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PubMed1997

Detection of Aryl Radicals in Hydrodediazoniations.

Iodoacetic acid, an effective aryl radical trapping agent, was employed to investigate the reactive intermediates in several hydrodediazoniations. Isolation of an aryl iodide constitutes a positive result in the test for aryl radicals. Equally as important is the lower yield of the reduction product when the trap diverts radicals from their usual reaction path. Hydrodediazoniations performed in MeOH, EtOH, i-PrOH, benzyl alcohol, THF, tetramethylurea, formamide, and hypophosphorous acid all involve aryl radical intermediates. Ferrocene was found to be an effective initiator in most of these reactions; through its action as an electron donor, it serves to shorten reaction times and to improve yields of hydrodediazoniation products. All hydrodediazoniations examined, whether initiated or not, involve radical intermediates.

Journal Article↗
PubMed1999

Synthesis of 2'beta-Deoxy-[8-(13)C;amino,9-(15)N(2)]adenosine: Unusual Annulation Conditions To Assemble the Purine Core.

Synthesis of 2'beta-Deoxy-[8-(13)C;amino,9-(15)N(2)]adenosine has been accomplished using a five-step process which employs a novel annulation of [(13)C]formamide [N-(4-[(15)N]amino-6-chloro-5-pyrimidinyl)] 2. To effect this dehydration, a complex of triethyl phosphite and TiCl(2)(i-OPr)(2) was used. 6-Chloro-[8-(13)C; 9-(15)N]purine was then converted in two steps to 2'beta-deoxy-[8-(13)C;amino,9-(15)N(2)]adenosine.

Journal Article↗
PubMed2000

New Insights into the Chemistry of Lithium Carbamoyls: Characterization of an Adduct (R(2)NC(O)CLi(OLi)NR(2)).

Studies of the reaction of lithium dicyclohexylamide with N,N-dibutylformamide, 1-formylpiperidine, and 4-formylmorpholine indicate that the equilibria resulting from these compounds are shifted toward the formation of an adduct, which quickly collapses to dicyclohexylamine and the lithiated carbamoyl anion derived from the initial disubstituted formamide. Further reactions of the lithium carbamoyl lead to a new adduct where a lithiated carbon is bounded to N, O, and a carbonyl functionality. The (13)C NMR analysis of the reaction mixtures showed the presence of similar intermediates in all cases: adducts of this type have not been reported before. These dilithiated intermediates were trapped with methyl iodide giving the corresponding doubly methylated derivatives. Isolation of substituted glyoxylamides and quantitative determination of the products yields constitute further evidence of the whole reaction scheme proposed.

Journal Article↗
PubMed2005

Density functional study of the mechanism of the Beckmann rearrangement catalyzed by H-ZSM-5: a cluster and embedded cluster study.

The mechanism of the Beckmann rearrangement (BR) catalyzed by the ZSM-5 zeolite has been investigated by both the quantum cluster and embedded cluster approaches at the B3LYP level of theory using the 6-31G(d,p) basis set. Single-point calculations were carried out at the MP2/6-311G(d,p) level of theory to improve energetic properties. The embedded cluster model suggests that the initial step of the Beckmann rearrangement is not the O-protonated oxime but the N-protonated oxime. The energy barriers derived from the proton shuttle of the N-bound to the O-bound isomer are determined to be approximately 99 and approximately 40 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The difference in the activation energy is due mainly to the effect of the Madelung potential from the zeolite framework. The next step is the rearrangement step, which is the transformation of the O-protonated oxime to be an enol-formed amide compound, formimidic acid. The activation energy, at the rearrangement step, is calculated to be approximately 125 and approximately 270 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. The final step is the tautomerization step which transforms the enol-form to the keto-form, formamide compound. The energy barrier for tautomerization is calculated to be 123 and 151 kJ/mol for the embedded cluster and quantum cluster approaches, respectively. These calculated results suggest that the rate-determining step of the vapor phase of the Beckmann rearrangement on H-ZSM-5 is the rearrangement or tautomerization step.

