DFT studies of the structure, vibrational and NMR spectra of 2-amino-pyridine betaine monohydrate

Four of the most stable conformers of 2-amino-pyridine betaine (1-carboxymethyl-2-amino-pyridinium inner salt) monohydrates, 2-NH₂PB·H₂O, and one anhydrous were analyzed by the B3LYP/6-31G(d,p) calculations and compared with the X-ray data. Two types of optimized conformers can be distinguished: (a) with NH₂ and COO groups and (b) an imino tautomer with NH and COOH groups. A common feature of the optimized molecules are intramolecular hydrogen bonds between the COO⁻ and H₂N or COOH and HN groups. In the crystal both NH₂ and COO groups participate in intermolecular hydrogen bonds. The probable assignments of the anharmonic experimental solid state vibrational frequencies of 2-NH₂PB·H₂O and 2-ND₂PB·D₂O (conformer 2) based on the calculated B3LYP/6-31G(d,p) harmonic frequencies have been made. Correlations between experimental chemical shifts for 2-NH₂PB, its hydrochloride and 1-carboxyethyl-2-amino-pyridinium inner salt (¹³C and ¹H in D₂O) and GIAO/B3LYP/6-31G(d,p) calculated isotropic shielding constants, δ_exp=a+bσ_calc, are reported. Good linear regression between experimental and theoretical results for ¹³C was obtained. Only in 2-NH₂PB the hydrogen at -position is outside the linear correlation.     

Rydberg states of the K2 molecule studied by laser spectroscopy in a supersonic beam

Rydberg states of potassium dimer have been studied in a crossed laser-molecular beam experiment. The K₂ molecules were formed in a supersonic expansion and excited by low-power cw dye laser. Two different excitation schemes have been used: The first scheme uses a single mode ring dye laser to induce near resonant two-photon transitions while in the second scheme stepwise excitation with two dye lasers is used. In each case excitation of Rydberg levels was detected by monitoring the ionization signal resulting from three-photon absorption. We report a detailed study of 700 two-photon resonances between 625 nm and 650 nm. Most of these signals can be assigned to transitions from the X¹σ _g ⁺ to¹σ _g ⁺ ,¹Π _g , and^1Δ _g states, which are all enhanced by the B¹Π _u intermediate state. Accurate rotational constants are given for the populated vibrational levels of these states. By stepwise excitation of Rydberg levels via theB ¹Π _u state we identify 3 series of Rydberg states as¹Δ _g (4S+nD),¹Σ _g ⁺ (4S+nD), and¹Σ _g ⁺ (4S+nS) with principal quantum numbers 7≦n≦20. Molecular constants of these and other observed but as yet unidentified states are given; quantum defects and dissociation energies are discussed.     

Evaluation of loop and pump injection systems in preparative liquid chromatography

The usefulness of pump or loop injectors in preparative liquid chromatography was evaluated. The concentration distribution of samples flowing through a UV detector cell, directly connected to the injector, was recorded at various flow velocities, at various diameters of loop tubing, and using various methods of injection [injection with sample volumes equal to a fraction or total loop volume, etc.]. The most advantageous methods were found to be either use of a loop injector to inject only a fraction of its total volume or use of a pump. Both of these methods ensure a almost rectangular concentration distribution.     

A Simple Method of the Determination of the Structural Heterogeneity of Microporous Solids

A new procedure for determining the structural heterogeneity of microporous solids is presented. Applying the concept of a general adsorption isotherm equation, a method for solving this equation for adsorption in micropores is developed, and the applicability of a new simple algorithm for determining the parameters of microporous structure of two commercial carbons is shown. Copyright 2001 Academic Press.     

Modeling of the hysteresis phenomena in finite-sized slitlike nanopores. Revision of the recent results by rigorous numerical analysis

The systematic investigation of the hysteresis phenomena in finite-sized slitlike nanopores via the Aranovich-Donohue (AD) lattice density functional theory (LDFT) is presented. The new reliable quantitative modeling of the adsorption and desorption branch of the hysteresis loop, through the formation and movement of the curved meniscus, is formulated. As a result, we find that our proposal, which closely mimics the experimental findings, can reproduce a rounded shape of the desorption branch of the hysteresis loop. On the basis of the exhausted commutations, we proved that the hysteresis loop obtained in the considered finite-sized slitlike geometry is of the H1 type of the IUPAC classification. This fundamental result and the other most important results do not confirm the results of the recent studies of Sangwichien et al., whereas they fully agree with the recent lattice studies due to Monson et al. We recognize that the nature of the hysteresis loops (i.e. position, width, shape, and the multiple steps) mainly depends on the value of the energy of both the adsorbate-adsorbate and adsorbate-adsorbent interactions; however, the first one is critical for the appearance of hysteresis. Thus, for relatively small adsorbate-adsorbate interactions, the adsorption-desorption process is fully reversible in the whole region of the bulk density. We show that the strong adsorbate-adsorbent interactions produce (also observed experimentally) multiple steps within hysteresis loops. Contrary to the other studies of the hysteresis phenomena in confined geometry via the LDFT formalism, we constructed both ascending and descending scanning curves, which are known from the experimental observations. Additionally, we consider the problem of the stability of both the obtained adsorption and desorption branches of the computed hysteresis loop in finite-sized slitlike nanopores.     

