Thermodynamically controlled synthesis of a chiral tetra-cavitand nanocapsule and mechanism of enantiomerization

The dynamic covalent synthesis, structure and conformational dynamics of a chiral polyimine nanocapsule 1a are reported. Reaction of four tetraformyl cavitands and eight H(2)N(CH(2))(2)NH(2) yields quantitatively 1a, which has a compact, asymmetrically folded, pseudo-C(2)-symmetric structure, as determined by X-ray crystallography, and encapsulates four CHCl(3) and three CH(3)OH guests in the solid state. In solution, 1a enantiomerizes by passing over a barrier of ΔG(298)(double dagger) = 21.5 ± 0.7 kcal mol(-1) via a refolding process.     

Determination of organophosphorus, triazine and 2,6-dinitroaniline pesticides in aqueous samples via solid-phase microextraction (SPME) and gas chromatography with nitrogen-phosphorus detection

Solid-phase microextraction (SPME) represents a very simple and rapid method for the extraction of organophosphorus, triazine and 2,6-dinitroaniline pesticides from aqueous samples without making use of any solvents. The same fiber can be used repeatedly. Moreover, a sample volume as small as 3 mL can be employed with no loss in sensitivity. 34 compounds have been extracted from aqueous samples by SPME using a 85 m polyacrylate fiber. For organophosphorus pesticides, a 100 m polydimethylsiloxane fiber has been used additionally for comparison. The fibers were directly introduced into the heated split/splitless injector of the gas chromatograph and determined using a nitrogen-phosphorus detector. The method was evaluated with respect to the limit of detection (LOD), linearity and precision. The limit of detection (LOD) depends on the compound and varies from 0.005–0.09 g/L. The method is linear over at least three orders of magnitude with coefficients of correlation usually >0.999. For triazines and 2,6-dinitroanilines the coefficient of variation (precision) is <8% while for organophosphorus compounds it may reach values up to 18% (however, if the latter compounds are extracted using the polydimethylsiloxane phase considerably higher precision is achieved). The partitioning of the analyte between the aqueous phase and the polymeric phase depends on the hydrophobicity of the compound as expressed by the octanol/water partitioning coefficient (P_ow). For triazines it was shown that there is a linear dependence of the logarithm of the analyte response on the log(P_ow) i.e. the higher the hydrophobicity, the higher the affinity of the analytes to the polymeric phase of the fiber and the higher the response. Salt addition has a strong effect on the extraction efficiency. This effect increases with decreasing hydrophobicity (increasing polarity) of the compound. The triazines ametryn, atrazine, propazine, simazine and simetryn have been identified in a ground water well sample by SPMEGC/NPD.     

Mapping the potential energy surface of the tolylcarbene rearrangement in the inner phase of a hemicarcerand

Photolysis of p-tolyldiazirine (6) in the inner phase of a hemicarcerand with four butane-1,4-dioxy linker groups (5) in C(6)D(5)CD(3) at 77 K yields the 5-methylcycloheptatetraene hemicarceplex 5 circle 3b in 41% together with innermolecular reaction products resulting from an insertion of transient p-tolylcarbene (1b) into an acetal C-H or linker C-O bond of 5 and from the addition of 1b to an aryl unit of 5. The yield of incarcerated 3b increased up to 67% if 6 is photolyzed inside a hemicarcerand with deuterated spanners and butane-1,4-dioxy linker groups (d(48)-5). Hemicarceplex 5 circle 3b is not formed if the photolysis is carried out in CDCl(3). Incarcerated 3b is stable at room temperature in the absence of oxygen and is characterized by 1D and 2D NMR spectroscopy. In the presence of oxygen, 3b reacts quantitatively to yield toluene and CO(2). Upon heating solutions of d(48)-5 circle 3b in C(6)D(5)CD(3), 3b rearranges to 1b and m-tolylcarbene (18). Both tolylcarbenes immediately react with the surrounding host. From a product analysis and the measured rate constants for the thermal decomposition of d(48)-5 circle 3b in the temperature range 70-102 degrees C, the activation parameters for the 3b to 1b and 3b to 18 rearrangements are calculated (3b to 1b: DeltaG(373)++ = 27.3 +/- 1.4 kcal/mol, DeltaH(373)++ = 26.4 +/- 1.0 kcal/mol, TDeltaS(373)++ = -0.9 +/- 1.0 kcal/mol; 3b to 18: DeltaG(373)++ = 27.8 +/- 1.4 kcal/mol, DeltaH(373)++ = 19.7 +/- 1.0 kcal/mol, TDeltaS(373)++ = 8.1 +/- 1.0 kcal/mol). These values are compared with those calculated by Geise and Hadad at the B3LYP/6-311+G** level of theory (Geise, C. M.; Hadad, C. M. J. Org. Chem. 2002, 67, 2532-2540). The slightly higher inner phase activation free energy of the 3b to 18 rearrangement is explained through steric constraints imposed by the surrounding hemicarcerand on the transition state. The enthalpy-entropy compensation observed for the 3b to 18 rearrangement is discussed and interpreted as a result of a hemicarcerand and solvent reorganization along the reaction coordinate. It is taken as indirect evidence for the intermediacy of 2-methylbicyclo[4.1.0]hepta-2,4,6-triene in the 3b to 18 rearrangement.     

