Solute Retention in Column Liquid Chromatography. VI. Enthalphy Entropy Compensation: The Column Temperature and Mobile-Phase Composition

Abstract

The concept of enthalpy-entropy compensation in “non-polar” reverse-phase (ODS) column liquid chromatography is examined in terms of the column temperature (10[ddot]-35[ddot]C) as well as mobile-phase composition (acidified water/tetrahydrofuran) for the solutes: phenol, p-nitrophenol, 2,4-dinitrophenol, o-chlorophenol, o-nitrophenol, 2,4-dimethylphenol, 2,4- dinitro-o-cresol, 4-chloro-m-cresol, and 2,4-dichlorophenol. Plots of log (solute capacity factor, k') against reciprocal column temperature, T^?1 exhibit approximate linearity only over the volume-fraction based mobile phase compositional range ?THF = 0 to 0.2. Substantial deviations occur beyond ?THF = 0.3. Plots of In k' against -ΔH[ddot] (compensation plots) yield a random distribution of data, indicating that what enthalpy-entropy compensation may exist for the systems studied is masked both in terms of temperature and mobile-phase composition. It is concluded that interaction(s) of the solutes with the stationary sorbent are more complex than has heretofore been supposed.     

Solute Retention in Column Liquid Chromatography. II. Optimization of Mobile-Phase Compositions

Abstract

Retentions are reported for nine phenol solutes with a reverse-phase column packing and acidified aqueous mobile phases containing tetrahydrofuran, acetonitrile, and methanol additives. Plots of log(k'_(M)) against additive volume fraction φ exhibit both positive and negative deviations from linearity. In contrast, plots of inverse capacity factor against φ are in every instance convex to the abscissa. The latter, as well as several systems comprising ionic solutes and/or solvents, are fitted exactly by a semi-empirical relation; however, only one of the fitted parameters appears to exhibit identifiable trends and then only very approximately so. It is deduced by simple inspection of the plots and without resorting to higher-component carriers, then verified experimentally, that the test mixture is completely separable with 30% methanol. Window diagrams of a, S_f, and R_s ordinates are used to identify alternative mobile phases and compositions in the instance that one of the solutes is present in large excess. The three methods of data reduction prove to be entirely equivalent since the solute capacity factors approach 10. When this is not the case, window diagrams constructed with the separation-factor parameter are said to be favored.     

Effect of pressure on retention factors in HPLC using a non-porous stationary phase

Summary Pressure and temperature have significant influence on retention in HPLC. This study investigates the effect of pressure and temperature on the retention behavior of aromatic hydrocarbons (toluene, ethyl benzene, butyl benzene, pentyl benzene) and polar, acidic and basic samples (phenol, acetophenone, N,N-dimethyl aniline, benzophenone) on a reversed phase column. The effect has been studied on non-porous, tetradecyl (C₁₄) coated silica particles. We found that the adsorption-induced decrease of the partial molar volume of the solutes investigated was between ΔV _m =5–15 cm³ mol⁻¹. The increment of the decrease of the partial molar volume due to the addition of one CH₂ group, for the homologous series of the aromatic hydrocarbons is approximately ΔV _CH2 =2.3 cm³mol⁻¹. Presented at Balaton Symposium '01 on High-Performance Separation Methods, Siófok, Hungary, September 2–4, 2001.     

Analysis of Thermal Decomposition Patterns of Aromatic and Heteroaromatic Amino Acids

Abstract

Differential enthalpic analysis of the thermal degradation of aromatic and heteroaromatic amino acids provides thermograms which are characteristic of specific amino acids (e.g. phenylalanine, tyrosine, tryptophan or histidine). Curve resolution of the resulting thermograms suggests that a minimum of three reactions are involved in the degradation (within the temperature limits of 500–620 [ddot]K) and provides T_M values for each of these concurrent reactions.     

