Electron affinity Electrophilicity

Monomer Reactivity. The poly(amic acid) groups are formed by nucleophilic substitution by an amino group at a carbonyl carbon of an anhydride group. Therefore, the electrophilicity of the dianhydride is expected to be one of the most important parameters used to determine the reaction rate. There is a close relationship between the reaction rates and the electron affinities, of dianhydrides (12). These were independendy deterrnined by polarography. Stmctures and electron affinities of various dianhydrides are shown in Table 1. 

Sequential addition of monomers 6 7-26-27-114) is the most obvious procedure. Once the first monomer has been polymerized, the resulting living species is used as a polymeric initiator for the polymerization of the second one. The monomers are to be added in the order of increasing electron affinity to provide efficient and fast initiation 26 U4). This condition is rather restrictive, and the number of monomer systems that can be used is limited (Table 5). Moreover, when the second monomer contains an electrophilic function (e.g. ester) which could lead to side reactions, it is necessary to first lower the nucleophilicity of the living site. This is best done by intermediate addition of 1.1-diphenylethylene25). The stabilized diphenylmethyl anions do not get involved in side reactions with ester functions, while initiation is still quantitative and fast. 

Elastic chains 163 Elastomers 27, 32, 33, 63, 65 Electron affinity, monomers 150 Electrophiles 155-157, 162 —, plurifunctional 162 Electrophilicity 149... 

The proposed subsequent reaction fits the fragmentation patterns observed in mass spectrometry where, even at 20 eV, group 14 centered radicals form in increasing order Sibasic data of this kind can provide estimates of kinetic behavior of such reactions, where M—M bonds are cleaved by electrophiles and which depend on the ionization potentials of the former as well as the electron affinity of the latter. 

Ionization Potential (IP), Electron Affinity (EA), Maximal Charge Acceptance ANmax, and Electrophilicity Index Ground State for the First and Second Row Atoms (Units in eV)... 

Addition of an alkyl nucleophile leads, due to the loss of one double bond, to a decrease of electron affinity and a concomitant negative shift of the reduction potential of about 100 to 150 mV per lost double bond. One possibility to compensate for this negative shift is the introduction of an electron-withdrawing substituent such as cyanide. Reaction of liCN or NaCN with Cjq at room temperature generates the monoadduct anion that can be quenched with various electrophiles [6]. 

The delocalization of the conduction electron onto the side chains would be expected if the pendant groups were replaced with more electrophilic substituents than the phenyl group. However, this is not the case. Figure 22 shows the absorption spectrum of poly-(methylnaphthylsilane) radical anion. The absorption spectrum is very similar to that of the naphthalene radical anion, which implies that the unpaired electron is localized on the pendant group. Increase of the electron affinity of pendant groups does not necessarily cause the delocalization. 

Sander applied DFT (B3LYP) theory to carbenic philicity, computing the electron affinities (EA) and ionization potentials (IP) of the carbenes." " The EA tracks the carbene s electrophilicity (its ability to accept electron density), whereas the IP represents the carbene s nucleophilicity (its ability to donate electron density). This approach parallels the differential orbital energy treatment. Both EA and IP can be calculated for any carbene, so Sander was able to analyze the reactivity of super electrophilic carbenes such as difluorovinylidene (9)" which is sufficiently electrophilic to insert into the C—H bond of methane. It even reacts with the H—H bond of dihydrogen at temperamres as low as 40 K, Scheme 7.2) ... 

The energy of the HOMO (EHomo) is directly related to the ionization potential and characterizes the susceptibility of the molecule to attack by electrophiles. On the other hand, EHOMO is directly related to the electron affinity and characterizes the susceptibility of the molecule toward attack by nucleophiles. Both the E, IOMO and LUMO energies are important in radical reactions. The concept of hard and soft nucleophiles and electrophiles has... 

The energy of the LUMO of a molecule can be approximated as its electron affinity. It measures the affinity of a molecule to accept electrons, and it acts as an electrophile or undergoes reduction. 

For those electrophiles (El) that undergo direct electron-transfer reduction at an inert electrode (glassy-carbon), the reduction potential ( red) is a measure of their electron affinity and electrophilicity [relative to that for H30+ (-2.10 V vs. NHE in aqueous media)] (the more positive the potential the more electrophilic the molecule see Chapter 1] ... 

Alkyl- and Aryl-Halides. Because the halo-groups of organic molecules have large electronegativities and electron affinities, all halo-carbon molecules are electrophilic. Their electrochemical reduction potential is a measure of their electrophilicity (and electron affinity), which is illustrated in Figure 12.1 for hexacWorobenzene (C6C16), 1,2,3,4-tetrachlorobenzene (1,2,3,4-C6H2C14), and n-butyl iodide (n-BuI).8,9 Table 12.1 summarizes the reduction potentials for several alkyl-halides and ary 1-chlorides.810... 

Quinones, Semiquinones, and Catechols. All molecules with unsaturated bonds (olefins, acetylenes, aromatics, carbonyls, quinones, etc.) have a degree of electrophilicity and electron affinity. Within a class, the extent of conjugation... 

Nucleophilic substitution reactions are driven by (1) the difference between the electron affinity of the electrophile ( E, one-electron reduction potential) and the electron-donating propensity of the nucleophile ( N, one-electron oxidation potential) (Ee - N) and (2) the bond energy of the newly formed bond.27 For example, the nucleophilic attack by HO- in MeCN ... 

If a given nucleophile does not react with an electrophile, it is because the bond energy and/or the electron affinity are too small (—AGreac < 0). 

To characterize the global readiness of molecules to donate or accept electron charge, the lowest ionization potential and greatest electron affinity (that are often simply called ionization potential and electron affinity) would be the best parameters when referring to Equation 6.37 and Equation 6.40 as (highly simplified) model reactions for nucleophilic and electrophilic interactions of a compound with endogenous reaction partners, and the associated MO energy values ... 

It was shown by reduction experiments with the corresponding thiacycloalkanes and cis-cycloalkenes that reduction starts at the triple bond and not at the sulfur. Thus, increasing deformation at the triple bond leads to larger electron affinity even in solution 182). The implications of these results also support the idea that linear alkynes are more electrophilic than related alkenes, because they can easily adopt a bent structure in reaction transition states. 

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