Etudes des sélénophènes mono et bisubstitués par Résonance Magnétique de 1H et de 13C

Proton and carbon magnetic resonance spectra of mono-and disubstituted selenophenes are investigated. The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents, with a view to studying the conformational equilibrium of the carbonyl group. π Electronic charges, computed by the PPP method, are correlated with the proton and carbon chemical shifts. The coupling constants between ¹³C and ¹H (1, 2 or 3 bonds) and ¹³C? ⁷⁷Se are shown to be good structural parameters and a set of substituent additivity constants is calculated.     

Etudes par la RMN du carbone-13 de quelques dérivés cyclopropaniques: Déplacements chimiques et paramètres structuraux

Measurements of the ¹³C chemical shifts of monosubstituted cyclopropanes (C₃H₅X, X = CH₃, Br, ? C?CH) enabled us to determine the direct additivity parameters which depend solely on the nature of the substituent. In the case of polysubstituted derivatives, complementary effects due to pairwise contributions of the substituents superpose upon the direct effects. A systematic study of cyclopropanes polysubstituted by bromine atoms, and methyl and ethynyl groups shows these different contributions and permits us to propose a simple interpretation for the majority of cases. Pairwise interaction has been ascribed either to electron transfer or to symmetric or dissymmetric steric interactions between the different substituents, as is shown by the comparison of the theoretical with the experimental shift values.     

Résonance magnétique nucléaire de 13C et 17O d'éthers aliphatiques. Effets γ entre les atomes d'oxygène et de carbone

The ¹³C- and ¹⁷O-chemical shifts of 31 aliphatic ethers are measured and discussed. The ¹⁷O-chemical shifts of the ethers ROR′ correlate with chemical shifts for the methylene groups of the corresponding alkanes RCH₂R′. The constant of proportionality can be related to the orbital expansion term 〈r^?3〉_2p. The δ_c for carbon atoms can also be correlated with δ_c for the corresponding alkanes. The origin of the correlation is discussed taking into account the conformational modifications resulting from introduction of an oxygen atom in an alkyl chain.     

17O-RMN. d'éthers,de cétones et d'esters cycliques. Détermination de constante de couplage quadrupolaire

¹³C- and ¹⁷O-relaxation times measured in cyclic ethers, cyclic ketones, and lactones allowed the determination of the quadrupolar interaction at the O-site. Values obtained for six-membered rings can be used as standard values for ethers (χ = 13 MHz), ketones (χ = 10.5 MHz), and esters (χ = 11.5 MHz for dicoordinated and 9.5 MHz for monocoordinated O-atom). Even within a series, no correlation is found between the ¹⁷O-chemical shift and the value of quadrupolar interaction.     

Substituent effects on carbon-13 chemical shifts in 2,6-disubstituted adamantanes

Carbon-13 NMR spectral data for a series of symmetrical 2,6-disubstituted adamantanes (O?, CH₂?, CH₃, OH, OCOCH₃) are presented. The substituent effects on ¹³C chemical shifts are additive, except for carbons 2 and 6 in 2,6-adamantanedione. The non-additivity of the substituent effect in the diketone can be interpreted in terms of a through-space interaction of the carbonyl π-electrons; the additivity in the other derivatives indicates that there is no appreciable interaction between the substituents.     

Etude par RMN du Proton et du Carbone-13 de Pyrimidines Substituées. I—Effets de Substituant dans les Dérivés Méthylés et Aminés

Substituent effects of methyl and amino groups on the chemical shifts of pyrimidine have been investigated by ¹H and ¹³C n.m.r. and compared with similar data obtained for benzene and pyridine. Taking into account pairwise interactions, the chemical shifts calculated by using an additivity relationship are in very good agreement with the experimental results, except for some hindered pyrimidines. This study enabled us to assign the ¹³C n.m.r. spectra of some trisubstituted pyrimidines.     