Contributions to the Mechanism of Isobutene Polymerization. VI. Effect of Halides

The effects of various allyl chlorides and alkyl halides on the overall yield of polymerization and molecular weight of polyisobutene have been investigated and expressed quantitatively by poison and transfer coefficients. The poison and transfer coefficients of halides have been compared with those obtained previously for corresponding hydorcarbons. The poison coefficients of halides and hydrocarbons can be treated formally in a similar manner (1/W_p vs. [X] plots linear for both classes); however, the appropriate transfer coefficients indicate fundamentally dissimilar transfer mechanisms in these systems (1/MW_p vs. [X] plots linear for hydrocarbons, whereas 1/MW_p vs. [X]^1/2 plots linear for halides). These results are discussed in terms of the allylic termination mechanism.     

一种新型羟基葫芦[6]脲气相色谱固定相性能的研究

本文首次成功地制备和利用羟基葫芦[6]脲((HO)12CB[6])作填充柱气相色谱固定相。研究表明,羟基葫芦[6]脲固定相(PSP)具有较宽的操作柱温、高度的化学和热稳定性,对多种类型的化合物展示较高的色谱分离选择性,这包括烷烃、芳烃、醇类、酯类、酮类和胺类等多种化合物。新柱对一些二取代苯环芳族位置异构体有较强的分离能力。实验发现,羟基葫芦[6]脲固定相对日用花露水中复杂的挥发性成分有高效快速分离能力。同时,初步探讨了新固定相的色谱分离机理,发现羟基葫芦[6]脲对溶质的部分包结作用,而不是完全包结作用,有利于提高其色谱分离选择性和柱效。此外,在极速程序升温色谱中,该固定相低流失基线漂移小,有利于实现宽沸点范围复杂样品的快速气相色谱分离分析。     

4-tert-Butylcalix[8]areme as a Stationary Phase in Gas-Solid Chromatography

Abstract

The behavior of 4-tert-butylcalix[8]arene as a stationary phase in gas-solid chromatography has been tested using organic substances of different chemical and physical properties. On a column packed with the macrocycle supported on Chromosorb W Silanized, the separations of alcohols, chlorinated hydrocarbons and aromatic compounds were achieved, although the mechanism of the interactions has not yet elucidated.     

Chemometric characterization of TLC systems of the type silica-binary non-aqueous mobile phase in the analysis of flavonoids

Summary Retention-mobile phase composition relationships have been determined for eighteen flavonoids chromatographed with non-aqueous systems of the type silicadiluent+polar modifier (S). The slopes of the plots depended on the molecular structures of the solutes, primarily on the number of hydroxyl groups. The selectivities and sequences depended on the diluent and modifier applied; plots ofR _M against log [S] (where [S] is the percentage concentration of S) were approximately linear, although deviations from linearity were observed occasionally. The plots enable selection of mobile phases of optimum composition for qualitative and quantitative analysis of given sets of flavonoids.     

Star-shaped block copolymers. III. Surface and interfacial properties

Surface and interfacial activities of A(B)₂ star-shaped block copolymers, where B is a polyoxirane block and A a polydiene or polyvinyl block, have been measured at 20°C. The surface tension of organic solvents is only slightly lowered by these copolymers, whereas a significant surface activity is noted in water. Interfacial tensions are dependent on both the nature of the organic solvent (aliphatic or aromatic hydrocarbons) and the molecular parameters of the copolymers; 50% polyoxirane seems to be the composition of maximum surface activity. The role played by the molecular architecture [A-B or A(B)₂] of the copolymers is demonstrated. The same limiting interfacial tension is obtained on increasing the concentration of diblock [A-B] or star-shaped block [A(B)₂] copolymer. The limiting value is, however, attained at significantly lower concentration with the star-shaped copolymers. Their ability to fill the interface is accordingly higher.     

A Simple Approach to the Synthesis of Fluoren-9-Ones

Cyclohexene-l-Carboxylic acid (I) undergoes reaction with various aromatic substrates (2a-i) in presence of Polyphosphoric acid (PPA) at 100[ddot]c to give cis-1,2,3,4,4a,9a - hexahydrofluoren-9-ones (3a-i) in good yield. Dehydrogenation of (3a-i) with selenium powder afforded corresponding fluoren-9-ones (4a-i) in high yield.     

