Pi electron donor

It should be noted here that the units of the F-nmr shifts compared to the log (kiko) values are such that sets 15, 16 and 17 are very highly weighted in the determination of the Or scale. In the statistical sense, this weighting is justified by the relatively high precision of the F-nmr shifts. In the physical sense, however, it is not clear that the high weighting is justified. Although F is the weakest pi electron donor of the united-atom-like first-row pair donor... 

The inclusion of p-substituted fluorobenzene F-nmr shifts in the basis sets suggests that weakly interacting Y pi electron donor groups are also permitted. 

Y is a strongly pi electron donor group. As previously noted in the results section, examples of Y from Table VI include centers of high pi electron charge density at carbon, sulfur, nitrogen, and oxygen. Also included in Table VI are examples of nucleophilic substitution transition states (cf. reactions 21 and 22) of the type... 

Bryce, M.R., Tetrathiafulvalenes as pi-electron donors for intramolecular charge-transfer... 

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... 

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). 

Khashaba et al. [34] suggested the use of sample spectrophotometric and spectrofluorimetric methods for the determination of miconazole and other antifungal drugs in different pharmaceutical formulations. The spectrophotometric method depend on the interaction between imidazole antifungal drugs as -electron donor with the pi-acceptor 2,3-dichloro-5,6-dicyano-l,4-benzoquinone, in methanol or with p-chloranilic acid in acetonitrile. The produced chromogens obey Beer s law at Amax 460 and 520 nm in the concentration range 22.5-200 and 7.9-280 pg/mL for 2,3-dichloro-5,6-dicyano-l,4-benzoquinone and p-chloranilic acid, respectively. Spectrofluorimetric method is based on the measurement of the native fluorescence of ketoconazole at 375 nm with excitation at 288 nm and/or fluorescence intensity versus concentration is linear for ketoconazole at 49.7-800 ng/mL. The methods... 

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... 

Horak, J., Maier, N.M., and Lindner, W., Investigations on the chromatographic behavior of hybrid reversed-phase materials containing electron donor-acceptor systems ii. Contribution of pi-pi aromatic interactions, J. Chromatogr. A, 1045, 43, 2004. 

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. 

Acyl and aroylcyanamides NCNC(0)R are also quite strong donors (Pi = —1.19 V) [40], similar to Cl , whereas organocyanamides NCNR2 are much weaker electron-donors [35, 36], being comparable to cyanamide itself. 

Protonation of a ligand normally leads to a significant increase in its net electron-withdrawing ability (or decrease of its net electron-donor character), as observed for the following pairs (Pi increase of 0.3-0.5 V) (Table 2) carbynes CCH2R versus vinylidenes C=CHR [14], aminocarbyne CNH2 versus isocyanide CNR [17],... 

Although the Pi parameter normally provides a reliable basis for the evaluation of the net electron donor/acceptor properties of ligands, as shown earlier, the comparison of their Pi values when ligating different metal centers has to be done with caution, in some cases, in view of the possible dependence of Pi on the type of the binding metal center (see in the following). 

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. 

Besides the electron richness, other electronic properties of the metal center can also affect Pi, as demonstrated for cyanamides N=CNR2, which tend to behave as considerable net electron-donors (a and tt). In fact (Table 6), at the trans- FeBr(depe)2 " " center Eg = 1.32 V), with the Br ligand [36], they act as weaker electron-donors than at trans- Mo(N2)(dppe)2 (much electron-richer. Eg = —0.13 V) [34], trans- Re(CNR)(dppe)2 " (electron-richer. Eg =... 

Examples of the former criteria are as follows metal centers with a low electron-richness (high Eg) bind preferably ligands that are strong electron donors (low Pi values) coordination of N2 is favored by a high electron-richness (low Eg) and a high polarizability (P) of the metal center [10, 11]. Tr ns-[ReCl(N2)(dppe)2] is an example of a rather stable N2 complex with such features (the Eg and p values of the binding metal center are 0.68 V and 3.4, respectively [21]). 

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. 

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

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. 

---