The NMR spectra of porphyrins—14: Self-aggregation of zinc(II) meso-nitro and meso-dinitro octaethylporphyrins

Complex formation in zinc(II) meso-nitro-octa-ethylporphyrin (1) and the corresponding α,β-dinitro (2) and α, γ-dinitro (3) zinc(II) chelates has been studied using proton NMR at 220MHz. This allows complete resolution of all the distinct groups in the proton spectrum, and the large concentration dependence of the spectra of 1 and 2 can be analysed to afford the monomeric and monomer-dimer shifts for all protons in these molecules. In contrast 3 shows no concentration dependence, nor any change upon addition of pyrrolidine, which immediately dissociates the aggregates of 1 and 2. The monomeric ¹H and ¹³C shifts are reported, together with those of zinc(II) octaethylporphyrin (4), and the complete assignment given allows the substituent shifts of the meso nitro groups in the porphyrin to be obtained. Analysis of the monomer-dimer shifts in terms of the ring current model gives the detailed geometries of the dimers, which have an inter-ring separation of ca 4.5 Å and a lateral displacement from the vertical of ca 1.0 Å. The results also allow the distinction between two different molecular complexes considered previously, and fully confirm our earlier suggestions that binding is due to metal-to-porphyrin, rather than metal-to-substituent, interactions.     

Search for a structural change in concentrated sodium sulfate solutions around the transition point temperature using the charge transfer to solvent spectrum of lodide ion as a probe

Claims have been made that aqueous sodium sulfate solutions exhibit an anomalous temperature dependence close to the transition point of the solid decahydrate (Na₂SO₄·10H₂O), viz., 32.38°C. In studies of dilute solutions these claims have been queried. Concentrated solutions, where this effect might be more readily detected, have not been investigated. A very accurate analysis of the shift of the charge transfer to solvent spectrum of iodide, a sensitive probe of its environment, (6×10^?5 M) in concentrated (1 and 2 M) sodium sulfate solutions, as a function of temperature has not evinced any support for a structural change in solution on traversing the transition point temperature.     

Hydroxyl-directed hydrogenation of homoallylic alcohols. Effects of achiral and chiral rhodium catalysts on 1,3 stereocontrol.

The hydroxyl-directed hydrogenation of homoallylic alcohols is examined with cationic rhodium-(I) and iridium-(I) catalysts. These reactions proceed with high levels of diastereoselection when either Rh(Diphos)⁺ or Ir(pry)Pcy₃⁺ are employed as catalysts.     

Identification and quantitation of N-acetyl metabolites of biogenic amines in the thoracic nervous system of the locust, Schistocerca gregaria, by gas chromatography-negative-ion chemical ionisation mass spectrometry

The N-acetylated metabolites of p-tyramine, p-octopamine and dopamine were identified unambiguously and quantitatively determined in a single ventral thoracic nerve cord of the locust, Schistocerca gregaria, by gas chromatography-negative-ion chemical ionisation mass spectrometry (GC-NICIMS). Deuterium-labelled analogues of each compound were added to a single ventral thoracic nerve cord in acetonitrile: the tissue was homogenised and the suspension centrifuged. The solvent was removed from the supernatant and the resultant residue was derivatised with trifluoroacetic anhydride. Under negative-ion chemical ionisation conditions, the trifluoroacetyl derivatives produced ions which were sufficiently abundant to be suitable for selected-ion monitoring. This method is highly specific and gave a limit of detection below the picogram levels. N-Acetyl-5-hydroxytryptamine was determined using a previously published GC-NICIMS technique [S.P. Markey, R.W. Colburn and J.N. Johannessen, Biomed. Mass Spectrom., 7 (1981) 301]. The concentrations of N-acetyltyramine, N-acetyloctopamine, N-acetyldopamine and N-acetyl-5-hydroxytryptamine in locust thoracic nerve cords were, respectively, 1.86 +/- 0.71, 1.13 +/- 0.34, 6.77 +/- 8.48 and 0.07 +/- 0.02 ng per tissue.     