Journal Article↗
PubMed2005

Anomalous microscopic dielectric response of dipolar solvents and water.

This paper reports measurements of static microscopic dielectric response of several dipolar solvents to charge redistribution in a fluorescent probe. Contrary to recent predictions of dielectric theories and computer simulations of bulk liquids, the observed dielectric response of most solvents conforms to the macroscopic continuum description even at atomic distances, as if these solvents had no spatial intermolecular structure. Such conformance is observed for several probes when the contribution of specific probe-solvent interactions to the response is negligible. However, water, formamide, and glycerol exhibit anomalous responses even though such a probe is used. We discuss a possible reason for the macroscopic-like behavior and a connection between the anomaly and fluctuating structures formed by anomalous solvents near the hydrophobic surface of the probe.

Electrons↗
PubMed2006

What can you learn from a molecular probe? New insights on the behavior of C343 in homogeneous solutions and AOT reverse micelles.

The behavior of C343, a common molecular probe utilized in solvation dynamics experiments, was studied in homogeneous media and in aqueous and nonaqueous reverse micelles (RMs). In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission bands showing that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the hydrogen-bonding properties of the solvent. Specifically, in the ground state the molecule displays a bathochromic shift with the polarity polarizability (pi) and the H-bond acceptor (beta) ability of the solvents and a hypsochromic shift with the hydrogen donor ability (alpha) of the media. The carboxylic acid group causes C343 to display greater sensitivity to the beta than to the pi polarity parameter; this sensitivity increases in the excited state, while the dependence on alpha vanishes. This demonstrates that C343 forms a stable H-bond complex with solvents with high H-bond acceptor ability (high beta) and low H-bond donor character (low alpha). Spectroscopy in nonpolar solvents reveals J-aggregate formation. With information from the Kamlet-Taft analysis, C343 was used to explore RMs composed of water or polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/isooctane using absorption, emission, and time-resolved spectroscopies. Sequestered polar solvents included ethylene glycol (EG), formamide (FA), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMA). Dissolved in the AOT RM systems at low concentration, C343 exists as a monomer, and when introduced to the RM samples in its protonated form, C343 remains protonated driving it to reside in the interface rather than the water pool. The solvathochromic behavior of the dye depends the specific polar solvent encapsulated in the RMs, revealing different types of interactions between the solvents and the surfactant. EG and water H-bond with the AOT sulfonate group destroying their bulk H-bonded structures. While water remains well segregated from the nonpolar regions, EG appears to penetrate into the oil side of the interface. In aqueous AOT RMs, C343 interacts with neither the sulfonate group nor the water, perhaps because of intramolecular H-bonding in the dye. DMF and DMA interact primarily through dipole-dipole forces, and the strong interactions with AOT sodium counterions destroy their bulk structure. FA also interacts with the Na+ counterions but retains its H-bond network present in bulk solvent. Surprisingly, FA appears to be the only polar solvent other than water forming a "polar-solvent pool" with macroscopic properties similar to the bulk.

Journal Article↗
PubMed2006

Solvothermal synthesis and photoluminescent properties of ZnS/cyclohexylamine: inorganic-organic hybrid semiconductor nanowires.

An inorganic-organic hybrid semiconductor, ZnS/CHA (CHA = cyclohexylamine) nanocomposites was successfully synthesized via a solvothermal method using CHA as solvent, which yielded uniform and ultralong nanowires with widths of 100-1000 nm and lengths of 5-20 microm. Changing the reaction conditions could alter the morphology and optical properties of the nanocomposites. The periodic layer subnanometer structures were identified by high-resolution transmission electron microscopy (HR-TEM) images, with thickness of approximately 2 nm. The composites exhibited a very large blue-shift in their optical absorption edge as well as an exciton excitation band due to a strong quantum confinement effect caused by the internal subnanometer-scale structures. The pure hexagonal wurtzite ZnS nanowires were also obtained by extracting the ZnS/CHA nanocomposites with dimethyl formamide (DMF). In addition, the luminescent properties of exciton and defect-related transitions in different samples of ZnS/CHA were discussed in detail.

