Resonant scattering of slow electrons from naphthalene vapour

Resonant scattering of low energy electrons from naphthalene has been investigated using electron transmission spectroscopy. The transmission spectrum of naphthalene yields a value of ?0.20 ± 0.05 eV for the first electron affinity. The first and second excited states of C₁₀H^?₈ are detected at 0.96 ± 0.1 eV and 1.55 ± 0.13 eV. Resonant structure is also observed at 5.29 ± 0.1 cV and 7.55 ± 0.05 eV; it is suggested that the former resonance is the previously unidentified peak in the threshold electron excitation spectrum of Compton and co-workers.     

Electrical transport properties of liquid alloys

Two complementary mechanisms have been proposed for relatively high temperature superconductor MgB₂. While the first is the electron–phonon mechanism of BCS theory, advocated strongly by Pickett and co-workers, the second, by Bianconi et al., invokes Feshbach shape resonances. While we cannot presently discount the second mechanism, and while both proposals exploit the multiband nature of the electronic structure of MgB₂, we show here that five body-centred cubic (bcc) transition metals, whose superconducting transition temperature correlate intimately with elastic constants and therefore are plainly BCS-like in character, lie on a curve which has MgB₂ at the high T _c end. Any alternative mechanism to electron–phonon interaction in MgB₂ will need to account quantitatively for this circumstance.     

Local inverse inelastic mean free path at interface

We calculated a local inverse inelastic mean free path (local-IIMFP) for electrons crossing a medium–medium interface, considering various incident electron energies, crossing angles and combinations of materials. We used an extension of a classical dielectric model developed by Li and co-workers for an electron crossing a surface (interface vacuum-medium). Moreover, the integration over the distance of the local-IIMFP allows to obtain the interface excitation parameter (or IEP) characterizing the change in excitation probability for an electron crossing an interface once caused by the presence of the interface in comparison with an electron for which only volume excitations are considered. We perform these calculations for angles between 0° and 80°, for electron energies between 500 and 2500 eV and for various pairs of materials, as Al/In for its academic interest or Au/Si and SiO₂/Si for their technological importance. Small but not negligible variations of the local-IIMFP and the IEP were observed for metal–metal or metal–semiconductor interfaces, while quite significant variations are obtained when one of the materials is a insulator.     

Synthesis of 2,5-Dimethylpyrazine-3,6-Dicarboxylic Acid Derivatives

The general synthetic method of pyrazines¹ has been established. The method for synthesizing symmetrically substituted pyrazine derivatives, however, has not been well-studied.² Especially for synthesis of the title pyrazines, it is only described in a few words by Adkins³ and co-workers that ethyl 2-aminoacetoacetate employed in process to preparing of the threonine synthesis is spontaneously autoxidized to give 2,5-dimethyl-3,6-dicarbetoxy pyrazine as a byproduct. On the basis of this finding of Adkins, we have established a convenient method for the synthesis of 2,5-di-methylpyrazine-3,6-dicarboxylic acid derivatives (D_1–5) using the corresponding acetoacetic acid derivatives (A_1–4) as the starting materials (Scheme).     

Energy transfer or electron transfer?—DFT study on the mechanism of [2+2] cycloadditions induced by visible light photocatalysts

The intramolecular [2+2] cycloaddition of 1,3-dienes under visible light irradiation investigated by Yoon and his co-workers shows remarkably high yield and stereoselective differences under different photocatalysts. The reaction was speculated to be induced by energy transfer. However, the origin for these phenomena is still unclear. In this scene, the detailed mechanism for the [2+2] cycloaddition of 1,3-dienes under visible light has been investigated using density functional theory B3LYP and TPSSTPSS methods. The result shows that the reaction not only can be induced by energy transfer between photocatalysts and reactants, but also can be induced by electron transfer between them. The [2+2] cycloaddition induced by energy transfer is carried out along the potential energy surface (PES) of triplet excited states (T₁) firstly, and then goes back to the singlet ground state (S₀) via MECPs (minimum energy crossing points) between the PESs of the S₀ and T₁ states, forming the product in the S₀ state. The [2+2] reaction induced by electron transfer proceeds along the doublet state PES of the cation radical reactant and the neutral four-membered ring product could be obtained by electron transfer from the corresponding reactant or reduced photocatalyst. The origin of stereoselectivity of the [2+2] reaction is attributed to the reaction mechanism difference under different photocatalysts.     