Déplacements Chimiques du Carbone 13, Constantes de Couplage 1J(CH) et 3J(CH) dans une série de dérivés trisubstitués du benzène

¹³C chemical shifts, 1-bond and 3-bond (meta) ¹³C? H coupling constants have been determined in a series of trisubstituted benzene; the substituents are Cl, NH₂, N(C₂H₅)₂, N(iC₃H₇)₂, N(C₂H₄)₂O. Chemical shifts are only in moderate agreement with the usual additivity rules. Additivity rules relative to the above mentioned coupling constants are proposed. With few exceptions, the difference between predicted and observed values is less than 10%.     

Oxygen-17 and Carbon-13 Nuclear Magnetic Resonance. Chemical shifts of unsaturated carbonyl compounds and acyl derivatives

The ¹⁷O and ¹³C chemical shifts (δ) of 14 α,β-unsaturated aldehydes and ketones and 33 acyl derivatives RXC = O (X = Cl, OH, OMe, OEt, NH₂ and R = H or alkyl) have been measured. In the unsaturated carbonyl series, a correlation exists between δ ¹⁷O and the π electron density at the β-carbon atom. From this correlation, an δ ¹⁷O of 530 ppm was extrapolated for the loss of one electron at the oxygen atom. In the acyl series, the δr ¹⁷O were also sensitive to changes in the polarity of the carbon-oxygen bond. A partial correlation between ¹⁷O-NMR. chemical shifts and the nuclear quadrupole coupling constants exists for aldehydes, ketones, esters and amides but not for acyl chlorides.     

Etude par RMN du 13C d'Hydroxy-4 Coumarines Substituées en 3. Analyse par Corrélation des Effets de Substituants

Carbon-13 chemical shifts and ¹³C-¹H coupling constants of 3-substituted 4-hydroxycoumarins and 4-hydroxy-7-methoxycoumarins are determined. A three-parameter correlation with ? and ? of Swain and Lupton and Q of Schaefer applied to these compounds provides linear relationships for the prediction of chemical shifts from substituent parameters.     

Etudes d'Hétérocycles Pentagonaux Polyhétéroatomiques par RMN du 13C. Effets de Substituant,de la Benzocondensation et Etude de la Tautomèrie Prototropique en Série Benzothiazolique

The ¹³C NMR spectra of 43 benzothiazoles have been recorded in DMSO-d₆. All carbon atoms have been attributed in an unambigous way owing to substituent effects in position 4, 5, 6 or 7. We discuss variations of chemical shifts as a function of the nature of the substituent in position 2 (equation of type: Δδ = aF+bR+cQ+d′), annelation in the benzoheterocyclic series, and prototropic tautomerism in the benzothiazolic series (in the case of the substituent in the 2-position being an OH, SH or NHR group).     

Etude par spectrométrie de masse de l'ionisation de benzonitriles,de phénylacétonitriles et de N,N-diméthylanilines substitués

Molecular ionization potentials for series of compounds of the type X? C₆H₄? CN, X? C₆H₄CH₂? CN and X? C₆H₄? N(CH₃)₂ have been measured using the retarding potential difference technique (RPD. technique). The effect of the various substituents X is better correlated through the electrophilic Brown σ_p⁺ constants than through Hammett's σ_p values. No meta-para orientation effect is observed. For all the disubstituted phenyl compounds studied, the effect of the second substituent is affected by the electron-releasing power of the original substituent. Ionization potentials calculated by using the semi-empirical method of equivalent orbitals are in good agreement with the experimental values.     

17O, 13C and 1H NMR and IR spectral study of crowded ketones: possible intramolecular C—H···O interactions

Unusual behaviour was observed in the study of the ¹⁷O, ¹³C and ¹H NMR and IR spectra of crowded (1‐adamantyl)alkyl ketones. As the size of the alkyl substituent is increased, abnormal upfield chemical shifts in the ¹³C NMR and downfield shifts in the ¹⁷O NMR of the carbonyl group, as well as downfield shifts in the ¹H NMR of the adamantyl γ'‐protons, are found. In the IR spectrum, the ν_C?O stretching frequencies of the ketones with bulky substituents show considerable red shifts. Correlation of the NMR shifts with the number of γ‐carbon atoms of the alkyl substituents and comparison with the IR results indicated that there is an intramolecular through‐space CH···O interaction in crowded ketones. This was supported by the results of ab initio calculations. Copyright © 2002 John Wiley & Sons, Ltd.     

