Application of the temperature dependence of vicinal spin-spin coupling constants to the investigation of the configuration of substituents in saturated 5-membered rings

Temperature dependence of cis- and trans-vicinal spin-spin coupling constants of substituted cyclic 5-membered compounds is discussed. The temperature dependence of trans-vicinal coupling constants is shown to be essentially stronger than that of cis-vicinal coupling constants if the energies of the conformers corresponding to potential energy curve minima are different. The temperature dependence of 33 vicinal spin-spin coupling constants has been studied for di- and trisubstituted thiophanes with a known configuration of substituents. Experimentally determined changes of trans-vicinal coupling constants with temperature are markedly larger than those for cis-vicinal coupling constants, whose values, in most cases, are practically temperature independent. Differences in the temperature dependence of cis and trans-vicinal spin-spin coupling constants can be applied for the determination of the configuration of substituents in saturated 5-membered rings.     

Calculation of gauche coupling constants from substituent electronegativities

The calculation of gauche coupling constants from substituent electronegativities by a method which takes into account the orientation of the substituents is described. This correlation has been derived from the coupling constants of disubstituted ethanes and has been applied to the calculation of coupling constants in cyclohexanes and 6-membered heterocycles. The detection of angular distortion in 6-membered rings from the difference between calculated and observed coupling constants is also described.     

On the configuration of five-membered rings: a spin-spin coupling constant approach

Five-membered rings are clearly among the most common structural motifs found in chemistry and biology. Nevertheless, the configuration of conformationally mobile five-membered rings is often difficult to assign from nuclear magnetic resonance (NMR) data. A simple, reliable, and efficient approach for the stereochemical analysis of five-membered rings based on the measurement of NMR coupling constants is presented. Density functional theory calculations using representative conformations of the full conformational space available to rings with different substitution patterns were used to identify differences between the accessible coupling constant values for cis and trans relative orientations of the substituents. The calculations were assessed experimentally using NMR data obtained from a number of models. This approach can be easily used to analyze different five-membered rings, such as oxolanes, cyclopentanes, furanosides and pyrrolidines, and their relative configuration can be determined without the need for making further conformational considerations.     

Effect of the substituents on the conformational behavior of five-membered rings: Application to the cis- and trans-2,5-dimethoxytetrahydrofuran

A model is proposed which assumes that the pseudorotational potential in five-membered rings is given by the combination of contributions from the unsubstituted ring, from the individual substituents and from interactions between pairs of substituents. The application of this model to the potentials calculated by the MM2 force field for the cis and trans-2,5-dimethoxytetrahydrofuran shows that the contributions from the individual substituents explain the main features of the potentials of these disubstituted rings. The pseudorotational analysis from vicinal proton spin-spin coupling constants ³J_HH confirms the realibility of the MM2 potentials.     

Additivity of substituent effects on the proton-proton coupling constants of disubstituted pyridines

From the spin-spin coupling constants of pyridine and monosubstituted pyridines the effects of several substituents have been calculated- Assuming an additivity relationship when two of these substituents are present in the same molecule, the spin-spin coupling constants for 11 disubstituted pyridines have been calculated.The NMR spectra of 13 disubstituted pyridines have been studied to obtain accurate values of their coupling constants. The experimental values of these constants are in very good agreement with those calculated using the additivity relationships.     

DFT calculations of 31P spin-spin coupling constants and chemical shift in dioxaphosphorinanes

One-bond heteronuclear spin-spin coupling constants (1)J(PX) (X=H, O, S, Se, C and N) between the phosphorus atom and axial and equatorial substituents in dioxaphosphorinanes are computed using density functional theory (DFT). The experimental values of these coupling constants for a variety of substituents can be applied to identify different diastereoisomers. The DFT calculations confirm the systematic trend observed in experiment, and indicate that the computed (1)J(PX) coupling constants are related to the length of the axial and equatorial bonds. A similar relation between the phosphorus chemical shift and the R(PX) bond length appears to be valid, with the exception of selenium substituents.     

One-bond CC spin—spin coupling constants in derivatives of benzene. Non-linearity of 1J(CC) vs substituent electronegativity

A set of one-bond CC coupling constants has been determined for mono- and disubstituted benzenes. Large ¹J(CC) values have been found within the benzene rings bearing highly electronegative substituents such as halogens, methoxy and nitro groups and small values for those with electropositive substituents. The total range of ¹J(CC) couplings observed in our work is larger than 50 Hz. These large variations of CC spin—spin couplings are interpreted in terms of Fermi-contact contributions and the redistribution of s-electrons within a CC bond under influence of substituents. Contrary to some previous findings the data obtained in the present work indicate that the relationship between ¹J(CC) and the substituent electronegativity is not linear.     

