Benzene back electron transfer

The /cbet value in Scheme 7 is expected to vary depending on the driving force (- AGbet) of back-electron transfer as shown in Fig. 7, where the dependence of log bet versus — AGbet in MeCN and benzene is drawn schematically... 

Table 1 Electron-Transfer Rate Constants (ked Determined from the Fluorescence Quenching of RAcrH" by BuTlNAH and R Sn, Determined from the Dependence of the Quantum Yields on [BuTlNAH] and [R Sn] in the PhotoaUtylation of RAcrH with Bu BNAH and R Sn in MeCN, CHCI3 and Benzene at 298 K, and the Driving Force of Back-Electron Transfer (—AGbet) from RAcrH" to Bu BNAH and R Sn in MeCN...

Fig. 4. Back electron transfer rates in photogenerated radical ion pairs in acetonitrile. In all cases cyanoanthracenes served as electron acceptors in their excited states, (a) Methylated benzene derivatives [60b] as donors (one-ring compounds), V = 11.5 cm"1, As = 1.63 eV. (b) Methylated biphenyls or naphthalenes [60b] as donors (two-ring compounds), V = 8.5 cm-1, /, = 1.48 eV. (c) Methylated phenanthrenes [60b] as donors (three-ring compounds) V = 8.0 cm-1, Xs = 1.40 eV. (dl Diphenylbutadienes [61] as donors, V = 8 cm-, X, = 1.55 eV. In all cases X, = 0.25 eV and V= 1500 cm-1...

Fig. 5. Back electron transfer rates in photogenerated radical ion pairs in acetonitrile (a) 9,10-Dicyanoanthracene in its excited state served as the acceptor. Aryl, alkyl, methoxy and amino benzene derivatives as well as aliphatic amines served as donors [62] (V = 23 cm-1, 2, = 0.97 eV, Aj = 0.64 eV). (b) Perylene, pyrene, benzperylene, and aromatic amines served as donors. Tetracyanoethylene (TCNE), pyromellitic dianhydride (PMDA), phthalic anhydride (PA), maleic anhydride, pyrene and perylene served as electron acceptors [63], Various combinations of donors or acceptors were excited (V = 20 cm , As = 1.45 eV, A, = 0.07 eV). The parabolas drawn are different from those offered in the original analysis. The parameters that were used were selected to emphasize the similarity to Fig. 4 (in all cases v = 1500 cm-1)...

Another mechanism for the 2n -f 27t ) photocycloaddition of alkenes via electron transfer is the reaction that proceeds via a triplet state which is produced by a back-electron transfer from a radical anion of the electron acceptor to a radical cation of the electron donor. The triplet state alkenes generated by this way can undergo the cyclodimerization (Scheme 22). Farid showed that the DCA-sensitized (2n 3- 2n) photocyclodimerization of 1,2-diphenylcyclo-propene-3-carboxylate occurs via the triplet state of the cyclo-propene in acetonitrile [84]. In this photoreaction, two types of the (An -(- 27t) photocycloaddition reactions take place between DCA and the cyclopropene depending upon solvents. One type of the cycloadduct is produced in benzene via exciplex and the other type of the photocycloadduct is produced in... 

The arrow represents the electron transfer excitation from the HOMO of the donor to the LUMO of the acceptor, iodine. (B) Schematic diagram of the back electron transfer process which leaves the molecular iodine electronically excited. (C) Pictorial representation of the HOMO in benzene and the LUMO of iodine. Adapted from Ref [55]. 

The decay of the parent is biexponential with a short time constant, typically 250 fs, and a longer one, 800 fs. The kinetics appear to be parallel, i.e., correspond to different and competing decay channels. Their pre-exponential factors are different, with the short one dominating. The short decay has been assigned to an ionic to covalent back electron transfer pictured in Figure 17, resulting in iodine dissociation. A back transfer from the iodine n orbital to the half-filled benzene n orbital leaves iodine in a dissociative state. The reaction channel forms (benzene) - -I-1, and corresponds to the minor decay component of the parent. 

