The 1H and 13C chemical shifts for some tetrakis(para-substituted phenyl)ethylenes

The proton and carbon chemical shifts for a series of tetrakis(p-substituted phenyl)ethylenes are described. Assignments followed routine substituent chemical shift trends. Both proton and carbon chemical shifts ortho to the varying substituent follow the empirical parameter, Q. The ethylene carbon chemical shifts are proportional to those at the position para to the varying substituent.     

Substituent and solvent effects on the 19F chemical shifts of 3- and 4-XCH2-substituted fluorobenzenes

Substituent effects on the ¹⁹F chemical shifts of a series of 3- and 4-XCH₂-substituted fluorobenzenes have been studied in five different solvents (dimethyl sulphoxide, acetone, chloroform, carbon tetrachloride and benzene). The results show that in both series substituent effects (other than inductive effects) contribute in at least some cases to the overall shifts, but that these showed little solvent dependence. The meta series showed a proportionately greater solvent dependence than the para series, and this is attributed to the greater importance of ‘through solvent’ direct field effects in the former.     

A unified correlation of substituent effects with carbon,proton and fluorine chemical shifts in aromatic and olefinic systems

At the present time no completely satisfactory quantum mechanical calculations exist for carbon, proton or fluorine chemical shifts in various substituted aromatic or olefinic systems. However, the chemical shifts in such systems have been shown to be well correlated by a linear multiple regression analysis with the Swain and Lupton field and resonance para meters ? and ?, and the semiempirical parameter Q. The utility of Q in testing substituent stereochemistry has been exemplified previously. Here the applications of the complete regression analysis to a wide variety of different systems for the three nuclei are given. The correlation is also shown to apply to selenium in substituted selenophenes. The ¹³C chemical shifts for a series of ortho substituted toluenes are presented and comparisons made with other ortho disubstituted benzenes.     

Carbon-13 NMR studies of a series of benzylphenols

Carbon-13 NMR spectra of a series of benzylphenols and their O-alkylated derivatives were recorded to find the substituent effects of the benzyl, hydroxybenzyl and alkoxybenzyl groups on the ¹³C chemical shifts. It was found that the methylene bridge carbons show signal shifts mainly due to the mesomeric effects of the OH and OCH₃ substituents, and that in the case of ortho-substituted benzyl compounds, the methylene carbon signals exhibit upfield shifts due to both mesomeric and steric effects.     

13C-, 14N- und 1H-kernresonanzspektroskopische Untersuchungen an m-/p-substuierten N-Methylpyridiniumjodiden

In a series of 21 m-/p-substituted N-methylpyridinium salts, N-methyl proton, carbon-13 and pyridinium nitrogen-14 chemical shifts were determined by heteronuclear double resonance, and partly in the case of the ¹⁴N nucleus and the methyl protons by direct measurement. In a few compounds the quadrupole relaxation times proved to be too short for the ¹⁴N coupling to be detected. This problem was overcome by adequately rising the sample temperature. For all three nuclei a marked dependence of the chemical shifts on the nature of the substituent could be established. Increased nitrogen π-electron density shifts the resonances towards higher fields. In the case of the ¹⁴N shieldings, this tendency is attributed to changes in the paramagnetic screening term, whereas for ¹³C and ¹H an interpretation is given in terms of a neighbour anisotropy contribution. The latter explanation is based on the observation that the calculated carbon and hydrogen charge densities show no significant variations throughout the series. Excellent shift correlations were obtained with Hammett substituent constants when σ⁺-values were used for donor substituents. A similar substituents, dependence could be observed for the direct ¹³CH methyl coupling constants, whose magnitude increases with decreasing nitrogen charge density.     

Solvent effects on the proton magnetic resonance spectra of thio- and dithio-oxamides

The chemical shifts of the N-methyl signals for a number of thio-oxamide and oxamide derivatives are unambiguously determined in the solvents chloroform, carbon tetrachloride, benzene, chlorobenzene, o-dichlorobenzene and nitrobenzene. The ASIS effect is discussed. The temperature dependence of the absolute shift and of the relative shift differences of both methyl groups of the N,N-dimethyl derivatives are studied.     

A correlation of substituent effects with proton chemical shifts in aromatic tetrazolic acids

The effect of substituents on the proton chemical shifts and spin–spin coupling constants in ortho-, meta- and para-substituted 5-phenyltetrazoles (tetrazolic acids) in DMSO–CH₃CN (1:1, v/v) was studied. With the meta- and para- substituted compounds the additivity rule of chemical shifts was obeyed, thereby enabling increments characterizing the effects of individual substituents in monosubstituted benzenes to be determined. By employing the Smith and Proulx equation, the chemical shifts of the aromatic protons were correlated with the F, R and Q substituent constants. The values of these constants are 1.02, ?0.004 and 5.49, respectively, for the tetrazolyl substituent.     

