Pi delocalized

The dual substituent parameter treatment attributes substituent effects to an additive blend of polar (/) and pi delocalization (R) effects, each of which may be represented as a op product (3) ... 

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). 

The pi delocalization parameters of Table XXVII show X=p /p/ values which generally follow the classical sequence > X". There are several... 

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. 

The parameters of Table V show many of the trends previously recognized (2i, 4, 10, 19). The substituents (-/ ) having a first row element with an unshared electron pair as first atom (F, OCsH , OM2, NHCOMe, NH2 and NMe2) show enhanced pi delocalization across the scales i.e., -a < °R increment of the scales is greater between... 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

Equation 6 would hold for a family of free radical initiators of similiar structure (for example, the frarw-symmetric bisalkyl diazenes) reacting at the same rate (at a half-life of one hour, for example) at different temperatures T. Slope M would measure the sensitivity for that particular family of reactants to changes in the pi-delocalization energies of the radicals being formed (transition state effect) at the particular constant rate of decomposition. Slope N would measure the sensitivity of that family to changes in the steric environment around the central carbon atom (reactant state effect) at the same constant rate of decomposition. 

More entries were used to test the valT3ity of equation 6 for reaction 1 than for the other families of initiators (Table I). Six of the thirteen entries for reaction 1 have AE(x) values bunched between -21 and -24 kcal/mole. It was felt that, by using all of these six entries, any bias in structure activity relationships would be decreased for this region of radical pi-delocalization energies. This group also includes those diazenes which are most used commercially, such as 2,2 -azobisisobutyronitrile (AIBN - entry 16 ) and dimethyl 2,2 -azobisisobutyrate (entry 14). 

The most important results of the linear free energy equations in this study (Table IV) are the applications to which they can be used. For a new free radical initiator, belonging to any of the four radical forming reactions of this study, equation 6 should be useful to predict the rate of decomposition with reasonable accuracy. All that is needed is an HMD calculation to obtain the pi-delocalization energy for the radical formed in the reaction (R ) and an estimate of the steric A values for groups bonded to the central carbon of R. ... 

Much of the interest in the polysilanes, polygermanes, and polystannanes involves their sigma delocalization and their sigma-pi delocalization when coupled with arenes or acetylenes. This is not unexpected since silicon exists as a covalent network similar to diamond. In exhibiting electrical conductivity, germanium and tin show more typical metallic bonding. Some polystannanes have been referred to as molecular metals. ... 

Pi Delocalization Effects, an Analysis of (Wells, Ehrenson, and Taft). 

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