Sigma electron donor

When a halogen is in close contact with another atom approximately 180° from the halogen-halogen sigma bond, it is acting as an electron acceptor. When the contact is approximately 90°, it is acting as an electron donor. 

Shumate WJ, Mattem DL, Jaiswal A, Burgess J, Dixon DA, White TR, Honciuc A, Metzger RM (2006) Spectroscopic and rectification studies of three donor-sigma-acceptor compounds, consisting of a one-electron donor (pyrene or ferrocene), a one-electron acceptor (perylenebisimide), and a C19 swallowtail. J Phys Chem B110 11146-11159... 

Fig. 5.22. Nitration of sulfanilic acid B and desulfonation of the nitration product G. The protonation product D of the substrate is a benzene derivative with two strong electron acceptors (S03H and H3N ) and therefore cannot form a sigma complex under the reacdtion conditions. The fact that an Ar-SE reaction occurs nonetheless is due to the small equilibrium amount of the neutral form B of sulfanilic acid. B is nitrated in ortho-position to the electron donor (H2N) and thus in meta-position to the electron acceptor (S03H).

Figure 2 shows the structures of the bifunctional model systems l(n), (n=4,6,8,10,12), investigatedThe synthesis of these systems and a discussion of their electrochemical properties as well as of their electronic absorption and emission spectra have been given elsewhere (Oevering et al., 1987). These molecules contain a 1, 4-dimethoxynaphthalene unit as the photoexcitable electron donor (E./2 =+1.1 V vs. see in acetonitrile) and a 1,1-dicyano-vinyl moiety as a moderately powerful electron acceptor (Ev=-. l V) separated by an array of at least n C-C sigma-bonds. The centre-to-centre separation (Rc) and the edge-to-... 

Chelation involves coordination of more than one sigma - electron pair donor group from the same ligand to the same central atom . Note that in this quoted description the choice of the word "involves" appears to be deliberately more nebulous than what one expects from a formal definition of a term. 

Loosely bound aggregates (chemical effects) are formed with the hydrocarbons acting as electron donors (Lewis base) and the solvents acting as electron acceptors (Lewis acid). The hydrocarbon that forms the most stable complex with the solvent experiences a decrease in volatility. Electron donors are rated by ionization potential, and electron acceptors are rated by their electron affinities. The selectivity will be higher, the larger the difference in ionization potential between the hydrocarbons and the larger the electron affinity of the solvent (9). While data on ionization potentials of hydrocarbons can be found (15, 16), electron affinities data are rare because of difficulties in their experimental determination. Prausnitz and Anderson (8) recommend that the sigma scale, proposed by Hammett (17), be used to determine approximately the solvents relative ability to form complexes with the two hydrocarbons. Attempts by this author, however, to use this scale were not conclusive. Prausnitz and Anderson (8) should be consulted to understand better the physical and chemical effects. 

See also activated complex adduct charge-transfer COMPLEX ELECTRON-DONOR-ACCEPTOR COMPLEX ENCOUNTER COMPLEX INCLUSION COMPOUND PI (n) ADDUCT SIGMA (o) ADDUCT, TRANSITION STATE. 

Examination of the chemical, electrochemical, and spectroscopic behaviour of R3EM(C0)3L (R = Ph or Me, E = Ge or Sn, M === Mn or Re, L = phen or bpy) reveals that the lowest state is one in which an electron has been transferred from the HOMO, which has sigma E—M bonding character to the LUMO which is mainly localized on The rhenium complexes emit in fluid solution at room temperature, and the luminescence may be quenched by both electron acceptors and electron donors, as in reactions (5) and (6). No net photochemistry is observed... 

Aviram and Ratner proposed that a unimolecular rectifier be a single D-a-A molecule, where D is a good one-electron donor with relatively first low ionization potential, o is some saturated covalent "sigma" bridge,... 

Uses Electron donor component of polyolefin polymerization catalyst complexes improves photoresist adhesion to silicon nitride coupling agent release agent lubricant blocking agent chemical intermediate Manuf./Distrib. ABCR http //www.abcr.de, AP Resources http //www.apr.co.kr, Acros Org. http //www.acros.be, Aldrich http //www.sigma-aldrich.com, Fluka http //www.sigma-aldrich.com... 

The electrophile bonds to one carbon atom of the benzene ring, using two of the pi electrons from the pi cloud to form a sigma bond with a ring carbon atom. This carbon atom becomes sp -hybridized. The benzene ring acts as a pi-electron donor, or nucleophile, toward the electrophilic reagent (Figure 4.4). 

Li QZ, Guo X, Yang X, Li WZ, Cheng JB, Li HB (2014) A sigma-hole interaction with radical species as electron donors does single-electron tetrel bonding exist Phys Chem Chem Phys 16(23) 11617-11625. doi 10.1039/c4cp01209g... 

Even so, there are examples that are consistent with the structure being of the metallocyclopropane variety. One ready measure of the amount of each type of bonding is the location of the alkyne stretching fiequency in the IR, This absorption appears about 2200 cm in alkyne complexes while the band appears about 1750 cm for the metallacyclopropene complexes. Alkyne complexes can also act as four-electron donors. Back-donation into the metallic n orbitals is negligible in these cases, but sigma-donation occurs. Finthermore, alkynes form a number of din-uclear complexes where each perpendicular n system can be considered to formally bond to a separate metal atom. Thus care must be exercised in assigning the type of bonding present. 

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