A comparison of electronic properties of various modifications of graphite

We have investigated various electronic band structures for different graphite modifications using the extended tight binding method. The specific crystal structures studied are the two naturally occurring graphite modifications (Bernal and rhombohedral), and a hypothetical configuration where the carbon atoms in consecutive layers are directly above each other. The latter structure has not been observed in pure graphite, but it is the backbone structure for different stages of intercalated graphite, e.g. Li-graphite compounds LiC_n. On comparing band structures for various graphite modifications we find important differences in the π-bands close to the Fermi energy, the region dominating transport and low energy excitation properties.     

The Baylis-Hillman chemistry in aqueous media: elucidation of mechanism for synthesis of ether side-product leads to an efficient approach to C-O bond formation

The formation of an ether from the Baylis-Hillman (BH) adduct during the BH reaction of 5-isoxazolecarboxaldehydes is a common phenomenon if the reaction is allowed to proceed for longer periods. The amount of formation of such ethers depends on the acrylates used and is most significant for tert-butyl acrylates. A study of the plausible mechanism for the formation of these side-products led to reactions of acetates of BH adducts with phenol in aqueous media to yield the corresponding 3-phenoxy alk-2-enoates in good yields. The successful translation of solution phase methodology to solid phase for application towards combinatorial chemistry is discussed.     

Symmetrization of the trimer model for Si(111) 7×7

A symmetrized extension of the trimer model for the Si(111) 7×7 structure is presented. It exhibits new features such as sixfold coordinated Si atoms which are not unusual in Si chemistry but have not been considered in previous structural models. Atomic coordinates are optimized by a Keating-type strain energy minimization and the structure factor for transmission electron diffraction is calculated.     

Responses of neurons to click-pairs as simulated echoes: auditory nerve to auditory cortex

When two identical sounds are presented from different locations with a short interval between them, the perception is of a single sound source at the location of the leading sound. This "precedence effect" is an important behavioral phenomenon whose neural basis is being increasingly studied. For this report, neural responses were recorded to paired clicks with varying interstimulus intervals, from several structures of the ascending auditory system in unanesthetized animals. The structures tested were the auditory nerve, anteroventral cochlear nucleus, superior olivary complex, inferior colliculus, and primary auditory cortex. The main finding is a progressive increase in the duration of the suppressive effect of the leading sound (the conditioner) on the response to the lagging sound (the probe). The first major increase occurred between the lower brainstem and inferior colliculus, and the second between the inferior colliculus and auditory cortex. In neurons from the auditory nerve, cochlear nucleus, and superior olivary complex, 50% recovery of the response to the probe occurred, on average, for conditioner and probe intervals of approximately 2 ms. In the inferior colliculus, 50% recovery occurred at an average separation of approximately 7 ms, and in the auditory cortex at approximately 20 ms. Despite these increases in average recovery times, some neurons in every structure showed large responses to the probe within the time window for precedence (approximately 1-4 ms for clicks). This indicates that during the period of the precedence effect, some information about echoes is retained. At the other extreme, for some cortical neurons the conditioner suppressed the probe response for intervals of up to 300 ms. This is in accord with behavioral results that show dominance of the leading sound for an extended period beyond that of the precedence effect. Other transformations as information ascended included an increased variety in the shapes of the recovery functions in structures subsequent to the nerve, and neurons "tuned" to particular conditioner-probe intervals in the auditory cortex. These latter are reminiscent of neurons tuned to echo delay in bats, and may contribute to the perception of the size of the acoustic space.     

Chemistry of 5(2H)-isoxazolones: Novel conversion of positional isomers

Reactions of 4-arylidene-3-methyl-5(4H)-isoxazolones with nucleophiles lead to 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones. Formation of 5(4H)- and 5(2H)-isoxazolone derivatives have been observed when 4-substituted-arylidene-3-rriethyl-5(4H)-isoxazolones are reacted with methyl magnesium iodide. Oxidation of 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones with manganic acetate and pyridinium chlorochromate, gives 4,4′-bis[4-substituted-benzyl-3-methyl-5(4H)-isoxazolone]. Oxidation of 4-substituted-benzyl-3-methyl-5(2H)-isoxazolones with metachloroperbenzoic acid gives 4-hydroxy-4-substituted-benzyl-3-methyl-5(4H)-isoxazolones and reactions of the same substrates with N-bromosuccinimide furnish 4-bromo-4-substitutedbenzyl-3-methyl-5(4H)isoxazolones.     

NAA determiantion of iron and silicon in USGS standards and bauxites using fast neutrons from241Am-Be source

Fast neutron-activation methods have been developed for the determination of iron and silicon in USGS and Indian standards and bauxites. Nuclear reactions⁵⁶Fe/n, p/⁵⁶Mn and²⁸Si/n, p/²⁸Al were carried out using²⁴¹Am-Be neutron source and cutting off thermal neutrons with a Cd shield. For Si a cyclic method was adopted due to short half life of²⁸Al /2.3 min/. The methods are non destructive, fast, economic and ideal for bulk analysis of rocks and process control.A part of this work was presented at the International Symposium on Nuclear Analytical Chemistry, Slowpoke Reactor Facility, Dalhousie University, Canada, June 5–7, 1985.     

Synthesis and X-Ray Structure of Bis{S-(1-lithio-3,3-diphenylprop-2-enyl)-N-methyl-S-phenylsulfoximine– Tetrahydrofuran (1/2)} Complemented by Model ab initio Calculations of α-Lithiosulfoximines

The gas-phase structure of N,S,S-trimethylsulfoximine 1 and of its monolithiated isomers 2–4 was calculated by ab initio methods. It was found that a Li? C? S? N four-membered chelate 2 is the most stable isomer. The second minimum 4 shows N? Li? O complexation and is only slightly higher in energy. Li-Contacts with the C(α) atom and the sulfoximine O-atom in 3 are energetically disfavored by 6.1 kcal/mol. The two transition states 5 and 6 suggest an interconversion mechanism of 2 to 3 with 4 as an intermediate. A comparison of 4 with the crystal structure of the THF solvate 7 , which was prepared by the addition of BuLi to (±)-S-(3,3-diphenylprop-2-enyl)-N-methyl-S-phenylsulfoximine ( 8 ) at low temperature in THF, demonstrates that the coordination geometry in the solid state is in good agreement with the calculated structure. The (1-lithioallyl)sulfoximine 7 crystallized as a centrosymmetric dimeric aggregate featuring an eight-membered ring with the atomic sequence (Li? N? S? O)₂. The O-atoms of two THF molecules and the sulfoximine O- and N-atoms are coordinated to the Li-atom in a tetrahedral orientation. After metallation, a significant shortening of the S? C(α) bond is observed. Remarkably, only one of the two possible diastereoisomeric enantiomer pairs is found in the solid state.