Reversal of order of chemical shifts. Differential solvent effects in the hydrogen shifts of toluene,m-xylene and o-xylene as determined by 320 MHz tritium NMR spectroscopy

Chemical shifts for the aromatic tritons of toluene decrease in the order meta>para>ortho for pure toluene, but in the order meta>ortho>para for dilute solutions of toluene in carbon tetrachloride, chloroform, cyclohexane and dimethyl sulphoxide; m-xylene shifts are 5>4,6>2 in the pure state and 5>2>4,6 in carbon tetrachloride and dimethyl sulphoxide; o-xylene shifts are 4,5>3,6 in the pure state and reversed in carbon tetrachloride.     

Lineare Korrelation der chemischen Verschiebung der Vinylprotonen mit den LCAO-MO-π-Elektronendichten in meta- und para-substituierten Styrolen und Zimtsäuren

A linear correlation was found between the proton chemical shifts of the vinyl protons in m/p-substituted styrenes and cinnamic acids and the π-electron densities of the corresponding carbon atom, calculated by the simple LCAO-MO procedure, using standard values for the perturbation parameters.     

Natural abundance 13C NMR investigation of substituted bromobenzene using noise decoupling of proton resonances

The natural abundance ¹³C NMR spectra of bromobenzene and ten derivatives have been obtained using the technique of noise modulated decoupling of the proton resonances. Assignments have been made for all ¹³C resonance signals using an additivity rule. The chemical shifts of the aromatic carbon nuclei in the para-substituted compounds are discussed in terms of the Taft parameters (σ_R, σ_I) and the electronegativity of the substituent.     

Proton NMR studies of a polybenzimidazole in solution

Molecular aggregation of poly(4,4′-diphenyl ether-5,5′-bibenzimidazole)(PBI) in solution has been studied by high resolution proton NMR. PBI and model compounds have been synthesized, purified, and characterized. Proton resonances in the NMR spectrum of PBI are assigned by comparison with the proton resonances of the model compounds. Spectra are studied by total line-shape analysis, assuming each absorption curve to be Lorentzian. For PBI in N,N-dimethylacetamide (DMAc), the resonance due to the proton of a hydroxyl group formed by proton exchange between the imino group of PBI and the carbonyl group of DMAc is observed. The activation energy for the proton exchange, obtained from Arrhenius plots of the temperature dependence of the chemical shifts of the hydroxyl proton and the imino proton, was found to increase in the order corresponding to dissociation energy of the N? H···O?C hydrogen bond. The chemical shifts in the NMR spectra of PBI-DMAc solutions on the addition of LiCl are strongly dependent on the polymer-salt ratio; and thereby the coordination position of LiCl to PBI is tentatively identified, assuming a pseudocontact LiCl-induced shift. The dependence of the chemical shifts of protons in PBI on the dielectric constant of the solvent is demonstrated by using polar solvents of varying dielectric constant, such as N-methylpyrrolidone, dimethylsulfoxide, and formic acid. The viscosity of the PBI-DMAc solutions is reported at various temperatures and concentrations of LiCl. The results from viscometry are explicable in terms of the NMR observations.     

The 13C NMR spectra of nine ortho-substituted phenols

The ¹³C chemical shifts of nine ortho-substituted phenols have been determined in cyclohexane and in dimethyl sulfoxide. All data were acquired in duplicate upon c.w. and FT instrumentation and both sets of data are presented. The chemical shifts at carbons 1, 2 and 3 are correlated with the parameter Q. Q has been defined as P/Ir³ where P is the polarizability of the adjacent C? X bond, I is the first ionization potential of the elements F, Cl, Br, I and H, and r is the C? X bond length. Experimental values of Q are available for other substituents. The field and resonance parameters of Swain and Lupton may be combined with Q to form a three part multiple regression correlation which is more exact than that with Q alone and which applies to all carbons in the aromatic ring. The results of this study suggest that only one value of the Q parameter is needed to characterize the behavior of the nitro group in these solvent systems. This conclusion is contrasted with earlier results of the effect of substituents on proton chemical shifts in these systems.     

Nuclear magnetic resonance spectra of carbanions—VI. Cumyl-, α-methylbenzyl- and benzyl carbanions

The proton magnetic resonance spectra of the three title carbanions have been observed in THF with potassium as a counter ion. The ortho-protons in the α-methylbenzyl carbanion are nonequivalent at room temperature. This shows that the α-carbon in this carbanion is in the near-sp² configuration. The aromatic proton chemical shifts of the benzylpotassium obtained here are at higher shielding than those of benzyllithium reported previously by Sandel and Freedman. This seems to arise from the different ionic nature of the bonds between carbon and metal in the carbanions.     

Benzene dilution shifts and steric interactions in N,N-dialkylamides

The NMR spectra of ten disubstituted amides have been recorded at 0°C in carbon tetrachloride and in benzene solutions. The benzene dilution shifts (ASIS) and proton spin decoupling were used to make the chemical shift assignments. A time-averaged solvent cluster model for the association between the amide and benzene is consistent with the observed ASIS values. The assignments for the N-methine and N-methyl resonance peaks in RCON[CH(CH₃)₂]₂, where R is methyl, ethyl or propyl are inverted from the assignments for N,N-diisopropylformamide (R ? H).     

