Selectivity in normal-phase liquid chromatography with binary mobile phase

A simple theoretical approach to normal-phase liquid chromatography with binary mobile phase is presented. In the model used the driving force for the retention is competitive adsorption of solutes and both solvents. A new expression for the separation factor is proposed and discussed. An influence of different parameters on the selectivity is shown. The theoretical concepts are illustrated by the selected experimental data.     

A New Application of Quasimonotone Sequences

We prove a general theorem dealing with the generalized absolute Cesàro summability factors of infinite series. This theorem also includes some new and known results.     

Optimisation and validation of liquid chromatographic and partial least-squares-1 methods for simultaneous determination of enalapril maleate and nitrendipine in pharmaceutical preparations

Simultaneous determination of enalapril maleate (ENA) and nitrendipine (NIT) in pharmaceutical preparations was performed using liquid chromatography (LC) and the partial least-squares-1 (PLS-1) method. In LC, the separation was achieved on a C8 column and the optimum mobile phase for good separation in a gradient elution programme was found to be acetonitrile-water (φ _r = 81: 19) and optimum flow-rate, temperature, injection volume, and detection wavelength were set at 1.0 mL min⁻¹, 25°C, 10 μL, and 210 nm, respectively. Dienogest was selected as an internal standard. In the spectrophotometry, a PLS-1 chemometric method was used. The absorbance data matrix related to the concentration data matrix was established by measurement of absorbances in their zero order spectra with an increment of Δλ = 1 nm in the 220–290 nm range for ENA and with Δλ = 1 nm in the 230–290 nm range for NIT in the PLS-1 method. Following this step, calibration was established by using this data matrix to predict the unknown concentrations of ENA and NIT in their binary mixture. These optimised methods were validated and successfully applied to a pharmaceutical preparation in tablet form and the results were subjected to comparison.     

Phase behavior of linear heterogeneous trimers on a square lattice

Monte Carlo simulations in the grand canonical ensemble, the multiple-histogram analysis and finite-size scaling techniques have been used to study a phase behavior of trimer BAB on a square lattice. The systems with the same energies u(AA) = u(BB) and different strengths of interactions between unlike segments are considered. The AB-contacts are energetically unprofitable. There are two phase transitions: the first-order vapor-liquid transition and the second-order structural transition in the supercritical fluid. The phase diagram topology depends on the energy u(AB). The crossover between the tricritical point phase diagram topology and the critical end phase diagram topology is found. It is demonstrated that the transition to the ordered strip-like phase is non-universal.     

On triazoles XLII . A new convenient method for the N‐alkylation of highly insoluble cyclic amides

A simple N‐alkylation method of highly insoluble cyclic amides based on the high solubility of their easily isolable tetraalkylammonium and tetraalkylphosphonium salts was elaborated. The method has a rather wide scope, it is not influenced by the identity of the different rings attached to the 1,2,4‐triazolo[1,5‐a]‐pyrimidinone moiety of isomers 1 and 2 , nor the presence of the triazole substituents. It proceeds smoothly without any unwanted by‐products, at relatively low temperatures, and is not sensitive to moisture. The method allows an easy isolation of all possible N‐alkylated derivatives 3, 7 , and 8 . Spectral analysis of isomers 3, 7 , and 8 showed that our previous results concerning the formation of 3 type N‐alkylated derivatives as main products of the N‐alkylations as well as the tautomeric structure of the non‐alkylated isomers 1 and 2 is correct. However, it also showed that the isolation of a single N‐alkylated isomer 3 and its comparison with the corresponding non‐alkylated derivative to prove its tautomeric structure may lead to mistakes.     

Cell rotation graphs of strongly connected orientations of plane graphs with an application

The cell rotation graph D(G) on the strongly connected orientations of a 2-edge-connected plane graph G is defined. It is shown that D(G) is a directed forest and every component is an in-tree with one root; if T is a component of D(G), the reversions of all orientations in T induce a component of D(G), denoted by T⁻, thus (T,T⁻) is called a pair of in-trees of D(G); G is Eulerian if and only if D(G) has an odd number of components (all Eulerian orientations of G induce the same component of D(G)); the width and height of T are equal to that of T⁻, respectively. Further it is shown that the pair of directed tree structures on the perfect matchings of a plane elementary bipartite graph G coincide with a pair of in-trees of D(G). Accordingly, such a pair of in-trees on the perfect matchings of any plane bipartite graph have the same width and height.     

Coloring of distance graphs with intervals as distance sets

Let D be a set of positive integers. The distance graph G(Z,D) with distance set D is the graph with vertex set Z in which two vertices x,y are adjacent if and only if |x−y|D. The fractional chromatic number, the chromatic number, and the circular chromatic number of G(Z,D) for various D have been extensively studied recently. In this paper, we investigate the fractional chromatic number, the chromatic number, and the circular chromatic number of the distance graphs with the distance sets of the form D_m,[k,k^′]={1,2,…,m}−{k,k+1,…,k^′}, where m, k, and k^′ are natural numbers with m≥k^′≥k. In particular, we completely determine the chromatic number of G(Z,D_m,[2,k^′]) for arbitrary m, and k^′.     

The Vinylic SN Reactions of Nitrodienes with Heteroatom‐Substituted Nuchleophilies and Their Structural Studies

Herein, we report the reactions of 1,1,2,4,4‐pentachloro‐3‐nitro‐1,3‐butadiene 1a and (1Z)‐1‐bromo‐1,2,4,4‐tetrachloro‐3‐nitro‐1,3‐butadiene 1b with nitrogen‐ and sulfur‐containing nucleophiles to obtain highly functionalized S‐, S,S‐, S,S,S‐, S,O‐ and N,S‐substituted‐polyhalodiene‐3‐nitro‐1,3‐butadiene derivatives. Most of these reactions turned out to be highly selective with good to very good yields. All new compounds have been characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and Fourier transform infrared spectroscopy spectroscopic data. The molecular structures of the 3a and 21a due to its exceptional substitution pattern were evidenced by the X‐ray single‐crystal diffraction method.