Electron absorption spectra and voltabsorptograms of poly-3-octylthiophene films

Steady-state absorption spectra of poly-3-octylthiophene films in different oxidation states and differential cyclic voltabsorptograms of poly-3-octylthiophene films in 0.1 M LiClO₄ solutions in acetonitrile are studied. Electron spectra of films demonstrate the following three absorption bands: a complex band with a pronounced maximum at λ = 450 nm, which corresponds to π → π* electron transitions in the reduced fragments of poly-3-octylthiophene films and two absorption bands (at λ_max = 780 and λ_max > 1100 nm), which correspond to the oxidized film fragments. It is concluded that two chemically and optically distinguishable oxidation products are formed during the polymer oxidation.     

An appraisal of molecular force fields for the representation of polypeptides

A number of force fields of the molecular mechanics type have been tested for their ability to represent as an energy minimum, the observed crystal structure for three cyclic hexapeptides, cyclo-(-Ala-Ala-Gly-Gly-Ala-Gly-), cyclo-(-Ala-Ala-Gly-Gly-Ala-Gly-), and cyclo-(-D-Ala-D-Ala-Gly-Gly-Gly-Gly-). The most effective force field tested was that recently proposed by Kollman and co-workers, notwithstanding its use of “united” atoms for CH, CH₂, and CH₃ groups. Fields proposed by Levitt, and adaptations of that of Scheraga and co-workers, were also effective. Force fields in which hydrogens bonded to electronegative atoms were not specified explicitly were less accurate in representation.     

Stereochemical applications of mass spectrometry: 1—The utility of electron impact and chemical ionization mass spectrometry in the differentiation of stereoisomeric benzoin oximes and phenylhydrazones

A study of the electron impact and chemical ionization (H₂, CH₄, and iso-C₄H₁₀) mass spectra of stereoisomeric benzoin oximes and phenylhydrazones indicates that while the former can be distinguished only by their chemical ionization mass spectra the latter are readily distinguishable by both their electron impact and chemical ionization mass spectra. The electron impact mass spectra of the isomeric oximes are practically identical; however, the chemical ionization spectra show that the E isomer forms more stable [MH]⁺ and [MH? H₂O]⁺ ions than the Z isomer for which both the [MH]⁺ and [MH? H₂O]⁺ ions are relatively unstable. In electron impact the Z-phenylhydrazone shows a lower [M]⁺˙ ion abundance and more facile loss of H₂O than does the E isomer. This more facile H₂O loss also is observed for the [MH]⁺ ion of the Z isomer under chemical ionization conditions.     

Spotlighting main group elements in polynuclear complexes

François Gabbaï, Cameron Jones and Connie Lu introduce the Chemical Science themed collection on the topic of main group elements in polynuclear complexes.

