Pi electron acceptor

Y is a strong interacting pi electron acceptor. In addition to carbonium and oxocarbonium ion centers, examples of Y groups from Table XIII include Nj and several electrophilic substitution transition states (cf. sets 10,21 and 23) of the type... 

The upper lim cone conformed calix[4]arene-zinc porphyrin (ZP) conjugate 180 derivatized at ZP with a p5u-omellitimide (PI) electron acceptor at the distal position on calixarene with a carotenoid unit was designed and S5mthesized for stud5ung the electric field effect of photogenerated ion pair on an adjacent chromophore. The photoinduced charge separation within a ZP-PI electron donor-acceptor pair having an 8.4 A center-center distance in a linear orientation has been found to... 

Finally, replacement of TFE by tetracyanoethylene (TONE) will adversely affect the metal-olefin bonding because TONE does not have the abnormally weak pi bond of TFE. TFE will tend to utilize carbon 2s AO s more than TONE because the sigma electronegativity of fluorine is greater than that of CN. Thus, only the greater pi electron acceptor ability of TONE will favor M-TCNE over M-TFE. 

Rule PI If a catalyst surface is predominantly covered by an electron acceptor adsorbate, then an electron acceptor (electronegative) promoter is to be recommended. 

To determine the distinctions (if any) between conjugative donor-acceptor interactions involving six and those involving four pi electrons, we now examine the cyclopentadienyl anion 19 and cation 20. 

Broo, A. Electronic structure of donor-spacer-acceptor molecules of potential interest for molecular electronics./. Donor-.pi. spacer-acceptor, Chem. Phys., 169(1993), 135-150... 

H20 served as the electron donor and methyl viologen as the electron acceptor. 0 consumption was measured with a Clark-type electrode and phosphorylation was measured colorimetrically. Data are presented as averaged I values SD obtained with three Isolations of thylakoids. Average specific activities were o9 3 ymoles 0, consumed and 171 16 ymoles Pi esterified/mg Chi h for the coupled reactions, and 223 i 5 ymoles 0 consumed/mg Chi h for the uncoupled reaction. 

Panda P, Veldman D, Sweelssen J, Bastiaansen J, Langeveld-Voss BMW, Meskers SCJ (2007) Charge transfer absorption for pi-conjugated polymers and oligomers mixed with electron acceptors. J Phys Chem B 111 5076... 

Aryl phosphines, for example, PPh3 and PMePh2, display a net electron-acceptor character comparable to those of linear organoisocyanides, and the latter phosphine has the same Pi value as phos-I--------------------1... 

The phosphaalkyne P=CBu bonded in the mode, has a Pi value (—0.04 V) [18] that is very close to that of CO (Pi = 0 V). Since the former is not expected to be a good cr-donor, this similarity is accounted for by considering that it is also not an efficient 7r-acceptor [18]. A similar case is that of the N2 ligand (Pi = —0.07 V [10]). Since Pi reflects the net tt-electron acceptor minus cr-donor character, similar values can be found for both a strong rr-acceptor/strong a -donor ligand and a weak rr-acceptor/weak cr-donor Kgand. 

Pickett et al. initial general Eqs. (4) and (5), Pi was considered to be independent of the binding metal center. However, one should be aware that the net tt-electron acceptance/a-donation of a ligand (measured by Pi) is not an intrinsic property of the ligand alone, but can also be determined by the jr-electron releasing and the (7-acceptance ability of the particular binding metal center. For instance, if an unsaturated ligand has, in principle, a considerable jr-electron acceptor character, this can be fulfilled only if the metal site is an effective jr-electron donor. 

A similar effect conceivably accounts for the higher Pi value (—0.74 V) (weaker net electron-donation) of the cyanide ligand estimated [15] at trans- FeH(dppe)2 with the strong donor trans-hydride, in comparison with that (—l.OV) [10] obtained at Cr(CO)5 and also proposed at trans- MoL(dppe)2) (L = CO, N2) with the strong net electron-acceptor L ligand, in spite of the lower electron-richness of the former Fe site Eg = 1.04 V) relatively to the latter (Eg = —0.11 or —0.13 V) molybdenum centers. 

The PnL value is the sum of the n individual Pi values (Eq. 15) when the n ligands exhibit independent electronic effects, usually being in mutually cis positions and behaving as electron acceptors or donors that are neither very strong nor very weak [2,51]. 

Aromatic compounds have a special place in ground-state chemistry because of their enhanced thermodynamic stability, which is associated with the presence of a closed she of (4n + 2) pi-electrons. The thermal chemistry of benzene and related compounds is dominated by substitution reactions, especially electrophilic substitutions, in which the aromatic system is preserved in the overall process. In the photochemistry of aromatic compounds such thermodynamic factors are of secondary importance the electronically excited state is sufficiently energetic, and sufficiently different in electron distribution and electron donor-acceptor properties, ior pathways to be accessible that lead to products which are not characteristic of ground-state processes. Often these products are thermodynamically unstable (though kinetically stable) with respect to the substrates from which they are formed, or they represent an orientational preference different from the one that predominates thermally. 

Further work used a similar system to inhibit the formation of a second ion pair completely, using the electric field of an initial ion pair. In compound 14, Zn3PN and 9-(N-pyrrolidinyl)perylene-3,4-dicarboximide (pyr-PMI) are the electron donors, while NI and PI are once again electron acceptors.11701 Photoinduced electron transfer from Zn3PN to PI with 416 nm laser pulses occurs with t = 27 ps however, if a 645 nm laser pulse is used to excite pyr-PMI first, this event is completely inhibited. 

The putative cation that would be generated by C—N bond cleavage in D would contain an O atom with a very weak pi electron donating ability (+M effect). This is because the z BUjAl group, which binds to this O atom, acts as a Lewis acid. Consequently, the cation in question would be an acceptor-substituted carbenium ion (with a C—O single bond) rather than an oxocarbenium ion (with a C=0 double bond). 

Ferredoxin is believed to function as the first electron acceptor of photoactivated chlorophyll Tagawa and Arnon 99) San Pietro 85)). In net electron flow, the electrons for reducing ferredoxin originate from water, pass to chlorophyll and then to ferredoxin. The reduction of ferredoxin is coupled to the evolution of oxygen Arnon, Tsujimoto, and McSwain (8)). It is emphasized that oxygen evolution end ferredoxin reduction are closely associated and, in the presence of ADP and Pi, they... 

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