Conformational effects on mesophase stability: numerical comparison of carborane diester homologous series with their bicyclo[2.2.2]octane,cyclohexane and benzene analogues

Three series of diesters of 4‐alkoxyphenols containing 12‐vertex p‐carborane (1A[n], n = 1–22), 10‐vertex p‐carborane (1B[n], n = 1–12) or bicyclo[2.2.2]octane (1C[n], n = 1–12) as the central structural element were prepared and investigated by optical and calorimetric methods. All carborane diesters exhibited exclusively nematic behaviour, whereas the carbocyclic analogues 1C[n] and also cyclohexane (1D[n]) and benzene (1E[n]) derivatives, showed early onset of SmA phase and complete disappearance of nematic behaviour. The isotropic transition temperatures, T _MI, for the five series of mesogens were analysed numerically using a three‐parameter exponential function. The resulting limiting values, T _MI(∞), provided a quantitative assessment of the central element ability to support the mesogenic state. They demonstrated that, whereas the T _MI(∞) values for the carbocycles, C, D, and E, are around 125°C, for carboranes A and B this value is 70±2°C and 49±19°C, respectively. Two types of comparative analysis of trends in T _MI relative to those of the terephthalate series 1E[n] demonstrated abnormal behaviour of both carborane series (1A[n] and 1B[n]) and also the cyclohexane series (1D[n]). The former showed progressive destabilisation of the mesophase, whereas the series 1D[n] exhibited increasing mesophase stability relative to 1E[n] with increasing chain length. Both of these effects were explained using conformational analysis of theoretical models and experimental molecular structures for 1A[3], 1B[4] and 1C[4]. The increasing relative destabilisation of the mesophase in the carborane derivatives was rationalised by the high order rotational axes in A and B and D _4d symmetry for B. The trend of the ΔT _MI values for series 1D[n] was explained with the existence of the equatorial‐axial conformational equilibrium for the cyclohexane derivatives. The clearing temperatures for the hypothetical pure diequatorial conformers 1D[n]‐ee were estimated.     

Electrostatic interactions in normal and reversed-phase high-performance thin-layer chromatography. Preliminary experiments

Summary The retention behaviour of five polycyclic aromatic hydrocarbons having nearly equal ionization potentials but different molecular polarizability values was investigated using normal and reversed-phase high-performance TLC systems. Homologous series of hydrocarbons and n-alcohols were applied as the mobile phases. The electrostatic interactions operating in these chromatographic systems were analysed by studying the relationship between the log 1/R_F values and the molecular polarizabilities of solutes and solvents.     

Solar State 1H and 13C NMR Studies of Melanoidins Synthesized from D-Xylose and Glycine

Parameters affecting ¹³C CP/MAS NMR spectra of solid melanoidins (in particular ¹H T₁, T₁₀ and the cross polarization time t_cp) have been obtained with the aim of determining the optimum conditions necessary for quantitative studies. Melanoidins were synthesized at 22[ddot], 68[ddot] and 100 [ddot]C, from molar solutions of D-xylose and glycine. The ¹H T₁₀ values for all melanoidins were similar. Single exponential ¹H T₁ and ¹H T₁₀ relaxation curves were observed for all three polymers, suggesting that the mace-rials were relatively homogenenous. An increase in T₁'s with an increase in unsaturation was also observed. The optimum conditions for quantifying the different types of C were found to be t_cp = 2 msec., recycling time 2 sec. and for these conditions, the melanoidins synthesized at 22[ddot]C showed 18% unsaturation, 18% carboxyl and amide C, while the melanoidin synthesized at 100 [ddot]C showed 28 and 11% respectively. Solid state ¹H NMR lineshapes were obtained and these consisted in all three melanoidins of a broad and a narrow component attributed to the protons of the polymer core and the protons of the mobile side chains or methyl groups respectively.     