Electrochemical and electrochromic properties of rare-earth metal diphthalocyanine complexes

The electrochemical and spectroelectrochemical properties of several new rare-earth metal diphthalocyanine complexes with different substituents containing both electron-withdrawing and -donating substituents in the phthalocyanine rings were studied. The influence of structural modification of the phthalocyanine complexes (viz., the nature of the central metal atom and substituents in the phthalocyanine rings and the number of phthalocyanine rings in the complexes) on the total number of redox transitions, their potentials, and spectral characteristics of anionic and cationic forms of the complexes was examined. The potentials of the first anodic and cathodic redox transitions of the diphthalocyanine complexes are in a good linear correlation with the ionic radii of lanthanides. The potentials of the observed redox transitions of the complexes under study correlate well with the sums of Hammett constants for the substituents and the minimum molecular electrostatic potential of the benzene rings in the phthalocyanine moiety, which serves as a measure of electron perturbations introduced by the substituents. The revealed regularities allow the prediction of the redox properties and structure of rare-earth element diphthalocyanine complexes, which are redox-active in a specified potential range.     

13C NMR spectra and three-dimensional structures of isomeric 3,4-substituted thiophans

The chemical shifts and the direct carbon-proton spin-spin coupling constants for a number of cis and trans isomers of 4-amino-substituted 3-hydroxy(acyloxy)-thiophans, the configurations of the substituents in which and the conformational states were previously established by an independent method, were studied. It was found that in the spectra of the cis isomers the signals of the vicinal ¹³C atoms, which bear the substituents, are shifted to strong field as compared with the trans isomers ( trans-cis 1.7–4.3 ppm). Conformational effects of the substituents on the chemical shifts of the ring ¹³C atoms were noted. It is shown that a relationship exists between the direct carbon-proton spin-spin coupling constants and the spatial orientations of the acyloxy and acylamino substituents for five-membered saturated rings.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1612–1616, December, 1981.     

NMR studies of substituted pyridines

Proton magnetic resonance chemical shifts and coupling constants for a series of 2-substituted-5-nitropyridines and 2-substituted-3-nitropyridines, both in the free base and protonated forms, are reported. The substituents were chloro, bromo, iodo, and hydroxyl. The effect of the substituents on the chemical shifts are substantial but not unusual. The coupling constants are less sensitive to the nature of the substituent and are perturbed only slightly when the ring nitrogen is protonated. This latter observation is consistent with the results of LCAO-MO calculations.     

Charge density and NMR parameters. Aliphatic derivatives

The charge distribution in several substituted aliphatic derivatives was determined by a parametric MOLCAO method. The charges were successfully employed to interpret NMR parameters, namely proton and carbon chemical shift and vicinal proton-proton coupling constants in ethyl derivatives and carbon-proton coupling constants. The linear correlations found between NMR parameters and charge densities are restricted to substituents of the same row of the periodic system. The decay along the aliphatic chain of the perturbation induced by substituents on proton chemical shift is also reproduced by charge distribution. Some results also seem to indicate that the substituents affect the hybridization of the attached carbon atom.     

Fluorine-containing carbocations. III. Alkylation of polyfluorobenzenes with polyfluorinated α,α-difluorobenzyl cations and generation of polyfluorinated α-fluorodiphenylmethyl cations

Alkylation of incompletely fluorinated benzenes with polyfluorinated α,α-difluorobenzyl cations in an SbF₅ medium results in the formation of polyfluorinated α-fluorodiphenylmethyl cations whose precursors are the primary products of alkylation, i.e. the corresponding polyfluorinated α,α-difluorodiphenylmethanes. The influence of substituents on the ¹⁹F chemical shifts and spin-spin coupling constants J(FF) has been analysed in terms of their effects on contributions of the resonance structures controlling the positive charge distrubution on the coplanarity of the phenyl rings and the plane of the α-carbon bonds.     

13C NMR study of some polychloro-isobutane and -isobutene compounds

The ¹³C chemical shifts and the carbon–proton coupling constants have been determined for some chlorinated isobutane and isobutene compounds. The one-bond coupling constants in isobutane derivatives showed a regular increase with an increasing number of γ-chlorine substituents. The three-bond coupling constant of the methyl carbon decreased from 4.2 to 2.0 Hz as the number of chlorine substituents in the γ-position increased. In the isobutene compounds, the vicinal coupling of C-1 was larger to protons in a group that is trans with respect to a chlorine substituent on C-1 than to those in the corresponding group cis to the chlorine. The vicinal coupling constants between atoms in geminal groups (on C-2) seem to be affected by the orientation of the chlorine substituent on C-1.     

13C,13C coupling constants from a mixture of isotopomers (13C) of substituted aromatic compounds; 1-nitronaphthalenes

¹³C Homonuclear decoupling experiments led to the assignment of the carbon-carbon coupling constants of ¹³C-enriched 1-nitronaphthalene obtained by nitration of [1-¹³C]naphthalene. The effects of substituents on coupling constants can be explained on the basis of the electronegativity of the first atom of the substituent, and the observed substituent effects are shown to be parallel to previous data for oxygen containing substituents.     

