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The near thermal conditions of an ion-trap mass spectrometer were used to examine the intrinsic gas-phase reactivity and selectivity of nucleophilic substitution reactions. The well-defined organocuprate anions [CH3CuR]- (R = CH3CH2, CH3CH2CH2, (CH3)2CH, PhCH2CH2, PhCH2, Ph, C3H5, and H) were reacted with CH3I. The rates (reaction efficiencies, φ) and selectivities (the product ion branching ratios) were compared with those of [CH3CuCH3]- reacting with CH3I. Alkyl R groups yielded similar efficiencies, with selectivity for C-C bond formation at the coordinated R group. Inclusion of unsaturated R groups curbed the overall reactivity (φ = 1 to 2 orders of magnitude lower). With the exception of R = PhCH2CH2, these switched their selectivity to C-C bond formation at the CH3 group. Replacing an organyl ligand with R = H significantly enhanced the reactivity (8-fold), resulting in the selective formation of methane. Unique decomposition and side-reactions observed include: (1) spontaneous β-hydride elimination from [RCuI]- byproducts; and (2) homocoupling of the pre-existing organocuprate ligands in [CH3CuC3H5]-, as shown by deuterium labeling. DFT (B3LYP-D/Def2-QZVP//B3LYP/SDD:6-31+G(d)) predicts that the alkylation mechanism for all species is via oxidative addition/reductive elimination (OA/RE). OA is the rate-limiting step, while RE determines selectivity: the effects of R on each were examined. [ABSTRACT FROM AUTHOR]

We have evaluated the effect of varying three key parameters for Fluorescence Correlation Spectroscopy analysis, first in the context of a one species/one environment system, and then in a complex system composed of two species, or conversely, two environments. We establish experimentally appropriate settings for the (1) minimum lag time, (2) maximum lag time, and (3) averaging times over which an autocorrelation is carried out, as a function of expected diffusion decay time for a particular solute, and show that use of appropriate settings plays a critical role in recovering accurate and reliable decay times and resulting diffusion constants. Both experimental and simulated data were used to show that for a complex binary system, to extract accurate diffusion constants for both species, decay times must be bounded by adequate minimum and maximum lag times as dictated by the fast and slow diffusing species, respectively. We also demonstrate that even when constraints on experimental conditions do not permit achieving the necessary lag time limits for both of the species in a binary system, the accuracy of the recovered diffusion constant for the one species whose autocorrelation function is fully time-resolved is unaffected by uncertainty in fitting introduced by the presence of the second species. [ABSTRACT FROM AUTHOR]

We have designed and constructed a continuous imaging reflectron time-of-flight mass spectrometer (TOFMS) that provides a mass spectrum at every pixel of a two-dimensional image with a 100% duty cycle. The technique is based on pseudorandom ion beam modulation and three-dimensional (x, y, t) ion imaging. We use a multichannel plate detector with a delay-line anode that provides x, y positions and flight times t of every ion arrival event. The precision of the peak heights in the 100% duty cycle mass spectra is shown to be enhanced even at short (10 ms) acquisition times, which should prove useful for the study of solution kinetics or fast chromatographic separations. As a demonstration of the system's capability, we have imaged the fragmented ions that underwent surface-induced dissociation inside the reflectron and the ions that fragmented spontaneously through postsource decay. [ABSTRACT FROM AUTHOR]

Time-gated techniques are useful for the rapid sampling of excited-state (fluorescence) emission decays in the time domain. Gated detectors coupled with bright, economical, nanosecond-pulsed light sources like flashlamps and nitrogen lasers are an attractive combination for bioanalytical and biomedical applications. Here we present a calibration approach for lifetime determination that is noniterative and that does not assume a negligible instrument response function (i.e., a negligible excitation pulse width) as does most current rapid lifetime determination approaches. Analogous to a transducer-based sensor, signals from fluorophores of known lifetime (0.5-12 ns) serve as calibration references. A fast avalanche photodiode and a GHz-bandwidth digital oscilloscope is used to detect transient emission from reference samples excited using a nitrogen laser. We find that the normalized time-integrated emission signal is proportional to the lifetime, which can be determined with good reproducibility (typically <100 ps) even for data with poor signal-to-noise ratios (∼20). Results are in good agreement with simulations. Additionally, a new time-gating scheme for fluorescence lifetime imaging applications is proposed. In conclusion, a calibration-based approach is a valuable analysis tool for the rapid determination of lifetime in applications using time-gated detection and finite pulse width excitation. [ABSTRACT FROM AUTHOR]

