Determination of rotational barriers of carbon?carbon bonds in 2-arylpiperidines

Carbon-carbon sp³-sp² rotational barriers of 3,3-dimethyl-2-(3,4,5-trimethoxyphenyl)-4-piperidones and their ethylene ketals have been evaluated using nmr techniques. The conformation of 1 hydrochloride has been studied by NOE determinations. Values found for the hydrochlorides of the title compounds are discussed in terms of equilibria with free bases and nitrogen inversion.     

Reactivity of dimetallated benzalazines

Summary The action of dpe or PPh₃ on dimetallated benzalazines in presence of NaClO₄ gives to ionic compounds only with dpe. can be obtained from [Pd(p-XC₆H₃CH=N-)Cl(PPh₃)₂]₂ and AgClO₄.     

Fourier transform infrared spectroscopy and the analytical study of works of art for purposes of diagnosis and conservation

This paper is a study of the analytical capacity of Fourier transform infrared spectroscopy (FT-IR) for any type of samples from works of art. The analytical information obtained with this technique on organic and inorganic compounds is extremely useful in the preliminary studies necessary for diagnosis and to decide on the conservation process. This paper reports the analysis and study with FT-IR on samples extracted from the microlayers in several works of art from different periods (16th to 18th century) comprising wall paintings and canvas. The most outstanding of these being the frescoes by A. Palomino from two ceilings in the Santos Juanes church. The analytical procedures for the different components of the works studied such as

1. (a) varnish applied to wood panels and canvas paintings,

2. (b) binding media and pigments used by the artists,

3. (c) inerts, mortars, stuccos and grounds have been optimized. The FT-IR technique offers a quick analysis of microsamples (less than 0.5 mg) and is able to characterise the different molecular groups which provide information on the nature of the different materials of organic and inorganic origin used by the artist and thus permits the diagnosis of pathologies requiring conservation treatment.

Precise atomic masses of147Eu,147Gd,and151Tb derived from their decay properties

Theβ-decay energies of¹⁴⁷Eu,¹⁴⁷Gd, and¹⁵¹Tb were determined by usingγ-spectroscopical methods. The comparison of experimental with calculatedK-capture probabilities yielded theQ _EC values 1.690( _?16 ⁺²¹ )MeV and 2.203( _?13 ⁺¹⁹ )MeV for¹⁴⁷Eu and¹⁴⁷Gd, respectively. By measuring the ratio of positron decay to electron capture for two branches in¹⁴⁷Eu decay, the decay energiesQ _EC=1.702(13) MeV andQ _EC=1.709(18)MeV were derived. Also fromEC/β ⁺ ratios the valuesQ _EC=2.225(75) MeV for¹⁴⁷Gd, andQ _EC=2.566(12)MeV for¹⁵¹Tb were obtained. Earlier discrepancies in the mass adjustment of these isotopes were removed. In course of the present studiesγ-decay properties of¹⁴⁷Eu and¹⁴⁷Gd were reinvestigated.     

A chemometric study of the simultaneous determination of calcium and magnesium in natural waters

A method for the simultaneous spectrophotometric determination of calcium and magnesium in mineral waters with an FIA system is tested. The method is based on the reaction between the analytes and arsenazo(III) at pH 8.5. The calculations of the amounts of both analytes in the samples are carried out with the H-point standard addition method (HPSAM) for ternary mixtures, and with a partial least squares (PLS) model after a proper variable selection. The results obtained for the determination of calcium were comparable using both methods. The employment of the HPSAM brings to our attention the influence of the calcium concentration in the sample to the development of the reaction between magnesium and arsenazo(III). HPSAM also permits to estimate the concentration of magnesium in the samples.     

