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.     

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.     

Interactions intramoleculaires. XXI—Constantes de couplage et interactions gauches avec un groupement t-butyle (molécules cyclohexaniques deutériées)

Different isotopic modifications of deuterated products of 1-t-butlyl-4-methoxycarbonyl cyclohexene (d₄-3,3,6,6), cis- and trans-3-methyl-4-cyanocyclohexene (d₃-3,6,6), cis- and trans-3-methyl-4-cyanocyclohexene (d₃,6,6), cis- and trans-3-t-butyl-4-methoxycarbonylcyclohexene (d₃,-3,6,6) are shown by nuclear magnetic resonance spectral analysis. By comparison of ³J and ⁴J coupling constants of model molecules and molecules with large gauche interactions, we obtain proof that the latter are in a chair conformation with moderate cycle deformations.     

Prediction of anti and gauche vicinal proton-proton coupling constants in carbohydrates: A simple additivity rule for pyranose rings

The effect of the relative orientation and electronegativity of substituents on the magnitude of ³J(aa), ³J(ae) and ³J(ee) is well predicated by a simple set of additivity constants, valid for pyranose rings in carbohydrates. The proposed set of parameters is used to calculate 327 coupling constants [³J(HH)] in a variety of pyranosides and related compounds. A comparison with experimental values taken from the literature shows that couplings in molecules which are conformationally pure and underformed can be predicted with a surprising accuracy. An overall root-mean-square agreement of 0.29 Hz is attained for a selected group of 305 coupling values. A statistical breakdown of ΔJ(aa) and ΔJ(ae) [ΔJ=J(exp)-J(calc)] along each carbon-carbon bond in the pyranose systems reveals an unexpected degree of geometrical homogeneity.     

INDO calculations of the solvent dependence of some spin–spin couplings of fluorocarbons

The results of some SCPT and SOS calculations of ¹J(CH), ¹J(FC), ²J(FH), ³J(FH), ³J(FF) and ¹J(CC) in a variety of solvents are reported. The calculations employ the solvaton model and INDO parameters. In almost all cases the coupling is predicted to increase as the dielectric constant of the solvent increases, the exception being ³J(FH)cis in trifluoroethylene which is supported by experimental results.     

Phosphates d'énols: XI—Etude des constantes de couplage 3J(P?O?C?H) et 4J(P?O?CC?H)

NMR studies on E and Z enolic phosphates with no substituent on carbon 1 provided evidence for the existence of planar and gauche conformations. When no bulky substituents are present in the cisoid position of the phosphates two planar and two gauche forms are possible. The most stable conformation is the planar form having the phosphate group trans to the double bond. With a bulky substituent in the cisoïd position of the phosphates the planar form with the group cis to the double bond could not be observed. A close agreement is obtained between the values of dihedral angles determined from observed coupling constants and those calculated with the CNDO/2 method. Changes observed in cisoïd and transoïd ⁴J(POCCH) coupling constants with temperature variation and the reversal of magnitude of these constants when a bulky substituent is on carbon 1⁵⁸ are explained.     

A detailed evaluation of the dependence of 3J(H?H) on bond angle in alkenes and cycloalkenes

Newly determined and accurate data for the magnitudes of cis vinyl proton-proton spin-spin coupling constants in cis-dialkylethylenes and cycloalkenes have been obtained. With these new data and also values taken from the recent literature, it has proved possible to make a critical determination of the correlation between ³J(H? H) and C?C? H bond angles in ethylenic systems. It is suggested that it is possible to obtain accurate estimates of bond angles using NMR coupling constants, even though much more data will be required to fully substantiate this proposal. Whereas cis-³J(H? H) decreases rapidly with increasing C?C? H bond angles, evidence is presented that the opposite is the case for trans-³J(H? H). A brief theoretical discussion of these trends in coupling constants is given.     

