Symmetry components analysis of nuclear spin–spin coupling constants

Molecular symmetry properties are used to define “normal” spin–spin coupling constants corresponding to some irreducible representations of the symmetry point group of the molecule. The relationship between these normal coupling constants and the measured ones is established in closed form for the most common cases. The Ramsey perturbation formula is analysed into symmetry components by means of the Winger–Eckart theorem. Both contributions predicted by the molecular-orbital method, i. e. direct coupling via σ electrons and indirect coupling via σ–π interaction are studied. Numerical calculations for the coupling constants of ethane, ethylene and acetylene were carried out without the mean excitation energy approximation by using SCF ? MO wave functions; overlap between atomic orbitals is systematically taken into account by calculating coupling constants. Theoretical and experimental results are compared in terms of symmetry components.     

Substituent effects on nuclear spin–spin carbon–carbon coupling constants in derivatives of acetylene

¹J(¹³C?¹³C) nuclear spin–spin coupling constants in derivatives of acetylene have been measured from natural abundance ¹³C NMR spectra and in one case (triethylsilyllithiumacetylene) from the ¹³C NMR spectrum of a ¹³C-enriched sample. It has been found that the magnitude of J(C?C) depends on the electronegativity of the substituents at the triple bond. The equation ¹J(¹³C?¹³C) = 43.38 E_x + 17.33 has been derived for one particular series of the compounds Alk₃SiC?CX, where X denotes Li, R₃Sn, R₃Si, R₃C, I, Br or Cl. The ¹J(C?C) values found in this work cover a range from 56.8 Hz (in Et₃SiC?Li) to 216.0 Hz (in PhC?CCI). However, the ¹J(C?C) vs E_x equation combined with the Egli–von Philipsborn relationship allows the calculation of the coupling constants in Li₂C₂ (32 Hz) and in F₂C₂ (356 Hz). These are probably the lowest and the highest values, respectively, which can be attained for ¹J(CC) across a triple bond. The unusually large changes of the ¹J(C?C) values are explained in terms of substituent effects followed by a re-hybridization of the carbons involved in the triple bond. INDO FPT calculations performed for two series of acetylene derivatives, with substituents varied along the first row of the Periodic Table, corroborate the conclusions drawn from the experimental data.     

Some INDO calculations of fluorine-nitrogen spin–spin coupling constants

INDO parameterized calculations of ⁿJ(¹⁹F¹⁵N) are reported where n=1, 2, 3, 4 and 5. The calculations are performed within the sum-over-states perturbation and self-consistent perturbation frameworks. In general, satisfactory agreement between both sets of calculated results and the available experimental data is obtained. All of the ¹ J(¹⁹F¹⁵N) and ⁴J(¹⁹F¹⁵N) couplings can be of either sign. Most of the couplings considered are dominated by the contact contribution but the non-contact interactions can be very important in certain cases.     

Signs of heteronuclear spin–spin coupling constants in 15N-acetamide

By using digital deconvolution to improve spectral resolution, earlier NMR studies on ¹⁵N-enriched acetamide have been revised and extended to determine the signs of the heteronuclear spin-spin coupling constants. ¹J(¹³CO¹⁵N), ²J(¹³CH₃¹⁵N) and ³J(C¹H₃¹⁵N) are negative while ³J(¹H¹³CH₃)>0. The results, interpreted on the basis of the ‘selective decoupling’ formalism, were confirmed by computer simulation of the double resonance spectra. It is shown that ²J(¹H-α¹³CO) is significantly larger than ²J(¹H^{N 13}CO). Thus, jointly with {¹H-β}-¹³C′ double resonance experiments, {¹H-α}-¹³C′ experiments ought to be most helpful when assigning peptide group carbonyl resonances. The study provides valuable information for the interpretation of heteronuclear coupling constants in polypeptides.     

The additivity of NMR carbon–carbon spin–spin coupling constants

Fluorene-9-¹³C, fluorenone-9-¹³C, acenaphthenone-11-¹³C, acenaphthenone-12-¹³C, 1-methylcyclopentanol-1-¹³C and 1-methylcyclopentene-1-¹³C were synthesized to obtain J(CC) values between the natural carbons and the labeled carbons. Each of these compounds possessed at least one asymmetric dual-path coupling, i.e., coupling between the labeled carbon and another carbon via simultaneous two- and three-bonded coupling paths. Model ¹³C-labeled compounds were synthesized where necessary to give expected values of the constituent mono-path couplings. Values of these dual-path couplings (²⁺³)J suggested that the observed value is the (at least approximate) algebraic sum of the two constituent J values.     