Efforts towards the incorporation of main group elements in polynuclear motifs or in the coordination sphere of transition metals have been a prevalent theme of coordination chemistry, and one that has delivered notable advances in the area of structure and bonding. In the past decade, this field has witnessed an increased emphasis on the influence of the main group moiety over the reactivity or physical properties of the resulting constructs. Through a collection of both invited and selected articles, this themed issue puts the spotlight on this developing field, while at the same time illustrating far-reaching applications in the areas of small molecule activation, catalysis and molecular magnetism.A number of papers in this themed issue highlight the significant recent progress that has been made in the development of homo- and heterometallic systems incorporating s- and p-block elements, both in low oxidation states (often element–element bonded) and normal oxidation states. These have found particular use as low toxicity, earth abundant alternatives to late transition metal complexes in stoichiometric and catalytic transformations of small molecule substrates to value added products. This theme is introduced in primary articles dealing with the reactivity of magnesium-based systems. As shown by Jones, Maron and co-workers, magnesium(i) dimers (LMg–MgL, L = β-diketiminate) are activated by coordination of simple Lewis bases, and are subsequently able to reductively couple carbon monoxide to form the deltate and transient ethenediolate dianions (C_nO_n²⁻, n = 3 and 2, respectively; DOI: 10.1039/D0SC00836B). In another contribution, β-diketiminato-stabilised magnesium diboronates are shown by Hill, McMullin and co-workers to act as rare “masked” sources of nucleophilic boryl anions for the synthetic transformation of imines (DOI: 10.1039/C9SC02087J). These two papers integrate with the content of two reviews that highlight the unique structures and reactivity of polynuclear complexes containing low valent group 2, 13 and 14 elements. One of these reviews, by Inoue and co-workers, focuses on the structures of ditetrelenes (R₂E^II E^IIR₂, E = group 14 element) and ditetrelynes (RE^I E^IR), and their remarkable reactivity towards small molecules (DOI: 10.1039/D0SC03192E). Another review by Crimmin and co-workers explores the role that magnesium(i) and aluminium(i) reductants play in C–H bond activation reactions, and the synergy that may arise when the main group reagent is combined with a transition metal (DOI: 10.1039/D0SC03695A). Showcasing the value that s-block metals may display in their normal valence, Williams and coworkers describe macrocyclic Mg^II/Zn^II heterodinuclear complexes as highly effective catalysts for epoxide/CO₂ ring opening co-polymerization (DOI: 10.1039/C9SC00385A). The broader significance of this concept is developed in a review on heterobimetallic complexes containing s-block metals, in which Hevia and her co-worker highlight the unique ability of such complexes to support cooperative catalysis (DOI: 10.1039/D0SC05116K). The Lewis acidity of s-block cations can also be harnessed to manipulate the covalency of metal–ligand interactions, as elegantly demonstrated by Arnold, Love, Vitova, Schreckenbach and co-workers, who investigate a series of uranyl(v) complexes featuring U O–E motifs (E = group 1 or 2 element, DOI: 10.1039/C8SC05717F).Reduced polynuclear main group complexes can also provide new platforms for the discovery of atypical reactivity as illustrated by Kong and co-workers, who report mono-base-stabilized 1,2-diboranylidenehydrazines, a set of compounds that feature an unprecedented BNN-1,3-dipole that readily adds to arenes or small molecules such as CO₂ (DOI: 10.1039/D0SC02162H). In keeping with the theme of reactive diboron-containing units, Braunschweig and co-workers show in another captivating report that B–B triply-bonded diborynes can add to diboranes to afford B₄ chains, a transformation that could pave the way to new polymers with polyboron units in the main chain (DOI: 10.1039/C9SC02544H). The synthetic potential offered by low oxidation state main group elements comes to the fore in two additional reports, both dealing with Si₆ clusters. In the first one, Scheschkewitz and co-workers show that these silicon clusters can be functionalised with tetrylene substituents, and can act as ligands towards group 9 metal fragments, yielding complexes which act as catalysts for alkene isomerisations (DOI: 10.1039/D0SC02861D). A second report by Lips and co-workers describes highly unsaturated and structurally dynamic Si₆R₄ species (R = amide) with exposed silicon vertices (DOI: 10.1039/D0SC01427C). Exposed silicon moieties can also be appended to classical ligands as demonstrated by Roesky and co-workers who report on cyclopentadienyl ligands substituted by a silylene (R₂Si:). These ligands not only act as two-electron Si donors towards transition metal fragments but also undergo isomerization or deprotonation reactions leading to sila-fulvenes (DOI: 10.1039/D0SC04174B). Reduced group 14 elements can also be directly incorporated in the five-membered ring of cyclopentadienyl-like ligands as illustrated by Müller, Albers and co-workers in a contribution dealing with the germacyclopentadienediyl [K₂(:GeC₄R₄)] as an η⁵-ligand and its conversion into the first germaaluminocene, [Cp*Al(η⁵-:GeC₄R₄)] (DOI: 10.1039/D0SC00401D).As stated in the introductory paragraph, positioning main group elements in the coordination sphere of transition metals provides access to unusual reactivities, as in a contribution by Ozerov and co-workers (DOI: 10.1039/D0SC04748A) who demonstrate the reversible addition of ethylene to a boryl-based bis(phosphine) iridium pincer complex. A unique aspect of this contribution is the concomitant participation of the iridium and boron centres in the coordination of the hydrocarbon ligand. The ability of boron to cooperate with an adjacent transition metal centre is again a leading theme in two additional contributions selected for inclusion in this issue. The first one concerns the reversible addition of H₂ across an Ni–B bond, as elegantly documented by Rodríguez, Lledós and co-workers (DOI: 10.1039/D0SC06014C), who also used a boryl-based pincer as a supporting ligand. Exploiting the somewhat counter-intuitive reality that gold is more electronegative than boron, Yamashita, Lin and co-workers show that gold(i) diarylboryl complexes react as gold-based nucleophiles with organic reagents bearing C O and C N bonds (DOI: 10.1039/D0SC05478J). The unique reactivity of late transition metal–boryl