Yields of Br* (4P1/2) as a function of wavelength in the photodissociation of Br2 and IBr

A flashlamp-pumped tunable dye laser has been used to study the photochemical production of Br* (4²P_1/2) atoms from Br₂ and IBr exc     

Transport processes in the sorption of colored ions by peat particles

A three-step model has been proposed for the adsorption of Astrazone Blue dye (Basic Blue 69) on peat. The initial rate of uptake of dye ions due to physical adsorption and chemisorption (ion exchange) has been correlated using a surface mass transfer coefficient. These coefficients have been determined and expressed in the dimensionless mass transfer form, Sh/Sc^0.33, as a function of agitation, initial dye concentration, peat particle size range, dye solution temperature, and mass of peat.     

1,2-Bis(2,5-diphenylphospholano)methane, a new ligand for asymmetric hydrogenation

1,2-Bis(2,5-diphenylphospholano)methane (Ph-BPM) has been prepared in good yield from 2,5-trans-diphenylphospholane-borane adduct. Rhodium and ruthenium complexes of this ligand have been prepared and their usefulness in asymmetric hydrogenation has been investigated. [Ph-BPM Rh(COD)]BF₄ showed high activity and selectivity for itaconate and dehydroamino acid hydrogenation. Ph-BPM RuCl₂(DPEN) was effective for imine hydrogenation.     

The synthesis of chiral functionalised morpholine N-oxides using a tandem Cope elimination/reverse-Cope elimination protocol

Hydroxylamine derivatives of (S)-prolinol have been generated using a Cope elimination. These undergo reverse-Cope elimination onto a pendant double bond to give morpholine N-oxides containing three contiguous chiral centres.     

Charge calculations in molecular mechanics. Part 8 Partial atomic charges from classical calculations

Summary The CHARGE2 programme, which involves the classical calculation of both the inductive and resonance contributions to the partial atomic charges in molecules is described, and the charges and electrostatic potentials obtained presented for some illustrative examples.In substituted methanes (CH₃X, CF₃X, CCl₃X) the effects of varying the electronegativity of the substituents and the - and -substituent contributions are clearly illustrated for a variety of substituent groups X.The problems involved in the inclusion of silicon into this scheme are detailed, together with the methods of overcoming them. The partial atomic charges ( and contributions) and electrostatic potentials for some silicon oxygen compounds are presented and discussed.The partial atomic charges from CHARGE2 for all the natural amino acids as their N-acetyl, N-methyl-amides are given and compared with those obtained from the AMBER and ECEPP/2 force fields. Considerable differences in these figures are observed, with the AMBER charges consistently much larger than those from the other two methods.The CHARGE2 partial atomic charges and electrostatic potentials for the four common nucleic acids, adenine, cytosine, guanine and thymine, are given and compared with those derived from other calculations. Again there is general similarity but also there are considerable differences, with those from the AMBER force field somewhat larger than the other methods.For previous parts in this series, see Refs. 1-7.     

New In-situ Sampling and Analysis of the Production of CeO<Subscript>2</Subscript> Powders from Liquid Precursors using a Novel Wet Collection System in a rf Inductively Coupled Thermal Plasma Reactor. Part 1: Reactor System and Sampling Probe

A new reactor and a novel in-situ sampling technique were developed for the study of the synthesis of CeO₂ powders produced from dissolved cerium nitrate salts. The conical reactor minimized particle recirculation and provided a highly symmetrical and undisturbed plasma flow suitable for the analysis of the phenomena affecting the formation of CeO₂ powders. Both a calorimetric study of the reactor and a thermodynamic analysis of CeO₂ formation were conducted. The sampling probe is described and near-isokinetic sampling was achieved. The sampled particles were collected using a miniature wet collection system, i.e. a mist atomizer and a custom-made spray chamber. A numerical simulation of the velocity and temperature fields of the plasma gas in the reactor was done using Fluent. A comprehensive droplet-to-particle formation mechanism presented elsewhere is revisited and expanded based on calorimetry, thermodynamics of CeO₂ formation, numerical simulations and collected particles. No traces of other oxidation states other than CeO₂ were found.     

Efficient unimolecular deprotonation of aniline radical cations

[reaction: see text] Deprotonation of the radical cations of aromatic amines, such as anilines, generally occurs much more slowly than other fragmentation reactions. Here we report a stereoelectronic effect involving twisting of the anilino group out of the plane of the benzene ring that results in a significantly increased rate of reactivity toward deprotonation. Quantitative studies of the rate constants for deprotonation as a function of aniline radical cation pKa (Br?nsted plots) demonstrate that the effect is not simply due to a change in the reaction thermodynamics. By combining this stereoelectronic effect with covalent attachment of carboxylate as a base, aniline radical cations that undergo unimolecular deprotonation with rate constants as high as 10(8) s(-1), even in unfavorable protic media, are described.     

Conformational substates modulate hydride transfer in dihydrofolate reductase

In earlier studies of the hydride-transfer reaction catalyzed by dihydrofolate reductase (DHFR) we identified features of the protein correlated with variations in the reaction barrier. We extend the scope of those studies by carrying out potential of mean force (PMF) simulations to determine the hydride-transfer barrier in the wild-type protein as well as the G121V and G121S mutants. While our prior studies focused on the reactant state, our current work addresses the full reaction pathway and directly probes the reactive event. The free energy barriers and structural ensembles resulting from these PMF calculations exhibit the same trends reported in our previous work. Fluctuations present in these simulations also exhibit trends associated with differences in the hydride-transfer barrier height. Moreover, vibrational modes anticipated to promote hydride transfer exhibit larger amplitudes in simulations that generate lowered barriers. The results of our study indicate that discrete basins (substates) on a potential energy landscape of the enzyme give rise to distinct hydride-transfer barriers. We suggest that the long-range effects of mutations at position 121 within DHFR are mediated by differentially preorganized protein environments in the context of distinct substate distributions, with concomitant changes to the dynamic properties of the enzyme.     

The photochemical reactions of α,β,γ,δ -unsaturated diazo compounds at low temperature

The α,β:γ,δ-unsaturated tosylhydrazone lithium salts (4) undergo photolysis at ?60°C to give vinylcyclopropenes (10) and [l,2]diazeto[l,4-a] pyrroles (11) and/or (12). The formation of (11, X=H) establishes the intermediacy of (8, X=H) in the formation of the 3$\text{H}$-1,2-diazepine (9). The diazoalkene (5, X=Me) showed opposite electrocyclisation periselectivity to its thermal cyclisation and gave (11, X=Me) $\text{via}$ (8, X=Me), rather than the pyrazoles (2) and (3).     

First Synthesis of 3-Mercapto-2(1H)-pyridinone, a Simple Disubstituted Pyridine Useful for Synthesis of the 4-Azaphenoxathiine Ring System and Its Novel Diazaphenoxathiine Analogs: 1,6-Diazaphenoxathiine and 2,6-Diazaphenoxathiine(1)

3-Mercapto-2(1H)-pyridinone (1) can be synthesized in three simple high-yielding steps from readily available 2-tert-butylthiazolo[4,5-b]pyridine (2). Its disodium salt condenses with o-chloronitrobenzene, 2-chloro-3-nitropyridine, and 3-chloro-4-nitropyridine 1-oxide to give respectively 4-azaphenoxathiine (10), 1,6-diazaphenoxathiine (12), and 2,6-diazaphenoxathiine 2-oxide (14) which reduces to 2,6-diazaphenoxathiine (15). The structures of these previously unreported azaphenoxathiine systems were confirmed by assignment of their respective (13)C NMR spectra.