Bimolecular QRRK analysis of methyl radical reactions

Reactions which proceed through energized adducts, including radical recombinations, insertions, and addition to unsaturates, frequently exhibit unusual kinetic behavior. The branching ratios among various product channels are often complex functions of both temperature and pressure. Four such reactions involving methyl radicals are analyzed by combining chemical activation distribution functions with QRRK methods to predict rate constants for each channel. These include three oxidation paths, CH₃ + O, CH₃ + O₂, CH₃ + OH, and the addition reaction CH₃ + C₂H₂. These predictions are compared to experiments wherever possible; generally, the agreement is quite satisfactory. Analysis of the energetics of the various reaction channels, using parameters which are readily available, provides a convenient framework for prediction. Suggested rate constants for the various channels for the four reactions are given at three pressures, 20, 760, and 7600 Torr, for the temperature range 300–2500 K. The approach used here can easily be applied to other reactions.     

Variations of the 1H spin-lattice relaxation in a fatty liver model as compared to normal liver

We report the results of an in vitro study on ethionine-injected rat liver (EI) and on normal rat liver (C) performed analyzing with iterative fitting procedures the 1H spin-lattice relaxation curves detected by IR pulse sequence at 20 MHz and at 37 degrees C on fresh excised samples. Single-exponential functions did not adequately describe the experimental curves both in EI and in C group. The analysis of the curves by two-exponential hypothesis showed a small portion of the signal characterizable by a time constant of about 80 ms, common both to EI and to C samples. A slowing of about 30% in the relaxation characterized the remaining portion of the curve (90-95%) in EI as compared to C samples. The hypothesis that the 1H of the triglycerides vacuoles present in EI livers had a relaxation curve additional to the remaining signal was checked by three-exponential analysis. The results were not in contrast with the known value of the triglycerides percentage content and with the spin-lattice relaxation time of the -CH2 group 1H obtained in different experimental conditions in the same fatty liver model. The negative results of the three-exponential analysis on normal liver curves as well as the favorable controls performed to test the analysis procedure supported further this hypothesis. The remaining signal after subtraction of the triglycerides contribution showed still the small fast portion and the increase of the relaxation time of the major portion (from approximately 300 ms up to approximately 400 ms) as compared to C samples.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spontaneous Assembly of Magnetic LEGO Bricks

This paper describes simple methods of demonstrating macroscale spontaneous assembly. These demonstrations can illustrate topics in college or high school chemistry courses, such as the thermodynamics of crystallization and the chelate effect. The assembling units are built from a combination of magnetic and conventional LEGO building blocks. Unlike many other spontaneous-assembly experiments, these assemblies do not require surface flotation to form.     

Microscopic reversibility and reciprocal relations for Brownian molecular machines

Chemists have made great progress in synthesizing molecules that emulate in part the remarkable properties of biological molecular motors, most especially the ability to use chemical energy to drive directed motion and do mechanical work. Here the mechanism of a molecular motor is treated as a renewal process in which the motor molecule fluctuates away from, and then returns to some arbitrary initial configurational state. During this excursion, some number of fuel molecules will have been catalytically converted to product, and the motor will have undergone some number of mechanical cycles in which work is done on the environment. The dependences of the number of catalytic and mechanical processes per renewal obey reciprocal relations for arbitrarily strong load force and chemical driving force. These relations characterize the behavior of the system far from thermodynamic equilibrium in the same way that the Onsager reciprocal relations characterize the system close to thermodynamic equilibrium.     

Towards aflatoxins: a formal synthesis of aflatoxin B2

The development of a formal synthesis of aflatoxin B2 is described, which utilizes a Dötz benzannulation reaction as a key step.     

