Electron polar mechanism

Blattler C, Jent F and Paul H 1990 A novel radical-triplet pair mechanism for chemically induced electron polarization (CIDEP) of free radicals in solution Chem. Phys. Lett. 166 375-80... 

The dielectric constant is a measure of the ease with which charged species in a material can be displaced to form dipoles. There are four primary mechanisms of polarization in glasses (13) electronic, atomic, orientational, and interfacial polarization. Electronic polarization arises from the displacement of electron clouds and is important at optical (ultraviolet) frequencies. At optical frequencies, the dielectric constant of a glass is related to the refractive index k =. Atomic polarization occurs at infrared frequencies and involves the displacement of positive and negative ions. 

The treatment of electrostatics and dielectric effects in molecular mechanics calculations necessary for redox property calculations can be divided into two issues electronic polarization contributions to the dielectric response and reorientational polarization contributions to the dielectric response. Without reorientation, the electronic polarization contribution to e is 2 for the types of atoms found in biological systems. The reorientational contribution is due to the reorientation of polar groups by charges. In the protein, the reorientation is restricted by the bonding between the polar groups, whereas in water the reorientation is enhanced owing to cooperative effects of the freely rotating solvent molecules. 

A full description of how a reaction occurs is called its mechanism. There are two general kinds of mechanisms by which reactions take place radical mechanisms and polar mechanisms. Polar reactions, the more common type, occur because of an attractive interaction between a nucleophilic (electron-rich) site in one molecule and an electrophilic (electron-poor) site in another molecule. A bond is formed in a polar reaction when the nucleophile donates an electron pair to the electrophile. This movement of electrons is indicated by a curved arrow showing the direction of electron travel from the nucleophile to... 

Most organic reactions take place by polar mechanisms, in which a nucleophile donates two electrons to an electrophile in forming a new bond. Other reactions take place by radical mechanisms, in which each of two reactants donates one electron in forming a new bond. Both kinds of reactions occur frequently in the laboratory and in living organisms. Less common, however, is the third major class of organic reaction mechanisms—pericyclic reactions. 

The delocalization of excessive a- (or P-) spins and the bond polarization can take place among radical orbitals, p and q, and the central n (or o) and n (or o ) orbitals, resulting in the electron transferred configurations (T) and locally excited configurations (E), respectively (Fig. 5a). The delocalization-polarization mechanisms are different between singlet and triplet states, as addressed in the following subsections. 

The delocalization-polarization mechanism in the singlet state is more complicated than that in triplet. Similar to the triplet state, there also exists a cyclic - G- T - E- T - configuration or -7t-p-7t -q- (-o-p-o -q-) orbital interaction in the singlet (Fig. 6). In the singlet state, however, the radical orbital q is an electron-accepting orbital (A) for the a-spin electron (rather than the donating orbital in triplet). Thus, there is an additional path of a-spin electron delocalization, - G- T - Tj- T - or... 

In a recent paper. Mo and Gao [5] used a sophisticated computational method [block-localized wave function energy decomposition (BLW-ED)] to decompose the total interaction energy between two prototypical ionic systems, acetate and meth-ylammonium ions, and water into permanent electrostatic (including Pauli exclusion), electronic polarization and charge-transfer contributions. Furthermore, the use of quantum mechanics also enabled them to account for the charge flow between the species involved in the interaction. Their calculations (Table 12.2) demonstrated that the permanent electrostatic interaction energy dominates solute-solvent interactions, as expected in the presence of ion species (76.1 and 84.6% for acetate and methylammonium ions, respectively) and showed the active involvement of solvent molecules in the interaction, even with a small but evident flow of electrons (Eig. 12.3). Evidently, by changing the solvent, different results could be obtained. 

Bajorath, J., J. Kraut, Z. Li, D. H. Kitson, and A. T. Hagler. 1991. Theoretical studies on the dihydrofolate reductase mechanism electronic polarization of bound substrates. Proc. Natl. Acad. Sci. USA 88, 6423. 