Diastereoselective Spirocyclization of C-(Alkyloxycarbonyl)formimines of 2-Substituted 1HIndole-3-ethanamines (= Tryptamines): Basic Studies. 5th Communication on Indoles,Indolenines, and Indolines

C-(Alkoxycarbonyl)formimines of type 15–18 were derived from the 2-substituted tryptamines 2 , 9 , 10 , and 11 and transformed with tosyl chloride into tricyclic 3-spiroindoles of types 19–22 (Scheme 3). The influence of the homochiral alkoxy moieties A–D on the stereochemical outcome of this reaction was studied. Good-to-excellent diastereoselectivities were observed with the (?)-8-(phenylmenth-3-yl)oxy group ( B ) as homochiral auxiliary. The structures of the tricycles 4 , (2′R,3S)- 19B , and (2′S,3R) 20C were established by X-ray analysis, the structures of the others by NOE and CD studies, and by chemical correlation. Possibilities to explain the steric course of the spirocyclizations are discussed.     

Molekulares S=GeCl2 Matrix-IR-Untersuchungen und ab initio SCF-Rechnungen

The Molecule S?GeCl₂. Matrix IR Investigation and Ab initio SCF Calculation Molecular S?GeCl₂ is found in a matrix reaction between the high-temperature molecule Ge?S and Cl₂. A structure analog to that of phosgene can be derived from the isotopical shifts (⁷⁰Ge/⁷²Ge/⁷³Ge/⁷⁴Ge/⁷⁶Ge and ³⁵Cl/³⁷Cl) within the IR spectra. The normal coordinate analysis results for the Ge?S force constant a value of 4.21 mdyn/Å. The spectroscopic results are confirmed by ab initio SCF calculations.     

Kinetic energy release and position of transition state during the intramolecular substitution of ionized 2-benzoyl pyridines

The loss of substituents X from molecular ions of ortho substituted 2-benzoyl pyridines has been investigated as a function of the dissociation energy of the C? X bond. Comparison of unimolecular and collisional induced decompositions of the resulting [M ? X]⁺ ions and reference ions arising from 3-hydroxypyrido[1,2-α]indole shows that cyclic fragment ions are formed in every case by an intramolecular substitution reaction with the exception of the parent compound (X = H), which gives rise to a mixture of [M ? H]⁺ ions with different structures. The heat of formation of the cyclic ion has been estimated experimentally and by calculation using thermochemical data, and from this value and the appearance energies, the activation energies of the reverse reactions have been evaluated for the different reaction systems. Measurement of the kinetic energy release during the substitution reactions shows that only part of the reverse activation energy is released as kinetic energy. The energy partitioning quotient varies from 0.37 to 0.08 depending on the dissociation energy of the C? X bond or the reaction enthalpy. A sudden change in the energy partitioning quotient is observed with increasing exothermicity of the reaction, paralleling the behaviour of similar reaction systems. These results are interpreted as a demonstration of the influence of the variation of transition state position on the energy partitioning quotient.     

Analysis of semiconducting materials by high-resolution radiofrequency glow discharge mass spectrometry

The analytical capabilities of a high-resolution mass spectrometer in combination with a 13.56 MHz glow discharge ion source for the analysis of semiconducting materials (silicon carbide and gallium arsenide) were studied. It was shown that single positively charged ions of sample material have about 10 eV higher average energy than the ions of the discharge and residual gas. Therefore effective energy separation of the ions of analyte from the ions of the discharge and residual gas was achieved by adjusting the ion transfer optics (breadth and position of energy slit), which improves the analytical capabilities of the developed method.Some analytical applications are presented to illustrate the performance of r.f. GDMS for the bulk analysis of semiconducting materials. The results of the trace element analysis of gallium arsenide and silicon carbide samples are compared with data of independent methods (LIMS, ICP-AES, SIMS).Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthdayOn leave from the Institute of Inorganic Chemistry, 630090 Novosibirsk, Russia     

Determination of 13C/12C-ratios of anthropogenic organic contaminants in river water samples by GC-irmMS

This study describes the application of a common analytical procedure adapted for compound-specific stable carbon isotope analyses of riverine contaminants. To evaluate the sensitivity of the analytical method and the precision of the isotopic data obtained, a set of numerous substances at different concentration levels were measured. For most of the anthropogenic contaminants investigated (including chlorinated aliphatics and aromatics, musk fragrances, phthalate-based plasticizers and tetrabutyl tin) acceptable carbon isotope analyses could be obtained down to amounts of approximately 5?ng absolutely applied to the gas chromatograph. These amounts correspond to concentrations in water samples at a natural abundance level of approximately 50–200?ng?L^?1 (low to medium contaminated river systems). However, it has to be considered that the precision and the sensitivity of the analytical method depend partially on the chemical properties of the substances measured. Five recovery experiments were conducted to assess changes in carbon isotope ratios during sample preparation and measurement. The compounds selected for these experiments are known riverine contaminants. Isotopic shifts or higher variations of the isotope ratios as a result of the analytical procedures applied were observed only for a couple of contaminants. Furthermore, compound-specific carbon isotope analyses were performed on eight water extracts of the Rhine river. By comparing the variation of the data of several individual compounds with the deviations obtained from the recovery experiments, it was possible to differentiate contaminants with unaffected isotope ratios and substances with significant alterations of the δ¹³C-values.