Dehydrogenation of piperazine into pyrazine on alumina-platinum catalysts

Dehydrogenation of piperazine on alumina-platinum catalysts at 360–400°C was investigated. It was found that the alumina-platinum catalyst has low selectivity in the formation of pyrazine (yield 43%), and dehydrogenation is accompanied by decomposition and coke formation and products of decomposition in the formation of alkylpyrazines. The alumina-platinum catalyst in In₂O₃ and Re₂O₇ additives permits obtaining pyrazine with a yield of up to 80%. Dehydrogenation of piperazine is accompanied by reactions of decomposition, dehydroisomerization, and alkylation to a small degree, resulting in the formation of pyrrole, methylimidazole, and alkylpyrazines.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1483–1488, July, 1990.     

Nuclear magnetic resonance spectrometry of chemically and physically altered porous silica surfaces under gas chromatographic conditions

Conventional nuclear magnetic resonance spectrometry in solution, in combination with (13)C labelling, has been utilized to investigate differences in freedom of motion of similar chemically altered and physically modified porous silica sorbents under dry conditions, and the results have been correlated with previously reported results for chemically altered solvated surfaces. Spin-lattice relaxation time of the labelled terminal methyl groups decreases in the order chemically modified solvated ligands, chemically bonded unsolvated ligands, and physically sorbed unsolvated molecules.     

Metallisation of gel surfaces under ambient conditions

A room temperature method to coat a non-conducting gel phase with a metal is described, which uses galvanic displacement. Electrolytes are dissolved in the gel phase to allow metal deposition from an immiscible liquid electrolyte solution. Conformal deposition was achieved by imprinting the gel, followed by galvanic displacement of gold.     

Modeling of complex-forming chromatographic systems under ionizing radiation conditions

Simulation of displacement complex-forming chromatography systems under irradiation conditions was performed for eluents based on diethylenetriaminepentaacetate. Acetates, iminodiacetates, formates, glycinates, and succinates were considered as radiolysis products. The model with an acetic acid salt as the radiolysis product best correlated with the experimental data.     

Nickel oxide reduction under nonisothermal conditions

The interaction of nickel(II) oxide with carbon, polyethylene, and polystyrene in the temperature range 20–1000°C has been investigated by thermal analysis. The interaction of NiO with carbon and polyethylene under these conditions results in complete nickel reduction. In the case of polystyrene, because of its low decomposition temperature, the reduction process is incomplete. As compared to reduction with polyethylene, reduction with carbon takes place at a much higher temperature (750 versus 370°C) and requires a higher activation energy (68 ± 3 versus 41 ± 1 kcal/mol).     

Angiotensin I retention behavior on Butyl-Sepharose under linear loading chromatographic conditions

Adsorption behavior of angiotensin I on a commercial Butyl-Sepharose support has been studied in function of temperature and ammonium sulphate concentration. Under isocratic elution conditions and at the higher salt concentrations, a characteristic of the chromatographic performance of angiotensin I was the broadness of the corresponding peak and in most of the cases the appearance of two peaks. These results have been interpreted in terms of on-column cis–trans isomerization of angiotensin I (a proline containing polypeptide) followed by its “re-conformation” after the interaction with the support. It has been proposed that the peak splitting phenomenon, a combined effect between temperature, salt concentration in the mobile phase and the ligand, is caused by slow kinetics of isomerization that is on the same time scale as the chromatographic separation. Salt concentration and temperature promote the conversion of the trans form of angiotensin I into its cis form, which has a bigger hydrophobic surface area, in the presence of Butyl-Sepharose. The retention of the cis form of angiotensin I increases with the increase in salt concentration in the mobile phase and seems to be little affected by temperature.     

Catalytic synthesis of pyrazine,piperazine, and 1,4-diaza[2,2,2]-bicyclooctane

A study is made of the deamination of diethylenetriamine over acidalkali catalysts, i. e., kaolin and alumina with promoters. Promoters which raise the acidity of the catalyst, affect the formation of triethylene diamine favorably. Increasing the amount of additive increases the amount of triethylenediamine, and cuts the optimum temperature at which it is formed. On kaolin or Al₂O₃+15% B₂O₃, the yield of triethylenediamine amounts to 30% theory. Addition of MoO₃ facilitates dehydrodeamination and hydrogenolysis of the diethylenetriamine. The optimum promoter for preparing pyrazine is MoO₃ along with a small amount of acid oxides. On the Al₂O₃+5% MoO₃+1% P₂O₅, the pyrazine yield is 27.5% theory. Triethylenediamine can be separated from mixtures of it with piperazine by azeotropic distillation with mxylene or a mixture of mesitylene and α-memylnaphthalene.     