linkages pervades in another contribution by Conejero, Lledós and co-workers who detail the highly choreographed addition of boranes such as HBpin and HBcat to a cationic, T-shaped, cyclometallated Pt(ii) bis-carbene complex (DOI: 10.1039/D0SC05522K). Isolated species include σ-BH Pt^II complexes, en route to the formation of T-shaped Pt^II bis-carbene complexes. Last, Tilley, Eisenstein and co-workers remind us of the importance of main group hydrides in catalysis in a contribution that pinpoints the intermediacy of dinuclear nickel–silyl species in an alkene hydrosilylation reaction mediated by a cationic nickel complex (DOI: 10.1039/D0SC00997K).Within the theme of heterometallic cooperativity, we highlight three articles where group 13 elements were introduced into transition metal complexes to promote small-molecule activation. In each report, a unique ligand design is used to juxtapose the transition metal centre with the group 13 element(s). Szymczak and co-workers appended two Lewis acidic borane groups to a pincer ligand via flexible linkers. The pendant boranes were critical for the stabilization of a rare high-spin Fe^II dihydride complex by forging Fe–H → B interactions (DOI: 10.1039/C9SC00561G). Upon exposure to an arylisocyanide, a good π-acid, the reductive elimination of H₂ ensued to form the iron(0) complex. Such a step is reminiscent of the E₄ intermediate in nitrogenase, which is proposed to release the obligatory H₂ equivalent upon binding of N₂ [see Chem. Rev., 2014, 114, 4041]. Envisioning a more active role for boranes, Harman and co-workers use the diboraanthracene platform, whose redox flexibility and dynamic Lewis acidity can be orchestrated to promote reactivity at the bound transition metal (DOI: 10.1039/C9SC02792K). The authors isolate a key Au borohydride intermediate that reduces CO₂ to formate, and close a synthetic cycle from CO₂ to formic acid using only proton and electron equivalents. Moving down the group 13 to the heavier congeners, Lu and co-workers show that the choice of the heavy group 13 ion (Al, Ga, or In) that is directly appended to a nickel(0) centre can significantly tune the Ni electronics (DOI: 10.1039/C9SC02018G). In comparing a triad of non-classical Ni(η²-H₂) adducts, the identity of the group 13 ion was found to perturb the free energy and activation energy of H₂ binding by ∼5 kcal mol⁻¹. Lastly, in a timely review, Takaya details the growing momentum of using main group/metalloid complexes as supporting ligands for transition metal-based catalysis (DOI: 10.1039/D0SC04238B). Takaya’s review presents illustrative examples to showcase the diverse main group elements (groups 13–15) and strategies that are being harnessed for transition metal catalysis.Moving down the periodic table to the f-elements, several articles explore heterometallic lanthanide and actinide complexes that fundamentally challenge our understanding of bonding and electronic structure. Using mixed arene π-ligands, Liddle and co-workers isolated an unusual bent Th “sandwich” complex that is stitched by K⁺ ions into a tetrathorium cluster (DOI: 10.1039/D0SC02479A). Diaconescu, Huang and co-workers report inverted sandwich complexes of Sm and Y featuring a bridging biphenyl ligand and bridging K⁺ ions (DOI: 10.1039/D0SC03555F). Depending on the lanthanide element, these inverted sandwiches feature Sm^III–arene–Sm^III or Yb^II–arene–K⁺ bonding interactions, where the biphenyl ligand is formally tetraanionic or dianionic, respectively. Freedman and co-workers conducted an in-depth study on the electronic structures of Sn-based heterometallics that contain a direct bond between Sn and a first-row transition metal that is varied from Mn to Ni (DOI: 10.1039/D0SC03777J). The authors make a striking comparison between the high-spin configurations of the 3d ions and those of typical Ln coordination complexes, wherein the coordinate bonds are more ionic. They rationalize that the Sn group behaves as an inverted, weak-field ligand due to the large energy mismatch between the Sn 5s/5p and 3d atomic orbitals [see Chem. Rev., 2016, 116, 8173]. Controlling spin states is only one of several requisites for the design of single molecule magnets (SMMs). Layfield, Mansikkamäki and co-workers report a triad of dinuclear dysprosium complexes, where the exogenous borohydride donor is varied in both number and coordination (terminal to bridging) (DOI: 10.1039/D0SC02033H). The authors observed a favourable increase in the effective energy barrier for a dinuclear dysprosium complex with a Dy : BH₄ ratio of 2 : 1. Lastly, Nippe, Chibotaru and co-workers explore magneto-structural relationships in a series of trigonal prismatic Ln^III complexes (Gd to Lu) that are scaffolded by three doubly deprotonated ferrocene (FeCp₂)²⁻ ligands and capped by Li⁺ ions (DOI: 10.1039/D0SC01197E). By virtue of its size and axis of anisotropy, the authors were able to engender SMM behaviour for the Ho^III complex. The authors demonstrate that the Ln size and the nature of the Li⁺ solvate both influence the twist angle, where the ideal trigonal prism geometry (twist angle of 0°) results in the large anisotropy that is conducive to SMM behaviour.To illustrate the diversity of the field, this themed issue also highlights several additional contributions dealing with atypical phosphorus-containing ligands. For example, Scheer and co-workers show that the four-membered cyclo-P₄ ligand of organometallic tantalum complexes can be used as a square building block for the construction of molecular capsules upon combination with silver cations and an appropriate template (DOI: 10.1039/D0SC03437A). Two additional contributions document recent trends at the confluence of traditional organophosphorus chemistry and coordination chemistry. Gessner and co-workers review the unique properties of phosphorus ylides and their ability to stabilize low-valent main group species, leading to the formation of new main group ligands for transition metal-based catalysis (DOI: 10.1039/D0SC03278F). The second contribution comes from Normand, Sosa Carrizo and co-workers who decipher the ambiphilic properties of bis(iminophosphoranyl)phosphide ligands and suggest that they be regarded as containing a triphosphenium coordinating unit (DOI: 10.1039/D0SC04736H).This themed issue was assembled with the intent of spotlighting the role played by main group elements in polynuclear complexes. We hope that those reading these articles will appreciate the topical diversity of this research field, its relevance to various areas of chemistry, and the numerous future research opportunities it presents.     

