Anab initio quantum-chemical study of proton addition to F-, Ch3-, and CF3-substituted ethylene derivatives

Protonated forms of the molecules of ethylene derivatives with the general formula C₂X₂Y₂ (X=Y=H) (1), F (2), CH₃ (3) CH₃ (4); X=F, Y=H:cis-(5)trans- (6)) were calculated by theab initio MP2/6-31 G^* method with full geometry optimization. The minima and saddle points located on the potential energy surface (PES) of the protonated ethylene molecule correspond to the stationary states and transition states of proton migration, respectively. The stationary states are characterized by a nonclassical geometry of carbocations similar to that of π-complexes, whereas the transition states have a classical structure. Unlike1, the carbocations of molecules2–6 have the classical structure. The saddle points on the PES of the ethylene derivatives correspond to the structures of the π-complex type, which are the transition states of proton migration between the C atoms of the ethylene bond. The barrier to rotation about, the C−C bond depends essentially on the substituent nature. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1333–1337, August, 2000.     

Evaluation of novel dendrimer chiral stationary phases using HPLC

Summary The reversed phase chromatographic properties of the [G1]-L-glutamic and ethyl ester-AC-silica (1), [G2]-L-glutamic acid ethyl ester-AC-silica (2) and the [G1]-L-glutamic acidt-butyl ester-AC-silica (3) dendrimer stationary phases were evaluated. Initial studies involved the comparison between these phases with a classic reversed phase (i.e. ODS1) by the separation of a standard reversed phase test mixture composed of dimethylphthalate, nitrobenzene, anisole, diphenylamine and fluorene. Separations were achieved with comparable performance to those obtained with the conventional reversed phase (ODS1). However, it was apparent that the chromatographic selectivity exhibited by the dendrimer stationary phases was different from that of the ODS1 phase. On a per mole basis, the dendrimers exhibited similar (and sometimes greater) affinity for these analytes compared with the ODS1 ligand. Subsequent chromatographic experiments were conducted upon the dendrimer chiral stationary phases using chiral analytes under reversed phase and normal phase conditions. Chiral resolution was not observed.     

Role of the Dissolution Mechanism in Determining Chromatographic Separation of Biogenic Amines from Their Precursors and Metabolites by HPLC-EC

Abstract

The current study was designed to elucidate the theoretical basis for chromatographic separation of biogenic amines on an octadecyl-silica (C-18) reverse phase column by determining the intermolecular forces between the solute and the stationary phase. The solutes mass transfer diffusion and the heat effect between solutes and stationary phase were assessed by a convenient method. This study demonstrates that the dissolution mechanism plays a major role in the process of chromatographic resolution of biogenic amines and their precursors and metabolites by HPLC-EC.     

High-Performance Liquid Chromatography of some Bases and Nucleosides on Alkylphosphonate-Modified Magnesia-Zirconia Column

ABSTRACT

The high-performance liquid chromatographic behavior of some bases and nucleosides was studied on a new reversed-phase stationary phase, alkylphosphonate-modified magnesia-zirconia. The effect of mobile phase variable such as methanol content, ionic strength and pH on their behavior was investigated. It was found that the retention behavior of the bases and nucleosides on the new stationary phase is similar to that on ODS stationary phase. The retention mechanism on the new stationary phase was also discussed. The separation of some bases and nucleosides was accomplished on the new stationary phase.     

High-Performance Liquid Chromatography of Copper,Mercury, Nickel,and Cadmium Bisdibenzyldithiocarbamate Complexes

Abstract

High-performance liquid chromatographic separations of Cu (II), Hg(II), Ni(II), and Cd(II) bisdibenzyldithiocarbamates at nanogram levels by adsorption chromatography on silica gel were reported. Lichrosorb Si 60 (10 um) was used as the stationary phase and benzene-cyclohexane mixtures as the mobile phase.     