Figure 3. The absorption of cyanine dye (Cy) radicals monitored at 430 nm following excitation of a benzene solution with an 18 ps laser pulse. The time dependence of the absorption changes of cyanine radical for the benzyltriphenylborate case is faster than its decay. For the vinyltriphe-nylborate, back electron transfer and the reaction that follows electron transfer have competitive rates. For the tetraphenylborate salt, the back electron transfer process dominates after electron transfer, therefore the boron-carbon bond cleavage does not occur and almost no cyanine dye radical formation is observed (data adapted from [25]).

Irradiation of electron deficient arenes in the presence of cis-l,2-diphenylcyclopropane leads to formation of the trans isomer by an electron transfer mechanism. The reaction occurs by way of the radical cation of the cyclopropane which isomerises prior to back electron transfer. It has now been examined using menthyl and bornyl esters of benzene tetracarboxylic acid as chiral electron transfer sensitisers. °° Slight excesses of one of the enantiomers of the trans-1,2-diphenylcyclopropane were observed. The dicyanoanthracene sensitised reactions of 1,1,2,3-tetra-arylcyclopropanes have been studied.Depending on the substituents present on the arene rings these compounds rearrange to 1,1,3,3-tetra-arylpropenes. The rearrangement occurs in a ring opened radical cation intermediate. 

Light induced electron transfer from an amine to an excited arene leading to a contact radical ion pair is proposed to account for the products observed when 9,10-dicyanoanthracene is irradiated in wet benzene in the presence of an a-aminoketone. The non-polar solvent maintains the proximity of the radical ion pair which would normally then undergo non-productive back electron transfer however, in this case the ion pair reacts, ultimately to furnish 9,10-dihydro-9,10-dicyanoanthracene along with products of fragmentation of the a-aminoketone. Alkyl borate and borohydride salts can also serve as electron donors in the photoreduction of aryl cyanides and aryl halides and an abstract of a report on this topic has appeared. ... 

The technique has been described in detail elsewhere. [26] In short, a pulse of high energy electrons induces a series of chemical reactions that can be monitored, e.g., using time resolved UV-vis spectroscopy. The reaction of interest is usually induced by a reaction between a radical formed from radiolysis of the solvent (usually water) and a solute molecule. The primary radiolysis products in aqueous solution are HO, e q", H, HjOj, H2 and The major radical species, HO and e q, are formed in equimolar concentrations, 0.28 ol/J each, on electron or y-irradiation.[27] As can be seen in reaction 2, the hydroxyl radical does not yield a benzene radical cation instantly upon reaction with a substituted benzene. For this reason, secondary oxidants, such as S04, Brj and N3, are usually used to generate benzene radical cations. To determine one-electron reduction potentials of radical cations, the redox equilibrium between the radical cation of interest and a redox couple with a known one-electron reduction potential is studied. The equilibrium constant can be derived from the rate constants of the electron-transfer reaction and the back reaction and/or the equilibrium concentrations of the two redox couples (reaction 6).[28]... 

R= Ru(CN)6 -, Mo(CN)8 -, Fe(CN)6 -, and so forth (Table 2)) in an aqueous phase have been surveyed. At the probe microelectrode surface, ZnPor+ was oxidized to ZuFor" ". When the probe is positioned close to the benzene-water interface, ZnPor+ is reduced back to ZnPor by accepting an electron from R in the aqueous phase at the liquid-liquid interface. In the experiment, the driving force was controlled with two parameters the difference in standard potentials of the redox mediators in benzene and in water (AE ), and the interfacial potential drop (A ), which is controllable by varying the concentration ratio of a base electrolyte such as Cl04 in the two Kquids. The driving force dependence on the electron transfer rate at the liquid-liquid interface has been shown in the literature in the absence and presence of the monolayer. The existence of the monolayer lowers the electron transfer... 

In the initial work, it was found that a catalytic amount of cathodic current at - 1.7 V vs SCE decomposes [FeCp(C6H6)] PF6 in 95 % ethanol on Hg cathode in basic media. Cyclopentadiene, benzene and Fe(OH)2 were characterized and the electron-transfer chain mechanism was proposed to account for the benzene exchange for solvent molecules. The driving force of the propagation cycle is the exergonic cross ET step since the three EtOH ligands do not allow back bonding whereas benzene does. ... 

---