1H and 13C NMR study of 8-hydroxyquinoline and some of its 5-substituted analogues

¹H and ¹³C NMR spectra of 8-hydroxyquinoline (oxine) and its 5-Me, 5-F, 5-Cl, 5-Br and 5-NO₂ derivatives have been studied in DMSO-d₆ solution. The ¹H and ¹³C chemical shifts and proton–proton, proton–fluorine, carbon–proton and carbon–fluorine coupling constants have been determined. The ¹H and ¹³C chemical shifts have been correlated with the charge densities on the hydrogen and carbon atoms calculated by the CNDO/2 method. The correlation of the ¹H and ¹³C chemical shifts with the total charge densities on the carbon atoms is approximately linear (r_H² = 0.85, r_C² = 0.84). The proton in peri position to the nitro group in 5-NO₂-oxine is an exception.     

A comparative study between para‐aminophenyl and ortho‐aminophenyl benzothiazoles using NMR and DFT calculations

Ortho‐substituted and para‐substituted aminophenyl benzothiazoles were synthesised and characterised using NMR spectroscopy. A comparison of the proton chemical shift values reveals significant differences in the observed chemical shift values for the NH protons indicating the presence of a hydrogen bond in all ortho‐substituted compounds as compared to the para compounds. The presence of intramolecular hydrogen bond in the ortho amino substituted aminophenyl benzothiazole forces the molecule to be planar which may be an additional advantage in developing these compounds as Alzheimer's imaging agent because the binding to amyloid fibrils prefers planar compounds. The splitting pattern of the methylene proton next to the amino group also showed significant coupling to the amino proton consistent with the notion of the existence of slow exchange and hydrogen bond in the ortho‐substituted compounds. This is further verified by density functional theory calculations which yielded a near planar low energy conformer for all the o‐aminophenyl benzothiazoles and displayed a hydrogen bond from the amine proton to the nitrogen of the thiazole ring. A detailed analysis of the ¹H, ¹³C and ¹⁵N NMR chemical shifts and density functional theory calculated structures of the compounds are described. Copyright © 2014 John Wiley & Sons, Ltd.     

Effets stériques des groupes alkyle sur le déplacement chimique des hydrogènes d'un méthyle: Correlation avec les termes Es de Taft

The effect of alkyl groups (Me, Et, i-Pr, t-Bu) on the ortho methyl proton chemical shifts is shown to be linearly correlated with the Taft (E_s) scale in model compounds 1 . Experimental results can be expressed by Δδ = δ_R – δ_Me = λ′E_s in which λ′ is solvent and model dependent. The calculations of the chemical shifts according to ApSimon and Buckingham are used to show the predominant influence of steric effects on the observed substituent effect.     

Structure elucidation with lanthanide induced shifts. 6—solvent effects on bound shifts and association constants

The NMR spectra of acetone, adamantanone and 1-adamantanecarbonitrile have been studied in the presence of Eu(fod)₃ in various solvents. A substantial solvent dependence is found for the association constants between shift reagent and substrate. The magnitude of the association constants is correlated with solvent polarity, and a tenfold decrease in K₁ is observed upon changing from carbon tetrachloride to the more polar dichloromethane. Only a very small solvent dependence is observed for the bound shifts. The relative bound shifts (and therefore the structures of the LS complexes) are found to be solvent independent. The small solvent dependence of the absolute magnitude of the bound shifts for the LS complexes is suggested to result from experimental errors.     

NMR chemical shift substituent effects: 2-α-monosubstituted N,N-diethylacetamides

¹H NMR chemical shifts for some α-hetero-substituted N,N-diethylacetamides were recorded. The resonance assignments for the syn- and anti-methylene and -methyl protons have been made unambiguously through their aromatic solvent induced shifts and are opposed to the literture assignments for the N-methylene protons. An empirical relationship between the Charton polar (σ_L) and steric (V) parameters and the α-methylene proton resonances was found. The N-methylene proton chemical shifts also showed a qualitative dependence on the α-substituent electronegativity, while the N-ethyl methyl proton chemical shifts were related to the α-substituent steric effects. The Paulsen and Todt anisotropic model and the more populated rotamers proposed seem to explain the results very well.     

On the alleged correlations between simple LCAO-MO reactivity indices and proton chemical shifts in planar,condensed, benzenoid hydrocarbons

Previous investigations into the proposed correlations between proton chemical shifts in conjugated hydrocarbons and the various LCAO-MO ‘reactivity indices’ (at the corresponding carbon atoms to which the peripheral protons are bonded), are here reassessed in the light of more-recently acquired experimental data, for the case of the planar, alternant, condensed, benzenoid hydrocarbons. A correlation coefficient of 0·73, statistically ‘significant’ at less than the ½% level, is obtained. Nevertheless, there are several unsatisfactory features of such proposed correlations (which are discussed), and, in the final analysis, no causative relation is expected between proton chemical shifts and reactivity indices in these molecules. Furthermore, the relative chemical shifts of the sterically unhindered protons in planar, alternant, benzenoid hydrocarbons can be accounted for by the ‘ring current’ effect alone, without the need to postulate a dependence of the proton chemical shifts on reactivity indices, which is in any case considered to be unlikely on physical grounds.