A CNDO/2 study on the additivity and the nature of the non-additivity of the substituent effects on 13C NMR shifts in chlorobenzenes and chlorophenols

The general correlation between the electron densities and the ¹³C NMR chemical shifts is found to be quite poor in the cases discussed. The non-additivities of the substituent effects on the chemical shifts and the CNDO/2 electron densities correlate only weakly. However, when the electron densities are made specific to different types of atomic orbitals, the s electrons have a pronounced effect in all the models tested. This is explained by an indirect effect on the 〈1/r³〉 term of the p electrons. Good correlations are found between the sums of the chemical shifts and the corresponding sums of the substituent charge excesses. The different behaviour of OH and Cl substituents in the additivity of the substituent effects can be explained by their different back-bonding properties.     

Interactions intramoleculaires: XXII—Constantes de couplage, équilibres conformationnels et interactions gauches avec un groupement t-butyle (molécules cyclohexéniques deutériées)

The NMR parameters are determined for a series of disubstituted (R = CH₃, (CH₃)₃C; X = COOCH₃, CN) and specifically deuterated cyclohexenes. The ³ J(HH), ³J(HD), ⁴J(HH) and ⁵ J(HH) coupling constants are used to evaluate the conformational equilibria. Three of the five compounds with an allylic t-butyl substituent, are conformationally heterogeneous; these equilibria, and those between stereoisomers are used for an analysis of this phenomenon, carried out within the framework of the hypothesis of additivity of conformational free energies and gauche interactions.     

Spectroscopie RMN du Carbone-13 de Thiocétones Aliphatiques: Propanethione et Dérivés,Thiocétones Bicycliques et α Cyclopropaniques

Carbon-13 NMR spectra of various aliphatic ketones and thioketones were determined and interpreted. As shown by the relation between ¹³C chemical shifts in C=A groups, the C=S is more sensitive to the substituents than the C=O group. Conjugative effects are more pronounced in α-cyclopropyl thioketones than in α-cyclopropyl ketones.     

RMN 13C d'Alcynes Aliphatiques: Analyse Topologique des Effets de Substituants Alkyles sur les Déplacements Chimiques des Carbones sp par la Méthode DARC PELCO

Alkyl substituent effects on the sp carbon chemical shifts of aliphatic alkynes have been analysed by the DARC/PELCO method estimating the perturbations linked to the introduction of localized sites in the Limited, Concentric, Ordered Environment of the carbons described by graduate generation from the centre –C?C^.–. A site correlation ‘Topology-δ’ is proposed and its ability to predict the chemical shifts is checked on a population of 55 linear to highly branched alkynes, over a range of 25.2 ppm. This work also shows (i) the independence of the effects of the alkyl groups linked to the C?C bond; (ii) their attenuation with increasing branching of the alkyl substituents; (iii) the small values of the γ_π effects; (iv) the additivity of site perturbation increments (without interaction terms) up to a frontier corresponding to the –C(H,Me,tBu) and –C(Me₂,tBu) groups. A simplified model is derived from a partially ordered generation graph and, for rapid estimation of δ, the global effects of alkyl radicals are expressed as group parameters by summation of their site parameters.     