5-Dethia-5-oxacephams: toward correlation of absolute configuration and chiroptical properties

The relationship between chiroptical properties of differently substituted 5-dethia-5-oxacephams and their respective molecular structures was investigated. The amide chromophore of the beta-lactam unit in these compounds was found to be nonplanar with a shallow pyramidal configuration at the nitrogen atom. Due to the nonplanarity, the beta-lactam system becomes inherently dissymmetric, which is supported by a high magnitude of the n --> pi* CD band. It was also found that the helicity of the lactam moiety in investigated oxacephams is controlled by the absolute configuration at the C(6) carbon atom. On this basis, a helicity rule correlating a positive (negative) sign of the n right arrow pi Cotton effect with a negative (positive) O [double bond] C [bond] N [bond] C(6) torsional angle for policyclic beta-lactam derivatives possessing a nonplanar amide chromophore was formulated.     

Synthesis of oligosaccharides as potential novel food components and upscaled enzymatic reaction employing the β-galactosidase from bovine testes

The β-galactosidase from bovine testes (EC 3.2.1.23) promotes the transfer of a galactose unit to glucose or galactose-containing residues in manifold derivatives, establishing β1→3 linkages.The synthesis of several potentially biologically important oligosaccharides β-d-Galp-(1→3)-α-d-Glcp-(1→2)-β-d-Fruf2, β-d-Galp-(1→3)-β-d-Galp-(1→4)-α,β-d-Glcp4, β-d-Galp-(1→3)-α-d-Glcp-(1→4)-d-Glcp-ol/Manp-ol 6, β-d-Galp-(1→3)-α-d-Glcp-(1→6)-β-d-Fruf8, β-d-Galp-(1→3)-α-d-Glcp-(1→6)-[α-d-Glcp-(1↔2)]-β-d-Fruf10, α-d-Galp-(1→6)-[β-d-Galp-(1→3)]-α-d-Glcp-(1↔2)-β-d-Fruf12, β-d-Galp-(1→3)-α-d-Glcp-(1↔2)-β-d-Fruf-(1↔2)-β-d-Fruf14 has been reached in yields between 7 and 44% by implementation of this specific enzyme. In addition, we found that it is feasible to gain high yields without an enzyme-specific buffer and even making upscaled preparation on a gram scale.     

Improvement of the Derjaguin-Broekhoff-de Boer theory for capillary condensation/evaporation of nitrogen in mesoporous systems and its implications for pore size analysis of MCM-41 silicas and related materials

In this work, we propose an improvement of the classical Derjaguin-Broekhoff-de Boer (DBdB) theory for capillary condensation/evaporation in mesoporous systems. The primary idea of this improvement is to employ the Gibbs-Tolman-Koenig-Buff equation to predict the surface tension changes in mesopores. In addition, the statistical film thickness (so-called t-curve) evaluated accurately on the basis of the adsorption isotherms measured for the MCM-41 materials is used instead of the originally proposed t-curve (to take into account the excess of the chemical potential due to the surface forces). It is shown that the aforementioned modifications of the original DBdB theory have significant implications for the pore size analysis of mesoporous solids. To verify our improvement of the DBdB pore size analysis method (IDBdB), a series of the calcined MCM-41 samples, which are well-defined materials with hexagonally ordered cylindrical mesopores, were used for the evaluation of the pore size distributions. The correlation of the IDBdB method with the empirically calibrated Kruk-Jaroniec-Sayari (KJS) relationship is very good in the range of small mesopores. So, a major advantage of the IDBdB method is its applicability for small mesopores as well as for the mesopore range beyond that established by the KJS calibration, i.e., for mesopore radii greater than approximately 4.5 nm. The comparison of the IDBdB results with experimental data reported by Kruk and Jaroniec for capillary condensation/evaporation as well as with the results from nonlocal density functional theory developed by Neimark et al. clearly justifies our approach. Note that the proposed improvement of the classical DBdB method preserves its original simplicity and simultaneously ensures a significant improvement of the pore size analysis, which is confirmed by the independent estimation of the mean pore size by the powder X-ray diffraction method.     

Synthesis of polyfunctional phosphorodithioates and structural analogues mediated by azetidinium ions and epoxides

The nucleophilic ring opening reactions of N,N-dialkyl-3-hydroxy(benzyloxy)azetidinium salts 1 and N,N-dibenzyl-2,3-epoxypropylamine 10 with anions of mono- and dithioacids of phosphorus 2 affords phosphorodithioates (and structural analogues) 3 and 11 possessing S-propyl substituents incorporating 2-hydroxy and 3-dialkylamino functionalities. A strong dependence of the stability of these compounds on structural factors is discussed.