Maximal overlap principles

This effect has been reproduced in ab initio calculations40 and rationalized.41 The principle of maximum overlap states that the preferred trajectory corresponds to the best molecular overlap between the reaction partners (rule 4). If the nucleophile adopts a perpendicular trajectory, the atomic overlap with the carbon will be maximized. However, a competing out-of-phase overlap between the nucleophile HOMO and the carbonyl LUMO (shown by the wavy line) reduces the overall frontier orbital interaction. If the nucleophile is displaced laterally (arrow), the small diminution in the overlap with C is outweighed by the reduction in the antibonding interaction with O. This increase in the overall overlap explains the preference for attack from an obtuse angle... 

This so-called stereoelectronic factor operates to maximize or minimize orbital overlap, as the case requires, to obtain the most favorable energy. This was evident from the three- and four-center systems we have discussed by the VB and HMO methods. It was also implicit in favored anti-1,2-additions, 1,3-cyclizations (Fig. 23), fragmentations (e.g. (174)), etc. Here we have selected several reaction types to illustrate the principle. In this and other sections, we show that the tendency for reaction centers to be collinear or coplanar stems largely from orbital symmetry (bonding), but may also derive from steric and electrostatic effects, as well as PLM. 

In this section we focus primarily on the stereochemistry of the concerted E2 mechanism. The most familiar examples are dehydrohalogenation and dehydrosul-fonylation reactions effected by strong bases. In principle, elimination can proceed with either syn or anti stereochemistry. For acyclic systems, there is a preference for anti elimination, but this can be overridden if conformational factors favor a syn elimination. The anti TS maximizes orbital overlap and avoids the eclipsing that is present in the syn TS. 

In structures where n = 7 or above, overlapping clusters are not observed, and maximum separation of the clusters occurs consistent with the composition, dimensions and symmetries of the unit cell. Furthermore, relaxation of the cation sublattice is observed to be maximized. In particular, although in the = 7 structures double vacancies do occur along <11 1) across metal atoms and where no metal atoms separate them, the latter are not found in any other known structures, even for = 9. The only intermediate compound reported for the actinide oxides is the iota phase, An70j2 (see sections 2 and 3). However, efforts to prepare other phases have not been extensive. They might occur in the heavier oxides of the series from Pu to Es since these may possess both the 3 -F and 4+ oxidation states. The structural principles appropri-... 

In principle, any couple of fluorophores can be used for FRET, provided that the emission spectrum of the donor overlaps with the absorption of the acceptor. For a review of FRET-couples (and RO values) of chemical dyes see [62]. Furthermore, donors with a high fluorescence quantum-yield and acceptors with a high molar absorbance will display increased FRET. For FLIM it will be important to tune the instrument-performance to ensure maximal sensitivity to small changes in lifetimes at the control donor lifetime. Usually this is easily achieved. Many FRET-pairs have been used for FRET-FLIM including chemical probes as Fluorescein-Rhodamine [54,93],calcein-sulforhodamine B [94], and Cy3-Cy5, [70]. Since 1996, the availability of genetic-encoded fluorophores such as CFP, GFP, YFP has boosted application of FRET-FLIM enormously [95]. Nowadays fluorescent-tagging of proteins no longer depends on laborious protein pu-... 

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