Determination of antimony in geochemical samples by graphite furnace atomic absorption spectrometry using different matrix modifiers

Determination of antimony by graphite furnace atomic absorption spectrometry using five different matrix modifiers, viz. nitric acid, copper, nickel, molybdenum and palladium, together with L'vov platform was studied. Without matrix modification, antimony was lost in a 1.2-M HCl solution when the thermal pretreatment temperature exceeded 700°C. By using 1.4 M HNO₃ or $\text{1μg}\text{20μl}$ copper solution the thermal pretreatment temperature could be increased up to 900°C. The matrix modification with $\text{2μg}\text{20μl}$ palladium, $\text{4μg}\text{20μl}$ molybdenum, or $\text{20μg}\text{20μl}$ nickel allowed the use of 1300°C as the pretreatment temperature. The best results were obtained with palladium when its concentration exceeded $\text{0.8μg}\text{20μl}$. Chemical interferences were studied with standard solutions spiked with different metal chlorides. The severe interference caused by iron chloride was avoided with ascorbic acid (2.5 %, $\text{w}\text{v}$). The method was tested on some geochemical reference samples.     

The synthesis and x-ray crystal structure of a tetracyclo[4.2.1.1.2,503,7]decane derivative. A rigid,twist boat-containing ring system.

Attempted intramolecular pinacolization of cage diketone $\text{4}$ unexpectedly gave ketol $\text{5}$ instead. Compound $\text{5}$ was shown to possess the rare tetracyclo[4.2.1.1.^2,50^3,7]decane ring system by means of an X-ray crystal structure of hydroxy-mesylate $\text{10}$ derived from $\text{5}$ by sodium borohydride reduction followed by mesylation. A possible mechanism for the transformation of $\text{4}$ into $\text{5}$ is presented, and certain noteworthy features of the crystal structure of $\text{10}$ are discussed. The base-catalysed fragmentation of $\text{10}$ to give enone $\text{11}$ is also described.     

Electron-transfer photooxygenation. 8. dicyanoanthracene-sensitized photooxidation of ortho-dimethoxybenzene

Two cleavage products, $\text{cis,cis}$-dimethylmuconate ($\text{1}$) and $\text{trans,trans}$-dimethyl-muconate ($\text{2}$), are inefficiently produced in the 9,10-dicyanoanthracene (DCA) sensitized photooxidation of $\text{ortho}$-dimethoxybenzene ($\text{o}$-DMB) in polar solvents. An electron transfer mechanism is proposed in which superoxide ion (O^?₂0 combines with ortho-dimethoxybenzene to form a dioxetane (3), which cleaves to form ($\text{1}$). The initial products is $\text{1}$, which is rapidly converted to $\text{2}$ and other products under the conditions; $\text{no}$ cis-trans isomer is formed.     

Neutron activation analysis of high-purity selenium : Part II. Determination of tellurium

Neutron activation determination of 15–25 p.p.b. of tellurium in selenium can be done by irradiating 100-mg samples for 4 days at a neutron flux of 6$\text{.}\text{?}$10¹² n. cm^-2.sec^-1. The daughter isotope of ¹³¹Te,¹³¹I is separated by distillation and extracted into carbon tetrachloride. Finally the silver iodide precipitate is measured by β,γ -coincidence counting. The standard deviation is 3–7%.     

Activation and transfer of oxygen XV evidence for the transient opening of the pyrazine ring in n1- and n5- alkylflavin models. 4a- to 1010- adduct isomerization and pyrazine- and pyrimidine- ring contractions.

The pyrazine ring opening is proven by rearrangements leading to benzimidazoles $\text{4}$, $\text{13}$ and $\text{14}$. A 4^a- to 10^a-adduct isomerization ($\text{6}$ → $\text{7}$) and the intermediacy of a carbonyl oxide $\text{15}$ are indicated by pyrimidine ring contractions into $\text{8}$ and $\text{9}$, respectively.     

