Applications of fast atom bombardment mass spectrometry (FAB MS) to organometallic and coordination chemistry

This article describes the technique of fast atom bombardment mass spectrometry (FAB MS) and its applications to the analysis of organometallic and coordination complexes. Sections on ion generation and matrices are followed by a Periodic Group-based review of FAB MS results obtained from Transition Metal and Main Group compounds, organometallic cluster carbonyls and their derivatives, and salts of poly-oxo-anions. The literature has been surveyed from 1981 to approximately mid-1986.     

Photochemistry of artificial photosynthetic reaction centers in liquid crystals probed by multifrequency EPR (9.5 and 95 GHz)

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C(60)(1) have been followed by multifrequency time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (X-band) and high field and frequency (W-band). The electron-transfer process has been characterized in the different phases of two uniaxial liquid crystals (E-7 and ZLI-1167). The triad undergoes photoinduced electron transfer, with the generation of a long-lived charge-separated state, and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. Both the photoinduced spin-correlated radical pair and the spin-polarized recombination triplet are observed starting from the crystalline up to the isotropic phase of the liquid crystals. The W-band TREPR radical pair spectrum has allowed unambiguous assignment of the spin-correlated radical pair spectrum to the charge-separated state C(.+)-P-C(60)(.-). The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum and the spin-selective recombination rates have been derived from the time dependence of the spectrum. The weak exchange interaction parameter (J = +0.5 +/- 0.2 G) provides a direct measure of the dominant electronic coupling matrix element V between the C(.+)-P-C(60)(.-) radical pair state and the recombination triplet state (3)C-P-C(60). The kinetic parameters have been analyzed in terms of the effect of the liquid crystal medium on the electron-transfer process. Effects of orientation of the molecular triad in the liquid crystal are evidenced by simulations of the carotenoid triplet state EPR spectra at different orientations of the external magnetic field with respect to the director of the mesophase. The order parameter (S = 0.5 +/- 0.05) has been evaluated.     

Thermal analysis and the scientific conservation of cultural materials

Prior to treatment, letters dating from 1665–66 were characterised using SEM, FTIR, ESCA or XPS, TG, TMA and DLTMA. All three papers were similar in composition and were basically cellulosic materials. Two which had been sent from Lisbon contained trace amounts of Fe and Cu; these impurities were also present in a letter sent from Dublin. All three letters had been written with iron gall ink. The letter from Dublin had an additional feature in the ink in the form of small bright specks of mica. The papers were examined after conservation treatment using the above same techniques in an attempt both to determine and quantify any changes which had occurred during the treatment process. Scanning electron micrographs showed that propounced changes had occurred to the surfaces of the treated papers. With XPS it was possible to measure the change in the surface composition of the papers which occurred on treatment. It was also found that the treatment reduced the thermal stability of the papers in all three cases.     

Noise in atomic absorption flame spectrometry—a comparison between single beam and double beam instruments

Noise levels have been measured under a variety of conditions using comparable single and double beam instruments. At low absorbance, in the vicinity of the detection limit, the major types of noise are photon noise, flame transmission noise and lamp flicker noise. The precision at low absorbance is slightly better with single than double beam providing a blank correction is made before and after each sample measurement. At higher absorbances, analyte absorption noise is dominant and there is no difference in precision between single and double beam.     

A combined 14N/27Al nuclear magnetic resonance and powder X-ray diffraction study of impurity phases in beta-sialon ceramics

Beta-sialons are ceramic phases occurring in the SiO(2)-Si(3)N(4)-AlN-Al(2)O(3) system. A series of samples with differing compositions has been investigated by magic-angle spinning nuclear magnetic resonance (NMR) spectroscopy and powder X-ray diffraction (XRD). Although the constituent nitrogen nuclei occupy positions of low symmetry in the beta-sialon structure, 14N NMR spectra could be recorded for the samples examined. The origin of the 14N signal could be traced to the presence of an aluminium nitride (AlN) impurity phase with the help of 27Al NMR and XRD results. Similarly, the existence of Al(2)O(3) grains could be readily detected for a number of samples. Thus, the combination of 14N and 27Al NMR is shown to be an especially effective tool in identifying and characterizing impurity phases in sialon ceramics, complementing the results obtained from standard XRD analysis.     