Ab initio structure determination of BiPb2VO6 from powder diffraction data

The crystal structure of BiPb2VO6 has been determined from powder diffraction data using a combination of direct methods and the novel approach of applying simulated annealing methods simultaneously to X-ray and neutron data; BiPb2VO6 is a polar, noncentrosymmetric, second harmonic generation active material and its crystal structure is one of the more complex to be solved ab initio from powder diffraction data.     

Derivatisation of carboxylic acid groups in pharmaceuticals for enhanced detection using liquid chromatography with electrospray ionisation tandem mass spectrometry

Tris(2,4,6-trimethoxyphenyl)phosphonium propylamine bromide (TMPP) has been used for the derivatisation of maleic, fumaric, sorbic and salicylic acids to facilitate determination using liquid chromatography/electrospray ionisation tandem mass spectrometry (LC/ESI-MS/MS) in positive ion mode. Detection limits, achieved using multiple reaction monitoring mode, were 2, 4, 0.4 and 540 fmol (5 muL injection) for derivatised fumaric, sorbic, maleic and salicylic acids, respectively. In comparison, detection limits achieved in negative ion mode for the underivatised acids were 24, 51, 2, and 117 fmol, respectively. The method was successfully used for the determination of sorbic acid in a sample of Panadol. The derivatisation of salicylic acid was not as successful, probably due to poor reaction efficiency.     

Voltammetry at spatially heterogeneous electrodes

Recent advances are overviewed which enable simulation of the voltammetric behaviour of surfaces which respond in an electrochemically spatially heterogeneous fashion. By use of the concept of a “diffusion domain” computationally expensive three-dimensional simulations may be reduced to tractable two-dimensional equivalents. In this way the electrochemical response of partially blocked electrodes and microelectrode arrays may be predicted, and are found to be consistent with experimental data. It is, furthermore, possible to adapt the “blocked” electrode analysis to enable the voltammetric sizing of inert particles present on an electrode surface. Finally theory of this type predicts the voltammetric behaviour of electrochemically heterogeneous electrodes—for example composites whose different spatial zones display contrasting electrochemical behaviour toward the same redox couple.     

Ab initio calculations and normal coordinate analysis of ruthenium tris-alpha-diimine complexes

For the first time, a full scaled quantum chemical normal coordinate analysis has been performed on [Ru(LL')(3)](2+) complexes, where LL' = 2,2'-bipyrazine (bpz) or 2,2'-bipyrimidine (bpm). Geometric structures were fully optimized using density functional theory and an effective core potential basis set. The infrared and Raman spectra were calculated using the optimized geometries. The results of the calculations provide a highly satisfactory fit to the experimental infrared and Raman spectra, and the potential energy distributions allow a detailed understanding of the vibrational bands therein.     

Polymorph exploration of bismuth stannate using first-principles phonon mode mapping

Accurately modelling polymorphism in crystalline solids remains a key challenge in computational chemistry. In this work, we apply a theoretically-rigorous phonon mode-mapping approach to understand the polymorphism in the ternary metal oxide Bi₂Sn₂O₇. Starting from the high-temperature cubic pyrochlore aristotype, we systematically explore the structural potential-energy surface and recover the two known low-temperature phases alongside three new metastable phases, together with the transition pathways connecting them. This first-principles lattice-dynamics method is completely general and provides a practical means to identify and characterise the stable polymorphs and phase transitions in materials with complex crystal structures.

Using a phonon mode-mapping approach, we recover the known experimental phases of the ternary oxide Bi₂Sn₂O₇ and identify three new metastable phases, highlighting the utility of the method for polymorph prediction on many other complex materials.     

The dielectric constant of a polar liquid by the simulation of microscopic drops

We report a new method of computing the static dielectric constant for a polar liquid using a natural system, a drop of liquid in its own vapour. Results for the Stockmayer potential agree with those computed for a homogeneous “cyclic” liquid with Ewald corrections. The method is quite general.