Résonance Magnétique Nucléaire de 17O. Aldéhydes et cétones aliphatiques: additivité des effets de substitution et corrélation avec la 13C-RMN

¹⁷O-NMR. Aliphatic aldehydes and ketones, additivity of substituent effects and correlation with ¹³C-NMR. The ¹⁷O-chemical shifts of 9 aldehydes, 22 aliphatic and 4 alicyclic ketones, in the natural abundance FT.-NMR. spectra followed a good correlation with the ¹³C-chemical shifts of the terminal C-atoms of corresponding methylene compounds. An additivity relation involving 6 parameters represents the ¹⁷O-shifts of 28 of the measured products with a standard deviation of 2.5 ppm. The additivity parameters are discussed with respect to the modifications of the polarity of the carbonyl group induced by the hyperconjugative interaction of π and π* orbitals with the πCH ₃ orbitals of the alkyl substituent groups.     

Proximity and the heteroatom effects on the carbon-13 chemical shifts of methyl-substituted phenols,anilines and thiophenols

Upfield substituent-induced ¹³C chemical shifts for aryl carbons of polymethyl substituted benzenes, phenols, anilines and thiophenols were investigated as a function of the proximity between substituents X and CH₃ (X = CH₃, NH₂, OH and SH). The results indicate that the induced shifts of the substituted aryl carbons are, in general, independent of the polar substituent but depend on the number of adjacent substituted aryl carbons. A ?2.0 ppm upfield shift was found for a substituted aryl carbon adjacent to one substituted aryl carbon and a ?3.8 ppm upfield shift for a substituted aryl carbon bound by two substituted aryl carbons. It is suggested that the near additivity of the upfield shifts is the result of changes in the bond order between the aromatic ring carbons in the region of the substituted aryl carbons due to distortion of the ring. The ¹³C chemical shifts of the methyl substituents for methyl substituted phenols, anilines and thiophenols were determined, and it was found that the values could be predicted from the additivity parameters reported for the analogous methylbenzenes plus an additional pair-interaction term associated with the through-space electronic influence of the heteroatom.     

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.     

1H, 13C and 77Se NMR spectra of substituted selenoanisoles

The ¹H, ¹³C and ⁷⁷Se chemical shifts and the ¹J[C(Me)H(Me)], ^1.2J(SeC) and ²J(SeH) coupling constants in 14 para- or meta-substituted selenoanisoles, R? C₆H₄? Se? CH₃, have been measured and the dependence of these parameters on the electronic effects of the substituent R is discussed. A significant (up to 6 ppm) deviation from additivity of the substituent influence on the shielding of the ¹³C ring carbons has been found.     

Pairwise effects of chlorine substituents on the 13C NMR chemical shifts of dichlorobicyclo[2.2.1]heptanes (norbornanes)

The ¹³C NMR spectra of nine dichlorinated bicyclo[2.2.1]heptanes (norbornanes) have been measured and assigned. The pairwise effects of chlorine substituents which cause deviations from the additivity of single-substituent effects were investigated and are discussed. The largest effect found is the high-field shift of carbons bearing vicinal cis substituents. In the case of geminal substitution deviations from additivity were found to be to low field and large in the γ, smaller in the β and negligible in the α chemical shifts. The observed deviations for 1,3-disubstituted cases vary from ?3.2 to +1.1 ppm at different carbons, allowing no simple explanation. Replacement of α-hydrogen in a diaxial 1,3-arrangement by CH₃, OH or CI causes the single substituent effect, namely the γ_a effect, to change considerably.     

Comprehensive parameter set for the prediction of the 13C-NMR chemical shifts of sp3-hybridized carbon atoms in organic compounds

A parameter set is described for prediction of the carbon-13 chemical shifts of sp³-hybridized carbon atoms based on a simple linear additivity relationship. It was tested against 88 692 known chemical shift values. Many of the previously reported parameters were amended and new parameters added. With this parameter set, the chemical shifts in 99.7% of the 88 692 cases can be estimated with an overall standard deviation of 5.6 ppm for the predicted values relative the the experimental ones. The additivity parameters reported here can be used per se or in connection with a recent computer program for the estimation of carbon-13 chemical shifts, which automatically selects and applies additivity rules appropriate for the individual carbon atoms of the chemical constitution entered.     