Photochemistry of phenyl alkyl ketones adsorbed on zeolite molecular sieves. Observation of pronounced effects on Type I/Type II photochemistry

The photolysis of phenyl alkyl ketones adsorbed on a number of commonly available zeolites (molecular sieves) can result in dramatic changes in Type I/Type II photochemistry.The photochemistry of ketones in ordered environments is a topic of current interest^2–4. Environmental effects can have important influence on the conformational flexibility of organic molecules, which in turn can affect the outcome of photochemical reactions⁵. Two recent reports^2,3 on the Norrish Type II reaction in ordered media prompts us to report our initial studies of the photochemistry of a number of phenyl alkyl ketones adsorbed in zeolites.Zeolites are crystalline aluminosilicates of usually well-defined structure⁶. Within the zeolite framework are a system channels and cavities of varying dimensions(2 – 13Å)⁶, some of which are capable of adsorption of large organic molecules (e.g., substituted benzenes). Thus the possibility that the internal spaces (or void volumes) of zeolites can exert topological control on organic photochemical reactions warrants investigation, since it is well-known that zeolites display shape-selective catalytic and adsorptive properties in important industrial chemical processes¹⁰. However, only a handful of reports of photochemical reactions on zeolites are known^4,7–9; the majority of these concern the catalytic splitting of water⁹. In this study, several commonly available zeolites were studied, and the results are compared to those obtained in homogeneous solution.Phenyl alkyl ketones $\text{1}$ - $\text{3}$ were employed in this study. The photochemistry of valerophenone $\text{1}$, octanophenone $\text{2}$, and α,α-dimethylvalerophenone $\text{3}$ is relatively well-understood in solution^11–13. For $\text{1}$ and $\text{2}$, the photobehaviour is characterized by Type II reaction, to give a triplet 1,4-biradical, which can either fragment, to give acetophenone, or cyclize, to give $\text{cis}$ and $\text{trans}$ cyclobutanols (eqn.1). Type I reaction in not observed. In solution, the ratio of fragmentation of cyclization (F/C) products is ～ 4 for $\text{1}$ and $\text{2}$. In general, the $\text{trans}$ isomer dominates, with t/c = 3 – 5 in benzene, and decreasing to a limit of 1 in more polar solvents (MeOH or micelles)^11-–13. For $\text{3}$. Type I reaction is observed in addition to Type II. The ratio of Type I/Type II product has been reported to be 0.3 in benzene without added thiol, and 0.6 with added thiol¹¹.Ketones $\text{1}$ and $\text{3}$ were deposited on Zeolites Na⁺-A, Na⁺-X, Na⁺-Y, Na⁺-Mordenite and resembling $\text{C}$ is also possible, from which cyclization is prohibited. For $\text{3}$, Type I reaction is known to compete with the Type II process¹¹. Adsorption of this ketone on Silicalite results in reaction via the least-motion pathway, namely Type I reaction, to give benzaldehyde as the aromatic product. Thus the behaviour of this ketone on Silicalite is quite consistent with the explanation offered for $\text{1}$ and $\text{2}$.Na⁺-Y is a large pore zeolite, with a pore diameter of ～8 Å and an internal cavity (supercage) of ～13 Å⁶. Additionally, we have found from related studies¹⁶ that of the zeolites capable of adsorbing benzene-type molecules studied in this work, Na⁺-Y allows the greatest degree of molecular mobility for photogenerated benzyl radical. Thus the observed F/C ratios of less than unity for this zeolite probably reflects the increased mobility of the photogenerated 1,4-biradical, allowing it to undergo ring closure readily. Interestingly, this zeolite also gave an $\text{inverse}$ t/c ratio for cyclobutanols of $\text{2}$.The results for the other zeolites are not readily distinguishable from those observed in solution, although two of these (Na⁺-X and Na⁺-Mordenite) are capable of adsorbing $\text{1}$ - $\text{3}$⁶. In any event, these two zeolites offer a medium for Type I/Type II reaction which essentially duplicate the behaviour in solution without the presence of solvent. Additional studies are in progress to further study the use of zeolites as a medium for photochemical reactions.     

Precise,non-destructive determination of fluorine by 14-MeV neutron activation analysis

A procedure is described for the precise determination of fluorine in organic and metal-organic compounds by 14-MeV neutron activation analysis using the reaction: ¹⁹F(n,2n)¹⁸F (β +, t_{$\text{1}\text{2}$} = 110 min).A relative standard deviation of better than ±0.5% is achieved by irradiating samples and standard simultaneously. Uniform neutron exposures were ensured by rotating the samples during irradiation. Positron emitters of short half-life are allowed to decay before counting. In metal-organic compounds, Sc, Zn, Ga and Ag cause the most serious interference; for organic compounds the method is rapid, and specific for fluorine.     

Synthesis of peptide alkaloids-III. Amino acids and peptides-XXXII. Synthesis of dihydro-zizyphine A and B

Recently we described² a new cyclisation method for the synthesis of ansa peptides by hydrogenolysis of Z-pentafluorophenyl ester and ring closure on the surface of the palladium catalyst. We applied this reaction to the synthesis of the 14-membered ansa peptide dihydro-zizyphine G¹. Here we wish to report the synthesis of the 13-membered ansa peptides dihydro-zizyphine A ($\text{8a}$) and B ($\text{9a}$)³.     

