Receiver gain function: the actual NMR receiver gain

The observed NMR signal size depends on the receiver gain parameter. We propose a receiver gain function to characterize how much the raw FID is amplified by the receiver as a function of the receiver gain setting. Although the receiver is linear for a fixed gain setting, the actual gain of the receiver may differ from what the gain setting suggests. Nevertheless, for a given receiver, we demonstrate that the receiver gain function can be calibrated. Such a calibration enables accurate comparison of separately acquired NMR signals in quantitative analysis, which frequently requires different receiver gain settings to avoid receiver saturation or achieve optimum sensitivity. The application of receiver gain function, along with the definition of receiving efficiency, allows easy concentration determination by a single internal or external concentration reference. Copyright © 2010 John Wiley & Sons, Ltd.     

Complexation of lanthanides, actinides and transition metal cations with a 6-(1,2,4-triazin-3-yl)-2,2':6',2'-terpyridine ligand: implications for actinide(III)/lanthanide(III) partitioning

The quadridentate N-heterocyclic ligand 6-(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2'?:?6',2'-terpyridine (CyMe(4)-hemi-BTBP) has been synthesized and its interactions with Am(III), U(VI), Ln(III) and some transition metal cations have been evaluated by X-ray crystallographic analysis, Am(III)/Eu(III) solvent extraction experiments, UV absorption spectrophotometry, NMR studies and ESI-MS. Structures of 1:1 complexes with Eu(III), Ce(III) and the linear uranyl (UO(2)(2+)) ion were obtained by X-ray crystallographic analysis, and they showed similar coordination behavior to related BTBP complexes. In methanol, the stability constants of the Ln(III) complexes are slightly lower than those of the analogous quadridentate bis-triazine BTBP ligands, while the stability constant for the Yb(III) complex is higher. (1)H NMR titrations and ESI-MS with lanthanide nitrates showed that the ligand forms only 1:1 complexes with Eu(III), Ce(III) and Yb(III), while both 1:1 and 1:2 complexes were formed with La(III) and Y(III) in acetonitrile. A mixture of isomeric chiral 2:2 helical complexes was formed with Cu(I), with a slight preference (1.4:1) for a single directional isomer. In contrast, a 1:1 complex was observed with the larger Ag(I) ion. The ligand was unable to extract Am(III) or Eu(III) from nitric acid solutions into 1-octanol, except in the presence of a synergist at low acidity. The results show that the presence of two outer 1,2,4-triazine rings is required for the efficient extraction and separation of An(III) from Ln(III) by quadridentate N-donor ligands.     

Stress-Softening and reinforcement of rubber

The first part of the paper is concerned with stress-softening in rubber vulcanizates. Recent theories are briefly discussed and results are presented to demonstrate the authors' theory that the phenomenon of stress-softening in filled vulcanizates is due to the gum phase of the composite. A simple model demonstrating stress-softening effects is also presented.

Following the authors' earlier work on the importance of hysteresis in determining the strength of rubber vulcanizates, in the second part of the paper, they apply this work to filler-loaded vulcanizates. It is found that when data are expressed independently of time and temperature, unfilled and filled results can be unified by the use of a simple hydrodynamic correction factor. On the other hand, when data are expressed in terms of time or temperature, an order of magnitude is obtained between the gum and filler-loaded vulcanizate results. The meaning of this anomaly is discussed and the authors' own conclusions on the most likely type of model for the reinforcement of rubbers is presented.     

A general procedure for the synthesis of isoxazolidin-5-ones

Conjugate addition of N-substituted hydroxylamines to α,β-unsaturated esters followed by cyclisation of the adducts with lithium bis (trimethylsily)amide provides the first general means of synthesising isoxazolidin-5-ones, the N-benzyl derivatives of which may be hydrogenolised to β-aminoacids.     

Two-dimensional NMR studies of 2-substituted thioxanthene sulfoxides

2-Fluoro-, 2-chloro-, 2-bromo-, 2-methyl-, and 2-methoxythioxanthene have been prepared by borane reduction of the corresponding thioxanthone. The corresponding sulfoxides were prepared by m-chloroperoxybenzoic acid oxidation of these sulfides. Proton and carbon chemical shifts have been assigned to these thioxanthene sulfoxides with the aid of LROCSCM and SROCSCM experiments. Carbon chemical shifts in the unsubstituted rings occur at approximately 125 ppm (C5); 128 ppm (C6); 130 ppm (C7); 128 ppm (C8); and 36 ppm (C9). The methylene protons appears as AB doublets at approximately 4.2 and 3.8 ppm. All sulfoxides have the same, pseudo-equatorial geometry.     

Chemical synthesis of cross-link lesions found in nitrous acid treated DNA: a general method for the preparation of N2-substituted 2'-deoxyguanosines

Treatment of DNA with nitrous acid results in the formation of DNA-DNA cross-links. Two cross-link lesions have previously been isolated and their structures assigned based on spectroscopic data. The major lesion has been proposed to consist of two deoxyguanosine (dG) nucleosides sharing a common N2 atom (1), while the structure of the minor lesion has been proposed to consist of a common nitrogen atom linking C2 of a dG nucleoside to C6 of deoxyadenosine (2). The chemical synthesis of 1 and 2, utilizing a palladium-catalyzed coupling, is described herein. It is demonstrated that the spectroscopic properties of synthetic 1 are identical to that of lesion 1 obtained from nitrous acid cross-linked DNA, thus providing a proof of its structure. Comparison of the limited spectroscopic data available for lesion 2 originating from nitrous acid cross-linked DNA to synthetic 2 supports its structural assignment. The synthetic approach used for synthesis of 1 and 2 is shown to be a general method for the preparation of a variety of N2-substituted dG nucleosides in good yields.     

Alternating copolymerization of methyl acrylate with donor monomers having a protected amine group

Attempts were made to copolymerize p-aminostyrene, p-acetamidostyrene, N-methyl-p-aceta-midostyrene, N-(4-vinylphenyl) phthalimide, N-vinyl succinimide, and N-vinyl phthalimide with methyl acrylate complexed with ethyl aluminum sesquichloride. Only reactions involving N-(4-vinylphenyl)phthalimide and N-vinyl phthalimide yielded alternating copolymers. N-vinyl succinimide gave nonalternating copolymers insoluble in common solvents and the other monomers did not copolymerize. In some cases, the conventional radical copolymers were prepared for comparison purposes. The reactivity ratios of the free-radical initiated copolymerization of methyl acrylate (I) with N-(4-vinylphenyl)phthalimide (II) were r₁ = 0.14 and r₂ 1.56. The alternating copolymers were studied by ¹H-NMR and ¹³C-NMR spectroscopy. The alternating copolymer of N-(4-vinylphenyl)phthalimide with methyl acrylate was hydrazinolyzed to form the alternating copolymer of methyl acrylate with p-aminostyrene. Hydrazinolysis of the alternating copolymer of methyl acrylate with N-vinyl phthalimide removed the phthalimide moiety and generated vinyl amine units which readily cyclized with neighboring methyl acrylate units to form copolymers that contained five-membered lactam rings. The infrared (IR) spectra of the hydrazinolyzed products contain bands due to amine or amide groups and are devoid of the characteristic bands of the phthalimide ring.