Catalytic synthesis of pyrazine, piperazine, and 1,4-diaza[2,2,2]-bicyclooctane

A study is made of the deamination of diethylenetriamine over acidalkali catalysts, i. e., kaolin and alumina with promoters. Promoters which raise the acidity of the catalyst, affect the formation of triethylene diamine favorably. Increasing the amount of additive increases the amount of triethylenediamine, and cuts the optimum temperature at which it is formed. On kaolin or Al₂O₃+15% B₂O₃, the yield of triethylenediamine amounts to 30% theory. Addition of MoO₃ facilitates dehydrodeamination and hydrogenolysis of the diethylenetriamine. The optimum promoter for preparing pyrazine is MoO₃ along with a small amount of acid oxides. On the Al₂O₃+5% MoO₃+1% P₂O₅, the pyrazine yield is 27.5% theory. Triethylenediamine can be separated from mixtures of it with piperazine by azeotropic distillation with mxylene or a mixture of mesitylene and -memylnaphthalene.     

Chiral recognition ability of alpha-cyclodextrin with regard to some monoterpenoids under gas-liquid chromatographic conditions

Gas-liquid chromatography was applied to investigate the mechanism of alpha-cyclodextrin (alpha-CD) complexation processes with some chiral monoterpenoids differing from each other in chemical properties and structure. They were chosen from hydrocarbons, alcohols, aldehydes and ketones of acyclic, monocyclic and bicyclic structure. The relationships between the retention factor, k, of a guest solute (G) and alpha-CD concentration were studied. The obtained data enabled the stoichiometry, the stability of individual complexes and the separation factor of enantiomers to be determined. It was found that almost all the investigated monoterpenoids, apart from the acyclic ones, form inclusion complexes with alpha-CD. Straight-line relations (r vs. [alpha-CD]) were observed for monocyclic alcohols and pulegone, without any trace of enantioselectivity. This behaviour indicates that the 1:1 stoichiometry of the G-CD complexes does not lead to chiral recognition. Parabolic relations arising from 1:2 stoichiometry were found for limonene, alpha-phellandrene, some monocycylic ketones and all the investigated bicyclic terpenoids. It appeared that only the second step of complexation displayed marked enantioselectivity. However, a loss of efficiency resulting from slower equilibration is then noticeable. Attempts are made to rationalize the chromatographic results with respect to the structure of the investigated compounds.     

Influence of pressure on the chromatographic behavior of insulin variants under nonlinear conditions

The effect of pressure on the chromatographic behavior of two insulin variants in RPLC was investigated on a YMC-ODS C18 column, under nonlinear conditions. The adsorption isotherm data of porcine insulin and Lispro were measured at average column pressures ranging from 52 to 242 bar. These data fit well to the Toth and the bi-Langmuir isotherm models. The saturation capacity increases rapidly with increasing pressure while the affinity (or equilibrium) constant and the parameter characterizing the surface heterogeneity decrease. It is noteworthy that the distribution coefficient of the insulin variants increases with increasing pressure whereas their equilibrium constant b decreases for porcine insulin and increases for Lispro. The association constant b(ds), which characterizes the adsorption and desorption equilibrium of insulin in the system, increases with increasing pressure. The excellent agreement between the experimental overloaded profiles recorded under different pressures and those calculated using the POR model suggests that the chromatographic behavior of insulin is controlled more by equilibrium thermodynamics than by the mass transfer kinetics. The latter seems to be nearly independent of the average column pressure. Thus, increasing the average column pressure is an efficient, albeit costly, way to increase the loading capacity of the column, hence the production rate in preparative chromatography.     

Isomerization of isopropenylcyclobutane on silica gel under the conditions of chromatographic adsorptional analysis

Summary It was shown that, under the conditions of chromatographic adsorptional analysis on silica gel, isopropenylcyciobutane is isomerized with enlargement of the ring.     