Cyclohexylamines↗
PubMed2006

Fabrication of upended taper-shaped cuprous thiocyanate arrays on a copper surface at room temperature.

A new strategy has been well designed to form upended taper-shaped cuprous thiocyanate (hereafter abbreviated as CuCNS) arrays on a copper substrate with use of a simple solution-phase method at room temperature. This method consists of a liquid-solid reaction between a solution of thiocyanate ammonium and the copper substrate itself in the assistance of formamide. Novel CuCNS arrays are approximately perpendicular to copper substrate surfaces. Every single crystal shows an upended taper-like morphology (i.e., the tip end points into the surface of copper substrate and the other big end of the taper exposes out, like a dart thrusting into the copper substrate). On the basis of structure and chemical bond analysis, CuCNS crystals tend to grow along the c-axis, which is essential for the formation of CuCNS arrays on a copper substrate. This approach also provides a facile strategy to produce different patterns on different copper substrates, which may be applicable to the synthesis of other inorganic materials with various potential applications.

Journal Article↗
PubMed2006

Solvent effects on the redox properties of thioethers.

The one-electron reduction potential of the radical cations of thioanisole (1), benzyl methyl sulfide (2) and 2-hydroxyethyl benzyl sulfide (3) in water, formamide, acetonitrile, acetone, 1,1,1,3,3,3-hexafluoropropan-2-ol, methanol and 2-propanol was investigated by cyclic voltammetry. For comparison the one-electron reduction potentials in water were also measured using pulse radiolysis. The redox potential is strongly influenced by the nature of the solvent and the solvent sensitivity increases with charge localization. The present results have been used to evaluate solvent effects in view of the Kamlet-Taft relationship. The Kamlet-Taft expression quantitatively describes the solvent effects on the redox properties of 1-3 and gives the relative importance of the different solvent properties. The dominating contribution to the solvent effects is the solvent dipolarity/polarizability pi*, whereas alpha appears to be of minor importance. Furthermore, the relationship between the pi* and reduction potential of radical cations of 1-3 appear to be linear. It was also possible to find the same trend between the solvent dipole moment and peak potential of 1-3. These facts indicate that the nature of solvation is mainly nonspecific.

Electrochemistry↗
PubMed2006

Fluoroolefins as peptide mimetics: a computational study of structure, charge distribution, hydration, and hydrogen bonding.

The design of peptide mimetic compounds is greatly facilitated by the identification of functionalities that can act as peptide replacements. The fluoroalkene moiety has recently been employed for that purpose. The purpose of this work is to characterize prototypical fluoroalkenes (fluoroethylene and 2-fluoro-2-butene) with respect to key properties of peptides (amides) including structure, charge distribution, hydration, and hydrogen bonding. The results are compared to those obtained for model peptides (formamide, N-methylacetamide). Calculations have been carried out at the MP2 and B3LYP levels of theory with the 6-311++G(2d,p) and 6-311++G(2d,2p) basis sets. The results suggest that the fluoroalkene is similar in steric requirements to a peptide bond but that there is less charge separation. Calculations of the hydration free energies with the PCM bulk continuum solvent model indicate that the fluoroalkene has much smaller hydration free energies than an amide but that the difference in solvation free energy for cis and trans isomers is comparable. In studies of complexes with water molecules, the fluoroalkene is found to engage in interactions that are analogous to backbone hydrogen-bonding interactions that govern many properties of natural peptides and proteins but with smaller interaction energies. In addition, key structural differences are noted when the fluoroalkene is playing the role of hydrogen-bond acceptor which may have implications in binding, aggregation, and conformational preferences in fluoroalkene peptidomimetics. The issue of cooperativity in hydrogen-bonding interactions in complexes with multiple waters has also been investigated. The fluoroalkene is found to exhibit cooperative effects that mirror those of the peptide but are smaller in magnitude. Thus, pairwise addivitity of interactions appears to more adequately describe the fluoroalkenes than the peptides they are intended to mimic.

