Proton magnetic resonance studies of tautomerism in substituted pyrimidines

Studies of line width and chemical shift vs. temperature for amide and hydroxyl proton magnetic resonance signals from: barbituric acid, dialuric acid, parabanic acid, alloxan and alloxan monohydrate dissolved in anhydrous dimethyl sulfoxide-d₆ are reported. The behavior of the amide signals shows that, between 20 and 65°C, none of the compounds listed exhibits lactim-lactam tautomerism. The amide proton resonance in uracil has also been investigated. The signal is a closely spaced, equal intensity, doublet due to the non-equivalence of H(1) and H(3). Again, no evidence of tautomerism is observed. Activation energies for the hydroxyl resonances in dialuric acid and alloxan monohydrate indicate hydrogen bonding between solute and solvent. The results of simple LCAO-MO calculations are in accord with the experimental conclusions concerning tautomeric equilibria.     

Mass spectrometry of 8-hydroxyquinoline derivatives

In contrast to the fragmentation of the corresponding alkyl aryl ethers, characteristic [M-H]⁺ and [M-H, -CO]⁺ fragments were observed in the fragmentation of 5-nitro(halo)-substituted 8-alkoxyquinolines. It was found by means of deuterium labeling that a hydrogen atom is split out primarily from the alkoxy group. It was demonstrated that an [M-H, -CO]⁺ fragment was formed from the [M-H]⁺ ion, which has a three-ring structure and a quaternary nitrogen atom. The formation of an [M-CO]⁺ fragment is characteristic for the fragmentation of 5(7)-nitro(halo)-substituted 8-hydroxyquinolines. The interrelationship between the intensities of the [M-H]⁺, [M-H, -CO]⁺, and [M-CO]⁺ ion peaks and the protonation constants (pK_a) of the investigated compounds is discussed.Communication 4 from the series Mass spectrometry of biologically active substances, See [1] for communication 3.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 786–791, June, 1984.     

Proton magnetic resonance studies of actinomycin-related peptides containing N-methyl groups

The PMR spectra of several cyclic and acyclic dipeptide derivatives representing portions of the actinomycin structure have been studied. In the spectra of the acyclic compounds temperature-dependent duplicity resulted from the equilibration of conformers possessing cis and trans peptide bonds. The lanthanide shift reagent Eu(FOD)₃ was utilized to distinguish N-methyl groups in the two conformations and observations were made on the steric dependence of the observed shifts. The origin of the wide variation in chemical shift of N-methylvalyl α-protons is discussed in relation to the conformation of actinomycin.     

Spectrophotometric determination of halogenated 8-hydroxyquinoline derivatives

A spectrophotometric method is proposed for the determination of 8-hydroxyquinoline and three of its iodinated derivatives: 5-chloro-7-iodo-8-hydroxyquinoline (clioquinol), 5,7-di-iodo-8-hydroxy-quinoline (iodoquinol) and 8-hydroxy-7-iodo-quinolone-5-sulphonic acid (chinoform). The suggested method depends on the reaction with 4-aminoantipyrine in the presence of an alkaline oxidizing agent. A red antipyrine dye with an absorption peak at 500 nm is produced in all cases. The reacting ratio has been determined and a reaction mechanism is presented. The proposed method can be applied to the analysis of pharmaceutical preparations containing the compounds studied, and the results obtained compare favourably with those obtained with the standard methods.     

Proton and boron-11 magnetic resonance studies of some potassium tetraarylborates

Proton and boron-11 magnetic resonance spectra for several potassium para-substituted tetraarylborate compounds [KB(C₆H₄-pX)₄, where X is H, OCH₃, CH₃, Br, Cl, F, CF₃] have been obtained. The chemical shift between the centers of the AA′ and XX′ multiplets for the ring proton multiplets, relative to a reference chemical shift of 0·39 ppm for potassium tetraphenylborate, correlated with the corresponding Hammett σ values for the para-substituent. Additionally, the boron-11 chemical shifts gave a good correlation with corresponding σ values for the substituents. Electronegativities of para-substituted phenyl rings were calculated and found to be approximately 2·70 for all compounds studied. It was shown that electronic substituent effects do not greatly influence the electron density surrounding the central boron atom in the tetraarylborate ions.     