Ionisation Effects in the Reversed Phase Liquid Chromatographic Separation of Thyromimetic Iodoamino Acids

Abstract

The influence of ionisation equilibria on the retention behaviour of iodoamino acids and related compounds on micro-particulate octadecylsilica supports has been examined. The chromatographic data for these ionogenic solutes have been discussed in terms of current concepts for reversible solvophobic interactions with the hydrocarbonaceous stationary phase. This treatment permits the conditional effects of the mobile phase composition and pH on solute retention to be assessed and the relationship between the molecular surface area of a solute and its retention to a non polar stationary phase evaluated.     

Preparation and Evaluation of P-tert-Butyl-calix[8]arene-bonded Silica Stationary Phase for High Performance Liquid Chromatography

ABSTRACT

A new p-fert-butyl-calix[8]arene-bonded silica gel stationary phase was synthesized through heterogeneous functionalisation of suspended porous silica. A characterization of its structure was carried out by using elemental analysis, FTIR and ¹³C solid state NMR spectroscopy. Chromatographic performance of the new packing material was investigated by employing polycyclic aromatic hydrocarbons (PAHs) as probes and using methanol-water as mobile phase. The investigations show that the new stationary phase behaves as a reversed phase stationary phase. The liquid chromatographic separation of PAHs solutes on the new bonded phase was compared with that on a p-tert-butyl-calix[4]arene-bonded silica stationary phase. The new p-tert-butyl-calix[8]arene-bonded phase exhibited higher retention and better separation selectivity, although the carbon content and coverage of the new packing material was lower than that of the p-tert-butyl-calix[4]arene bonded silica stationary phase. A possible retention mechanism for the new packing material was also proposed.     

Resolution of naftopidil and bufuralol enantiomers by high-performance liquid chromatography on cellulose tris-(3,5-dimethylphenyl carbamate) column

The liquid Chromatographic resolution of the racemic cardiovascular drugs naftopidil and bufuralol to their corresponding enantiomers was achieved on cellulose tris-(3,5-dimethylphenyl carbamate) chiral stationary phase known as Chiralcel OD. The chiral recognition mechanism(s) involved between the chiral stationary phase and these drugs, which include hydrogen bonding, intercalative interactions, and steric interactions, among other factors, were discussed.     

HPLC of aminoacids on silica coated with ammonium tungstophosphate

Summary The chromatographic behaviour of aminoacids on columns of silica coated with different percentages of ammonium tungstophosphate at several buffer phosphate concentrations is reported. The stationary phase shows a high selectivity towards aminobutyric acids and methyl derivatives of tryptophan due to the distance between the carboxylic group and the –NH ₃ ⁺ group involved in the cation-exchange process and/or to the steric hindrance by a methyl substituent. A relationship between HPLC and HPTLC data has been verified and applied to the prediction of the separation of six aminoacids.     

Centrifugal Partition Chromatography. V. Octanol-Water Partition Coefficients,Direct and Indirect Determination

Abstract

Octanol-water partition coefficients (K_oct) are one of the accepted physico-chemical parameters for predicting the biological effects of organic chemicals. It is demonstrated that centrifugal partition chromatography (CPC) can be used to determine directly the K_oct values from about 0.003 to 300. The mobile phase must be water and the stationary phase must be octanol for accurate determination of the larger K_oct values. To reduce retention times and volumes, the stationary phase volume can be decreased. This can be done using an original mode: the “underload” mode. The “underloading” procedure is described. To extend the K_oct range which can be determine by CPC, the system hexane (60%) octanol (40%)-water was used. A single linear relationship between log K in this system and log K_oct was established allowing to determine the K_oct values up to 1000 (log K_oct=3).     

Electrochemically Controlled Liquid Chromatography on Conducting Polymer Stationary Phases

Abstract

Electrochemically controlled high performance liquid chromatography (EC/HPLC) using a conducting polymer stationary phase, has been investigated in the present paper. Polypyrrole coated reticulated vitreous carbon (RVC) particles have been employed as the stationary phase. Chromatographic characterization has been carried out using various molecular probes. The results indicate that chromatographic retention can be altered by the imposition of small electrical potentials on the stationary phase and that such effects can be used to improve the separation of certain compounds.     