Group IV organometal derivatives of pyridine. I—A 1H and 13C NMR investigation of trialkylmetal derivatives of pyridine

The proton and carbon-13 NMR spectra of thirteen trialkylmetal derivatives of pyridine, several of which were previously unknown, have been recorded and analysed. The proton NMR spectra show variations in proton chemical shifts but not in proton-proton coupling constants when the metal substituent is changed; the ring proton-metal coupling constants ⁿJ(M? H) in the tin and lead derivatives correspond closely with the corresponding proton-proton couplings ⁿJ(H? H) in pyridine. The carbon-13 chemical shifts of the carbons bound to the metal can apparently be correlated with the electron-donating ability of the trialkylmetal group. In the trimethylstannylpyridines the value of ¹J(Sn? C_ring) varies greatly with the position of the Me₃Sn group.     

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.     

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.     

Interpreting 2hJ(F,N), 1hJ(H,N) and 1J(F,H) in the hydrogen‐bonded FH–collidine complex

Ab initio EOM‐CCSD calculations were performed to determine ¹⁹F,¹H, ¹⁹F,¹⁵N and ¹H,¹⁵N spin–spin coupling constants in model complexes FH–NH₃ and FH–pyridine as a function of the F—H and F—N distances. The absolute value of ¹J(F,H) decreases and that of ^1hJ(H,N) increases rapidly along the proton‐transfer coordinate, even in the region of the proton‐shared F—H—N hydrogen bond. In contrast, ^2hJ(F,N) remains essentially constant in this region. These results are consistent with the recently reported experimental NMR spectra of FH–collidine which show that ^1hJ(H,N) increases and ¹J(F,H) decreases, while ^2hJ(F,N) remains constant as the temperature of the solution decreases. They suggest that the FH–collidine complex is stabilized by a proton‐shared hydrogen bond over the range of experimental temperatures investigated, being on the traditional side of quasi‐symmetric at high temperatures, and on the ion‐pair side at low temperatures. Copyright © 2002 John Wiley & Sons, Ltd.     

Stereochemical studies: XVII—the proton coupling constants and conformational equilibria of trans-1,2-disubstituted cyclohexenes

The vicinal ³J_aa and ³J_ee spin-spin coupling constants of a number of deuterated trans-1,2-disubstituted cyclohexenes and the ΔH and ΔS values of the conformational equilibria of these compounds have been determined by computer optimisation of the ³J(HH) = f(T) function. Compounds with —CF₃ and CCl₃ substituents were shown to have an enhanced proportion of the diaxial conformer.     

Proton magnetic resonance study of rotational isomerism in vinylcycloalkanes

The vicinal coupling constant, J(12), between the vinyl CH and the ring CH protons in vinylcyclohexane was calculated from a ‘partial molecule’ six-spin system. The 100 and 270 MHz results are in good agreement; those at 60 MHz were, however, still inaccurate in this approximation. J(12) increases with increasing solvent polarity and decreasing temperature. The energy difference between the s-trans and gauche conformers in both C₂Cl₄ and perdeuterioacetone solvents is 879 ± 167 J mol^?1 (210±40 cal mole^?1). The s-trans conformer is the most stable, in contrast to the isoelectronic cyclohexylcarboxyaldehyde where the gauche rotamers are lower in energy.     

Sélénophènes substitués. Additivité de l'influence des substituants sur les couplages J(H?H) et J(Se?H)

The spin-spin coupling constants J(H? H) and J(Se? H) of 2- and 3-substituted selenophenes, whose signs have been obtained by double resonance experiments, have been correlated with the reactivity constants F and R of Swain and Lupton by means of linear equations J = i + fF + rR. The relative inductive and mesomeric contributions to the coupling constants are discussed. Substituent effects on J(H? H) and J(Se? H) are found to be additive in 2,4-disubstituted selenophenes. In agreement with experimental results, this additivity relationship indicates that ³J(Se? H) becomes negative in 2,4-dinitroselenophene. Evidence is given from long range coupling constant data that 2-formylselenophene exists almost exclusively in the Se? O cis conformation and 3-formylselenophene in the Se? O trans conformation.     