Quinone-imides : regiospecificity of nucleophilic attack on n-alkanesulphonyl-n'-alkanoyl 14-benzoquinone-imines.

Substrates such as ($\text{2}$) undergo regiospecific attack by azide and thiocyanate ions at the terminus of the CC-CN-CO system. In the case of addition of azide, this is proved by the detailed analysis of the ¹H and ¹³C NMR spectra of the products ($\text{11}$) derived from the quinone-imides ($\text{8}$). The structure of the products obtained by the addition of thiocyanate to the quinone-imides and ($\text{tilde10}$)($\text{tilde20}$) is proved by their facile cyclisation to the 2-aminobenzothiazoles ($\text{18}$) and ($\text{21}$) respectively.     

Fluorocarbon derivatives of nitrogen. Part 10. Preparation of heptafluoro- and 1-chlorohexafluoro-2-nitrosopropane,and conversion on the latter to chloropentafluoroacetone oxime

Details are given for the preparation of heptafluoro-2-nitrosopropane and 1-chlorohexafluoro-2-nitrosopropane $\text{via}$ the routes CF₃CFCF₂ → [with CsFR_FCO₂Ag (R_F = CF₃, nC₃F₇)] (CF₃)₂CFAg → (with NOCl) (CF₃)₂CFNO and CF₃CFCF₂ → (with CsClNOCl) CF₃(CF₂Cl)CFNO, respectively, and for conversion of the latter nitroso-compound to chloropentafluoroacetone oxime $\text{via}$ reduction with aqueous potassium hydrogen sulphite.     

General approach for the synthesis of polyquinenes via the weiss reaction. Preparation of the chiral tetracyclo [6.6.0.01,5.08,12]tetradecane-3,6,10,13,-tetraene

The aldol approach to construct the tetracyclic polyquinane skeleton of $\text{12}$, followed by hydroboration and the HMPA mediated dehydration of four hydroxyl groups has resulted in the facile synthesis of the chiral polyquinene, tetracyclo[6.6.0.0^1,5.0^8,12]-tetradecane-3,6,10,13-tetraene $\text{2}$. This method completely avoids the troublesome fragmentation reactions earlier encountered when the strained tetraone $\text{9}$ was reacted with nucleophilic reagents.     

Structural investigation of an antibiotic sporaviridin IV. Structural revision of viridopentaoses

Structures of viridopentaose A, B and C are revised as $\text{4A}$, $\text{4B}$ and $\text{4C}$, respectively, by ¹H-NMR (360 MHz) spectra and chemical degradations.     

Determination of cadmium, copper and lead in natural waters after anion-exchange separtion.

A method is described for the determination of cadmium, copper and lead in samples of natural non-saline waters. After acidification with hydrobromic acid, the water sample is filtered and, following the addition of ascorbic acid, passed through a column of the strongly basic anion-exchange resin Dowex 1-X8 (bromide form). On this exchanger cadmium(II). copper(I) and lead(II) are adsorbed as anionic bromide complexes. After elution of these elements with 1 M nitric acid, the determinations by atomic absorption spectrometry are carried out in a medium consisting of 90% ($\text{v}\text{v}$) methanol and 10% ($\text{v}\text{v}$) 1.5 M hydrobromic acid. The procedure was used for the routine determination of cadmium, copper and lead in water samples collected in Austria.     

The determination of molybdenum and tungsten in sea and surface water

A simple method for the determination of molybdenum and tungsten in sea and surface water is presented. Molybdenum and tungsten are concentrated on activated charcoal by adsorption as the ammonium pyrrolidine dithiocarbamate complex; the optimal pH for adsorption is 1.3. Mo and W are then determined by thermal neutron activation, forming ⁹⁹Mo (T_{$\text{1}\text{2}$} = 66.7 h) and ¹³⁷W (T_{$\text{1}\text{2}$} = 23.8 h), respectively. The ^99mTc daughter of ⁹⁹Mo is measured as soon as the equilibrium between ^99mTc(T_{$\text{1}\text{2}$}= 6 h) and ⁹⁹Mo is established. The detection limits are 0.05 μg Mo l^-1 and 0.05 μg W l^-1 (or 0.001 μg W l^-1 after a simple chemical separation).     

