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- Larsson, Per-Erik
(författare)
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Modelling Chemical Reactions : Theoretical Investigations of Organic Rearrangement Reactions
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Chemical reactions are ubiquitous and very important for life and many other processes taking place on earth. In both theoretical and experimental studies of reactivity a transition state is often used to rationalise the outcome of such studies. The present thesis deals with calculations of transition states in radical cation rearrangements, and a principle of least motion study of the rearrangements in the barbaralyl cation. In particular, alternative quadricyclane radical cation (Q∙+) rearrangements are extensively studied. The rearrangement of Q∙+ to norbornadiene is extremely facile and is often used as a prototype for one-electron oxidations. However, electron spin resonance (ESR) experiments show that there are additional cations formed from Q∙+. Two plausible paths for the rearrangement of Q∙+ to the 1,3,5-cycloheptatriene radical cation are located. The most favourable one is a multistep rearrangement with two shallow intermediates, which has a rate-limiting step of 16.5 kcal/mol. In addition, a special structure, the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, is identified on these alternative paths; and its computed ESR parameters agree excellently with the experimental spectrum assigned to another intermediate on this path. Moreover, this cation show a homoconjugative stabilization, which is uncommon for radical cations. The bicyclopropylidene (BCP) radical cation undergoes ring opening to the tetramethyleneethane radical cation upon γ-irradiation of the neutral BCP. This rearrangement proceeds through a stepwise mechanism for the first ring opening with a 7.3 kcal/mol activation energy, while the second ring opening has no activation energy. The dominating reaction coordinate during each ring opening is an olefinic carbon rehybridization. The principle of least motion is based on the idea that, on passing from reactant to product, the reaction path with the least nuclear change is the most likely. By using hyperspherical coordinates to define a distance measure between conformations on a potential energy surface, a possibility to interpret reaction paths in terms of distance arises. In applying this measure to the complex rearrangements of the barbaralyl cation, a correct ordering of the conformations on this surface is found.
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| 2. |
- Zetterström Evins, Lena
(författare)
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Geochronology of the Oklo and Bangombé fossil natural fission reactors : tracing the effects of geological events
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thesis to be publicly defended for the degree of Doctor of Philosophy, in lecture room De Geersalen, Svante Arrhenius väg 8a, Stockholm, Friday February 1, 2002 at 10.00 am. Two billion years ago, natural fission reactions took place in the Oklo and Bangombé uranium ores in Gabon, West Equatorial Africa. Uraninite (UO 2+x) in the reactor zones is studied as a natural analogue to spent nuclear fuel. Since uraninite accumulates radiogenic Pb, U-Pb isotope systematics may be used to establish when uraninite alteration, and consequently migration of elements contained in the uraninite, occurred. The aim of this study is to establish when uraninite alteration and related radiogenic Pb loss occurred in the Oklo and Bangombé natural fission reactors.The isotopic compositions of U, Pb and S in uraninite, galena (PbS) and zircon (ZrSiO 4) were analysed using an ion microprobe. The ion microprobe technique requires the use of standards of the same mineral as the sample in order to determine instrumental fractionation. One part of this study involved describing and evaluating potential uraninite and galena standards for U and Pb isotopic analysis by ion microprobe. The galena standard analyses showed that Pb isotopic fractionation during galena analyses was ca 0.5% per atomic mass unit (a.m.u.) in favour of the lighter isotope. The uraninite standard study showed that U isotopic fractionation was ca 1.4% per a.m.u. in favour of the lighter isotope and that uraninite ion microprobe analysis may be influenced by significant amounts of hydrides. Hydride interferences during Pb isotopic analysis of uraninite were found to be of greater importance than Pb isotopic fractionation.A dolerite dyke swarm in the Francevillian basin, where the Oklo and Bangombé U deposits are located, was previously dated to sometime between ca 1000 and 700 Ma. In this study, zircon U-Pb ion microprobe analyses of one of the dolerite dykes yielded an age of 860±39 Ma (2). This age is in accordance with single-stage Pb-Pb model ages of large, mm-sized galena crystals that formed during a major Pb loss from uraninite in the reactor zones. Smaller galena crystals, ca 10 µm in diameter, are common in reactor zone 16 at Oklo. Variable Pb isotopic compositions of these crystals indicate that they formed sometime after the ca 860 Ma old major Pb loss from uraninite. An age of ca. 500 Ma is suggested for this second event of Pb loss and galena crystallisation. The existence of a second event of radiogenic Pb loss from uraninite is also indicated by the alteration of dolerite dykes that cross-cut the Oklo deposit. One of these altered dykes contains galena with Pb originating from the uranium ore as well as S from the surrounding sediment. Hence, the galena crystals in the dyke are not magmatic but a result of a later element migration. Pb-Pb and U-Pb systematics of uraninite also indicate that recrystallisation of uraninite occurred sometime between 600 to 500 Ma ago at both Oklo and Bangombé. Thus, this study has shown that at least two episodes of Pb loss from uraninite occurred after the formation of fission reactors; one at ca 860 Ma and one 600-500 Ma ago. Either one of these events might be related to regional Pb mobility in the Francevillian basin.
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