Characterization of Tin(IV) Oxide Thin Films Prepared by Atmospheric Pressure Chemical Vapor Deposition of cis‐[SnCl4{OC(H)OC2H5}2]

A new single‐source precursor, [SnCl₄{OC(H)OC₂H₅}₂], prepared by treating tin tetrachloride with ethyl formate (1:2 ratio) was developed for the deposition of tin oxide thin films on glass substrates. The compound [SnCl₄{OC(H)OC₂H₅}₂] is highly volatile and provides very high growth rates (up to 100Å s^?1 at 560 °C) in an atmospheric pressure chemical vapor deposition (APCVD) reactor. More significantly, the compound does not decompose to tin oxide below 320 °C, thereby minimizing the formation of particles in the vapor above the growing tin oxide film. To prepare highly conducting fluorine doped tin oxide (SnO₂:F) films 2,2,2‐trifluoroethyl trifluoroacetate was used as the source of fluoride. High quality SnO₂:F films were deposited at 560 °C with a flow rate of 2 mL fluoride reagent hr^?1; typical film properties are resistivity of 5.9 X 10^?4 Ω cm, Hall mobility of 27.3 cm² V^?1 s^?1, carrier concentration of 3.9 X 10²⁰ cm^?3 and percent transmission ranging from 86 to 88 %. The best films of SnO₂:F possess transparencies as high as 90 % (750 nm), sheet resistances as low as 7 Ω sq^?1 and Haacke's figure of merit as high as 29 X 10^?3 (750 nm). The newly developed APCVD reactor and the chemistry were optimized with respect to structural, electrical and optical properties of the films by adjusting the substrate temperature, gas flow rates and the amount of fluoride present in the vapor stream. Growth rates with respect to deposition time, substrate temperature and flow rates of precursors were found to be similar for both undoped (SnO₂) and doped (SnO₂:F) samples. The SnO₂:F films possess larger grains than the SnO₂ which may account for the lower resistivity and the higher mobility in the SnO₂:F samples.     

NMR Characterization of Lanthanide(3+) Complexes of Tetraazatetrakisphosphinato and Tetraazatetra­kisphosphonato Ligands

The three‐dimensional structures in aqueous solution of the entire series of the Ln³⁺ complexes [Ln(DOTP*‐Et)]^? (formed from the free ligand P,P′,P″,P′′′‐[1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetrayltetrakis(methylene)]tetrakis[P‐ethylphosphinic acid] (H₄DOTP*‐Et) were studied by NMR techniques to rationalize the parameters governing the relaxivity of the Gd³⁺ complex and evaluate its potential as MRI contrast agent. From the ¹H‐ and ³¹P‐NMR lanthanide‐induced‐shift (LIS) values, especially of the [Yb(DOTP*‐Et)]^? complex, it was concluded that the [Ln(DOTP*‐Et)]^? complexes adopt in solution twisted square antiprismatic coordination geometries which change gradually their coordination‐cage structure along the lanthanide series. These complexes have no inner‐sphere‐H₂O coordination, and preferentially have the (R,R,R,R) configuration of the P‐atoms in the pendant arms. Self‐association was observed in aqueous solution for the tetraazatetrakisphosphonic acid ester complexes [Ln(DOTP*‐OEt)]^? (=[Ln(DOTP‐Et)]^?) and [Ln(DOTP*‐OBu)]^? (=[Ln(DOTP‐Bu)]^?) at and above 5 mM concentration, through analysis of ³¹P‐NMR, EPR, vapor‐pressure‐osmometry, and luminescence‐spectroscopic data. The presence of the cationic detergent cetylpyridinium chloride (CPC; but not of neutral surfactants) shifts the isomer equilibrium of [Eu(DOTP*‐OBu)]^? to the (S,S,S,S) form which selectively binds to the cationic micelle surface.     

Condensation and freezing of droplets on superhydrophobic surfaces

Superhydrophobic coatings are reported as promising candidates for anti-icing applications. Various studies have shown that as well as having ultra water repellency the surfaces have reduced ice adhesion and can delay water freezing. However, the structure or texture (roughness) of the superhydrophobic surface is subject to degradation during the thermocycling or wetting process. This degradation can impair the superhydrophobicity and the icephobicity of those coatings. In this review, a brief overview of the process of droplet freezing on superhydrophobic coatings is presented with respect to their potential in anti-icing applications. To support this discussion, new data is presented about the condensation of water onto physically decorated substrates, and the associated freezing process which impacts on the freezing of macroscopic droplets on the surface.