One-bond carbon–carbon spin–spin coupling constants: A data summary

A summary of all the one-bond carbon–carbon spin–spin coupling constants, Known up to the beginning of 1980, is given in diagrammatic form.     

A note on the calculations of spin–spin coupling constants involving carbon

Sum-over-states perturbation and self-consistent perturbation calculations of ⁿJ(CC) using standard INDO parameters are discussed. Calculated values of ¹J(OC) for acetone are reported. In general it seems that the sum-over-states calculations are the more reliable. The importance of including non-contact contributions in the calculation of couplings between carbon and nuclei with larger values of 〈r^?3〉 is stressed.     

C and H nuclear magnetic shielding and spin–spin coupling constants of C-enriched bromomethane in the gas phase

¹³C and ¹H NMR spectral parameters are investigated for ¹³CH₃Br in gaseous matrices. It is found that both the ¹³C and ¹H chemical shifts of ¹³CH₃Br are linearly dependent on solvent density. Similar dependence is also detected for one-bond spin–spin coupling, ¹J(CH). For the first time the ¹³C and ¹H magnetic shielding constants and ¹J(CH) spin–spin coupling are obtained for an isolated ¹³CH₃Br molecule together with the coefficients responsible for solute–solvent molecular interactions in gaseous matrices. The present experimental results confirm the accuracy of some recent ab initio calculations of nuclear magnetic shielding performed for bromomethane.     

Carbon-13, proton spin–spin coupling constants in some 2-substituted propanes

Carbon-13, proton coupling constants have been measured in eighteen different 2-substituted propanes. ¹J(C-2,H) shows variations similar to those observed previously for monosubstituted methanes. ²J(C-2,H) is essentially independent of the substituent at C-2, while ²J(C-1,H) varies over a range of at least 5 Hz. The latter coupling constant becomes more positive as the electronegativity of the substituent increases while ³J(CH) decreases as the electronegativity of the substituent increases. The observed trends in ⁿJ(CH) are compared with those calculated using semi-empirical molecular orbital theory at the INDO level of approximation.     

Long-range nuclear spin–spin coupling between selenium-77 and phosphorus-31 in biphosphorus compounds

The coupling constants ⁿJ(⁷⁷Se³¹P), n = 1–4, have been measured in the proton-decoupled ³¹P NMR spectra of a range of diphosphorus selenides and diselenides. ³¹P–{¹H, ³¹P} and ³¹P–{¹H, ⁷⁷Se} triple resonance experiments have been used to establish the signs of the coupling constants, and it is found that both the magnitudes and signs depend upon the stereochemical relationship of the coupled nuclei.     

The conformational dependence of vicinal 13C13C spin–spin coupling constants in alicyclic compounds

A series of alicyclic compounds with dihedral angles of 0°, 60°, 90°, 120° and 180° between a ¹³C-labelled carbon atom and a carbon atom separated by three bonds from the label has been synthesized. The vicinal ¹³C¹³C spin coupling constants were measured, and from the results a Karplus-type relationship between ¹³C¹³C spin coupling and dihedral angle is proposed.     

A theoretical study of medium effects on the transmission mechanisms of the fermi contact term of spin–spin coupling constants in the acetamide molecule

Solvent effects on different transmission mechanisms of spin–spin coupling constants are analyzed from a theoretical viewpoint. Medium effects are introduced using the solvaton model, and the decomposition of coupling constants in σ- and π-electron transmitted components is accomplished with the PRMO method. Trends thus obtained are in fairly good agreement with experimental findings reported in the literature. In all types of couplings studied in this work, σ and π components show opposite behavior when increasing the polarity of the solvent.     

Theoretical nuclear spin–spin coupling constants using atom–atom polarizabilities. 13C?H and H?H′ coupling constants of some [2.2.1] bicyclic compounds using the INDO approximation

Theoretical nuclear spin–spin coupling constants are calculated using mutual and self atom–atom polarizabilities according to a theory where no semi-empirical parameters are used, except Slater exponents which can be obtained from other sources. As an application, the ¹³C? H and H? H′ couplings of some [2.2.1] bicyclic compounds are calculated with the aid of INDO molecular orbitals and compared with the experimentally obtained coupling constants.     