Preparation and Characterization of Amine Modified Porous Polymer for Rplc

Abstract

The retention behaviour and selectivity of various aromatic compounds, including halobenzenes and a IkyIbenzenes, were examined on the stationary phase of chloromethylated polystyrene(CMS) beads, of which the chloride group was substituted by the alkylamines: n-hexyl(C₆), n-octyl(C₈), n-decyl (C₁₀),n-dodecyl (C₁₂), n-tetradecyl (C₁₄) or n-hexadecy1 (C₁₆). Changes in the distribution coefficients of benzene derivatives are demonstrated not only by the changes of the eluent compositions but also by the changes of chain length in the stationary phase. In general, a longer alkyl chain in the stationary phase gives longer retention time and different selectivity. The effects of MeOH/H₂O ratios on retention and selectivity was also demonstrated on the C₁₄ stationary phase. Good separations of halobenzenes and alkyIbenzenes were obtained with C₁₄ stationary phase. Moreover, the retention time for samples was shortened by about a half compared to polystyrene stationary phase. These results show the chemical modification of CMS gel with long chain alkyl amines decreases tailing and provides new supports with different selectivities.     

Enthalpy-Entropy Compensation of Octylsilica Stationary Phase in Reversed-Phase HPLC

Abstract

Enthalpy-entropy compensation has been investigated in reversed-phase high performance liquid chromatography with octylsilica stationary phase. The compensation temperatures were determined for this system, and the results show that their change with the composition of the mobile phase was almost similar to that with octadecylsilica stationary phase. It is concluded that the retention mechanism of the separation of alkylbenzenes in both systems with the mobile phase exceed 20% water content is the same.     

Synthesis and Silica‐Based Immobilization of Monofunctionalized Heptakis(2,6‐di‐O‐methyl‐3‐O‐pentyl)‐β‐cyclodextrin for Enantioselective Capillary‐HPLC

Heptakis(2,6‐di‐O‐methyl‐3‐O‐pentyl)‐β‐cyclodextrin was monofunctionalized by the regioselective introduction of exactly one ω‐epoxyoctyl group at the primary site of the cyclodextrin. The site‐specifically substituted cyclodextrin was immobilized to commercially available aminopropyl silica by nucleophilic opening of the epoxy function of the spacer substituent resulting in a lipophilic chiral stationary phase with broad applicability for enantiomer separations in capillary‐HPLC under reversed‐phase conditions.     

Separation of Free Amiimo Acids on Reverse Stationary Phases Using an Alkyl Sulfonate Salt as a Mobile Phase Additive

Abstract

Alkylsulfonate (RSO₃ ^?) salts were evaluated as mobile phase additives for the separation of free amino acids on reverse stationary phases using an acidic mobile phase where the amino acids are cations. The enhanced amino acid retention is the result of two major interactions, one being retention of the RSO₃ ^? salt on the stationary phase and the other an ion exchange selectivity between the amino acid analyte cation and the RSO₃ ^? countercation, or other countercations in the mobile phase. Major mobile phase variables are: type and concentration of RSO₃ ^? salt (the studies focused on C₈SO₃ ^? salts), presence of organic modifier, type of countercation present, and mobile phase pH and ionic strength. Alkyl modified silica and polystyrenedivinyl-benzene copolymeric reverse stationary phases were compared. A mobile phase gradient, increasing per cent organic modifier was shown to be best, is necessary for separating complex mixtures of polar and nonpolar or basic amino acids. The procedure is applicable to the identification and/or determination of amino acids in mixtures or in peptides after hydrolysis.     