Various diorganozinc compounds (ZnR2 R = Me, Et, Pr, Pr1, Buc, Ph) reacted with o-quinones by two mechanisms, namely (i) a single-electron transfer from ZnR2 to the quinone to yield, after hydrolysis, alkyl(phenyl)oxyphenols, and (ii) a polar 1,2- and 1,4-addition of ZnR2 similar to those of conjugated ketones.201 Diorganozinc compounds with low ionization potentials favor a polar mechanism. 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

Silicon is a model for the fundamental electronic and mechanical properties of Group IV crystals and the basic material for electronic device technology. Coherent optical phonons in Si revealed the ultrafast formation of renormalized quasiparticles in time-frequency space [47]. The anisotropic transient reflectivity of n-doped Si(001) featured the coherent optical phonon oscillation with a frequency of 15.3 THz, when the [110] crystalline axis was parallel to the pump polarization (Fig. 2.11). Rotation of the sample by 45° led to disappearance of the coherent oscillation, which confirmed the ISRS generation,... 

Recently, electron-mediated, scalar couplings which are active between magnetic nuclei on both sides of the hydrogen bridge have been discovered in nucleic acids [28-41], proteins [42-54] and their complexes (Tabs. 9.1-9.3) [54—56]. These couplings are closely related to similar inter- and intramolecular couplings across H-bonds in smaller chemical compounds [57-60]. It is well established [31, 58, 61-74] that such trans H-bond scalar couplings follow the same electron-mediated polarization mechanism as any covalent... 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

A second mechanism (the polarization mechanism) arises due to the polarization of the fully occupied (bonding) crystal orbitals formed by the eg. oxygen 2p. and Li 2s atomic orbitals in the presence of a magnetic field. A fully occupied crystal (or molecular) orbital in reality comprises one one-electron orbital occupied by a spin-up electron and a second one-... 

The relative sizes of the polarization and delocalization mechanisms will depend on the orbitals involved in the overlap and the occupancies of the metal orbitals. For example, the polarization mechanism appears to be relatively weak for Ni. in contrast to Mn + and Cr +. even when both mechanisms involve eg orbitals. This is most likely due to the smaller exchange interaction expected for the Ni + system, which contains fewer unpaired electrons. Some earlier studies neglected the polarization mechanism, assuming that it was much weaker than the other possible shift mechanisms, which lead to incorrect assignments. ... 

Feng et al. (1986) performed quantum-chemical calculations of aromatic nitration. The resnlts they obtained were in good accordance with the IPs of N02 and benzene and its derivatives. The radical-pair recombination mechanism is favored for nitration whenever the IP of an aromatic molecule is much less than that of N02. According to calculations, nitration of toluene and xylene with N02 most probably proceeds according to ion-radical mechanism. Nitration of nitrobenzene and benzene derivatives with electron-acceptor substituents can proceed through the classical polar mechanism only. As for benzene, both mechanisms (ion-radical and polar) are possible. Substituents that raise the IP of an aromatic molecule to a value higher than that of N02 prevent the formation of this radical pair (one-electron transfer appears to be forbidden). This forces the classical mechanism to take place. It shonld be nnderlined that a solvent plays the decisive role in nitration. 

In this study, benzaldehyde and benzaldehyde-methyllithium adduct were fully optimized at HF/6-31G and their vibrational frequencies were calculated. The authors used MeLi instead of lithium pinacolone enolate, since it was assumed that the equilibrium IBs are not much different for the MeLi addition and lithium enolate addition. Dehalogena-tion and enone-isomerization probe experiments detected no evidence of a single electron transfer to occur during the course of the reaction. The primary carbonyl carbon kinetic isotope effects and chemical probe experiments led them to conclude that the reaction of lithium pinacolone enolate with benzaldehyde proceeds via a polar mechanism. 

The addition of RLi and other nucleophiles to carbonyl functions in general proceeds via one of the two possible reaction pathways, polar addition (PL) and electron transfer (ET)-radical coupling (RC) sequence (equation 5). Current reaction design for the synthetic purpose of additions of common nucleophiles to aldehydes and ketones is mostly based on the polar mechanism, but apparently the ET process is involved in some reactions of, for example, Grignard reagents Mechanistically there are three possible variations the PL pathway, the ET rate-determining ET-RC route and the RC rate-determining ET-RC route. 

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