Competition between configurations

This competition, manifested in the structure of the actinide atomic spectra, also appears as a competition between configurations there is an accumulation of many configurations, both even and odd, at about the same height, leading to a high density of energy levels (in fact, we had arrived at the same conclusion by inspecting Fig. 1 in Sect. A.I.2). 

More than 50 years ago, Flory and Huggins [13-17] formulated a lattice model which captures the essential features of this competition between configurational entropy of mixing and enthalpy contributions, and even today this extremely simplified model is the basic ground on which most of the discussion... 

The competition between the polar and steric dipoles of molecules may also lead to internal frustration. In this case, the local energetically ideal configuration cannot be extended to the whole space, but tends to be accomodated by the appearance of a periodic array of defects. For example, the presence of the strong steric dipole at the head of a molecule forming bilayers will induce local curvature. As the size of the curved areas increases, an increase in the corresponding elastic energy makes energetically preferable the... 

In the discussion of the stereochemistry of aldol and Mukaiyama reactions, the most important factors in determining the syn or anti diastereoselectivity were identified as the nature of the TS (cyclic, open, or chelated) and the configuration (E or Z) of the enolate. If either the aldehyde or enolate is chiral, an additional factor enters the picture. The aldehyde or enolate then has two nonidentical faces and the stereochemical outcome will depend on facial selectivity. In principle, this applies to any stereocenter in the molecule, but the strongest and most studied effects are those of a- and (3-substituents. If the aldehyde is chiral, particularly when the stereogenic center is adjacent to the carbonyl group, the competition between the two diastereotopic faces of the carbonyl group determines the stereochemical outcome of the reaction. 

The atomic spectra of the actinides are very complex and it is difficult to identify levels in terms of quantum numbers and configurations (6). The chemical behaviour of the elements is dictated by the configurations of the electrons around the nucleus and in the case of the actinides it is the competition between the 5/ 1 7 s2 and the 5 /n 1 6 d 7 s2 levels that dictates these chemical properties. A comparison of the /-energy levels of the lanthanides and the actinides shows that less energy is required for the promotion of the 5 / -> 6 d levels than for the 4/ -> 5 d levels in the lanthanides. As a result of this lower energy requirement by the actinides they have the tendency to display higher valences since the bonding electrons are more readily available. It is only at the commencement of the second half of the actinides that there is commencement of properties which echo those of the lanthanides. 

In Chapter 5, we discuss in a simple way static (crystalline field) and dynamic (coordinate configuration model) effects on the optically active centers and how they affect their spectra (the peak position, and the shape and intensity of optical bands). We also introduce nonradiative depopulation mechanisms (multiphonon emission and energy transfer) in order to understand the ability of a particular center to emit light in other words, the competition between the mechanisms of radiative de-excitation and nonradiative de-excitation. 

Proton transfer will be prevented when the initial fragments separate too rapidly for the partners to rotate into a suitable configuration. Here, the lower limit for the intermediate s lifetime can be estimated to be 10 " s. The competition between the... 

Solvolysis of optically active 5-tosyloxypenta-l, 2-dienes leads to methylenecyclobutanols with inversion of configuration at the carbon bearing the leaving group, without loss of optical activity.13 Once again there is competition between the homoallyl and 1234-1243 rearrangement. 

The competition between elimination and substitution channels when an alkyl halide is allowed to react with a nucleophile in the gas phase is a difficult problem to tackle, since in most gas-phase experiments only the ionic products of reaction are monitored (a few exceptions are reported below). Thus, for example, when w-propyl bromide is allowed to react with methoxide ion in the gas phase, the bromide ion produced can arise either by elimination (a) or by substitution (b) and the two pathways cannot be distinguished from the ions alone (Scheme 34). In this specific case it was possible to establish that the reaction follows exclusively the elimination channel through collection and analysis of the neutral products246. The experiments were performed on a FA apparatus configured with a novel cold finger trap coupled to a GC/MS system. Material collected by the trap was separated by capillary gas chromatography and the individual components identified by their retention times and El mass spectra246. 

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