Carbon-13 nuclear magnetic resonance spectra. I—Monosubstituted adamantanes and adamantanones

The chemical shifts of 4_ax and 4_eq substituted adamantanones are compared with those calculated with the aid of a given additivity rule. It is found that both δ carbons are shielded with respect to the substituent. In contract to the situation for the y-atoms, however, the δ_syn atom is less shielded than the δ_anti atom. In most cases the additivity rule predicts the chemical shifts with an accuracy of approximately 1 ppm. Exceptions are carbon atoms 4 and 9 of the 4_eq substituted adamantanones. In both cases the measured values are 4 to 6 ppm upfield. This can possibly be explained by an electronic interaction between the carbonyl group and the substituent.     

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.     

13C-n.m.r. spectra of acrylophenone (1-phenylprop-2-en-1-one) and ring-substituted acrylophenones

¹³C-n.m.r. chemical shifts of 10 acrylophenones (1-substituted phenylprop-2-en-1-ones) are reported. The additivity parameters for the substituent effect of the acryloyl group in the aromatic ring and of the benzoyl group in ethylene were calculated. Comparison of ethene chemical shifts in chalcones (1,3-diphenylpropenones) with calculated values yielded good results, showing that cross conjugation in these molecules is of little importance. Chemical shifts in acrylophenones and chalcones can therefore be calculated as linear combinations of the mutual effects of the constituent groups.     

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.     

15N nuclear magnetic resonance spectroscopy. Natural abundance 15N NMR of monosubstituted indoles

¹⁵N chemical shifts of twenty-four substituted indoles have been determined in natural abundance (in organic solvents) using Fourier transform NMR. The overall chemical shift range is 27 ppm, with groups in the 2-, 3- and 5-ring positions showing the largest substituent effects. Substituents capable of resonance interaction with the indole nitrogen give shifts in the expected directions but they cannot be correlated with known substituent parameters. Compounds measured in DMSO give 0·2 to 10·2 ppm downfield shifts with respect to the same compound measured in CDCl₃. ¹³C NMR data for previously unreported compounds are also reported.     

Carbon-13 NMR spectra of DDT,its analogues and related compounds

Carbon-13 NMR spectra of mono- and disubstituted aromatic compounds including DDT, its analogues, homologues, derivatives and certain model compounds have been studied. The Savitsky scheme of carbon chemical shifts in disubstituted benzenes is applicable to these compounds. The data obtained show that in mono- and disubstituted aromatic compounds containing two different substituents in the α- and β-positions of the side chain, the substituted ring carbon atom shifts follow the additivity rule and can be calculated from substituent increments. Mutual effects of substituents in the ring and in the side chains are analysed. The chlorine atoms in α-position to the phenyl ring give rise to an additive α-effect of about 25 ppm, as in perchloroalkanes. The influence of a β-chlorine atom in the side chain on the substituted carbon atom in the ring is, however, only 3 ppm as against the usual value of about 10 ppm for the β-effect in alkyl chains. Moreover, the first β-chlorine substituent has no noticeable influence on the substituted ring carbon chemical shift: the effect of 3 ppm is transferred to the para-carbon atom almost without attenuation. The ring substituted carbon atom signal shifts caused by the γ-effect of chlorine in the side chain are similar to those observed in aliphatic chains. The ortho-chlorine substituents shift the side chain α-carbon atom signal by 3.6-5.2 ppm to high field compared to para-chlorophenyl compounds. This is similar to the chlorine γ-effect in aliphatic chains.     

13C-NMR-Untersuchungen an Steroiden. IV—Substituenteneffekte an 16-mono- und 16,17-disubstituierten 3-Methoxyöstra-1,3,5(10)-trienen

The ¹³C NMR spectra of some stereoisomeric 16-mono- and 16,17-disubstituted 3-methoxyestratrienes (16-substituents: OH, Br, N₃, NHAc, SCN, SeCN, SH, NCS; 17-substituent: OH) are reported and a complete assignment is given. For the 16-monosubstituted steroids the shift effects induced by the 16-substituent are almost independent of the steric arrangement of the substituent (16α or 16β). The investigation of the substituent induced shifts of 16,17-disubstituted compounds shows that the vicinal disubstitution leads to considerable deviations from additivity, particularly for the substituted carbon atoms which can amount to 14 ppm.     