Estimation of chromatographic parameters on two silica-based columns connected in series under nonaqueous reversed phase liquid chromatographic conditions

Empirical equations were produced to relate important chromatographic parameters on two silica-based columns serially linked, in isocratic nonaqueous RP HPLC, to retention times and peak widths of the separated compounds on the individual columns. These equations were derived because the experimental data seemed to deviate from the values expected, applying basic chromatographic theoretical equations. The chromatographic parameters studied were retention time, peak width, resolution, number of theoretical plates, capacity factor, and separation factor. In addition, empirical linear relationships were produced for the estimation of the above mentioned parameters of the serial systems, in direct and reverse order, relating them to those obtained on each column, separately. The experimentally obtained values were in good agreement with those estimated by the derived equations.     

"Non-evaporating" microdroplets on self-assembled monolayer surfaces under ambient conditions

A unique "non-evaporation" phenomenon, i.e., the unusually slow evaporation process of sessile microdroplets on self-assembled monolayer (SAM) surfaces, is reported. It has been observed that only droplets containing a certain proportion of a volatile and a less-volatile component undergo non-evaporation, which is characterized by hours-long existence of the droplets maintaining constant contact angle, contact area, and volume. We propose that for alcohol-water binary mixtures on SAM surfaces, the highly orientated and closely packed hydrophobic 1-decanethiol molecules induce a concentration gradient of alcohol in water, with a higher concentration of alcohol near the SAM surface. Initial evaporation of the alcohol (more volatile) increased the contact angle until the establishment of a new composition, which contains a strong hydrogen-bonding network among the water molecules in the presence of the alcohol alkyl chains. There is a lessened tendency for the alcohol to evaporate in the presence of a concentration gradient due to such interactions, which results in the observed "non-evaporating" phenomenon. This type of unusual evaporating profile was not observed on conventional substrates, such as polycarbonate sheets and microscope glass slides modified with alkyltrichlorosilanes.     

Highly rapid and direct synthesis of monoacylated piperazine derivatives from carboxylic acids under mild conditions

A series of monoacylated piperazine derivatives were obtained by the reaction of carboxylic acids with 2-chloro-4,6-dimethoxy-1,3,5-triazine in dichloromethane at room temperature. Good to excellent yields, short reaction times and mild reaction conditions are important features of this methodology.     

Effect of the topology of benzotriazole derivatives on their chromatographic retention under RP-HPLC conditions

Sixth order connectivity indices are calculated for 29 benzotriazole derivatives. Correlations between the connectivity indices, physicochemical characteristics, and retention factors of the mentioned compounds are obtained for reversed-phase high-performance liquid chromatography conditions. Based on the correlations obtained, a predictive calculation of the retention factors of some benzotriazole derivatives is performed.     

Using ferroelectric poling to change adsorption on oxide surfaces

Adsorption has been invoked to explain many phenomena in ferroelectric materials including the unanticipated stability of ultrathin ferroelectric films; however, the intrinsic surface properties of ferroelectric oxides have been largely unexplored. Therefore, the effect of ferroelectric poling on the adsorption/desorption of two polar molecules, acetic acid and 2-propanol, and one nonpolar molecule, dodecane, on LiNbO3(0001) was compared. The two polar molecules were found to adsorb significantly more strongly on the positive surface. Temperature-programmed desorption (TPD) data yielded desorption pre-exponentials of the two polar molecules more than 11 orders of magnitude lower than expected. Ferroelectric materials are also intrinsically pyroelectric, and it is shown that the unusually low desorption pre-exponentials can be explained by temperature dependent heats of adsorption that result from changes in the surface dipole as the samples are heated. This conclusion was supported by dodecane adsorption/desorption, which was independent of polarity with normal desorption pre-exponentials. The differences between the polar and nonpolar molecules indicate that interactions between polar molecules and ferroelectric surfaces are dominated by electrostatics. It is shown that adsorption energy differences between positive and negative surfaces are large enough to switch the polarity of ferroelectric thin films.     

Force measurements on platelet surfaces with high spatial resolution under physiological conditions

Investigations on platelets are essential to understanding the regulation of hemostasis and thrombosis. Activated platelets undergo dramatic conformational and morphological changes mediated by numerous plasma proteins. AFM techniques can combine high spatial resolution with measurements of the mechanical properties of platelet surfaces. Here, we demonstrate two-dimensional force mapping over a human platelet adsorbed on glass under physiological buffer. The best resolution of platelet membrane elasticity we obtained was at 15.6×15.6 nm² pixel⁻¹. In addition, quantitative information on platelet surface charge density was extracted from individual force curves with the aid of DLVO theory.