Proton magnetic resonance spectra of some vinyl and ethyl derivatives of five-membered N-heterocycles

Conclusion On the basis of the chemical shifts of the signals in the PMR spectra of the N-vinyl and N-ethyl derivatives of indole, benzimidazole, benzotriazole, and imidazole a theory was advanced regarding the character of the influence of a second and third nitrogen atom on the vinyl and ethyl radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 475–476, February, 1972.We express our gratitude to Yu. L. Frolov for his helpful evaluation of the experimental data.     

Naphthopyrones,Part I Proton magnetic resonance spectral studies of some methylnaphthopyran-2-one and methylnaphthopyran-4-one derivatives

The 220 MHz¹H-NMR spectra of four methylnaphthopyrone derivatives are presented and discussed. The chemical shift changes which occur upon protonation in trifluoroacetic acid solution are much more significant for the benzochromones than for the benzocoumarins. The particularly large effects at the methyl (0.6 ppm) and at the residual pyrone ring proton (1.2 ppm) provide a useful technique for differentiation between the two series of compounds.

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Naphthopyrone, 1. Mitt.:¹H-N M R einiger Methylnaphthopyran-2-one und Methylnaphthopyran-4-one

Zusammenfassung Es werden die 220 MHz¹H-NMR-Spektren von vier Methylnaphthopyronen diskutiert. Die Änderungen der chemischen Verschiebungen, die sich mit Trifluoressigsäure als Lösungsmittel ergeben, sind für die Benzochromone bedeutend signifikanter als für Benzocumarine. Die besonders großen Effekte für Methyl (0,6 ppm) und das verbleibende Pyronringproton (1,2 ppm) stellen eine brauchbare Methode für die Unterscheidung der beiden Reihen dar.

     

Proton magnetic resonance spectra of cinnolines

The ¹H NMR spectra of cinnoline and some 8-substituted derivatives 1 in DMSO-d₆ are reported. A previous assignment of the chemical shifts of the heterocyclic ring protons H-3 and H-4 is confirmed by deuteration studies. The variations in the chemical shifts of H-3 to H-7 are discussed in terms of (a) solvent effects, (b) the perturbation effect of the heterocyclic ring, and (c) the electronic and proximity effects of the 8-substituents, using single-parameter (SCS_o, SCS_m, SCS_p, σ_o, σ_m, σ_p, σ_p⁺, σ_4,1, σ_5,1, σ_6,1Q) and dual-parameter (F, R; σ₁,σ_R) treatments to correlate the data. Marked proximity effects on H-7 are noted for the nitro and acetamido groups, and are correlated with conformational changes due to interaction of the groups with the N-1 lone pair. Variations in the coupling constants are attributed to partial double-bond fixation.     

Proton magnetic resonance studies of the phase structure of bulk-crystallized linear polyethylene

The broad-line proton NMR spectra of melt-crystallized samples of polyethylene covering a very wide range in molecular weight have been analyzed in terms of contributions from three components: (1) a crystalline region with crystals of orthorhombic form; (2) a noncrystalline region with liquid-like character which produces a Lorentzian contribution to the spectrum; and (3) an intermediate region in which the rotation of methylene groups about C—C bonds is partly hindered. The relative mass fractions as well as the character of these components depend greatly on the molecular weight. Samples of low molecular weight (e.g., ?30,000) are predominantly composed of lamellar crystalline regions with a minor amount of interfacial regions and no liquid-like interzonal regions. As the molecular weight increases beyond 45,000, an interzonal region with a liquid-like character associated with a higher molecular mobility is produced. Above a molecular weight of 100,000, this liquid-like character becomes pronounced with an increase in molecular mobility in the interfacial region.