Use of isopycnic plots in designing operations of supercritical fluid chromatography: I. The critical role of density in determining the characteristics of the mobile phase in supercritical fluid chromatography

In Supercritical Fluid Chromatography (SFC), the key chromatographic parameters of any compound, its retention and efficiency, are known to strongly depend on the density of the mobile phase. This indicates that iso-density, also called isopycnic, plots drawn on the pressure-temperature plane can provide an effective tool to analyze how SFC systems may operate under different combinations of inlet and outlet pressures and column temperature. To effectively use these isopycnic plots in designing the operations of SFC systems, however, a deeper understanding of the factors behind the dependence of the performance of these systems on the mobile phase density is required. The nature of this density dependence is explored with reference to the key physical properties of the mobile phase, its viscosity, diffusivity and solubility. This study is focused on the use of pure carbon dioxide as the mobile phase, but this method of investigation is applicable for other mobile phase combinations as well.     

Use of isopycnic plots in designing operations of supercritical fluid chromatography. III: reason for the low column efficiency in the critical region

This paper discusses the origins of efficiency loss in supercritical fluid chromatography (SFC) when analyses are carried out in the low pressure supercritical region of carbon-dioxide, close to its critical point. Recent publications have shown strong evidence of radial thermal heterogeneity inside an SFC column and suggested that it leads to peak-shape distortion and greatly reduces the column efficiency. We demonstrate that the physico-chemical properties of CO(2) close to the critical point are such that formation of thermal heterogeneity inside the column is highly probable and could cause the observed efficiency loss. Consideration of isopycnic plots of CO(2) permits clear identification of the problematic region and explains why these properties of CO(2) are primarily responsible for the often perplexing efficiency losses taking place during the SFC operations.     

Photolysis of (3-methyl-2H-azirin-2-yl)-phenylmethanone: direct detection of a triplet vinylnitrene intermediate

The photoreactivity of (3-methyl-2H-azirin-2-yl)-phenylmethanone, 1, is wavelength-dependent (Singh et al. J. Am. Chem. Soc. 1972, 94, 1199-1206). Irradiation at short wavelengths yields 2P, whereas longer wavelengths produce 3P. Laser flash photolysis of 1 in acetonitrile using a 355 nm laser forms its triplet ketone (T(1K), broad absorption with λ(max) ~ 390-410 nm, τ ~ 90 ns), which cleaves and yields triplet vinylnitrene 3 (broad absorption with λ(max) ~ 380-400 nm, τ = 2 μs). Calculations (B3LYP/6-31+G(d)) reveal that T(1K) of 1 is located 67 kcal/mol above its ground state (S(0)) and has a long C-N bond (1.58 ?), and the calculated transition state to form 3 is only 1 kcal/mol higher in energy than T(1K) of 1. The calculations show that 3 has significant 1,3-carbon iminyl biradical character, which explains why 3 reacts efficiently with oxygen and decays by intersystem crossing to the singlet surface. Photolysis of 1 in argon matrixes at 14 K produced ketene imine 7, which presumably is formed from 3 intersystem crossing to 7. In comparison, photolysis of 1 in methanol with a 266 nm laser produces mainly ylide 2 (λ(max) ~ 380 nm, τ ~ 6 μs, acetonitrile), which decays to form 2P. Ylide 2 is formed via singlet reactivity of 1, and calculations show that the first singlet excited state of the azirine chromophore (S(1A)) is located 113 kcal/mol above its S(0) and that the singlet excited state of the ketone (S(1K)) is 85 kcal/mol. Furthermore, the transition state for cleaving the C-C bond in 1 to form 2 is located 49 kcal/mol above the S(0) of 1. Thus, we theorize that internal conversion of S(1A) to a vibrationally hot S(0) of 1 forms 2, whereas intersystem crossing from S(1K) to T(1K) results in 3.     

Spectroscopy and dissociation of I<Subscript>2</Subscript>–Rg (Rg = Kr and Xe) van der Waals complexes

The spectroscopy and dissociation of I₂–Rg (Rg = Kr and Xe) van der Waals complexes have been studied in detail using MP2 and CCSD(T) methods in conjunction with the correlation-consistent triple-ζ and quadruple-ζ quality basis sets. The large-core Stuttgart–Dresden–Bonn (SDB) relativistic pseudopotential is used for all heavy elements. The dissociation energy and depth of the potential well have been calculated using potential method and supermolecular approach in order to remove the discrepancy among the existing theoretical and experimental values. Most of the spectroscopic properties are first reported, and the rest agree very well with the theoretical and experimental values wherever available.     

Synthesis of gold nanoparticles in niosomes

This work outlines a novel method for the synthesis of stable gold nanoparticles within the spatially confined region of vesicles. For the first time, Span/cholesterol based niosomes have been used for nanoparticle synthesis. The restricted geometry within niosomes prevents nanoparticle aggregation. The results have important implications for controlled delivery of nanoparticles for therapeutic applications.     