Conformational analysis of aromatic derivatives of 2,3-dihydro-1H-1,5-benzodiazepine

The PMR spectra and dipole moments of some 2,4-disubstituted 2,3-dihydro-1H-1,5-benzodiazepines have been examined. It is shown that seven-membered rings with an aromatic or heterocyclic substituent in the 2-position have the boat configuration, the substitutent preferentially being equatorially oriented. It has been found that the changes in the chemical shifts and spin coupling constants for the protons of the CH-CH₂ fragment in the dihydrobenzodiazepine ring are mainly due to differences in the electronegativity of the substituents in the 2- and 4-positions. Some dipole moment vectors have been calculated for the dihydrobenzodiazepine ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1662–1666, December, 1984.     

Exploring the Palladium? and Platinum?Bis(pyridine) Complex Motif by NMR Spectroscopy,X‐ray Crystallography, (Tandem) Mass Spectrometry,and Isothermal Titration Calorimetry: Do Substituent Effects Follow Chemical Intuition?

A series of ten palladium? bis(pyridine) complexes, as well as their corresponding platinum complexes, have been synthesized. The pyridine ligands in each series carried different σ‐donor and/or π‐acceptor/donor substituents at the para‐position of their pyridine rings. These complexes were analysed by NMR spectroscopy, X‐ray crystallography, (tandem) MS, and isothermal titration calorimetry (ITC) to validate whether these methods allowed us to obtain a concise and systematic picture of the relative and absolute thermodynamic stabilities of the complexes, as determined by the electronic effects of the substituents. Interestingly, the NMR spectroscopic data hardly correlated with the expected substituent effects but the heteronuclear platinum? phosphorus coupling constants did. Crystallographic data were found to be blurred by packing effects. Instead, tandem MS and ITC data were in line with each other and followed the expected trends.     

13C n.m.r. spectra of 2-substituted pyrimidines

¹³C n.m.r. chemical shifts and carbon-proton coupling constants of 2-substituted pyrimidines are reported. The carbon chemical shifts are correlated with π-electron densities. Substituents which cause deshielding at the directly bound carbon (e.g. NH₂, OCH₃ and F) exert a more powerful effect in the benzene series than in the pyridine or pyrimidine series. The carbon-proton coupling constants do not correlate with the electronegativity of the substituents. Carbon-proton coupling constants and proton-proton coupling constants over the same number of bonds do not obey the Karabatsos relationship. The changes in the carbon-proton coupling constants in 2(1H)-pyrimidinone and 2(1H)-pyrimidinethione which accompany anion and cation formation are reported.     

Configurational and conformational analysis Axial-axial and axial-equatorial coupling constants in six-membered ring compounds

Axial-axial and axial-equatorial coupling constants (J_aa and J_ae) have been obtained directly from the NMR spectra of simple AB systems in a number of cis- and trans-l-(substituted)-2-arylcyclohexanes-3,3,6,6-d₄ measured in several solvents. Variations in J_aa and J_ae have been observed with various substituents. No simple correlation is found between electronegativity of substituents and substituent effects on coupling constants. A review of J_aa and J_ae in six-membered ring compounds is given.     

Fluorine NMR spectra of pentafluorophenyl derivatives

The fluorine chemical shifts and spin-spin coupling constants of 65 pentafluorophenyl derivatives with widely varying organic substituents were examined. Useful correlations of the three meta coupling constants with the chemical shifts of the para fluorine were found. It is suggested that these relationships be extended to all compounds of the type considered in order to determine the signs and approximate values of meta coupling constants. Equations for correlation of the fluorine chemical shifts with the Taft constants are presented. The possibility of calculating the Taft constants from the ¹⁹F NMR spectra of pentafluorophenyl compounds is being discussed.     

1H-NMR Spectra of Cyclohexa-1,4-dienes and Cyclohexenes Annellated to Bicyclo[2.2.1]hept-2-enes. The Inter-Ring Homoallylic H,H Coupling Constants as Stereochemical Probes

The 360-MHz-¹H-NMR spectra of cyclohexa-1,4-dienes and cyclohexenes annellated to bicyclo[2.2.1]hept-2-enes and 7-oxabicyclo[2.2.1]hept-2-enes show inter-ring homoallylic coupling constants between the bridgehead protons of the bicyclo[2.2.1]heptenes and the exo-protons of the allylic methylene groups (0.8 ± 0.15 Hz for bicyclo[2.2.1]hept-2-enes; 0.8–1.4 Hz for 7-oxabicyclo[2.2.1]hept-2-enes). Contrastingly, the corresponding coupling between the bridgehead protons and the endo-protons is absent. The observed values are compared with those calculated by the INDO and CNDO/2 methods and discussed in the light of the bicyclo[2.2.1]hept-2-ene bond π-anisotropy. Vicinal as well as intra-ring homoallylic coupling constants are consistent with a small puckering of the cyclohexa-1,4-diene rings toward the endo-face. The allylic exo-methylene protons are more deshielded than the endo-protons independent of the nature of the substituents, the nature of the bridges, and the degree of unsaturation of the annellated systems. These results constitute a probe for the configuration of cyclohexa-1,4-dienes and cyclohexenes annellated to these bicyclic skeletons.