Alkenes↗
PubMed2006

Proton-transfer reaction of 4-methyl 2,6-diformyl phenol in cyclodextrin nanocage.

We report here on the steady-state and time-resolved fluorescence studies on proton-transfer (PT) reaction of 4-methyl 2,6-diformyl phenol (MFOH) in confined nanocavities in three solvents, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), and water. Though DMSO and DMF individually interact with MFOH in a similar fashion, their modes of interaction get significantly modified in the presence of cyclodextrin (CD) nanocages. In DMSO, in the ground state, the solvated molecular anion of MFOH forms 1:1 inclusion complex with beta- or gamma-CD and attains greater stability compared to the normal form. In DMF, the solvated molecular anion gets converted to the H-bonded complex within the CD cavity resulting in a 50-nm blue shift in the absorption spectra. In the excited state, the anionic species gets more stabilized in DMSO while in DMF it is significantly destabilized in the presence of CDs. However, in case of water, MFOH gets trapped inside the water cages so that the CDs fail to complex with it effectively. There are also no changes in the excited-state lifetimes in water in the presence of CDs, but in case of DMSO and DMF, because of restricted rotation of the formyl group within the CD cavity, the contribution of the shorter lifetime components reduce significantly increasing the larger components. Some theoretical calculations at the AM1 level of approximation have also been carried out to demonstrate how the dipolar nature of the solvent influences excited-state PT in confined media.

Journal Article↗
PubMed2006

Anodic growth of highly ordered TiO2 nanotube arrays to 134 microm in length.

Described is the fabrication of self-aligned highly ordered TiO(2) nanotube arrays by potentiostatic anodization of Ti foil having lengths up to 134 mum, representing well over an order of magnitude increase in length thus far reported. We have achieved the very long nanotube arrays in fluoride ion containing baths in combination with a variety of nonaqueous organic polar electrolytes including dimethyl sulfoxide, formamide, ethylene glycol, and N-methylformamide. Depending on the anodization voltage, pore diameters of the resulting nanotube arrays range from 20 to 150 nm. Our longest nanotube arrays yield a roughness factor of 4750 and length-to-width (outer diameter) aspect ratio of approximately 835. The as-prepared nanotubes are amorphous but crystallize with annealing at elevated temperatures. In initial measurements, 45 mum long nanotube-array samples, 550 degrees C annealed, under UV illumination show a remarkable water photoelectrolysis photoconversion efficiency of 16.25%.

Electrodes↗
PubMed1998

The hydrophobic effect. 1. A consequence of the mobile order in H-bonded liquids.

The hydrophobic effect has an entropic nature that cannot be explained by classical multicomponent treatments that do not explicitly take into account both the mobility and the nonergodicity of the H-bonds in amphiphilic liquids. The nonergodic thermodynamics of mobile order in H-bonded liquids based on time fractions rather than on concentrations provides a novel qualitative and quantitative explanation for the molecular origin of the hydrophobic effect. Chiefly, this effect corresponds to the loss of the mobile order entropy of associated molecules by dilution with foreign substances. Not being a unique property of water, the propensity of an amphiphilic solvent to induce a solvophobic effect increases primarily as its structuration factor increases, and secondarity as the solute/solvent molar volume ratio increases. On this basis, it can be expected that in the absence of strong solute-solvent specific interactions, the solubility of nonelectrolytes will generally decrease in the following order: butanol > propanol > ethanol > methanol > propylene glycol > ethylene glycol > formamide > water.

Chemical Phenomena↗
PubMed2004

Synthesis and size control of luminescent ZnSe nanocrystals by a microemulsion-gas contacting technique.