Substituent Conformational effects in vicinal 13C?13C spin–spin coupling constants

A series of 7-¹³C-labeled o-substituted toluene derivatives and carboxyl-¹³C-isocrotonic and crotonic acid were synthesized and studied by ¹³C NMR spectroscopy to obtain ¹³C? ¹³C spin-spin coupling constants involving the labeled carbon. The cis ³J(CC) values were different from those in previous studies in that these J(CC) values were relatively small and the usual dependence of ³J(CC) on the s-character of the terminal carbon was reversed. Further, a strong dependence of ³J(CC) on the conformational orientation of a terminal carbonyl group was shown to exist. Through-space interactions of the two coupling carbons were shown to contribute to these ‘anomalous’ results, and thus it was shown the cis carbon-carbon couplings may not be directly related to geometrically equivalent proton-proton couplings, as are other carbon-carbon couplings.     

Molecular structure and the indirect spin–spin coupling constants in azulene—a proton NMR study in the nematic phase

The proton NMR spectrum of azulene has been investigated in the nematic phase of a liquid crystal. Spectral analysis provided the direct dipole-dipole coupling constants which were used to derive the structural information. It was found that the relative proton-proton distances in the 7- and 5-membered rings deviate significantly from those for regular heptagons and pentagons. Indirect spin-spin couplings were also obtained. Many of the inter-ring and long-range couplings have magnitudes between 0.2–0.8 Hz.     

Computer-assisted pseudorotation analysis of five-membered rings by means of proton spin–spin coupling constants: Program PSEUROT

A computer method for the calculation of the pseudorotational parameters in five-membered rings from vicinal proton spin–spin coupling constants is described. Some typical problems met in practice are discussed. Applications of the program in the conformational analysis of some substituted cyclopentanes are presented.     

Influence of the INDO parameterization on the indirect spin–spin coupling constants as calculated by the FPT INDO method

Alterations have been introduced in the semi-empirical INDO parameters in order to study their influence on the Fermi contact term of the indirect spin–spin coupling constants as calculated by the finite perturbation theory (FPT). For this purpose a set of molecules containing hydrogen, carbon and/or fluorine has been selected. In general, most coupling constants are found to be much more sensitive than other molecular properties to small changes in the INDO parameters. This sensitivity depends strongly on the particular calculated coupling constant. In most cases the uncertainty in the INDO parameters leads to uncertainties in the coupling constants which are much greater than their experimental errors.     

Correlations between molecular parameters and the spin–spin coupling constants of vicinal protons: Criticism of the karplus equations

Conformationally rigid systems such as xylopyranose 1,2,4-ortho esters ( 1a ) and ( 1b ) and 10-methoxy-6-aza-isoadamantane ( 2 ), for which the identity of conformations both in the crystalline state and in solution can reasonably be assumed, provide good models for the study of experimental correlations between the spin–spin coupling constants (J) of the vicinal protons and the dihedral bond angles (DBA) determined from an X-ray study in the H? C? C? H bond system. The Karplus equations and their modifications (with and without corrections for the electronegativity of adjacent groups) were found to be unable to provide satisfactory correlations between these parameters. Optimum coefficients for the equations connecting the J and DBA values with corrections for electronegativity were calculated by the least squares method. The same procedure was used to obtain an equation connecting the J with DBA values using the sum of the chemical shifts as a measure of the electronic factors affecting the J ∝ DBA dependence. The accuracy of the equation thus obtained lies within ±18% as opposed to ±48% for the original Karplus equations. A similar correlation was obtained for the angles between the intercrossing lines formed by the directions of the vicinal C? H bonds (ILA). When combined with the sum of chemical shifts, ILA provides a better correlation with the coupling constants J than the conventional parameter DBA.     

Carbon-13, 1H spin–spin coupling VI—biphenylene

¹³C, ¹H coupling constants for biphenylene have been obtained from the analysis of the ¹³C NMR spectrum of the natural abundance α-¹³C- and β-¹³C-isotopomers. The various mechanisms responsible for the observed results are discussed.