Energy level structure summary

A number of sources present extensive development of the energy level structure theory in lanthanide spectra, but works by Wyboume (1965), Judd (1963), and Dieke (1968) are particularly useful. Only a brief summary that includes recent developments in the theoretical model will be included here. 

Figures 1 and 2 show the dependence of polymer microstructure on the molecular weight of the polymer and therefore on the initial initiator concentration. The polymerization temperature also has an effect on the microstructure as can be seen in Figure 3 for polybutadiene. The overall heat activation energy leading to 1,2 addition is greater than that leading to 1,4 addition.2 IZ In summary, the stereochemistry of polymerization of butadiene and isoprene is sensitive to initiator level, polymerization temperature and solvent. The initiator structure (i.e., organic moiety of the initiator), the monomer concentration and conversion have essentially no effect on polymer microstructure.

In summary, the existence of these three different structure types suggests a rather delicate balance between the different minima on this interesting potential energy surface. Molecular orbital calculations at the highest levels currently practical are certainly quite useful in elucidating the nature of these structures. This is also an area where one may safely predict continuing efforts and where we anticipate additional insights by both ESR and CIDNP spectroscopy. 

Aquatic microcosms and mesocosms offer the ideal situation to investigate populations of species interacting in their natural environment (i.e., to study communities stressed in structured systems see Section 4.5.1). It is, however, only recently that these experiments have been analyzed at the community level (Van Wijngaarden et al. 1995 Sparks et al. 1999 van den Brink and Ter Braak 1999). Until 10 years ago, experiments were evaluated at the population level, largely ignoring species interactions and energy flows in the systems. The development of community-level endpoints offered the possibility to evaluate the experiments on a community level (i.e., they offered the opportunity to scale up the level of evaluation Kedwards et al. 1999). Summary community-level endpoints calculated from the results of these experiments are mostly structural ones measures of diversity (e.g. numbers of species, and the Shannon-Weaver diversity index) and similarity of the treated systems compared to the untreated controls (e.g., the principal response curves method, Bray-Curtis dissimilarity, or Stander s index see van den Brink and Ter Braak 1998 for a comparison). 

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