Carbon-13 NMR studies of quinolines and isoquinolines. II. Chlorine–nitrogen interactions and N-oxide effects

Carbon-13 chemical shift assignments are reported for four chloroquinolines, six chloroisoquinolines, one dichloroquinoline, four dichloroisoquinolines, four methylchloroquinolines, two methylchloroisoquinolines, quinoline N-oxide, isoquinoline N-oxide, five methylquinoline N-oxides, two methylisoquinoline N-oxides and three chloroisoquinoline N-oxides. Chlorine substituent chemical shift (SCS) effects are reported for the alpha, ortho, meta, para and peri positions. Consistent patterns are observed for the para and peri positions, a vinylogous ortho pattern is reported and the additivity of these SCS effects is demonstrated. Alpha SCS effects vary widely from 1.1 ppm upfield in 1-chloroisoquinoline to 6.7 ppm downfield in 4-chloroquinoline. These results, together with those in the literature, permit the definition of steric and nitrogen lone-pair contributions which modify the ‘normal’ chlorine SCS effect, and these modifying contributions are shown to be roughly additive. Large (6–16 ppm) upfield shifts are observed for the carbons ortho and para to the N-oxide group. The individual magnitudes of these shifts and their sum are constant and the effects are additive in substituted systems. A 9.5 ppm upfield shift is also observed for C-8 in quinoline N-oxides which is attributed to a space–charge interaction. Substituent chemical shift (SCS) effects for the chloro and methyl groups and the chemical shifts of the methyl carbons are essentially the same in the N-oxides as in the parent heterocycles and are additive, except for those molecules where the substituent is adjacent to the N-oxide moiety, in which cases substantial interactions are observed.     

Carbon-13 nuclear magnetic resonance spectra of some methyl and phenyl substituted 2H-azirines

Carbon-13 n.m.r. spectra have been obtained for some methyl and phenyl substituted 2H-azirines. The higher field resonance of C-2 than that of the corresponding aziridine carbon is interpreted in terms of ring strain. Substituent effects on the chemical shifts of the azirine ring carbons are discussed. A set of additivity parameters for the methyl and phenyl groups are obtained which can be used for the calculation of the chemical shifts of the azirine ring carbons. The substituent effect of an azirine ring on the chemical shift of benzene is also discussed in comparison with those of some other substituents. A high degree of s character (48.5%) in the exocyclic orbital of C-3 is indicated by a large J(¹³C-3,H) value (242.5 Hz).     

13C-NMR-Spektren von Tetracyclo[4.1.0.02,4.03,5]heptanen,Tetracyclo[5.1.0.02,4.03,5]octanen und Tricyclo[4.1.0.02,7]hept-3-enen. Ungewöhnliche δ- und γ-Substituenteneffekte

Carbon-13 NMR spectroscopic data of eleven tetracyclo[4.1.0.0^2,4.0^3,5]heptanes, two tetracyclo-[5.1.0.0^2,4.0^3,5]octanes and twelve tricyclo[4.1.0.0^2,7]hept-3-enes are reported. In the tetracycloheptanes, halogens located at the 7-position cause large δ substituent effects. endo-Halogens shift the C-4 signal to lower field by about 6 ppm, while exo-haolgens produce upfield shifts of the C-3 signal, which are dependent on the nature of the halogen and reach a maximum of 9.1 ppm in the case of fluorine. An orbital model is proposed to explain the δ upfield shifts. The compounds containing fluorine reveal a connection between the δ substituent effects and the corresponding ¹³C? ¹⁹F coupling constants. Substituents in the 5 position of tricycloheptenes are γ-substituents of C-1, C-3 and C-7 and produce downfield shifts of the absorptions of these nuclei. Their dependence on the nature of the substituent follows approximately those in 1-substituted adamentanes; in the case of C-7, however, their magnitude by far exceeds the adamantane values, bromine (15.5 ppm) being most effective.     