Effect of position of the lateral fluoro substituent on the mesophase behaviour of aryl 4-alkoxyphenylazo benzoates in pure and binary mixtures

Five groups of 4-substituted phenyl 4?-(2?- (or 3″-) substituted-4?-alkoxyphenylazo) benzoates (In_a-c to Vn_a-c) were investigated in which, within each group, the length of the terminal alkoxy group varies between 8 and 16 carbons, while the other terminal substituent, X, is a polar group that alternatively changed between the electron-donating CH₃O and the electron-withdrawing Br group, in addition to the un-substituted analogue (X = H). The lateral substituent (Y) in the five groups I–V varies, respectively, between H, 3-CH₃, 2-CH₃, 3-F and 2-F. Their mesophase stabilities were determined by differential scanning calorimetry and phases identified by polarised light microscopy. The two newly prepared groups of compounds (IVn_a-c and Vn_a-c) are structurally characterised by infrared, ¹H-NMR, mass spectroscopy, thermogravimetric and elemental analyses. Binary phase diagrams were constructed for each pair of isomers from groups IV and V bearing the same wing substituents but the lateral F is attached to different positions (2? or 3″).     

HPLC Enantioseparation of 1-(α-Aminobenzyl)-2-naphthol and 2-(α-Aminobenzyl)-1-naphthol Analogs on a β-Cyclodextrin-Based Chiral Stationary Phase

The enantiomers of 1-(α-aminobenzyl)-2-naphthol and 2-(α-aminobenzyl)-1-naphthol analogs were separated isothermally on a 3,5-dimethylphenylcarbamoylated β-cyclodextrin-based chiral stationary phase (Cyclobond DMP), with an n-hexane/alcohol modifier as mobile phase. Optimization of the separation was achieved by variation of combinations of the polar mobile phase additives ethanol and methanol. The nature and position of the α-aminobenzyl substituent of the 1- and 2-naphthol analogs influenced the retention and the selectivity.     

Chitin as Stationary Phase in Thin-Layer Chromatography–Application of Chromatographic Process Optimization Theory for Chitin Layers

Abstract

This paper describes initial of two basic theories of adsorption thin-layer chromatography potimization namely thermodynamic adsorption theory and theory based on mass action law, so called Snyder s theory in chromatographic systems containing chitin as stationary phase on wchich various amino acids were chromatographed. The results obtained were comparable to those obtained for analogous chromatographic systems containing silica gel as stationary phase. Comparison of applicability of these theories for well investigated silica gel and for not enough investigate chitin will permit to draw introductory conclusions about the processes carrying in the chromatographic systems containing chitin as stationary phase.     

Theory and Use of Centrifugal Partition Chromatography

Abstract

Centrifugal Partition Chromatography (CPC) is a variant of countercurrent chromatography (CCC). As in CCC, two immiscible liquids are used. The first liquid is the stationary phase, the second is the mobile phase. The liquid stationary phase is held in channels engraved in seveal polychlorotrifluoroethylene (PCTFE) plates. One hundred channels are engraved on each PCTFE plate. Four PCTFE plates are assembled together in a cartridge. Up to 12 cartridges (4800 channels) can be loaded in the rotor of a centrifuge. The centrifugal field, generated by the spinning rotor, holds the stationary phase sufficiently that a mobile phase can be pumped through in (Figure 1). The system is analyzed in detail.     

Liquid Chromatographic and Mass Spectral Analysis of N,N-Disubstituted 3,4-Methylenedioxyamphetamines

Abstract

The tertiary amine N,N-disubstituted derivatives of 3,4-methylenedioxyamphetamine (MDA) were prepared and their liquic chromatographic and mass spectral properties compared to other analogues of 3,4-methylenedioxyamphetamine. The N-methyl-N-ethyl, N-methyl-N-i-propyl and N-methyl-N-n-propyl MDA derivatives were separated in a reversed-phase system consisting of a C₁₈ stationary phase and a mobile phase of pH 4 phosphate buffer-acetonitrile (55:45). The mass spectral fragmentation of these amines is similar to that observed for the secondary amine MDA derivatives and can be used for their specific structural identification.