The Economic Synthesis of Pyridinium Fluorochromate(VI), C5H5NH[CrO3F] (PFC), and Solvent‐Free Oxidation of Organic Substrates with PFC

Abstract

A 1:1:1 stoichiometric reaction among CrO₃, aqueous HF and pyridine affords orange crystalline pyridinium fluorochromate(VI), C₅H₅NH[CrO₃F] (PFC), in 99.2% isolated yield. The reagent under solvent‐free conditions readily converts benzylic, secondary, and allylic alcohols to the corresponding carbonyls and selectively oxidizes secondary alcohols in the presence of primary alcohols, polycyclic hydrocarbons to cyclic ketones, benzoin to benzil, PPh₃ to O?PPh₃, methylphenyl sulfide to sulfoxide, cyclohexanone oxime to cyclohexanone, an allylic Δ⁵‐steroid to the corresponding α,β‐unsaturated ketone and deprotects dioxolanes and dithiolanes to aldehydes; the economic synthesis of PFC, its ease of reaction without solvent, versatility, and high isolated yields of the products are the significant features of the protocol.     

Hydrqalumination of Diphenylacetylene

It was found that diphenylacetylene reacts at 55[ddot]C with i-Bu₂AlH to form cis-vinylalane. On the other hand, i-Bu₂AlH adds mostly in anti-manner to diphenylacetylene at 110[ddot] C.     

Measurement of activity coefficients at infinite dilution in N-methyl-2-pyrrolidone and N-formylmorpholine and their mixtures with water using the dilutor technique

With the help of the dilutor technique activity coefficients at infinite dilution have been measured for 24 solutes (alkanes, alkenes, cyclic hydrocarbons, aromatic hydrocarbons, ketones, ethers and alcohols) in N-methyl-2-pyrrolidone (NMP), N-formylmorpholine (NFM) and their mixtures with water in the temperature range between 303.15 and 333.15 K (in a few cases for temperatures up to 423.15 K). The influence of water on the activity coefficient at infinite dilution is presented for different solutes. Furthermore, the selectivities at infinite dilution (S_ij^∞=γ_i^∞/γ_j^∞) for the separation of aliphatics from aromatics are discussed.     

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.     

Two Stabilized Calix[6]Cryptand Stereo Isomers used as Stationary Phases for Capillary Gas Chromatography

ABSTRACT

A couple of stabilized calix[6]cryptand steroisomers were synthesized by capping 1,3,5-trimethoxy-p-tert-butylcalix[6]arene with 1,1,1-tri(tosyloxyethoxyethoxymethyl)-propane and used as stationary phases for capillary gas chromatography. Their chromatographic characteristics and separation mechanism for aromatic positional isomers and polycyclic aromatic hydrocarbons are discussed.     

Efficient dichloromethylation of some aromatic hydrocarbons catalyzed by a new ionic liquid [C12minPEG800]Br under homogeneous catalysis in aqueous media

A series of new imidazolium-type ionic liquids based on polyethylene glycol have been prepared. The new recyclable temperature-dependant phase-separation system comprised of [C₁₂minPEG₈₀₀]Br and methylcyclohexane was also developed and successfully applied to the dichloromethylation of some aromatic hydrocarbons to prepare dichloromethyl-substituted hydrocarbons in excellent yields. The ionic liquid could be excellent recycled without any apparent loss of catalytic activity and little loss of weight even after 8 times recycling.     

Facile Protocol for Catalytic Frustrated Lewis Pair Hydrogenation and Reductive Deoxygenation of Ketones and Aldehydes

A series of ketones and aldehydes are reduced in toluene under H₂ in the presence of 5 mol % B(C₆F₅)₃ and either cyclodextrin or molecular sieves affording a facile metal‐free protocol for reduction to alcohols. Similar treatment of aryl ketones resulted in metal‐free deoxygenation yielding aromatic hydrocarbons.