Mass spectrometry of nitroazoles: 2—the mass spectra of methyl substituted nitroimidazoles

The mass spectra of six isomers of methylnitroimidazoles are reported and discussed. All compounds exhibit strong molecular ions, along with the characteristic fragmentations of aromatic nitro compounds. In some cases ortho effects—losses of OH^., H₂O, CHO^., CH₂O—are observed, due to interactions of adjacent substituents.     

The effect of preparation conditions on the activity and stability of copreciptitated Ni/Al2O3 catalysts for the methanation of carbon monoxide

The activities and stabilities of coprecipitated Ni/Al₂O₃ methanation catalysts depend markedly on their preparation and pretreatment. The results are discussed in relation to the structure of the precipitate and to changes which occur during calcination and reduction.

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Ni/Al₂O₃ . , .

     

On non-analytic thermodynamic behaviour of the classical linear chain

The free energy ? per particle and the two-particle distribution function g have been expressed as functionals of the nearest-neighbour interaction Φ(x) for a variety of boundary conditions. For some boundary conditions and potentials Φ(x), ? and g behave singularly at a critical density

$\text{ρ}{}_{\mathrm{<mtext>c</mtext>}}\text{=}\text{lim}\text{s→0}{}^{\mathrm{+}}\text{∫}\text{0}\text{∞}\text{e}{}^{\mathrm{?sx?\beta \Phi (x)}}\text{d}\text{x?}\text{∫}\text{0}\text{∞}\text{x}\text{e}{}^{\mathrm{?sx?\beta \Phi (x)}}\text{d}\text{x.}$

     

PICOSECOND LASER SPECTROSCOPY AND OPTICALLY DETECTED MAGNETIC RESONANCE ON A MODEL PHOTOSYNTHETIC SYSTEM

Fluorescence-detected magnetic resonance of triplets in zero magnetic field (FDMR), fluorescence fading (FF) due to triplet-formation, both at 4.2 K, and prompt fluorescence decay kinetics (FDK) at room temperature have been measured for free pheophorbide- a (f-Pheo) and bound (b-Pheo) to a synthetic polypeptide (L-L ys -L-A la -L-A la )_n, dissolved in dimethylformamide (DMF). Fluorescence decay kinetics measurements of f-Pheo in DMF yielded 1-5 ns lifetimes, for b-Pheo in DMF a ~ 50 ps decay-component was found emitting at 730–750 nm. Zero-field splitting parameters |D| and |E| of the lowest triplet state T₁ were determined from FDMR spectra as (337 and 24) 10^-4 cm^-1 for f-Pheo and (359 and 25) 10^-4 cm^-1 for b-Pheo, both in DMF. Decay rate constants of the three spin levels of T¹ of b-Pheo ( K _x= 1200 50 s^-1, k _y= 440 25 s^-1, k _z= 80 5 s^-1) and relative steady-state populations (N_x= 28 2%, N_y= 47 2%, N_z= 26 2%) determined from FF curves predict a fluorescence decrease at the D–E and D + E FDMR transitions, whereas experimentally a fluorescence increase is observed. The FDMR sign-inversion results from singlet-singlet energy transfer from b-Pheo monomers to their aggregates, followed by fast intersystem crossing to T₁. These results indicate that aggregates are formed by two or more b-Pheo molecules at different positions on the folded polypeptide chain. This situation resembles that in chlorophyll-proteins, containing low-lying traps, resulting from interaction of chromophores with other chromophores and with the protein environment.     

A hard sphere model for order-disorder transitions in colloidal dispersions

Use of a hard sphere model and the concept of an effective hard sphere diameter of a colloidal particle with its associated double layer is reported. This method allows rapid determination of the order-disorder transition in colloidal dispersions and yields reasonable estimates of the osmotic pressures compared with “exact” Monte Carlo calculations.     

Covalent amination of 1-alkyl- and 1-aryl-3-carbamoylpyridinium chlorides as “model” for enzymic activity of rabbit liver aldehyde oxidase

The site of animation for 1-alkyl-3-carbamoylpyridinium chlorides in liquid ammonia is dependent on the identity of the 1-alkyl substituent. For the methyl, ethyl and n-propyl derivatives exclusively 6-adducts are found. Adduct formation takes place at C-6 and C-4, when the 1-substituent is an i-propyl or t-butyl group. The adduct ratio for the latter compounds is determined by the size of the substituent. 1-Aryl derivatives exhibit amination at C-2 and C-6 and the adduct ratios are dependent on the temperature. When the aryl substituent is a 2,4,6-trimethylphenyl group the 4-adduct is detected as well. A comparison is made between the sites of oxidation of these compounds by rabbit liver aldehyde oxidase and the covalent amination pattern in liquid ammonia. It is shown that covalent animation as a “model” for the enzymic activity of aldehyde oxide1 is particularly valuable in cases where the enzyme reaction is controlled by steric factors.     

Convergent synthesis of noncovalent metallodendrimers containing hydrophobic dendrons at the periphery.

The noncovalent synthesis of "layer-block" metallodendrimers containing hydrophobic shells of covalent dendritic wedges at the periphery is described. Starting from first- and second-generation Fréchet wedges having phosphines at their focal point, convergent dendritic growth yields third- and fourth-generation metallodendrimers in which the coordination of nitriles, pyridines, and phosphines to SCS Pd(II) pincers is used as the assembly motif. In this convergent growth, the number of terminal hydrophobic phosphine wedges increases with generation. The solubility of the dendritic structures in apolar organic solvents such as chloroform and dichloromethane increases accordingly, in contrast to previously reported metallodendrimers. All dendritic structures were characterized by (1)H and (31)P NMR spectroscopy, elemental analysis, and MALDI-TOF mass spectrometry.     

Substitution Reactions of cis- and/or trans-Bromomesitylbis(triethylphosphine) Platinum (II) with Thiourea in Ethanol,Dimethylsulfoxide, and Acetone

The pressure and/or temperature dependencies of the rates of substitution of thiourea in cis- and/or trans-[PtBr(2, 4, 6-Me₃Ph)(Et₃P)₂] were studied in ethanol, DMSO, and acetone. The rate constants for both the nucleophile independent and nucleophile dependent reactions were measured. The rate and activation parameters are discussed in reference to data reported earlier for these reactions in methanol.     

Tuning chirality of single-wall carbon nanotubes by selective etching with carbon dioxide

CO2 molecule chemisorbs selectively on the zigzag tube edge without an activation barrier, whereas it physisorbs on the armchair edge of nanotubes. In addition, carbon nanotubes can be etched by an adsorbed oxygen atom of CO2 molecule. From our results, we suggest a selective etching mechanism for tuning the chirality of the mass-produced carbon nanotubes.