Effect of Light on the Electrokinetic Behavior of TiO(2) Particles in Contact with Cr(VI) Aqueous Solutions

The electrokinetic behavior of titanium dioxide particles (Degussa P25, mainly composed of anatase) put in contact with chromium(VI) aqueous solutions is highly sensitive to light exposure under normal laboratory conditions. In the dark, adsorption of Cr(VI) gives rise to substantial decrements in the mobilities, especially in the acidic branch, and, at higher concentrations, to shifts in the isoelectric point (pH(piep)) to lower values, as expected for anionic chemisorption. A two-mode adsorption model accounts qualitatively for the results. Under light, pH(piep) shifts in the opposite direction as the Cr(VI) concentration increases. A maximum value is attained at [Cr(VI)] approximately 10(-3) mol dm(-3), pH(piep) 8.2, which coincides with values reported for hydrous chromium(III) oxides. At higher concentrations, Cr(VI) adsorption defines a shift of pH(piep) to lower values. It is concluded that light absorption by titanium dioxide promotes the reduction of Cr(VI) and the surface precipitation of the hydrous Cr(III) oxide. The values of pH(piep) for partially covered surfaces are well described by a simple model of surface ionization derived earlier. FTIR/ATR analysis of the surfaces supports this interpretation, and further suggests that one of the modes of Cr(VI) adsorption implies surface dimerization to yield adsorbed dichromate. Copyright 2000 Academic Press.     

Does Hydrogen Bonding Matter in Crystal Engineering? Crystal Structures of Salts of Isomeric Ions

The preparation and structure determinations of the crystalline salts [3,3'-H(2)bipy][PtCl(4)] (2), [2,2'-H(2)bipy][PtCl(4)] (3) and [1,4'-Hbipy][PtCl(4)] (4) and [3,3'-H(2)bipy][SbCl(5)] (6) and [1,4'-Hbipy][SbCl(5)] (8) are reported. In addition a redetermination of the structure of the metastable salt [4,4'-H(2)bipy][SbCl(5)] (5 b) in the corrected space group Pbcm is described. These structures are compared to those of the known salt [4,4'-H(2)bipy][PtCl(4)] (1), the stable triclinic form of [4,4'-H(2)bipy][SbCl(5)] (5 a) and [2,2'-H(2)bipy][SbCl(5)] (7). In the case of the salts of the rigid [PtCl(4)](2-) ion, structures 2, 3 and 4 are essentially isostructural despite the differing hydrogen-bonding capability of the cations. Similarly, among the salts of [SbCl(5)](2-) ions, structures 7 and 8 are essentially isostructural. Structure 6 differs from these in having a differing pattern of aggregation of the [SbCl(5)](2-) ions to form polymeric rather than tetrameric units. It is evident that local hydrogen-bonding interactions, although significant, are not the only or even the decisive influence on the crystal structures formed by these salts. These observations are not in good accord with the heuristic "sticky tecton" or supramolecular synthon models for synthetic crystallography or crystal engineering.     

Analyses of tetracyline antibiotics by reversed-phase high-performance liquid chromatography

Summary A high-performance liquid chromatographic (HPLC) separation was developed that is generally applicable to nine commercially important tetracyline antibiotics. The general method uses an isocratic system and mobile phase consisting of 0.001MEDTA, pH 6.6, and methanol and a Vydac C₁₈ reversed-phase column. Quantitation of the particular tetracyline in some commercial preparations is accomplished by adjusting the mobile phase composition. Quantitative assays were developed for small amounts of 4-epitetracycline in tetracycline (TC) preparations, demeclocycline in minocycline preparations and TC in chlortetracycline (chlor-TC) preparations. A fast HPLC assay for potency was also developed for chlor-TC and minocycline in these commercial preparations.     

Liquid Crystals in Education

Abstract

The advancement of liquid crystal science and technology has been a truly interdisciplinary effort which combines basic principles of physics, chemistry, and engineering. The success of liquid crystal materials in flat panel displays and electro-optic applications can be attributed to the collaborations of scientists in various fields and the unique interface between academia and industry. Many scientists and engineers engaged in liquid crystal research are also educators in their respective disciplines andlor area of expertise. Those in academia routinely teach undergraduate and graduate courses in physics, chemistry, and engineering, and those in industry often find themselves teaching technical short courses and seminars. Many of us have visited elementary and high schools in the USA to speak to students about careers in science and engineering, and to present some eye-catching demonstrations that excite students about science and technology.     

