Neutral metal−benzene complexes, M n (benzene) m (M = Sc to Cu), are produced for all of the 3d transition metals in the gas phase by using the laser vaporization method. These species are characterized by mass spectrometry, photoionization spectroscopy, and chemical probe experiments. Depending on the metal, there are two types of structures for M n (benzene) m :  multiple-decker sandwich structures and metal clusters fully covered with benzene molecules (rice-ball structures). The former sandwich structure is characteristic of the complexes for early transition metals (Sc−V), whereas the latter is formed for late transition metals (Fe−Ni). Electronic structures of M1(benzene) x (x = 1, 2) complexes are investigated through systematic measurements of ionization energies (E i's).

A molecular beam of multilayer vanadium-benzene organometallic complexes Vn(C6H6)m was produced by a laser vaporization synthesis method. The magnetic moments of the complexes were measured by a molecular beam magnetic deflection technique, and were found to increase with the number of vanadium atoms in the cluster, showing that the unpaired electrons, which occupy the nonbonding dsigma orbitals localized on the metal atoms, couple ferromagnetically. These sandwich species represent a new class of one-dimensional molecular magnets in which the transition metal atoms are formally zerovalent.

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Ap spectral shift. cm-' Ap dimer 349 0.020 hexamer 771 0.040 trimer 544 0.029 heptamer 798 0.041 tetramer 660 0.035 octamer 814 0.042 pentamer 729 0.038Results are from Shipman-PCM-based exciton calculations which include monomer Soret band states. All shifts given are Fed shifts relative to the monomer Q, band. bGiven as Af = (1/M) floligomer) -f(monomer), where M is the number of monomers in the oligomer. flmonomer) = 0.65.aggregates studied here represent a somewhat limited model of a crystal, exploratory calculations on more extensively aggregated systems indicate that one-dimensional stacking is the major factor responsible for the observed spectral shifts, and only perhaps a 10% increase in shifts would be expected to arise from other modes of aggregation.Changes in oscillator strength are seen to increase steadily with aggregation, the oscillator strength of the octamer being approximately 6% larger than that of the monomer. As discussed previously, it is expected that these changes are underestimated by the present "limited" exciton calculations, and thus significant hyperchromism would be expected of the larger Me-BPheo-a aggregates. ConclusionsThe present results as well as those obtained in the previous study of related dimer systemsz9 reveal severe problems associated with application of the point-dipole approximation in exciton studies of aggregates of spatially-extended monomers. Unacceptable errors in both spectral shifts and oscillator strengths are produced by this method, regardless of whether experimentally or theoretically determined dipoles are employed. The method appears totally unreliable and should be avoided.The present results also demonstrate the importance of including non-nearest-neighbor interactions in exciton calculations of aggregated systems. Computations in which only nearest-neighbor interactions were considered seriously underestimated the magnitude of spectral shifts, although oscillator strengths were largely unaffected. Neglect of interactions between molecules with center-to-center distances >20 8, appear to produce acceptable errors in spectral shifts.There are insufficient experimental data to determine the accuracy with which Shipman-PCM-based calculations can predict spectral shifts, but it is clear that red-shifts are underestimated by this method. It is possible that larger spectral shifts would be obtained from exciton calculations in which higher quality monomer wave functions were employed. The present FSGObased wave functions contain basis functions which do not extend into space as much as do larger atomic basis sets; consequently, the present exciton calculations might be expected to underestimate the intermolecular interactions responsible for spectral shifts. Beyond this, it has been suggested" that charge-transfer effects may be significant, and consideration of this as well as monomeric charge redistribution may be necessary in order to obtain accurate spectral shift values.Exciton calculations on the present aggregates using the Shipman-PCM appear to be u...

The production of potent toxins by bloom-, scum-and mat-forming cyanobacteria, in fresh-, brackish and marine waters, appears to be a global phenomenon. Cyanobacterial toxins can also be produced by cyanobacteria from terrestrial sources. The range and number of known cyanobacterial toxins are increasing apace as associated poisoning incidents are investigated, and increasingly powerful analytical methods are applied to complement toxicity-based studies on both natural samples and laboratory isolates of cyanobacteria. Water quality management to reduce toxic cyanobacterial mass developments, and schemes to mitigate the potential effects of cyanobacterial toxins, require an understanding of the occurrence and properties of the toxins and of the exposure routes via which the toxins present risks to health. Here, we review advances in the recognition of cyanobacterial toxins and their toxicity, and of the exposure routes with reference to human health, namely via skin contact, inhalation, haemodialysis and ingestion (the oral route).

In the gas phase, novel network structures were found in organometallic clusters between metal atoms produced by laser ablation and organic ligand molecules. For 3d metal-benzene, M n Bz m , two kinds of structures of multiple sandwich and rice-ball were formed, depending on the metal elements. Early transition metals (M E ) of Sc, Ti, and V form the multiple-decker sandwich structure of (n, m) ) (n, n + 1) in which metal atoms and benzene are alternately piled up, while late transition metals (M L ) of Fe, Co, and Ni form the rice-ball structure in which central metal clusters were fully covered by benzene molecules. The ionization energy of M E -Bz drops significantly with increasing layers, which can be explained by delocalization of d electrons along the molecular axis. M-C 60 binary clusters were also generated by a two-laser vaporization method; M E -C 60 clusters efficiently form a chain or a ring structure consisting of a dumbbell unit, in which metal atoms and C 60 are alternately connected. For M L -C 60 clusters, however, the metal atom is tricapped by C 60 and a face-centered tetrahedron structure is formed at (n, m) ) (4, 4). A similar multiple-decker sandwich structure is formed also between lanthanide metal atoms (Ln) and an organic ligand of cyclooctatetraene (COT). The Ln-COT cluster is a charge transferred cluster consisting of positively charged Ln and negatively charged COT. Their electronic structure is fairly ionic and is localized around each metal atom. These novel structures of organometallic clusters should inspire new thoughts in material science because it is hoped that the regular arrangement of metal ions can introduce useful properties such as electroconductivity and magnetism.

The production of potent toxins by bloom-, scum-and mat-forming cyanobacteria, in fresh-, brackish and marine waters, appears to be a global phenomenon. Cyanobacterial toxins can also be produced by cyanobacteria from terrestrial sources. The range and number of known cyanobacterial toxins are increasing apace as associated poisoning incidents are investigated, and increasingly powerful analytical methods are applied to complement toxicity-based studies on both natural samples and laboratory isolates of cyanobacteria. Water quality management to reduce toxic cyanobacterial mass developments, and schemes to mitigate the potential effects of cyanobacterial toxins, require an understanding of the occurrence and properties of the toxins and of the exposure routes via which the toxins present risks to health. Here, we review advances in the recognition of cyanobacterial toxins and their toxicity, and of the exposure routes with reference to human health, namely via skin contact, inhalation, haemodialysis and ingestion (the oral route).

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