The development of zeolite-entrapped organized molecular assemblies

A brief summary is presented of the development of organized molecular assemblies entrapped within the supercages of Y-zeolite. Emphasis is placed on work originating in the author's laboratory, although a discussion of some of the important contributions made by other workers, which inspired and facilitated this work, are included. Following pioneering studies by Lunsford and co-workers, which demonstrated the feasibility of encapsulating the common photosensitizer [Ru(bpy)3]2+ within the Y-zeolite supercage, Dutta and co-workers documented efficient photoinduced electron transfer to viologen acceptors occupying neighboring supercages. We have extended the range of available materials by developing synthetically versatile methods to permit the incorporation of heteroleptic complexes, including constituent ligands which contain peripheral nitrogen donor groups; for example, 2,2'-bipyrazine. In an impressive study employing zeolite-excluded acceptors, Dutta and co-workers showed that the reducing equivalents available from photoinduced electron transfer from the zeolite entrapped sensitizer to intra-zeolite acceptors could be transferred to the extra-zeolite acceptors in aqueous suspensions, although the net charge-separation efficiency was low, presumably because of a persistent relatively efficient back-electron transfer process involving the primary photoproduct; that is, the entrapped sensitizer-acceptor dyad. Exploiting the susceptibility of certain heteroleptic complexes to add reactive ruthenium reagents, methods were developed to construct spatially organized donor-sensitizer-acceptor triads within the supercage framework of Y-zeolite. Such assemblies exhibit dramatically improved net charge-separation efficiencies, presumably as a consequence of inhibiting the wasteful back-electron transfer reaction between the initial sensitizer-acceptor couple.     

