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121.
Marcos Mandado Christian Van Alsenoy Paul Geerlings Frank De Proft Ricardo A Mosquera 《Chemphyschem》2006,7(6):1294-1305
A Hirshfeld decomposition scheme of the Hartree-Fock total molecular energy into atomic energies is presented. The calculations are performed by direct numerical integration and the results are compared for a set of 28 molecules containing different kinds of atoms. The calculated atomic energies show a strong dependency on changes of atomic electron population and hybridization. Linear correlations are found between the energy and the population for H, these being related to the electronegativity of this atom and to the external potential created by the remaining atoms. The proposed energy partitioning scheme appears to be useful for studies such as proton acidity, the anomeric effect and group transferability, and allows atomic virial ratios to be obtained. Finally, the atomic potential energies are found to mimic trends based on exact expressions as well as trends displayed by molecular quantities, thus lending credibility to the partitioning scheme used. 相似文献
122.
Ferreira SL dos Santos HC de Jesus DS 《Fresenius' Journal of Analytical Chemistry》2001,369(2):187-190
A procedure is proposed for the separation and determination of molybdenum in iron matrices by a batch process. It is based on the solid-phase extraction of the molybdenum(V) ion as thiocyanate complex on polyurethane (PU) foam. The extraction parameters were optimized. Using 0.20 mol L-1 hydrochloric acid, a thiocyanate concentration of 0.10 mol L-1, 100 mg of polyurethane foam and shaking time of 10 min, molybdenum (5-400 micrograms) can be separated and preconcentrated from large amounts of iron (10 mg). Desorption was carried out instantaneously by conc. nitric acid or acetone. Distribution coefficients, sorption capacity of the PU foam and coefficients of variation were also evaluated. The effect of some ions on the separation procedure was assessed. Iron(III) should be reduced to iron(II). The proposed procedure was used to determine molybdenum in standard iron matrices such as steel and pure iron. The achieved results did not show significant differences with certified values. 相似文献
123.
The use of aerosol produced in a nebulization chamber is proposed as an alternative to gas sample capture in flow systems. This paper describes the coupling of a sampling interface with a flow system, for in situ gas monitoring. Aspects related with the behavior of aerosol formation and gas solubilization in liquid drops are discussed. The method is applied to the determination of residual lime in acidic soils. Aliquots of 5.0 ml of 1.0 mol l−1 HCl were mixed with soil samples (1 g). The CO2 released from these samples was captured by a nebulized aerosol and determined conductivity. The analytical curve from 1.0×10−2 to 5.0×10−2 mol kg−1 CaCO3 was ploted applying the matrix matching approach. This proposition, allowed an increase in the sensibility with detection limit of 6.0×10−3 mol kg−1. The precision was good (R.S.D. <3%) for an analytical frequency of 22 determinations per hour. A fair agreement, at 95% confidence level, was found between the results from the proposed method and certified values of the investigated samples. 相似文献
124.
[reaction: see text] Racemic oxodiester 1 undergoes stereoselective cyclocondensation with (S)-tryptophanol, (S)-(3,4-dimethoxyphenyl)alaninol, or the corresponding amino acids, in a process involving a tandem dynamic kinetic resolution/desymmetrization of diastereotopic groups, to give bicyclic lactams, which are cyclized to substituted indolo[2,3-a]- and benzo[a]quinolizidines. 相似文献
125.
The L-dopa is the immediate precursor of the neurotransmitter dopamine. Unlike dopamine, L-dopa easily enters the central nervous system and is used in the treatment of Parkinson’s disease. A sensitive and selective method is presented for the voltammetric determination of L-dopa in pharmaceutical formulations using a carbon paste electrode modified with trinuclear ruthenium ammine complex [(NH3)5RuIIIORuIV(NH3)4ORuIII(NH3)5]6+ (Ru-red) incorporated in NaY zeolite. The parameters which influence on the electrode response (paste composition, potential scan rate, pH and interference) were also investigated. The optimum conditions were found to an electrode composition (m/m) of 25% zeolite containing 6.7% Ru, 50% graphite and 25% mineral oil in acetate buffer at pH 4.8. Voltammetric peak currents showed a linear response for L-dopa concentration in the range between 1.2×10−4 and 1.0×10−2 mol l−1 (r=0.9988) with a detection limit of 8.5×10−5 mol l−1. The variation coefficient for a 1.0×10−3 mol l−1 L-dopa (n=10) was 5.5%. The results obtained for L-dopa in pharmaceutical formulations (tablet) was in agreement with compared official method. In conclusion, this study has illustrated that the proposed electrode modified with Ru-red incorporated zeolite is suitable valuable for selective measurements of L-dopa. 相似文献
126.