A scalable method for controlled synthesis of luminescent compound semiconductor nanocrystals (quantum dots) using microemulsion-gas contacting at room temperature is reported. The technique exploits the dispersed phase of a microemulsion to form numerous identical nanoreactors. ZnSe quantum dots were synthesized by reacting hydrogen selenide gas with diethylzinc dissolved in the heptane nanodroplets of a microemulsion formed by self-assembly of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer in formamide. A single nanocrystal is grown in each nanodroplet, thus allowing good control of particle size by manipulation of the initial diethylzinc concentration in the heptane. The ZnSe nanocrystals exhibit size-dependent luminescence and excellent photostability.

Letter↗
PubMed2005

On the maximum spreading diameter of impacting droplets on well-prepared solid surfaces.

This paper presents a systematic study of liquid droplet impact on three polymer surfaces: poly(methyl methacrylate), poly(methyl methacrylate/n-butyl methacrylate), and poly(n-butyl methacrylate). Changing from one surface to the next represents an incremental variation in solid surface tensions of 5-6 mJ/m2. These surfaces were prepared through careful experimental procedures that were used for the determination of solid surface tensions from contact angles. Our data for the maximum spreading diameter of water and formamide impacting on these surfaces were compared with those predicted from literature models. Of the models selected, we modified the model of Pasandideh-Fard et al. [Phys. Fluids 1996, 8, 650] and the results yielded a least error of only 5.09 +/- 5.05% in the determination of the maximum spreading diameter. The improved model was also compared with literature data, and good agreement was found. Of course, any such comparisons would rely on accurate experimental impact dynamics data on carefully prepared surfaces.

Journal Article↗
PubMed2004

Effect of structure on the reduction potentials of films of constitutional isomers of iron-sulfur cluster core dendrimers.

The thermodynamic redox potentials of films of constitutional isomers of iron-sulfur cluster core dendrimers were measured and compared. It was determined that the primary structure of the dendrimer influences its reduction potential. Dendrimers containing so-called backfolded linkages were more difficult to reduce than their extended analogues. This behavior is rationalized by suggesting that the backfolded isomers pack more tightly around the iron-sulfur cluster, creating a more hydrophobic local microenvironment. Also, all of these molecules are easier to reduce in the film than in dimethyl formamide solution. The variation in redox potential between film and solution environment was compared to that of dendrimers of differing generations and correlated with the amount of hydrophobic dendron surrounding the cluster.

Journal Article↗
PubMed2006

Topography and surface free energy of DPPC layers deposited on a glass, mica, or PMMA support.

An investigation of energetic properties of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) layers deposited on glass, mica, and PMMA (poly(methyl methacrylate)) surfaces was carried out by means of contact angles measurements (advancing and receding) for three probe liquids (diiodomethane, water, and formamide). DPPC was deposited on the surfaces from water (on glass and mica) or methanol (on PMMA) solutions. The topography of the tested surfaces was determined with a help of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Using the measured contact angles, the total apparent surface free energy and its components of the studied layers were determined from van Oss et al.'s (Lifshitz-van der Waals and acid-base components, LWAB) and contact angle hysteresis (CAH) approaches. It allowed us to learn about changes in the surface free energy of the layers (hydrophobicity/hydrophilicity) depending on their number and kind of support. It was found that the changes in the energy greatly depended on the surface properties of the substrate as well as the statistical number of monolayers of DPPC. However, principal changes took place for first three monolayers.

Journal Article↗
PubMed2006

PVP-coated iron nanocrystals: Anhydrous synthesis, characterization, and electrocatalysis for two species.

We report the synthesis of Fe nanocrystals (approximately 9 nm) in an anhydrous media, formamide, using poly(N-vinyl-2-pyrrolidone) (PVP) as a protecting agent. The morphology, structure, and composition of the PVP-coated Fe nanocrystals are studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and FT-Raman spectroscopy. The surface properties of the PVP-coated Fe nanocrystals are studied by electrochemistry and micro-surface-enhanced Raman scattering (mSERS) using pyridine as a probing molecule. The PVP-coated Fe nanocrystals, when immobilized on an electrode substrate, display very good electrocatalytic activities in the selective reduction of H2O2 in the presence of oxygen and in the oxidation of NO.

Catalysis↗