Magnetic resonance studies of aniline oligomers formed in the reaction of aniline with 1,1-diphenyl-2-picrylhydrazyl

Aniline oligomers, which are formed in the reaction of aniline with 1,1-diphenyl-2-picrylhydrazyl (DPPH) in methanol and chloroform as solvents, are investigated by means of magnetic resonance techniques. The electron paramagnetic resonance (EPR) spectrum of the products of the reaction of aniline with DPPH (mole ratio, 2∶1) in chloroform solution, and of aniline with DPPH (mole ratio, 1∶2) in methanol solution, were recorded as the reaction progressed, and after the reaction was finished. The spectra of the products after reaction were resolved into the spectra of two species. For the acidified methanol solution the spectrum of one of the species shows hyperfine coupling to two nitrogen atoms with a coupling constanta(2·¹⁴N)=5.8 G, in good agreement with previously reported observations for the radical cation of model aniline trimers. A species is observed in the chloroform solution witha(1·¹⁴N)=11.4 G, which suggests the presence of aniline dimers. Both solutions also show a second signal which is attributed to higher oligomer radical cations with unresolved¹⁴N hyperfine structure.¹H nuclear magnetic resonance (NMR) spectra of aniline and aniline-DPPH solutions in CDCl₃ with the peaks due to DPPHH, the product of reduction of DPPH, were observed. The¹H NMR experiments show that only 15% of the available aniline was consumed, in agreement with the theoretical calculations for the formation of polyaniline-emeraldine base salt under these conditions rather than the sole formation of dimers, which would be expected to consume 33% of the available aniline. This is probably due to formation of partially oxidized long-chain aniline oligomers as the main reaction product.     

Transient oscillations under strongly mismatched Hartmann-Hahn conditions

Based on the standard 2D Polarization inversion experiment, a new pulse sequence (PI-TAPF) is proposed. It represents a combination of Polarization Inversion and TAPF (time averaged precession frequency) sequences. The depolarization period consists of phase-alternating intervals of different duration in the I channel. The pulse sequence yields transient oscillations under the strongly mismatched HH conditions where the required power for the dilute spins is reduced by a factor of 5. Experimental results recorded for a sample of ferrocene powder are well reproduced by numerical simulations.     

Spectral tuning of shortwave-sensitive visual pigments in vertebrates

Of the four classes of vertebrate cone visual pigments, the shortwave-sensitive SWS1 class shows some of the largest shifts in lambda(max), with values ranging in different species from 390-435 nm in the violet region of the spectrum to < 360 nm in the ultraviolet. Phylogenetic evidence indicates that the ancestral pigment most probably had a lambda(max) in the UV and that shifts between violet and UV have occurred many times during evolution. In violet-sensitive (VS) pigments, the Schiff base is protonated whereas in UV-sensitive (UVS) pigments, it is almost certainly unprotonated. The generation of VS pigments in amphibia, birds and mammals from ancestral UVS pigments must involve therefore the stabilization of protonation. Similarly, stabilization must be lost in the evolution of avian UVS pigments from a VS ancestral pigment. The key residues in the opsin protein for these shifts are at sites 86 and 90, both adjacent to the Schiff base and the counterion at Glu113. In this review, the various molecular mechanisms for the UV and violet shifts in the different vertebrate groups are presented and the changes in the opsin protein that are responsible for the spectral shifts are discussed in the context of the structural model of bovine rhodopsin.     

Structural and spectroscopic studies on mercury(II) tribenzylphosphine complexes

The tribenzylphosphine (PBz3) complexes of mercury(II), [Hg(PBz3)2](BF4)2, [Hg(PBz3)2(NO3)2] and [HgX(NO3)(PBz3)](X = Cl, Br, I and SCN), have been synthesised and their structures determined by single-crystal X-ray crystallography. [Hg(PBz3)2](BF4)2 contains [Hg(PBz3)2]2+ cations with linear P-Hg-P coordination, the first example of a truly two-coordinate [Hg(PR3)2]2+ complex. The mercury coordination in [Hg(PBz3)2(NO3)2] can be described as distorted tetrahedral, with a significant deviation of the P-Hg-P angle from linearity as a result of coordination of the nitrate groups. Nitrate coordination is also observed in [HgX(NO3)(PBz3)](X = Cl, Br, I), resulting in significantly non-linear P-Hg-X coordination. The thiocyanate complex is a centrosymmetric thiocyanate-bridged dimer with distorted trigonal-pyramidal mercury coordination to the P atom of PBz3, to the S and N atoms of two bridging thiocyanate groups, and to the O atom of one nitrate group. For all the nitrato complexes, secondary mercury-nitrate interactions (Hg-O 2.7-3.1 A) effectively raise the coordination number of the Hg(II) centres to six. High-resolution 31P solid-state NMR spectra of the six tribenzylphosphine mercury(II)-complexes, obtained by combining magic-angle spinning, proton dipolar decoupling and proton-phosphorus cross-polarization (CP-MAS), have been recorded. The spectra of [Hg(PBz3)2](BF4)2 and [HgX(NO3)(PBz3)](X = Cl, Br, I and SCN) exhibit a single line, due to species that contain non-magnetic isotopes of mercury, and satellite lines, due to 1J(31P-199Hg) coupling. The asymmetric unit of [Hg(PBz3)2(NO3)2] contains two molecules with four phosphorus environments, resulting in two AB spectra with 2J(31P-31P) coupling, due to species that contain non-magnetic isotopes of mercury, and satellite lines consisting of two ABX spectra, due to 1J(31P-199Hg) coupling. These spectra have been analysed to yield all of the chemical shifts and coupling constants involved. A remarkable increase in 1J(31P-199Hg) is observed from [Hg(PBz3)2](BF4)2 to [Hg(PBz3)2(NO3)2] as a consequence of the incorporation of the nitrate group into the Hg coordination sphere in the latter case. Several of the spectra also exhibit broader satellites due to the presence of scalar spin-spin coupling between 31P and the quadrupolar 201Hg nucleus. Slow-spinning methods have been used to analyze the spinning-sideband intensities of the NMR spectra, in order to obtain the 31P shielding anisotropy and asymmetry parameters Deltasigma and eta. The 199Hg and 31P NMR shielding tensors of PMe3 models of the above six compounds have been calculated using relativistic density functional theory. The 31P results are in good agreement with experiment and assist in the assignment of some of the signals.     

ChemInform Abstract: Crystal Structures and Vibrational Spectroscopy of [NBu4][Cu(CN)X] (X: Br,I) and [NBu4][Cu3(CN)4]×CH3CN.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.