The relationship between proton–proton NMR coupling constants and substituent electronegativities. II—conformational analysis of the sugar ring in nucleosides and nucleotides in solution using a generalized Karplus equation

The relationship between vicinal NMR proton–proton coupling constants and the pseudorotational properties of the sugar ring in nucleosides and nucleotides is reinvestigated. Compared with our earlier study several important improvements are introduced: first, a new empirical generalization of the classical Karplus equation is utilized, which allows an accurate correction for the effects of electronegativity and orientation of substituents on ³J(HH); second, empirical correlations between the parameters governing the conformation of β-D -furanosides (taken from an analysis of 178 crystal structures) were used to define proton–proton torsion angles as a function of the pseudorotation parameters P and Φ_m; and, third an iterative least-squares computer program was devised to obtain the best fit of the conformational parameters to the experimental coupling constants. NMR data for the sugar ring in the following compounds were taken from the literature and analysed: 3′,5′-cyclic nucleotides, a base-stacked ribonucleotide, 2′-anhydroarabinonucleosides, α-D -2′,2-O-cyclouridine, 2′- and 3′-aminosubstituted ribonucleosides, 2′- and 3′-deoxyribonucleosides. The present results confirm that the conformational properties found in the solid state are, on the whole, preserved in solution.     

Conformational Analysis of Fluorinated Pyrrolidines Using 19F–1H Scalar Couplings and Heteronuclear NOEs

A series of 3‐fluoropyrrolidines have been studied to investigate the influence of the stereoelectronic fluorine gauche effect on ring conformations in the solid state by single‐crystal X‐ray analysis and in solution phase by NMR spectroscopy. As part of these studies 1D ¹⁹F–¹H heteronuclear NOE (HOESY) experiments have been optimised for applications to small molecules and are described in detail. These have been employed to estimate ¹⁹F–¹H internuclear distances and were combined with vicinal ³J(F,H) and ³J(H,H) scalar coupling constants to analyse the ring conformations. Where possible the derived solution‐phase structural data have been compared with those of the crystalline state. The results demonstrate the influence of the gauche effect in stabilising C^γ‐exo conformations of the fluorinated pyrrolidines. It was further shown that when steric interactions were also present, this conformational bias was diminished and the contribution of the alternative C^γ‐endo conformation was seen to increase in solution at lower sample temperatures.     

Simultaneous determination of the magnitude and the sign of multiple heteronuclear coupling constants in 19 F or 31P‐containing compounds

The presence of a highly abundant passive nucleus (Z = ¹⁹ F or ³¹P) allows the simultaneous determination of the magnitude and the sign of up to three different heteronuclear coupling constants from each individual cross‐peak observed in a 2D ¹H‐X selHSQMBC spectrum. Whereas J(HZ) and J(XZ) coupling constants are measured from E.COSY multiplet patterns, J(XH) is independently extracted from the complementary IPAP pattern generated along the detected F2 dimension. The incorporation of an extended TOCSY transfer allows the extraction of a complete set of all these heteronuclear coupling constants and their signs for an entire ¹H subspin system. ¹H‐X/¹H‐Y time‐shared versions are also proposed for the simultaneous measurement of five different couplings (J(XH), J(YH), J(XZ), J(YZ), and J(ZH)) for multiple signals in a single NMR experiment. Copyright © 2015 John Wiley & Sons, Ltd.     

Application of 13C and 15N NMR spectroscopy to structural studies on nitramines

¹⁵N and ¹³C chemical shifts and the ¹J(¹⁵N¹⁵N), ¹J(¹⁵N¹³C) and ¹J(¹³CH) coupling constants have been determined for a number of ¹⁵N-enriched cyclic and acyclic secondary nitramines. The results are interpreted in terms of both electronegativity effects and conformational factors.     

High resolution 13C NMR spectroscopy. IV—stereochemical assignments in butenedioic acids and 3-pentene-2-ones

Vicinal ¹³C, H coupling constants ³J(CO, H) for butenedioic acids and ³J(CH₃, H) for 3-pentene-2-ones have been determined and are correlated with the configuration of the corresponding C?C double bond. For both types the relationship ³J(CH) trans > ³J(CH)cis holds; in the case of the CH₃, H couplings, however, the ³J(CH₃, H) trans values are reduced because of steric reasons, so that configurational assignments seem possible only when both isomers are present. Additionally, the coupling constants ³J(COC H₃,H ) and the chemical shifts δ have been evaluated for the pentenones and it is shown that these parameters give information about the predominating conformation of α, β-unsaturated methyl ketones.