Oxidative coupling of porphyrins using copper(II) salts

A method for radical coupling of porphyrins using copper(II) salts as one-electron oxidants was developed. A Zn(II)-porphyrin bearing an aminophenyl group yielded porphyrin oligomers, and two tri-arylporphyrins were oxidized to form doubly and triply linked dimers. Bromination of doubly linked dimers gave macrocycles with twisted skeletons.     

Photoinduced electron transfer in a hexaphenylbenzene-based self-assembled porphyrin-fullerene triad

A hexaphenylbenzene-based zinc porphyrin dyad forms a 1:1 complex with a fullerene bearing two pyridyl groups via coordination of the pyridyl nitrogens with the zinc atoms. The fullerene is symmetrically located between the two zinc porphyrins. The binding constant for the complex is 7.3 x 10(4) M(-1) in 1,2-difluorobenzene. Photoinduced electron transfer from a porphyrin first excited singlet state to the fullerene occurs with a time constant of 3 ps, and the resulting charge-separated state has a lifetime of 230 ps. This self-assembled construct should form a basis for the construction of more elaborate model photosynthetic antenna-reaction center systems.     

NMR spectra of carotenoporphyrins. Computer-assisted conformational analysis

A computer-assisted method of conformational analysis for porphyrin molecules bearing flexible side-chains has been developed. The method utilizes the ring current-induced chemical shift changes of the side-chain protons which arise from the porphyrin macrocycle and any attached aryl rings. The treatment has been applied to a series of carotenoporphyrin molecules, which are important as models for a variety of photophysical processes in biological systems. Chemical shift data of sufficient accuracy for the conformational analysis were obtained from 500 MHz NMR experiments. The conformations of the carotenoporphyrins varied from extended ones with the carotenoid well away from the porphyrin ring to tightly folded species, depending on molecular constitution. The analytical method can be extended to other porphyrin-based systems.     

Energy transfer and spin polarization of the carotenoid triplet state in synthetic carotenoporphyrin dyads and in natural antenna complexes

A series of carotenoporphyrin dyads, in which the carotenoid is covalently linked to a tetraarylporphyrin at the ortho, meta or para position of one of the meso aromatic rings, has been studied using Time-Resolved Electron Paramagnetic Resonance (TREPR) spectroscopy. In parallel, an investigation has been carried, on two different photosynthetic antenna systems, the B800–B850 complex ofR. acidophila and the LHCII complex of higher organisms. The initial spin polarization of the carotenoid triplet-state, populated indirectly by laser excitation, has been detected. It has been demonstrated that the initial polarization is not a characteristic property of the carotenoid triplet-state, as previously stated, but depends on the donor-acceptor mutual orientation. The triplet energy transfer to the carotenoid from a chlorophyll or porphyrin triplet state is discussed on the basis of the observed spin polarization.     

Driving force and electronic coupling effects on photoinduced electron transfer in a fullerene-based molecular triad

Tuning thermodynamic driving force and electronic coupling through structural modifications of a carotene (C) porphyrin (P) fullerene (C60) molecular triad has permitted control of five electron and energy transfer rate constants and two excited state lifetimes in order to prepare a high-energy charge-separated state by photoinduced electron transfer with a quantum yield of essentially unity (> or = 96%). Excitation of the porphyrin moiety of C-P-C60 is followed by a combination of photoinduced electron transfer to give C-P(.+)-C60.- and singlet-singlet energy transfer to yield C-P-1C60. The fullerene excited state accepts an electron from the porphyrin to also generate C-P(.+)-C60.-. Overall, this initial state is formed with a quantum yield of 0.97. Charge shift from the carotenoid to yield C(.+)-P-C60.- is at least 60 times faster than recombination of C-P(.+)-C60.-, leading to the overall quantum yield near unity for the final state. Formation of a similar charge-separate species from the zinc analog of the triad with a yield of 40% is also observed. Charge recombination of C(.+)-P-C60.- in 2-methyltetrahydrofuran yields the carotenoid triplet state, rather than the ground state. Comparison of the results for this triad with those for related triads with different structural features provides information concerning the effects of driving force and electronic coupling on each of the electron transfer steps.