In this paper, we introduce the use of fluorescence lifetime correlation spectroscopy to study the metal-fluorophore interactions in solution at the single-fluorophore level. A single-stranded oligonucleotide was chemically bound to a 50-nm-diameter single silver particle, and a Cy5-labeled complementary single-stranded oligonucleolide was hybridized with the silver particle-bound oligonucleotide. The distance between the fluorophore and silver particle was maintained by a rigid hybridized DNA duplex of 8 nm in length. The single Cy5-DNA-Ptg particles showed more than 10-fold increase in fluorescence intensity and a 5-fold decrease in emission lifetimes as compared with Cy5-DNA free molecules in the absence of metal. The decrease of lifetime for the Cy5-DNA-Ag particle allowed us to resolve the correlation functions of the two species based on the intensity decays. The increased brightness of the Cy5-DNA-Ag particle as compared to free Cy5-DNA resulted in an increased contribution of Cy5-DNA-Ag to the correlation function of the mixture. These results show that the effects of metal particles on fluorophores can be used to detect the small fractional populations of the metal-bound species in the presence of a larger number of less bright species. Our results also suggest that these bright fluorophores conjugated to silver particles could be used as the fluorescent probes for clinical detection in the biological samples with the high background. [ABSTRACT FROM AUTHOR]

Localized surface plasmons of metallic particles of subwavelength sizes strongly modify the spectral properties of nearby fluorophores. The enhanced radiative decay rate leads to high fluorescence efficiencies and decreased fluorescence lifetimes. In this report we show that metal-enhanced fluorescence generated by the presence of the silver islands on the glass substrate displays high depolarization. Intensities, lifetimes, and emission anisotropies of several fluorophore protein conjugates have been studied in the absence and presence of metallic nanostructures. Despite highly decreased lifetimes of about 10-fold and immobilization of conjugates on the solid substrate, the observed emission anisotropies for all fluorophores on the metal-enhanced substrate decreased 300-500% compared to that in solution. This observation implies a new generation of fluorescence polarization immunoassays with broad applications because of no restrictions to the lifetime of the probe and the size of labeled biomolecules. The changes in polarization are due to binding that occur on the bioactive surface localized near the metal particles. [ABSTRACT FROM AUTHOR]

We have studied the dissociative ionization behavior of Na 2molecules using two-color, three photon optical-optical double resonance enhanced excitation via the A 1Σ u+and the 2 1Π gstates. Excess energy ranges from about 150 to about 1500 cm −1above threshold for dissociative ionization into ground-state Na and Na +. Slow atomic Na +fragments and Na 2+molecular ions are detected using a linear time-of-flight spectrometer operated in low field extraction, core sampling mode. To explain the observed energy dependence of the Na +/Na 2+branching ratio, we introduce a semiclassical model for the underlying decay dynamics. Franck−Condon overlap densities for bound−free transitions starting in 2 1Π gvibrational levels indicate that atomic Na +fragments are primarily produced via Rydberg states, with principal quantum number n between 5 and 12, converging to the repulsive 1 2Σ u+first excited-state potential of Na 2+. Dynamics along these Rydberg curves involves competition between electronic (autoionizing) and nuclear (dissociative) degrees of freedom. Within the model, the autoionization lifetime τ autois the only one free parameter available to fit calculated Na +/Na 2+branching ratios as a function of excess energy to the observed values. The lifetime is assumed to be the same multiple cof the Bohr period of each Rydberg potential. A χ 2-minimization procedure yields, for the range of principal quantum numbers involved, a most likely value of c= 1.5 ± 0.3, implying that on average the Rydberg electron completes only 1 to 2 orbits before interaction with the excited core electron leads to autoionization. [ABSTRACT FROM AUTHOR]

