Electron transfer isomers

The use of infrared spectroscopy, either through fingerprint characterisation or by functional group identification, is well established. IR vibrational spectroscopy has thus been applied in spectroelectrochemistry for quite some time. ° The possibility to establish the symmetry of a molecule has made IR-SEC a most valuable tool for mixed-valence chemistry, ° allowing intramolecular electron-transfer rates in the picosecond region to be assessed and electron-transfer isomers to be established. ... 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Reaction of 2-chloromethyl-4//-pyrido[l,2-u]pyrimidine-4-one 162 with various nitronate anions (4 equiv) under phase-transfer conditions with BU4NOH in H2O and CH2CI2 under photo-stimulation gave 2-ethylenic derivatives 164 (01H(55)535). These alkenes 164 were formed by single electron transfer C-alkylation and base-promoted HNO2 elimination from 163. When the ethylenic derivative 164 (R = R ) was unsymmetrical, only the E isomer was isolated. Compound 162 was treated with S-nucleophiles (sodium salt of benzyl mercaptan and benzenesulfinic acid) and the lithium salt of 4-hydroxycoumarin to give compounds 165-167, respectively. 

Electron transfer to vinylaziridines results in ring-opening reactions, yielding allyl amines. Treatment of 268 with SmI2/DMEA (N,N-dimethylethanolamine) provided allyl amine 269 as a 2 1 mixture of olefmic isomers in 88% yield (Scheme 2.66) [97]. 

At this point, special mention37 should be made of the behaviour of highly conjugated ethylenic sulphones in weakly acidic media. For example, in the case when R1 =Ph (Z isomer), a fairly stable anion radical was obtained in dry DMF. However, either in aprotic (consecutive two one-electron transfer) or in protic media (ECE process, occurrence of the protonation step on anion radical), C—S bond cleavage is observed. The formation of the corresponding olefins by C—S bond cleavage may occur in high yield, and is nearly quantitative when R1 = H and R2 = Ph for an electrolysis conducted in... 

Au in 19 metals and semiconductors versus gold metal absorber Electron transfer from isomer shifts, correlation between isomer shift and host electronegativities... 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

Open-chain vicinal dihalides are apparently reduced conceitedly via a conformation in which the two halogen atoms are anti to each other. This conclusion is based upon the fact that meso isomers of structures 47 ait generally more easily reduced than the dl isomers, presumably because steric repulsions are less for meso than for dl in the transition state for electron transfer. The stereochemistry of the products of reduction of simple open chain vicinal di-... 

The first photophysical investigation performed on stereochemically pure metal-based dendrimers having a metal complex as the core is that concerning the tetranuclear species based on a [Ru(tpphz)3]2+ core (tpphz=tetrapyrido[3,2-a 2, 3 -c 3",2"-h 2",3"j]phenazine) [67]. Dendrimer 45 is an example of this family. In this compound, two different types of MLCT excited states, coupled by a medium- and temperature-dependent photoinduced electron transfer, are responsible for the luminescence behavior. However, the properties of all the optical isomers of this family of compounds are very similar. This finding is also in... 

An approximately linear relationship also exists between Rv and the Fe2+ Mossbauer isomer shift, i.e., between shortening of Fe2+—X bonds and increased electron transfer to the 4s orbitals of Fe2+. 

For the first time, the primary nitrone (formaldonitrone) generation and the comparative quantum chemical analysis of its relative stability by comparison with isomers (formaldoxime, nitrosomethane and oxaziridine) has been described (357). Both, experimental and theoretical data clearly show that the formal-donitrones, formed in the course of collision by electronic transfer, can hardly be molecularly isomerized into other [C,H3,N,0] molecules. Methods of quantum chemistry and molecular dynamics have made it possible to study the reactions of nitrone rearrangement into amides through the formation of oxaziridines (358). 

Laser flash photolysis was also applied to study the anion radicals of /ram-isomers of 4-nitro, 4,4 -dinitro- and 4-nitro-4 -methoxystilbenes, that are generated by triplet state quenching with l,4-diazabicyclo[2.2.2]octane (DABCO) in polar solvents at room temperature7. The study shows that electron transfer competes against the tram —> cis... 

As happens for other physico-chemical techniques, one must first ask if an electrochemical investigation is able to distinguish geometric isomers of the type cisjtrans or facjmer metal complexes. In principle, this is possible since, as mentioned previously, the redox potential of an electron transfer is influenced also by steric effects. For instance, we have seen in Chapter 5 that some octahedral complexes of the scorpiand diammac display different electrochemical responses, depending on whether the two outer amino groups assume cis or trans arrangements. One must keep in mind, however, that the differences in the electrochemical response of isomeric complexes can sometimes be quite small, so may escape a first examination. 

The simple theory of electronegativity fails in this discussion because it is based merely on electron transfer energies and determines only the approximate number of electrons transferred, and it does not consider the interactions which take place after transfer. Several calculations in the alkali halides of the cohesive energy (24), the elastic constants (24), the equilibrium spacing (24), and the NMR chemical shift 17, 18, 22) and its pressure dependence (15) have assumed complete ionicity. Because these calculations based on complete ionicity agree remarkably well with the experimental data, we are not surprised that the electronegativity concept of covalency fails completely for the alkali iodide isomer shifts. 

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