Spontaneous disproportionation of NO2 (N2o4) by aromatic hydrocarbons. Formation of nitrosonium EDA complexes as chemical intermediates

Strong (orange/red) colourations resulting immediately upon the exposure of nitrogen dioxide and its equilibrium dimer (dinitrogen tetroxide) to various aromatic hydrocarbons (ArH) are shown to arise from the nitrosonium EDA or electron donor-acceptor complexes [ArH, NO⁺NO ₃ ^? ]. The latter exhibit diagnostic charge-transfer absorptions and characteristic N-O stretching bands in the UV-vis and IR spectra, respectively, that relate directly to ArH/NO⁺ interactions extant in the EDA complexes previously derived from the authentic nitrosonium salt, NO⁺PF ₆ ^- . Time-resolved picosecond spectroscopy establishes the charge-transfer excited state of [ArH, NO⁺NO ₃ ^? ] to be essentially identical to that from [ArH, NO⁺BF ₄ ^? ]. Furthermore, the same temporal decay of the spectral transients (ArH^+?) from both systems indicates minimal ion-pairing effects of the counterions (NO ₃ ^? and BF ₄ ^? ) on the kinetics of back electron transfer.     

Certification of reference materials and accreditation of reference material producers: Questionable terminology leads to confusion

In the view of the Deutscher Kalibrierdienst (DKD) , a certifying body for reference materials can be considered to be a calibration laboratory. Therefore, accreditation of calibration laboratories in accordance with ISO/IEC 17025 is the most appropriate way to establish confidence in certificates for reference materials. If necessary, the criteria of ISO/IEC 17025 can be tailored to specific cases. There is no need to provide any new kind of reference-material specific accreditation. However, in view of the variety of reference materials and the practice existing in other countries, accreditation of testing laboratories and product certification bodies may optionally be acceptable as long as the same stringent principles with respect to traceability and measurement uncertainty are applied. Such accreditations but not accreditations of reference material producers (ISO Guide 34) are also covered by existing international mutual recognition arrangements (MRA).     

Enantioselective synthesis of 2-arylpiperidines from chiral lactams. A concise synthesis of (-)-anabasine

Cyclodehydration of achiral or racemic aryl-delta-oxoacids with (R)-phenylglycinol stereoselectively affords chiral non-racemic bicyclic lactams, from which the enantiodivergent synthesis of (R)- and (S)-2-phenylpiperidine, the diastereodivergent synthesis of cis- and trans-3-ethyl-2-phenylpiperidine, and the enantioselective synthesis of the piperidine alkaloid (-)-anabasine is reported.     

Comparison of the acidity of residual silanol groups in several liquid chromatography columns

The silanol acidity of Waters Resolve C18, Waters Resolve silica, Waters Symmetry C18, Waters Symmetry silica, Waters XTerra MS C18 and underivatized XTerra columns has been measured from the retention of LiNO3 with a methanol/water (60:40) mobile phase buffered to different pH values. The Li+ cation is retained by cationic exchange with the background cation of the mobile phase (Na+) through the ionized silanols. The number of active silanols increases in the order: XTerra MS C18 < Symmetry C18 < underivatized XTerra < Resolve C18 < Resolve silica approximately equal to Symmetry silica. XTerra MS C18 does not present any residual silanol acidity up to s(s)pH 10.0 (pH in 60% methanol) as measured by LiNO3. The underivatized XTerra packing and Symmetry C18 present active silanols only at s(s)pH values higher than 7.0. For the other three columns, two different types of silanols with different acidity (s(s)pKa values about 3.5-4.6 and 6.2-6.8, respectively) have been observed. Symmetry C18 shows evidence of the presence of active basic sites that retain NO3- by anionic exchange.     

Bildung siliciumorganischer Verbindungen. LVI. Reaktionen Si- und C-chlorierter 1,3,5-Trisilapentane mit CH3MgCl

Formation of Organosilicon Compounds. LVI. Reactions of Si- and C-Chlorinated 1,3,5-Trisilapentanes with CH₃MgCl (Cl₃Si? CCl₂)₂SiCl₂ (1) reacts with an excess of meMgCl (me = CH₃) forming me₃Si? C?C? Sime₃ (2), Sime₄, H₂C?C(Sime₃)[CH(Sime₃)₂] (3) as main products and (me₃Si)₂C? CH(Sime₃) and as by-products. The cleavage reaction of (1) to (2) and (3) does not occur when the meMgCl-concentration is lowered. The reaction is started by the formation of a GRIGNARD reagent at a CCl-group in compound (1). Cl₃Si? CCl₂? SiCl₂? CH₂? SiCl₃ forms with ; me₃Si? CCl₂? SiCl₂? CHCl? SiCl₃ forms (me₃Si)₂C?CH(Sime₃). A reaction sequence is given.