MP2 calculation of 77Se NMR chemical shifts taking into account relativistic corrections

The main factors affecting the accuracy and computational cost of the Second‐order Möller‐Plesset perturbation theory (MP2) calculation of ⁷⁷Se NMR chemical shifts (methods and basis sets, relativistic corrections, and solvent effects) are addressed with a special emphasis on relativistic effects. For the latter, paramagnetic contribution (390–466 ppm) dominates over diamagnetic term (192–198 ppm) resulting in a total shielding relativistic correction of about 230–260 ppm (some 15% of the total values of selenium absolute shielding constants). Diamagnetic term is practically constant, while paramagnetic contribution spans over 70–80 ppm. In the ⁷⁷Se NMR chemical shifts scale, relativistic corrections are about 20–30 ppm (some 5% of the total values of selenium chemical shifts). Solvent effects evaluated within the polarizable continuum solvation model are of the same order of magnitude as relativistic corrections (about 5%). For the practical calculations of ⁷⁷Se NMR chemical shifts of the medium‐sized organoselenium compounds, the most efficient computational protocols employing relativistic Dyall's basis sets and taking into account relativistic and solvent corrections are suggested. The best result is characterized by a mean absolute error of 17 ppm for the span of ⁷⁷Se NMR chemical shifts reaching 2500 ppm resulting in a mean absolute percentage error of 0.7%. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Carbon-13 NMR spectra of a series of para-substituted N,N-dimethylbenzamides. Comparisons of linear free energy relationships for carbon-13 and nitrogen-15 chemical shifts and rotation barriers

All carbon-13 chemical shifts for 11 para-substituted N,N-dimethylbenzamides in 1 mole % chloroform solution are reported, with assignments based upon double resonance experiments, analogy to chemical shifts of benzamide, and self-consistency between experimental and calculated values using recognized substituent parameters. In contrast to earlier reports, the aryl carbon chemical shift assignments for N,N-dimethylbenzamide are C-2, 127.0; C-3, 128.7; C-4, 129.4, and for p-chloro-N,N-dimethylbenzamide are C-1, 134.6; C-4, 135.5 ppm, relative to internal TMS. Good Hammett correlations (σ_p) are reported for ¹³C chemical shifts of C-1 (σ = 11.9 ppm) and even for the carbonyl group (σ = ?2.3 ppm) but are markedly improved if correlated with σ_p+ (σ = 9.5 ppm) and Dewar's F (σ = ?1.9 ppm), respectively. Excellent Swain–Lupton F and R correlations were found for some of the ¹³C chemical shifts and yielded values for percent resonance contributions to transmission of substituent effects as follows, C-1, 75 ± 4%; C-2, 51 ± 3%; C?O, 31±2%. These are compared to similar values calculated from the C?O of benzoic acids of 34±10%, and from the nitrogen-15 chemical shifts of benzamides of 56±2%. Correlations of these ¹³C δ values and ¹⁵N δ values with rotation barriers (ΔG) for N,N-dimethylbenzamides were examined, and it was found that while C?O δ values correlated only poorly the C-1 δ values correlated very well, but the best correlation was for ¹⁵N δ values of benzamides. It is suggested that Δ G and δ ¹⁵N are intrinsically related due to their numerical correlation, and the close similarity in percent resonance contribution of substituent influence on these parameters.     

13C NMR study on the methoxy carbon chemical shifts in chloro-substituted anisoles and guaiacols

The ¹³C NMR chemical shifts of methoxy carbons in chlorinated anisoles and guaiacols have been measured for acetone-d₆ solutions. Multiple linear regression analysis, and also ‘simple sum rule’ calculations, have been used to estimate the effects of the chlorine atoms (the position and degree of substitution) on the chemical shifts. The most important effects have shown to be due to the chlorine atoms adjacent to the methoxy and hydroxy substituents. For chlorinated guaiacols, the greatest effect is due to the chlorine atom adjacent to the methoxy group. For chlorinated anisoles, the substituents adjacent to the methoxy group (2,6-disubstitution) cause large effects. For both groups of compounds, the chemical shifts are also greatly influenced by the number of chlorine substituents. Using the three most important independent variables, the average differences between the observed and calculated chemical shifts are ca 0.2 ppm for anisoles and 0.1 ppm for guaiacols. For chloroguaiacols, the corresponding difference was only 0.1 ppm when calculations were performed using single substituent effects.