Delocalized effect

Other limitations of the reaction are related to the regioselectivity of the aryl radical addition to double bond, which is mainly determined by steric and radical delocalization effects. Thus, methyl vinyl ketone gives the best results, and lower yields are observed when bulky substituents are present in the e-position of the alkene. However, the method represents complete positional selectivity because only the g-adduct radicals give reductive arylation products whereas the a-adduct radicals add to diazonium salts, because of the different nucleophilic character of the alkyl radical adduct. ... 

Of the variety of quantum effects which are present at low temperatures we focus here mainly on delocalization effects due to the position-momentum uncertainty principle. Compared to purely classical systems, the quantum delocalization introduces fluctuations in addition to the thermal fluctuations. This may result in a decrease of phase transition temperatures as compared to a purely classical system under otherwise unchanged conditions. The ground state order may decrease as well. From the experimental point of view it is rather difficult to extract the amount of quantumness of the system. The delocahzation can become so pronounced that certain phases are stable in contrast to the case in classical systems. We analyze these effects in Sec. V, in particular the phase transitions in adsorbed N2, H2 and D2 layers. 

Since Woodward s work on the synthesis of chlorophyll a (60JA3800) it is known that the intrinsic unstable thioformyl moiety can be stabilized by the delocalization effect of heterocyclic systems. Recently the synthesis of 2-amino- and 3-aminothioformylthiophenes (and furans) and the corresponding benzo derivatives (Scheme 19) has been reported (96S1185). These compounds exist as amino tautomers (91S609 96S1185). 

X values clearly reflect the poorer transmission of pi delocalization effects from the meta than para position. For the select meta sets of Table XI, for example, X" is typically about. 4, whereas for corresponding reactions, X is around unity. Evidence is meager with respect to the ortho position, but it appears that in accord with the classical ideas of pi electron transmission in the benzene ring, generally X > X > X"" for corresponding reactions (cf. subsequent discussion). 

Evidence is provided by this analysis that (a) structural considerations discriminate among at least four practical classes of pi delocalization behavior, each of which has limited generality (b) the blend of polar and pi delocalization effect contributions to the observed effect of a substituent is widely variable among different reaction or data sets (the contributions may be opposite as well as alike in direction), depending upon structural considerations and the nature of the measurement (c) solvent may play an important role in determination of the observed blend of effects (d) it is the first three conditions which lead to the deterioration of the single substituent parameter treatment as a means of general and relatively precise description of observed electronic substituent effects in the benzene series. 

In view of the above arguments, we have continued to use the CTr substituent constants as a delocalized effect parameter. Correlations in this paper have been made therefore with Oj and or constants. The ai values were generally taken from the compilation of Charton (29), and Or values were obtained from the equation... 

With regard to the composition of the electrical effect, examination of the p values reported in Table XVII shows that in six of the sets which gave significant correlation, the localized effect is predominant (in these sets, either Pr < 50 or / is not significant). Thus it would appear that in so far as substituent effects are concerned, there are two major classes of electrophilic addition to the carbon-carbon double bond predominance of the localized effect or predominance of the delocalized effect. This behavior may well be accounted for in terms of the reaction mechanism. The rate-determining step in the electrophilic addition reaction is believed to be the formation of an intermediate which may be either bridged or a free carbonium ion. 

Of the five sets which were correlated with eq. (2), four gave significant correlations. Values of Pr are in the range 50 to 54, which indicates approximately equal contributions of the localized and delocalized effects. Thus, dipole moments of substituted acetylenes could be correlated successfully with the Up constants. 

There are almost no studies of substituent effects on additions to carbon-carbon triple bonds extant in the literature. Bowden and Price (208) have reported a correlation of rates of addition of hydrogen bromide to 3-substituted propiolic acids with the Hammett equation using the Op constants. Unfortunately, there are only three substituents in the set. Sufficient data are available for a single set of 1,3-dipolar cycloaddition. The set studied is shown in Table XXXIII, and the results of the correlation are in Table XXXIV. The correlation was significant the delocalized effect is predominant in this set. 

For the last few years we have tried to synthesize larger two-dimensional silicon frameworks. We wanted to find out how large a molecule must be in order to show delocalization effects of electrons as in elemental silicon. It is well known that radical anions are possible in simple cyclosilanes in which the additional electron is completely delocalized in the cycle [11]. 

Pd The percent delocalized effect. It too is a descriptor of the nature of the electrical effect. It is represented in subscripts as k . 

The mathematical criterion for resonance description of delocalization effects... 

As suggested by the original resonance theory of Pauling and Wheland,51 such delocalization effects appear to represent some type of average of multiple resonance structures. A general goal of resonance theory is to represent each property (P)true of the true delocalized system in resonance-averaged form... 

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