Cation and anion diversity in [M(dithiooxalate)2]2- salts: structure robustness in crystal synthesis

A series of crystalline salts based on the [M(dto)2]2- (dto = 1,2-dithiooxalate, M = Ni, Pt, Cu) dianion with hydrogen-bond donor cations have been synthesised following a molecular tectonics approach. The chelating M(dto)[dot dot dot]HN supramolecular synthon has been exploited in a systematic study of its robustness. The effects of competition between hydrogen-bond acceptors, of the shape and functionality of the cations and of varying the metal in the anion are discussed. The preparation and structural characterisation of the new crystalline phases [4,4'-H(2)bipy][Pt(dto)2] (2), [HNC5H4CO2H-4]2[Pt(dto)2] (5), [HNC5H4CO2H-3]2[Pt(dto)2] (6), [HNC5H4CH2CO2H-4]2[Ni(dto)2] (7), [HNC(5)H(4)CH(2)CO(2)H-3]2[Ni(dto)2] (8), [HNC5H4CONH2-4]2[Ni(dto)2] (9), [HNC5H4CHNOH-4]2[Ni(dto)2] (10), [HNC5H4CHNOH-3]2[Ni(dto)2] (11), [4,4'-H2bipip][Ni(dto)2] (12), [H2NC5H9CO2H-4]2[Pt(dto)2] (12), [H2NC5H9CO2H-4]2[Cu(dto)2] (14), [H2NC5H9CO2H-3]2[Ni(dto)2][H2O]2 (15), [H2NC5H9CO2H-3]2[Pt(dto)2][H2O]2 (16), [H2NC5H9CO2H-3]2[Cu(dto)2][H2O]2 (17), [H(Me)NC5H9CO2H-4]2[Ni(dto)2][H2O]2 (18) is reported. The charge-assisted NH[dot dot dot]dto synthon is formed in each of compounds 1-20, and is apparently much more robust than the conventional synthons used (such as the carboxylic acid dimer), which have a much lower rate of occurrence. The NH[dot dot dot]dto synthon may be generalised to 3- and 4-pyridinium species and 3- and 4-piperidinium derivatives. In the latter cases branching of the hydrogen-bond networks through the NH2 groups arises. The robustness of the NH...dto synthon allows structures of the form [NH cation]2[M(dto)2] to be regarded as being formed by the packing of neutral supermolecules. Cases of isomorphism (as in 16-18) and latent polymorphism (e.g. in 4 and 6) are noted.     

Measurement of nitrogen dioxide in cigarette smoke using quantum cascade tunable infrared laser differential absorption spectroscopy (TILDAS)

Although nitrogen dioxide (NO(2)) has been previously reported to be present in cigarette smoke, the concentration estimates were derived from kinetic calculations or from measurements of aged smoke, where NO(2) was formed some time after the puff was taken. The objective of this work was to use tunable infrared laser differential absorption spectroscopy (TILDAS) equipped with a quantum cascade (QC) laser to determine if NO(2) could be detected and quantified in a fresh puff of cigarette smoke. A temporal resolution of approximately 0.16s allowed measurements to be taken directly as the NO(2) was formed during the puff. Sidestream cigarette smoke was sampled to determine if NO(2) could be detected using TILDAS. Experiments were conducted using 2R4F Kentucky Reference cigarettes with and without a Cambridge filter pad. NO(2) was detected only in the lighting puff of whole mainstream smoke (without a Cambridge filter pad), with no NO(2) detected in the subsequent puffs. The measurement precision was approximately 1.0 ppbVHz(-1/2), which allows a detection limit of approximately 0.2 ng in a 35 ml puff volume. More NO(2) was generated in the lighting puff using a match or blue flame lighter (29+/-21 ng) than when using an electric lighter (9+/-3 ng). In the presence of a Cambridge filter pad, NO(2) was observed in the gas phase mainstream smoke for every puff (total of 200+/-30 ng/cigarette) and is most likely due to smoke chemistry taking place on the Cambridge filter pad during the smoke collection process. Nitrogen dioxide was observed continuously in the sidestream smoke starting with the lighting puff.     

Simple coupled-cluster singles and doubles method with perturbative inclusion of triples and explicitly correlated geminals: The CCSD(T)R12 model

To approach the complete basis set limit of the "gold-standard" coupled-cluster singles and doubles plus perturbative triples [CCSD(T)] method, we extend the recently proposed perturbative explicitly correlated coupled-cluster singles and doubles method, CCSD(2)(R12) [E. F. Valeev, Phys. Chem. Chem. Phys. 8, 106 (2008)], to account for the effect of connected three-electron correlations. The natural choice of the zeroth-order Hamiltonian produces a perturbation expansion with rigorously separable second-order energy corrections due to the explicitly correlated geminals and conventional triple and higher excitations. The resulting CCSD(T)(R12) energy is defined as a sum of the standard CCSD(T) energy and an amplitude-dependent geminal correction. The method is technically very simple: Its implementation requires no modification of the standard CCSD(T) program and the formal cost of the geminal correction is small. We investigate the performance of the open-shell version of the CCSD(T)(R12) method as a possible replacement of the standard complete-basis-set CCSD(T) energies in the high accuracy extrapolated ab initio thermochemistry model of Stanton et al. [J. Chem. Phys. 121, 11599 (2004)]. Correlation contributions to the heat of formation computed with the new method in an aug-cc-pCVXZ basis set have mean absolute basis set errors of 2.8 and 1.0 kJmol when X is T and Q, respectively. The corresponding errors of the standard CCSD(T) method are 9.1, 4.0, and 2.1 kJmol when X=T, Q, and 5. Simple two-point basis set extrapolations of standard CCSD(T) energies perform better than the explicitly correlated method for absolute correlation energies and atomization energies, but no such advantage found when computing heats of formation. A simple Schwenke-type two-point extrapolation of the CCSD(T)(R12)aug-cc-pCVXZ energies with X=T,Q yields the most accurate heats of formation found in this work, in error on average by 0.5 kJmol and at most by 1.7 kJmol.     