Theoretical investigation on the spectroscopic properties of furylfulgide with different substituents and design of novel bis‐furylfulgimide photochromes

Density functional theory and time‐dependent density functional theory were employed to theoretically analyze the effect of different substituents on the spectroscopic properties of furylfulgide. The result shows that the absorption spectra of ring‐closed isomer which substituted by an electron‐donating group (NH₂) at the R₃‐position of furylfulgide has an evident bathochromic shift compared with the others. Due to the steric hindrance effect, the difference of absorption wavelength was evidently enlarged by introducing several representative electron‐withdrawing groups at the R₆‐position of furylfulgide. In addition, we also designed a series of novel dimers which combined two furylfulgimide monomers into one new molecule. The relevant frontier molecular orbitals, energy levels and absorption properties were analyzed in detail by the calculation of low‐lying excited states. Finally, taking BFF‐6 (bis‐furylfulgimide) for an example, we discussed the transformation mechanism of four stable isomers in the toluene solution. Our conclusions manifest that the asymmetrical BFF‐6 can act as a potential multifunctional molecular switch in consideration of its distinguishable absorption bands and reversible conversion process. We hope that this research will be beneficial to design more practical and efficient molecular switch for further applications.     

Cover: Journal of the Chinese Chemical Society 5/2020

In this figure , the authors have used B3LYP/6-311++G^** level of theory to access the effects of 1 to 4 halogens on four unprecedented sets of cyclopentasilylene-2,4-dienes ( 1_X−4_X ). The results show that in going from electron withdrawing groups (EWGs) to electron donating groups (EDGs), the singlet-triplet energy gap (ΔE_s-t) decreases while nucleophilicity (N) increases. More details about this figure will be discussed by Prof. Mohamad Z. Kassaee and his co-workers on page 692–702 in this issue.

     

Structure of poly(maleic anhydride)

The bulk polymerization of maleic anhydride initiated with acylperoxides, di-tert-butyl peroxide, AIBN, or pyridine proceeds with evolution of CO₂. The amount of CO₂ generated depends on the nature and the concentration of the initiator. With peroxide initiators, less than 5% of the polymerized maleic anhydride is decarboxylated. ¹H-NMR spectra, obtained on the benzoyl peroxide-initiated polymer and its methyl ester, are consistent with the unrearranged poly(maleic anhydride) structure and rule out the polycyclopentanone structure proposed by Braun and co-workers. Base-initiated polymaleic anhydride is substantially decarboxylated, and the resulting polymer has anhydride and carboxyl groups. Elemental analyses and ¹H-NMR spectra obtained on the pyridine-initiated polymer and its methyl ester refute both the cis-poly(vinylene ketoanhydride) structure suggested by Schopov and the polycylopentanone structure proposed by Braun and co-workers.     

Manipulating Electron-Transfer Events in [Fe4Co4] Cubes via a Mixed-Ligand Approach: The Impact of Elastic Frustration

The engineering of intermolecular interaction is challenging but critical for magnetically switchable molecules. Here, we prepared two cyanide-bridged [Fe₄Co₄] cube complexes via the alkynyl- and alcohol-functionalized trispyrazoyl capping ligands. The alkynyl-functionalized complex 1 exhibited a thermally-induced incomplete metal-to-metal electron transfer (MMET) behaviour at around 220 K, while the mixed alkynyl/alcohol-functionalized cube of 2 showed a complete and abrupt MMET behaviour at 232 K. Remarkably, both compounds showed a long-lived photo-induced metastable state up to 200 K. The crystallographic study demonstrated that the incomplete transition of 1 was likely due to the possible elastic frustration originating from the competition between the anion-propagated elastic interactions and inter-cluster alkynyl-alkynyl & CH-alkynyl interactions, whereas the latter are eliminated in 2 as a result of the partial substitution by the alcohol-functionalized ligand. Additionally, the introduction of chemically distinguishable cobalt centers within the cube unit of 2 did not lead to a two-step but a one-step transition, possibly because of the strong ferroelastic intramolecular interaction through the cyanide bridges.     

Electrochemical and Spectroscopic Characteristics of Artemisinin Antimalarial Drug: Charge Transfer Redox Process

Artemisinin is a plant extract and popular in treating many diseases including malaria. It is a sesquiterpene lactone with several binding modes to several molecules. Oxidation and reduction of this molecule at different electrode materials was studied in tetrabutyl ammonium bromide in acetonitrile solution. The apparent standard heterogeneous rate constant for electron transfer (k_h) was found to be 23 cm s^?1. The value of the electron transfer coefficient (αn_a) was found to be 0.496. The diffusion coefficient (D) was found to be 3.34 × 10^?6 cm s^?1. Gold (Au), glassy carbon, and pyrolytic graphite electrodes exhibited distinguishable current densities. A value of E _1/2 = –0.81 V versus Ag/AgCl was obtained, and a value of n = 1.52 (2) was estimated.     

