Bond localization

This arises because as the temperature in increased from ambient, the main initial effect is to loosen the hydrogen-bonded local stmcture that iitiribits reorientation. Flowever, at higher temperatures, the themial motion of the water molecules becomes so marked that cluster fomration becomes iitiiibited. 

In the [3,2-c ] series the spectra of the parent compound and some closely related analogues have been compared with those predicted from MO calculations of electron densities. The results confirm the predicted occiurence of a considerable degree of bond localization in these compounds <80KGS54l), as was previously found in their deaza (cinnoline) analogues <67JCS(B)1243). Other comparisons of NMR values with those expected from MO calculations for a series of azanaphthalenes have included some results with pyridopyridazines <75MI21500). 

One should also classify the antibonding orbitals. Since these are much higher in energy they mix only weakly with the bonding localized orbitals. We neglect here this small perturbation. 

The ending caine stems from cocaine, the first clinically employed local anaesthetic. Procaine and tetracaine are ester-linked substances, the others are amides. Amide bonded local anaesthetics usually contain two i s in their name, ester-bonded only one. In the structure drawings, the lipophilic portion of the molecule is depicted at the left, the amine at the right. The asterisk marks the chiral centre of the stereoisomeric drugs. Lipid solubility is given as the logarithm of the water octanol partition coefficient, log(P). 

FIGURE 6.14 Oxidation of PMC-derivatives with different ring strains to mixtures of two possible ortho-qaiaoae methides the oxidation behavior and the ratio of the formed o-QMs agreed fully with the theory of strain-induced bond localization (SIBL). 

Rosenau, T. Ebner, G. Stanger, A. Perl, S. Nuri, L. From a theoretical concept to biochemical reactions strain-induced bond localization (SIBL) in oxidation of vitamin E. Chem. Eur. j. 2005, 11(1), 280-287. 

Figure 1. Benz[a]anthracene with regions of low bond localization energy (K-region) and low para localization energy (L-region) indicated.

A particularly attractive group of substrates are the annulenes, since electron transfer allows an interconversion of (4n + 2)n- and (4n)rr-systems, and thus a switch between 7>bond delocalization and rc-bond localization (Mullen, 1984). 

The PE spectra of cyclobutadiene 281192, tetra-t-butylcyclobutadiene 282193, 1,3,5-tri-t-butylpentalene 283194 and aceheptylene 284195, all of which are subject to second-order bond localization, have been described in the literature. Further examples are 1,3,5,7-tetra-t-butyl-s-indacene 285 (which exhibits a first double band 1,2 at I 2 = 6.75 eV, followed by two bands at I = 8.50 eV and I = 9.30 eV) and the tetracyclic hydrocarbon 1,3,6,8-tetra-t-butylpentaleno[2,l-a]pentalene 286 (the first two bands of which are observed at 6.40 eV and 7.65 eV)196. 

The weak aromaticity of oxazole is reflected by its chemical behavior, demonstrating a high degree of bond localization (illustrated by the propensity for cycloaddition reactions),137 and is supported by theoretical calculations (ring current indices, i.e., the bond... 

Double bond localization in dienes and polyenes has also attracted much attention but the examples are less widespread than with simple olefins. However, the specific reactivity of 1,3-dienes deserves special notice and recent work will be presented in detail to some extent in the following sections. 

As mentioned above, gas-phase coordination of unsaturated C—C bonds to metal cations constitutes another means for double-bond localization. Peake and Gross260 determined the fragmentation of Fe+/olefin complexes by CID spectrometry and reported highly characteristic differences for several constitutional octadiene isomers. High-resolution mass spectrometry proved to be necessary in certain cases due to the isobaric masses of Fe and even two units of C2H4 and/or CO261. Although not based on gas-phase ion/molecule... 

Coordination of vinylarenes to an Fe(CO)3 group gives rise to a complex (95) in which the metal is bound to the vinyl carbons and two of the carbons within the six-membered ring (equation ll)107. Crystal structure analysis indicates substantial bond localization in the uncomplexed portion of the ring. This has been interpreted as a loss of aromatic character due to participation of some of the 7r-electrons in coordination to iron. 

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