydro-tris(3-cumenyl-1-methylpyrazolyl)borate ancillary ligand, Zn = diamagnetic zinc(ii), SQ = semiquinone (S = 1/2), and NN = nitronylnitroxide radical (S = 1/2). We find that there is a nonlinear functional relationship between the biradical magnetic exchange coupling, J_D→A, and the computed conductance, g_mb. Although different bridge types (monomer vs. dimer) do not lie on the same J_D→Avs. g_mb, curve, there is a scale invariance between the monomeric and dimeric bridges which shows that the two data sets are related by a proportionate scaling of J_D→A. For exchange and conductance mediated by a given bridge fragment, we find that the ratio of distance dependent decay constants for conductance (β_g) and magnetic exchange coupling (β_J) does not equal unity, indicating that inherent differences in the tunneling energy gaps, Δε, and the bridge–bridge electronic coupling, H_BB, are not directly transferrable properties as they relate to exchange and conductance. The results of these observations are described in valence bond terms, with resonance structure contributions to the ground state bridge wavefunction being different for SQ–Bridge–NN and Au_n–S–Bridge–S–Au_n systems.

Calculated conductance through Au_n–S–Bridge–S–Au_n constructs are compared to experimental magnetic exchange coupling parameters in Tp^Cum,MeZn(SQ–Bridge–NN) complexes, where SQ = semiquinone radical and NN = nitronylnitroxide radical.