Multicolor fluorescence correlation spectroscopy has been recently developed to study chemical interactions of multiple chemical species labeled with spectrally distinct fluorophores. In the presence of spectral overlap, there exists a lower detectability limit for reaction products with multicolor fluorophores. In addition, the ability to separate bound product from reactants allows thermodynamic properties such as dissociation constants to be measured for chemical reactions. In this report, we utilize a spectrally resolved two-photon microscope with single-photon counting sensitivity to acquire spectral and temporal information from multiple chemical species. Further, we have developed a global fitting analysis algorithm that simultaneously analyzes all distinct auto- and cross-correlation functions from 15 independent spectral channels. We have demonstrated that the global analysis approach allows the concentration and diffusion coefficients of fluorescent particles to be resolved despite the presence of overlapping emission spectra. [ABSTRACT FROM AUTHOR]

The article focuses on the research designed to identify the misconceptions of high school students in concepts related to nuclear chemistry in Turkey. Researchers admit that the students hold a series of misconceptions in areas that include nuclear stability, half-life, binding energy, practical applications of nuclear chemistry, radioactive decay rate, among others. Moreover, they also enumerate two obstacles to effective learning of concepts concerning nuclear chemistry. Therefore, researchers recommend some things that will help students learn more about the subject.

Properties of the higher excited triplet states (Tn) of oligo(p-phenylenevinylene)s (OPVn, n denotes the number of phenyl rings, n = 3, 4) were investigated with use of the ns−ps and ns−ns two-color two-laser flash photolysis techniques. The lowest triplet excited states of OPVn were generated through the triplet energy transfer (ENT) process by the first 355-nm laser irradiation to the triplet sensitizer. With the delay time of 4 μs after the first laser irradiation, OPVn(T1) was selectively excited with the second 532-nm laser. Bleaching of the absorption of OPVn(T1) was observed during the second ps laser irradiation, indicating the generation of OPVn(Tn). The bleaching completely recovered at a few tens of picoseconds after the second picosecond laser irradiation, because of the internal conversion from OPVn(Tn) to OPVn(T1). The lifetimes of Tn were estimated to be 35 and 30 ps for OPV3 and OPV4, respectively. In the presence of the triplet energy acceptor (Q), the quenching and recovery of the T1-absorption bands of OPVn were observed because of ENT from OPVn(Tn) to Q and Q(T1) to OPVn, respectively. On the basis of the ENT from OPVn(Tn) to a series of Q, the T2−T1 energy gaps of OPV3 and OPV4 were estimated to be 1.3 and 1.1 eV, respectively. [ABSTRACT FROM AUTHOR]

The reaction N(2D) + O2 → NO(2Π,v,J) + O(3P,1D) has been considered for many years as a possible source of O(1D) in the thermosphere. We have used the Cold Chemical Infrared Simulation Experiment (COCHISE) facility to determine an upper limit to the O(1D) branching fraction of this reaction by observing the absence of the radiative decay of O(1D) and O2(b1Σ) produced by efficient energy exchange with O2. By carefully modeling the kinetic and fluid dynamics of the experiment, an upper limit of 0.02 ± 0.02 was found for the branching fraction into the O(1D) product channel. [ABSTRACT FROM AUTHOR]

Describes the generalized rank annihilation method and the direct exponential curve resolution algorithm for direct quantitative analysis of nuclear magnetic resonance free induction decay signals. Performance assessment of the two methods; Applications and advantages of each method.

Examines the influence of strong magnetic fields on the decay kinetics of nickel-bacteriochlorophyl. Probe on the triplet mechanism on the picosecond time scale; Effect of absorption of Ni-BChl in toluene and pyridine; Influence of magnetic fields on the decay kinetics of Ni2-bacteriochlorophyl; Investigation on the field dependence process.

Highlights the results of the study conducted by E. Nordhoff et al, published in the `Journal of Mass Spectrometry.' Prompt secondary decay of high-energy fragment ions; Use of prompt decay to sequence oligodeoxynucleotides; Intramolecular proton-catalyzed 1,2-elimination of individual bases.