Aminolysis of a model nerve agent: a computational reaction mechanism study of o,s-dimethyl methylphosphonothiolate

The mechanism for the aminolysis of a model nerve agent, O,S-dimethyl methylphosphonothiolate, is investigated both at density functional level using M062X method with 6-311++G(d,p) basis set and at ab initio level using the second-order M?ller-Plesset perturbation theory (MP2) with the 6-311+G(d,p) basis set. The catalytic role of an additional NH(3) and H(2)O molecule is also examined. The solvent effects of acetonitrile, ethanol, and water are taken into account employing the conductor-like screening model (COSMO) at the single-point M062X/6-311++G(d,p) level of theory. Two possible dissociation pathways, methanethiol and methyl alcohol dissociations, along with two different neutral mechanisms, a concerted one and a stepwise route through two neutral intermediates, for each pathway are investigated. Hyperconjugation stabilization that has an effect on the stability of generated transition states are investigated by natural bond order (NBO) approach. Additionally, quantum theory of atoms in molecules analysis is performed to evaluate the bond critical (BCP) properties and to quantify strength of different types of interactions. The calculated results predict that the reaction of O,S-dimethyl methylphosphonothiolate with NH(3) gives rise to parallel P-S and P-O bond cleavages, and in each cleavage the neutral stepwise route is always favorable than the concerted one. The mechanism of NH(3) and H(2)O as catalyst is nearly similar, and they facilitate the shuttle of proton to accelerate the reaction. The steps involving the H(2)O-mediated proton transfer are the most suitable ones. The first steps for the stepwise process, the formation of neutral intermediate, are the rate-determining step. It is observed that in the presence of catalyst the reaction in the stepwise path possesses almost half the activation energy of the uncatalyzed one. A bond-order analysis using Wiberg bond indexes obtained by NBO calculation predicts that usually all individual steps of the reactions occur in a concerted fashion showing equal progress along different reaction coordinates.     

Copper(II) complexes with neutral Schiff bases: Syntheses, crystal structures and DNA interactions

The synthesis and X-ray structural characterisation of a new Cu(II) complex, [Cu(L¹)Cl](ClO₄)·CH₃OH (1) [L¹ = N,N′-bis((pyridine-2-yl)phenylidene)-1,3-diaminopropan-2-ol], has been described in this work. The structural study reveals that the Cu(II) centre in 1 has a square pyramidal geometry with a trigonality index τ = 0.43, being coordinated by the organic ligand and a chloro group. The interaction of complex 1 and another complex previously reported by our group, [Cu(L²)](ClO₄)₂ (2) [L² = N-(1-pyridin-2-yl-phenylidene)-N′-[2-({2-[(1-pyridin-2-ylphenylidene)amino]ethyl}amino)ethyl]ethane-1,2diamine], with calf thymus DNA (CT-DNA) has been investigated using absorption and emission spectral studies. The binding constant (K_b) and the linear Stern-Volmer quenching constant (K_sv) have been determined.     

Magic sized ZnS quantum dots as a highly sensitive and selective fluorescence sensor probe for Ag+ ions

A green and simple chemical synthesis of magic sized water soluble blue-emitting ZnS quantum dots (QDs) has been accomplished by reacting anhydrous Zn acetate, sodium sulfide and thiolactic acid (TLA) at room temperature in aqueous solution. Refluxing of this mixture in open air yielded ZnS clusters of about 3.5 nm in diameter showing very strong and narrow photoluminescence properties with long stability. Refluxing did not cause any noticeable size increment of the clusters. As a result, the QDs obtained after different refluxing conditions showed similar absorption and photoluminescence (PL) features. Use of TLA as a capping agent effectively yielded such stable and magic sized QDs. The as-synthesized and 0.5 h refluxed ZnS QDs were used as a fluorescence sensor for Ag(+) ions. It has been observed that after addition of Ag(+) ions of concentration 0.5-1 μM the strong fluorescence of ZnS QDs was almost quenched. The quenched fluorescence can be recovered by adding ethylenediamine to form a complex with Ag(+) ions. The other metal ions (K(+), Ca(2+), Au(3+), Cu(2+), Fe(3+), Mn(2+), Mg(2+), Co(2+)) showed little or no effect on the fluorescence of ZnS QDs when tested individually or as a mixture. In the presence of all these ions, Ag(+) responded well and therefore ZnS QDs reported in this work can be used as a Ag(+) ion fluorescence sensor.     

Theoretical simulation of photovoltaic response of graphene-on-semiconductors

Using the fundamental models for voltage and current, we report on the photovoltaic behavior of graphene-on-semiconductor-based devices. The graphene-n-Si and graphene-n-GaAs systems are studied for open-circuit voltage (V _OC) and short-circuit current density (J _SC) under low- and high-level injection conditions. The effects of semiconductor doping density and surface recombination velocity on the V _OC of both systems are investigated. The V _OC for graphene-n-Si under low- and high-level injection conditions are found to be 0.353 V and 0.451 V, respectively, whereas the V _OC for graphene-n-GaAs under low- and high-level injection conditions are 0.441 V and 0.471 V, respectively. The J _SC for graphene-n-Si under low- and high-level injection conditions are calculated as 3 mAcm^?2 and 4.78 mAcm^?2, respectively, whereas the J _SC for graphene-n-GaAs under low- and high-level injection conditions are 5.2 mAcm^?2 and 6.68 mAcm^?2, respectively. These results are in good agreement with the reported experimental work.     

A study of magnetism in disordered Pt-Mn, Pd-Mn and Ni-Mn alloys: an augmented space recursion approach

In this paper we shall study three binary alloy systems, one constituent of which is Mn. The other constituents are chosen from a particular column of the periodic table: Ni(3d), Pt (4d) and Pd (5d). As we go down the column, the d-bands become wider, discouraging spin-polarization. In a disordered alloy, the situation becomes more complicated, as the exchange interaction between two atoms is environment dependent. We shall compare and contrast their magnetic behaviour using robust electronic structure techniques. In all three alloy systems conjectures are made to explain experimental data. In this paper we shall examine whether there is any basis to these conjectures.