Electroreduction of aromatic oximes: diprotonation, adsorption, imine formation, and substituent effects

Aromatic oximes are reduced in aqueous solution in a four-electron process. The reducible species in the pH range 5-8 is a diprotonated form of the oxime. This species is generated in the course of electrolysis in the vicinity of the electrode surface from the adsorbed neutral form of the oxime. The reduction is initiated by a cleavage of the N-O bond. The diprotonation facilitates the reduction process by the preformation of OH2+ as a good leaving group and by a positive charge on the azomethine nitrogen. Diprotonation has been proven based on shapes of i = f(pH) plots, by observed shifts of half-wave potentials with pH and by comparison with the reduction of nitrones. Some observed deviations from theoretical i = f(pH) plots were attributed to the role of adsorption on the rate of protonation. Adsorption is also responsible for dips on some of the i-E curves. Adsorption plays a role at concentrations as low as 1 x 10(-5) M, when the electrode surface is still not fully covered. This indicates that catalyzed protonation occurs on islets of adsorbed materials. At pH 2-5 the studied oximes in the vicinity of the electrode are predominately present in a protonated form, which is less strongly adsorbed. In this pH range the protonation takes place in a homogeneous reaction layer of the electrode. It yields a monoprotonated form, which is reduced. The separation of two two-electron waves observed for some oximes in acidic media serves as an experimental proof of the formation of imines as reduction intermediates. This separation is caused by the differences in pKa values of protonated forms of oximes and imines. The effects of substituents in the para position on the benzene ring are characterized by correlation with the Hammett substituent constant sigmax. This has been proven at pH 1.5 for substituted benzaldehyde oximes and at pH 5.0 for substituted acetophenone oximes.     

Determination of Total Phenols in Foods by Boron Doped Diamond Electrode

A robust electrochemical method to measure the total phenol content in food samples is presented. Under optimal condition, BDD electrode showed excellent performance to detect the oxidation of several phenols and does not present the drawback due to electrode fouling. The analytical method used to perform such measurement has been optimized and successfully applied in different food samples. The results obtained were compared with the standard Folin–Ciocalteau method.     

A Ramsey-Type Result for Convex Sets

Given a family of n convex compact sets in the plane, one canalways choose n of them which are either pairwise disjoint orpairwise intersecting. On the other hand, there exists a familyof n segments in the plane such that the maximum size of a subfamilywith pairwise disjoint or pairwise intersecting elements inn^log2/log5 n^0·431.     

Non empirical LCAO SCF MO investigations of electronic reorganizations accompanying core ionizations

Calculations have been carried out on an extensive series of molecules for both the neutral species and core ionized states. Substituent effects on C_1s , N_1s , O_1s , and F_1s levels have been investigated and where available comparison has been drawn with experiment. Comparison with Koopmans' theorem has allowed a relatively detailed study of change in relaxation energies as a function of substituent effect on a given core level. Whilst for C_1s levels the computed shifts in core binding energies are approximately linearly related to differences in relaxation energies for the N_1s , O_1s , and F_1s levels, the relative electronegativity of the substituent can invert this correlation. The empirical correction of Koopmans' theorem for differences in relaxation energies at different sites has been investigated for large molecules. The results compare well with direct hole state calculations.     

X-ray crystallographic and physical investigation of tricyanomethanide and dicyanamide complexes of cobalt(II) with imidazole and its methyl derivatives

Summary Reaction of Co^II with N(CN) _inf2 ^sup– or C(CN) _inf3 ^sup– in the presence of imidazole (iz) or its methyl derivatives (2-meiz and 4-meiz) gave eight compounds of Co^II: ligand stoichiometry 12, including two isomeric pairs ( and ) for the complexes [Co{C(CN)₃}₂(2-meiz)₂] and [Co{C(CN)₃}₂-(4-meiz)₂]. The complexes were studied by electronic and i.r. spectroscopies. For -[Co{C(CN)₃}₂(2-meiz)₂] singlecrystal X-ray analysis was performed; its crystal structure consists of one-dimensional chains, formed by C(CN) _inf3 ^sup– anions bridging between the Co^II atoms. The Co^II atom is nearly octahedrally coordinated by two tertiary nitrogens of 2-meiz and four nitrogens of C(CN) _inf3 ^sup– . The spectra of these compounds and of the complexes with iz, as well as that of -[Co{C(CN)₃}₂(4-meiz)₂], indicate all these compounds to have basically the same bridging polymeric octahedral structure. However, the spectra indicate distorted tetrahedral structures for the remaining compounds.     

Electron localizability indicator for correlated wavefunctions. I. Parallel-spin pairs

The electron localizability indicator (ELI) is based on a functional of the same-spin pair density. It reflects the correlation of the motion of same-spin electrons. In the Hartree–Fock approximation the ELI can be related to the electron localization function (ELF). For correlated wavefunctions the ELI formula differs from the one for the ELF.     

A convenient synthesis of trisubstituted pyrido[2,3-d]pyrimidin-7-ones

A novel, highly efficient and scalable route for the synthesis of trisubstituted pyrido[2,3-d]pyrimidin-7-ones was developed. The target compounds were synthesized in five steps from readily available reagents in about 40% overall yield.     

The transfrmation of difluorocarbene into 3,3-diflurocyclopropene

The addition of difluorocarbene to benzobarrelene gives the syn and anti 1,2 mono-adducts. Heating of either of them at 200°C in hexadeuteriobenzene generates naphthalene and 3,3-difluorocyclopropene.     

Mass spectrometry of 1,3- and 1,5-dicaffeoylquinic acids

Samples of 1,3- (1) and 1,5-dicaffeoylquinic acid (2) and their hexaacetate derivatives were examined using positive and negative electrospray ionization mass spectrometry and tandem mass spectrometry. Differences in the various spectra allow the discrimination of each of the isomers. Specific losses in the spectra of 2 also permit the identification of the site of substitution of one of the caffeic acid moieties as being at the 5-position. The spectra of 3,5- (3) and 4,5-dicaffeoylquinic (4) acids and their hexaacetate derivatives were compared with those of 1 and 2 and their derivatives, and differences in ion abundances or the presence/absence of specific ions can be used to identify uniquely each of the compounds.     

A measure of electron localizability

A functional of the same‐spin electron pair density is proposed as a measure of electron localizability. This functional yields the average number of same‐spin electron pairs in a region Ω enclosing a fixed charge. The functional equals zero if the fixed charge in Ω originates from one electron only, with all other same‐spin electrons outside the region Ω. Then, the correlation of the electronic motion in Ω and thus the localizability of an electron is high. If the motion of the same‐spin electrons becomes less correlated, more electrons participate in the fixed charge contained in Ω, the average number of same‐spin electron pairs (the functional) increases. In the Hartree–Fock approximation the Taylor expansion of the proposed localizability functional can be related to the electron localization function of Becke and Edgecombe without using an arbitrary reference to the uniform electron gas. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004