Maintenance of the slope of linearized calibration curves in Zeeman graphite furnace atomic absorption spectrometry

L'vov and co-workers developed a theoretical model and computational procedure (B.V. L'vov, L.K. Polzik and N.V. Kocharova, Spectrochim. Acta Part B, 47 (1992) 889 and B.V. L'vov, L.K. Polzik, N.V. Kocharova, Yu.A. Nemets and A.V. Novichikhin, Spectrochim. Acta Part B, 47 (1992) 1187) that linearized calibration curves in Zeeman graphite furnace atomic absorption spectrometry by taking into account the presence of stray light. The calculations of L'vov and co-workers were based on three parameters: the rollover absorbance A_r, Zeeman sensitivity ratio R, and the original background corrected peak absorbance values A_z. In order to simplify the calculations, R was assumed to be unity. In the studies reported here, this simplification is shown to be unsatisfactory because the slope obtained in the upper portion of the calibration curve, after linearization, is found to be different from the slope obtained in the normal linear region. Deviations between these slopes were found to be as high as 30%. The present work also shows that the theoretical model of L'vov and co-workers does not have a mathematical solution at high values of A_z. This failure of the model prevents its use at high A_z values. The physical nature of this failure is still unclear, which points to the necessity for further work to understand the inadequacies of the present theory. In the present studies, calculations based on the Newton method of successive approximations (A.I. Yuzefovsky, E.G. Su, R.G. Michel, W. Slavin and J.T. McCaffrey, Spectrochim. Acta Part B, 49 (1994) 1643), allow incorporation of the experimental value of R at the rollover point R′, which better linearizes the calibration curves. By use of this approach, a satisfactory result is obtained for lead (R′ = 0.67) up to the point of failure of the model at high values of A_z.     

Centrifugal distortion analysis of the rotational spectrum of cyclopropenylidene

A detailed centrifugal distortion analysis of the rotational spectrum of cyclopropenylidene C₃H₂ has been deduced from its millimeter wave spectrum. C₃H₂, a ring molecule recently discovered by Thaddeus and co-workers, has been efficiently produced by a very weak rf discharge in allene, C₃H₄. Fractional abundance as high as one percent has been obtained, which is explained by the long lifetime of the molecule, measured to be about 300 ms at 30 mTorr.     

Unexpected transformations of 3-(bromoacetyl)coumarin provides new evidence for the mechanism of thiol mediated dehalogenation of α-halocarbonyls

The mechanism for the thiol mediated dehalogenation of α-halogenated carbonyls has remained an unresolved problem, despite its ongoing application in synthetic organic chemistry. Nakamura and co-workers first proposed that net dehalogenation occurs via sequential nucleophilic substitutions, while Israel and co-workers concluded that the rate at which dehalogenation occurred suggested that dehalogenation proceeds in a single concerted step. In this study, we investigated the debromination and nucleophilic substitution of 3-(bromoacetyl)coumarin with a variety of thiophenols, whose electron donating or withdrawing natures resulted in large variations in the degree of nucleophilic substitution and dehalogenation products, respectively. Results from these experiments, in addition to an unexpected formation of thioether containing dibenzo[b,d]pyran-6-ones from a Robinson annulation, has provided new evidence for this disputed mechanism.     

Improved CNDO/S calculation of electronic spectra of organic compounds. I. New CNDO/S calculation by using an improved method of one-center electron repulsion integral

The NM-gamma CNDO/S program previously developed by our group was modified by the introduction of a new one-center electron repulsion integral gamma(AA)(new) approximation, namely, the gamma(AA)(new)-CNDO/S method. The value of this gamma(AA)(new) was evaluated according to the product values of the coefficient C with the gamma(AA) value proposed in our previous paper. This method using a new gamma(AA) was also found to improve the two-center electron repulsion integral gamma(AB) value with respect to the chemical softness proposed by Nishimoto and co-workers, together with the difference between HOMO and LUMO orbital energies. The results calculated by the present improved gamma(AA)(new)-CNDO/S method demonstrated that not only the calculated absorption maxima wavelengths and ionization potentials, but also the order and the assignment of orbitals coincided very well with those based on the results of experiments investigating a variety of polyenes, cyanynes, and polycyclic aromatic hydrocarbons.