Flow birefringence and conformational characteristics of polyamide hydrazide molecules in dimethylsulphoxide

Flow birefringence (FB) and intrinsic viscosity of 19 samples of aromatic polyamide hydrazide (PAH) in dimethylsulphoxide (DMS), previously characterized by their weight-average molecular weights by the light scattering method, have been investigated. The molecular-weight dependence of reduced birefringence according to theory [12] was used to determine the optical anisotropy of a monomer unit Δa = (200 ± 20) 10^?25cm³ and the length of the Kuhn segment $\text{A = (250 ± 30)}\text{A}\text{?}$ of PAH molecules. The second independent evaluation of rigidity of the PAH chain $\text{A = (240 ± 30)}\text{A}\text{?}$ was obtained according to the theory of rotational friction of rigid wormlike chains by using the coefficients of rotational diffusion of PAH molecules determined from the characteristic values of orientation angles of FB. The value of rigidity of the PAH chain obtained by this method is in good agreement with the data on molecular dimensions obtained by light scattering.     

C-Metallated reformatsky intermediates. Structure and reactivity.

¹³C-NMR analysis of the Reformatsky intermediates from t-b$\text{u}$tyl-α-bromo acetate, t-b$\text{u}$tyl-α-bromo isobutyrate and t-b$\text{u}$tyl-α-bromo pr$\text{o}$pionate have evidenced C-metallated species. Their ability to act as n$\text{u}$cleophilic reagents under mild conditions and their selectivity towards h$\text{a}$logen-and oxygen containing electrophiles is reported.     

The effect of the solvent upon the rates and mechanisms of organometallic reactions: VI. The complexation of dibenzylmercury in solutions studied by PMR spectroscopy

The concentration and temperature dependence of J(¹¹⁹HgC¹H) values for solutions of dibenzylmercury in various solvents show dearly that if the solvent is of the monodentate electron-donating type $\text{1}\text{1}$ complexes are formed predominantly, the $\text{1}\text{2}$ complexes existing only at very low temperatures. The results suggest that the $\text{1}\text{1}$ complexes are very weak and have planar T-shaped structures, whereas the $\text{1}\text{2}$ complexes are probably tetragonal pyramids. In complexes of the former type the relative content of s-electrons in the HgCH site is higher than in the $\text{1}\text{2}$ complexes.     

Synthesis of chiral tripeptides by asymmetric hydrogenation of dehydrotripeptides. Remarkable effects of N-protecting groups on stereoselectivity and reactivity

Dehydrotripeptides, X-$\text{C}\text{*}$CH(Rⁱ)-ΔPhe-$\text{C}\text{*}$CH(R^k)COOMe ($\text{3}$: X = ^tBOC-NH,CBZ-NH, CF₃CONH and N₃), were employed for the asymmetric hydrogenation catalyzed by chiral rhodium complexes and it was found that ^tBOC-$\text{3}$ brought about by far the best results. Stereoselective dideuteration of a ^tBOC-$\text{3}$ was successfully performed.     

Mechanisms for reactions of halogenated compounds. Part 4.[1] activating influences of ring-nitrogen and trifluoromethyl in nucleophilic aromatic substitution

Rate constants have been measured for the reactions of ammonia with various fluorinated pyridines and diazines in aqueous dioxan at 25°. From the results the activating effects of ring-nitrogen (relative to CH) and of trifluoromethyl (relative to -H) have been determined. Ring-nitrogen activates the system at points $\text{ortho}$-, $\text{meta}$- and $\text{para}$- to the point of substitution, in the ratios $\text{ortho}$- 6.2 × 10⁴, $\text{meta}$- 8.5 × 10², and $\text{para}$- 2.3 × 10⁵. Similarly a trifluoromethyl substituent is activating by a factor of 2.4 × 10³$\text{ortho}$- and 4.5 × 10³$\text{para}$- to the point of substitution.     

Addition products of hydrazine derivatives to azo-alkenes - III. α-phenylhydrazino-phenylhydrazones

The addition of phenylhydrazine to phenylazo-alkenes $\text{4}$ yields α-(1-phenylhydrazino)-phenylhydrazones $\text{1}$. The reaction of phenylhydrazine with α-halogenated carbonyl compounds $\text{5}$ affords either $\text{1}$ or the isomeric α-(2-phenylhydrazino)-phenylhydrazones $\text{2}$. Structures $\text{1}$ and $\text{2}$ (>N-NH₂ and-NH-NH- groups, respectively) can be differentiated by ¹H NMR in DMSO-D₆ solution. Possible pathways of the reactions leading to either $\text{1}$ or$\text{2}$ are discussed. Compounds $\text{1}$ are found to be precursors of phenylosazones $\text{6}$.