Born-Oppenheimer symmetry breaking in the C state of NO2: importance of static and dynamic correlation effects

A systematic theoretical treatment is performed with highly correlated ab initio theoretical methods to establish the structural nature of the C state of NO2. We predict the C state to have an asymmetric structure (point group C(s)). Spin-restricted and spin-unrestricted configuration interaction (CISD), coupled cluster [CCSD and CCSD(T)], multireference complete active space self-consistent field (CASSCF), and internally contracted multireference configuration interaction (ICMRCI) methods were used in conjunction with very large correlation-consistent polarized valence zeta cc-pVXZ and aug-cc-pVXZ [X = T, Q, 5] basis sets. The asymmetric C 2A' state is predicted to lie T(e) = 47.5 kcal/mol (2.06 eV, 16,600 cm(-1)) above the X 2A1 state at the aug-cc-pV5Z/UCCSD(T) level of theory, with T0 = 46.0 kcal/mol (2.00 eV, 16,100 cm(-1)), in good agreement with the experimental values of 46.77 kcal/mol (2.028 eV, 16,360 cm(-1)) by Weaver and 46.42 kcal/mol (2.013 eV, 16,234 cm(-1)) by Aoki. The symmetric structure (in C(2v) symmetry) with re(NO) = 1.274 A and theta(e) (ONO) = 109.9 degrees is a transition state between the two equivalent asymmetric (in C(s) symmetry) structures and is located only 1.53 kcal/mol (0.066 eV, 540 cm(-1)) above the asymmetric structure. The asymmetric structure is predicted to have structural parameters r(e)(NOl) = 1.489 A, r(e)(NOs) = 1.169 A, and theta(e)(ONO) = 109.7 degrees with the same method, aug-cc-pV5Z/UCCSD(T). The averaged NO bond distance is 1.329 A, and the difference between the two NO bond distances is 0.320 A. The three harmonic vibrational frequencies for the C 2A' state are 1656 (in-phase stretch), 759 (bend), and 378 (out- of-phase stretch) cm(-1). While these theoretical results further corroborate the previous predictions concerning the asymmetric nature of the C state, there remains discrepancy between the theoretical and experimental symmetric stretching mode omega1 (1656 and 923 cm(-1), respectively). It is possible, however, that this disagreement could be resolved by a reassignment of the corresponding lines in the experimental spectrum, though additional vibronic simulations of the spectrum are required to confirm this proposition.     

Single‐Step Microwave‐Mediated Synthesis of Oxazoles and Thiazoles from 3‐Oxetanone: A Synthetic and Computational Study

The direct microwave‐mediated condensation between 3‐oxetanone and primary amides and thioamides has delivered moderate to good yields of (hydroxymethyl)oxazoles and (hydroxymethyl)thiazoles. The reactions use a sustainable solvent and only require short reaction times. These are highly competitive methods for the construction of two classes of valuable heteroarenes, which bear a useful locus for further elaboration. Electronic structure calculations have shown that the order of events involves chalcogen atom attack at sp³ carbon and alkyl–oxygen cleavage. The critical role of acid catalysis was shown clearly, and the importance of acid strength was demonstrated. The calculated barriers were also fully consistent with the observed order of thioamide and amide reactivity. Spontaneous ring opening involves a modest degree of C? O cleavage, moderating the extent of strain relief. On the acid‐catalysed pathway, C? O cleavage is less extensive still, but proton transfer to the nucleofuge is well advanced with the carboxylic acid catalysts, and essentially complete with methanesulfonic acid.     

Alignment and ordering mechanisms at a liquid crystal–solid interface

Abstract

The role of surface coupling agents on the aligning and ordering mechanisms at a liquid crystal–solid interface are examined with deuterium nuclear magnetic resonance. The cylindrical channels of alumina membranes 0·2 μm in diameter are chemically modified using an aliphatic acid (C _n H_2n+1 COOH) as a surface coupling agent and filled with the liquid crystal compound 4′-pentyl-4-cyanobiphenyl deuteriated in the α position of the hydrocarbon chain (5CB-αd ₂). The preferred anchoring direction at the cavity wall and its strength are found to depend on the length of the aliphatic chain of the surface coupling agent which determine the nematic director field in the pores. The planar polar configuration with homeotropic anchoring conditions is stable for agents with n ≥7 while chain lengths n ≤6 support a uniform axial configuration with planar anchoring at the cavity wall. The pretransitional orientational ordering at the cavity wall above the clearing temperature is strongly reflected in the spectra. The radical changes in the quadrupole splitting as the length of the aliphatic chain of the surface coupling agent is varied indicates strong coupling between the 5CB molecules and the n = 15 surface, while shorter chain lengths reveal substantially reduced degrees of coupling.