Intramolecular Hydrogen Bonds of the CO…︁H?O Type as Studied by 17O-NMR

The ¹⁷O-NMR spectra of 1,4-naphthoquinone and 5-hydroxy-1,4-naphthoquinone (juglone) have been recorded in CDCl₃ solution at 40°. In juglone the ¹⁷O resonance of the carbonyl peri to the OH group was displaced by 70 ppm to low frequency relative to the resonance in the para-position. It is shown that this chemical shift arises mainly from intramolecular H-bonding, the substituent and steric effects being one order of magnitude smaller. Large carbonyl ¹⁷O chemical shifts between ?34 and ?100 ppm were also observed in a series of aromatic aldehydes and ketones where intramolecular H-bonds of the C?O…?H? O type are formed. The H-bond-induced carbonyl ¹⁷O chemical shifts were linearly correlated with both the ¹⁷O and ¹H chemical shifts of the OH groups. They represent a most sensitive measure of the strength of intramolecular H-bonds. The ¹⁷O resonances of the OH groups were directed to high frequency on H-bonding. Analysis of the ¹⁷O chemical shifts in 2,2′-dihydroxy-benzophenone showed clearly that the two OH groups build H-bonds simultaneously to the single carbonyl group. The ¹⁷O linewidths decreased strongly on H-bonding; the linewidth of the H-bonded carbonyl O-atom in juglone, for example, was reduced by 25% with respect to that of the free carbonyl O-atom. The carbonyl O-atom quadrupole coupling constants in juglone, evaluated from the combined use of ¹³C and ¹⁷O relaxation times, were 9.5 and 11.0 MHz, respectively. No correlation was observed between the H-bond-induced ¹⁷O chemical shifts and the variations in ¹⁷O quadrupole coupling constants.     

13C-Chemische verschiebungen und substituenteneffekte in polycyclisch konjugierten π-elektronensystemen mit fünf- und siebengliedrigen ringen1

In order to study the substituent influence on the ¹³C chemical shifts in polycyclic conjugated π-electron systems with five and seven membered rings we have studied the aceheptylene system with markedly alternating bond length— in contrast to azulene investigated earlier—as well as 5-azaazulene as a hetero-analogue of azulene. The substituent induced ¹³C chemical shifts Δδ_c observed in the monomethylazulenes, monomethylaceheptylenes and in 5-azaazulene are correlated with the atom-atom polarizibilities π_ij of the corresponding unsubstituted compound. For 5-aza-azulene it was further tested, if the ¹³C chemical shifts could be predicted using the ¹³C chemical shifts of 4-substituted derivatives (RCN, Cl, OEt) and the correlations given in the literature for the substituent induced ¹³C chemical shifts in the system benzene/butadiene. The results show that the influence of the π-electrons is markedly overshadowed in the methylazulenes and -aceheptylenes by other effects, but is clearly discernible in 5-azaazulene as a derivative of azulene; so the data can be used to predict the ¹³C chemical shifts of other azaazulenes.     

Polyhaloheterocyclic compounds. Part XXXI—carbon-13 NMR spectra as a structural probe for polychloroaromatic compounds

Carbon-13 spectra of twelve chlorofluoropyridines (C₅CI_nF_5?nN), of known structure, have been assigned and an assessment has been made of the degree of additivity of substituent chemical shifts (SCS). Reasonable additivity is observed for pyridine derivatives but little regulatiry can be inferred from the spectra of perchlorinated-diazines and -triazines. There is an approximately linear relationship between ¹³C and ¹⁹F chemical shifts.     

Sur le calcul a priori des déplacements chimiques de 1H et de 19F

We propose a new method for calculating ¹H and ¹⁹F chemical shifts in saturated organic compounds. Results show that the shifts are a balance of several contributions and allow an evaluation of these effects: a diamagnetic contribution d(—?) proportional to the net charge ? of the electronic environment of the nucleus considered and paramagnetic contributions p_X of each of substituents, constant attached to the substituent X in any molecule. For both nuclei, d values were deduced from a theoretical calculation of the screen in the free atom. This method, using a restricted number of parameters obtained from experiment, lead to the observed chemical shifts with a mean accuracy of ± 0,32 ppm for ¹H and ± 11 ppm for ¹⁹F. Therefore this method gives a simple quantitative relation between chemical shift and structure from which precise indications about the origin of the contributions from neighbouring electronic surroundings can be obtained.