Rodinei Augusti Marcos N. Eberlin Concetta Kascheres 《Journal of heterocyclic chemistry》1995,32(4):1355-1357
The reactions of carbethoxycarbene (:CH2-CO2Et, 2) with several acyclic enaminones (RCOCH=CR1NHR2, 3) lead to the unexpected formation of 2-Me, 3-CO2Et, 4-H, 5-R1-pyrroles 4 . Structural variations of the enaminones show that the structural fragments C(3)-CO2Et and C(2)-Me are provided by 2 and that the fragment C(5)-R1NHR2 originates from the enaminones 3 , while the RCO group from 3 is eliminated during the course of reaction. Reactions with cyclic and nitrogen-hindered enaminones do not lead to pyrrole formation but occur by simple insertion of 2 to the Cα-H bond. 相似文献
127.
New AP Wolff JC Crabtree S Freitas do Santos L Okafo G Lee J Divan K 《Journal of chromatography. A》2001,913(1-2):205-208
We have recently investigated the biodegradation of a number of acidic aromatic compounds that give excellent chromatography using trifluoroacetic acid (TFA) based HPLC methods. Unfortunately HPLC methods using TFA are not usually compatible with detection by negative ion mass spectrometry as TFA suppresses ionisation of the analyte during the electrospray process. We present a preliminary investigation of the use of an anion-exchange micro-membrane suppressor to remove TFA on-line post column with the aim of improvement of mass spectral data using an aromatic acid as an example, Thus LC-MS using a TFA based HPLC method with negative ion mass spectral detection is shown to be possible with good sensitivity. 相似文献
128.
Rosario Pardo Marcos Zayat David Levy 《Journal of Sol-Gel Science and Technology》2006,40(2-3):365-370
A photochromic naphthopyran derivative was embedded in sol-gel prepared thin ormosil films. The resulting samples show high
transparency and exhibit a strong red colouration upon irradiation with UV light. The photostability of the photochromic molecules
is strongly related to the nature of the embedding ormosil matrix. The introduction of organic functional groups into the
inner pore surface of the matrix allows tailoring the chemical environment where the dye molecules will be allocated, in terms
of the effectiveness of the interaction between the photochromic molecules and the Si-OH groups on the surface of the pores,
affecting the stability of the molecules upon prolonged exposition to UV light.
The photostability of the molecules was increased in matrices functionalized with larger organic groups, or with larger amount
of modifying groups. In this way the photodegradation of the photochromic molecules could be reduced by a factor of 5, as
compared with the photodegradation of the molecules in unfunctionalized silica matrix. 相似文献
129.
In this work a laboratory polymerization scale process was studied for the production of polybutadiene with high content of cis-1,4 repeating units. A Ziegler-Natta catalytic system based on neodymium versatate (catalyst), diisobutylaluminium hydride (cocatalyst) and tert-butyl chloride (chlorinating agent) was used. The influence of solvent nature (pure grade) and possible contaminants (electron donors) in a recovered solvent from a butadiene-styrene anionic polymerization industrial plant on the stereoselectivity and catalytic activity, molecular weight and molecular weight distribution of the resultant polybutadienes was studied. The polymers were characterized by infrared spectroscopy and size exclusion chromatography. Polybutadienes with cis-1,4 units content in the range of 99-98% were produced. The polymers weight-average molecular weight, , varied from 2.23 × 105 to 4.47 × 105 and the molecular weight distribution, MWD, from 3.1 to 5.1. 相似文献
130.
Neutral oxorhenium(V) complexes with thiosemicarbazones derived from 2‐pyridine formamide, HL1, are formed when [ReOCl3(PPh3)2] reacts with equimolar amounts of the ligands. Reduction of the metal and the formation of rhenium(III) complexes of the composition [Re(L1)2]+ occurs when an excess of thiosemicarbazones is used and the reaction is performed in boiling toluene for a prolonged period of time. The thiosemicarbazones deprotonate and act as tridentate ligands as has been confirmed by an X‐ray structure of [ReOCl2(L1b)], where HL1b is 2‐pyridineformamide‐N(4)‐ethylthiosemicarbazone and the ligand occupies the equatorial coordination sphere of the complex together with one of the chloro ligands. 相似文献