[en] The electronic structure of actinide bis(cyclopentadienyl)dialkyl, -butadiene, -metallacyclopentadiene, and -cyclobutadiene complexes is examined. Some Cp*₂Th(alkyl)₂ complexes display an interesting structural deformation in which a Th-C-C(alkyl) angle (α) opens up considerably. A molecular orbital analysis of Cp₂Th(C₂H₅)₂ traces the deformation to the characteristic shape of the d/sub σ/ fragment orbital of Cp₂Th(C₂H₅)⁺. The potential energy curve for the ethyl pivoting was found to be very soft, with a shallow minimum at α ∼ 160⁰. Nonrigidity of the ethyl orientation indicates that not only an α hydrogen but also a β hydrogen can come close to Th without a loss of Th-C bond strength; the discussion should provide a theoretical basis for understanding important aspects of C-H activation chemistry involving actinide centers. The Cp₂An(s-cis-C₄H₆) complex was calculated to be 0.74 eV (Th) or 0.42 eV (U) more stable than its s-trans-C₄H₆ isomer, in contrast to the analogous Cp₂Zr(C₄H₆) complex where the stability of the two geometrical isomers was well-balanced. The σ₂,π character of the s-cis-C₄H₆ coordination to An is somewhat less pronounced than that in the Zr congener, in harmony with the X-ray structures, while highly negative charges are accumulated on the s-cis-C₄H₆ terminal carbons

[en] Results of studies of the coordination of sigma-methyl and π-cyclopentadienyl (Cp) groups to iron and uranium are reported. The discussion focuses on the 3d⁶4s⁰4p⁰ Fe(II) and 5f⁰6d⁰7s⁰79⁰ U(VI) electron counts with the charge on the hypothetical molecules set at +1 for FECp and +5 for UCp. Changes in overlap population were studied as a function of the M-Cp and M-CH₃ (M=metal) separation, and the observed results are explained on the basis of the nature of the M-Cp and M-CH₃ bonds. The small U-Cp overlap population were explained by assuming that the weak π bond of the U-Cp carries very weak covalent character while covalency of the Fe-Cp bond is substantial. The U-CH₃ sigma bond is strongly covalent. From the analysis of the character of the bonding of actinides to Cp and CH₃, synthesis of actinide compounds with neutral π ligands such as olefins dienes, etc. is seen as very difficult while the possibility of neutral sigma ligand bonding with actinides seems highly possible. The most likely possibility of complex formation is coordination with CO

No abstract available

[en] As a part of China-Japan cooperative research on radiation epidemiology, we carried out dose-assessment study in the HBRA since 1991 to 1994. Particularly, we have examined quantitatively on the characteristics of distribution of the numerical measurements and analyzed the variables concerned, using different kinds of dosemeters in order to obtain the information with regard to accuracy of the data for dose estimation. (author)

[en] Lanthanides studied include lanthanum, cerium, praseodymium, neodymium, and gadolinium; actinides are thorium and uranium. The results of their x-ray photoelectron spectroscopy and the satellites accompanying lanthanide 3d/sub 5/2/ and actinide 4d/sub 5/2/ signals are reported. Interpretation is based on the hypothesis that half-occupied f orbitals play an important role in the striking change of the satellite intensity throughout the f transition metal series. The binding energies of the main 3d/sub 5/2/ and 4d/sub 5/2/ peaks and their satellites are reported along with the intensity ratio of the satellie to the main line. 1 figure, 1 table

[en] Ionizing radiation has been shown to induce specific-locus mutations resulting from two general types of genetic damage. Single gene mutations are attributed to intragenic alterations ranging from point mutations to intra-locus deletions. Multigene mutations result from extragenic alterations including chromosomal or multilocus deletions and chromosomal rearrangements such as non-disjunction and crossing-over. Although the involvement of these two general types of genetic alteration in radiation-induced mutations has been illustrated in mammalian cells including human cells, the precise mechanisms determining the proportion of the two types still remain to be fully characterized. (author). 11 refs., 1 fig., 1 tab

[en] Human cells respond to carcinogen-induced damage in their DNA in at least two ways. The first response, excision repair, proceeds by at least three variations, depending on the nature of the damage. Nucleotide excision results in relatively large repair patches but few free DNA breaks, since the endonuclease step is limiting. Apurinic repair is characterized by the appearance of numerous breaks in the DNA and by short repair patches. The pathways behave as though they function independently. Lymphoic cells derived from a xeroderma pigmentosum complementation group C patient are deficient in their ability to perform nucleotide excision and also to excise 6 methoxyguanine adducts, but they are apurinic repair competent. Organisms may bypass damage in their DNA. Lymphoblastoid cells, including those derived from xeroderma pigmentosum treated with ³H-anti-BPDE, can replicate their DNA at low doses of carcinogen. Unexcised ³H is found in the light or parental strand of the resulting hybrid DNA when replication occurs in medium with BrdUrd. This observation indicates a bypass reaction occurring by a mechanism involving branch migration at DNA growing points. Branch migration in DNA preparations have been observed, but the evidence is that most occurs in BrdUrd-containing DNA during cell lysis. The measurement of the bifilarly substituted DNA resulting from branch migration is a convenient method of estimating the proportion of new synthesis remaining in the vicinity of the DNA growing point. Treatment with carcinogens or caffeine results in accumulation of DNA growing points accompanied by the synthesis of shortened pieces of daughter DNA

[en] Following the earlier theoretical study of monocarbonylation of Cp₂M(R)X complexes (M = group 4 metals and actinides), the authors present here a molecular orbital analysis of this unique double carbonylation chemistry, and a mechanistic crossover experiment using isotopic labels. The aim of the theoretical part of this paper is an understanding of the stepwise migratory CO insertion, focusing on the following issues: the nature of bonding in the unusual 20-electron bis-n² structures of 4 and 6, and the mode of their coupling reactions. The authors will also suggest that there is an electronic reason for the different reactivity between 4 and 6. The molecular orbital calculations are of the extended Hueckel type, including f orbitals for actinides, and computational details are described in the Experimental Section

[en] The x-ray crystal and molecular structure for a series of U(IV) ylide complexes that contain more metal-carbon sigma-bonding interactions than any other reported organoactinide complex are reported. The U-C sigma distances, 2.66 A, are the longest yet reported. Extended Hueckel molecular orbital calculations indicate significant covalent character to the U-C sigma bonds in the complex, and an analysis of overlap populations provides an explanation for the observed structural parameters. Alternatively, the long U-C sigma bonds can be rationalized in terms of an ionic bonding model

[en] Single crystalline Cu and dilute Cu-alloys (with S, Fe, Mn, Au, Be) were isochronically annealed (up to 920 K) after irradiation (3 MeV - γ-Bremsstrahlung) or deformation (compression) at RT. Following each annealing step the ultrasonic attenuation (10-300 MHz) was measured at RT and evaluated in terms of dislocation resonance damping with respect to changes of dislocation loop-length and density. The different alloys exhibit characteristic solute influences on defect annealing in the Stages IV to VI, i.e. dislocation pinning, depinning, formation of Cottrell atmospheres and dislocation density recovery