Etude conformationnelle de la dimethyl-3,4 cyclopentanone trans: II—Spectres de RMN

The NMR spectra of trans-3,4-dimethylcyclopentanone and trans-3,4-dimethyl-2,2,5,5-tetradeuteriocyclopentanone are analysed. The coupling constants, calculated for the deuterated compound, are in good agreement with a diequatorial conformation of the methyl groups.     

Equation de Karplus et l'Hybridation du Carbone. Correlation de la Constante de Couplage Vicinale 3J(Hi,Hj) et la Somme de Constantes de Couplages 1J(C,H)

The vicinal coupling constant ¹H? ¹H exhibits a marked hybridization dependence. The coupling constants ¹³C? ¹H could be used to correlate the Karplus type equation for calculation of ³J(Hi, Hj). The correction term to this equation is proposed.     

1H and 13C NMR studies of 7-azaindole and related compounds

The ¹H and ¹³C chemical shifts, proton-proton coupling constants, and one-bond carbon-hydrogen coupling constants have been obtained for 7-azaindole, 1-methyl-7-azaindole, their corresponding methyl iodide salts, and the related compound 7-methyl-7H-pyrrolo [2,3-b]pyridine. are different from those of either 7-azaindole or 1-methyl-7-azaindole.     

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.     

Constantes de Couplage en Serie Pyridinique. Application de la Theorie des Orbitales Moleculaires

Coupling constants of 35 substituted pyridines are discussed and related to the electronic properties of the substituents and to the mechanism of coupling. A calculation of J is performed using Pople and Santry's theory and mutual polarizabilities obtained by CNDO/2 or Hoffmann methods. The influence of bicentric integrals on the theoretical values of J is examined.     

The structural dependence of two-bond 13C?13C coupling constants

Two-bond ¹³C? ¹³C coupling constants are discussed on the basis of INDO-SCPT calculations. The dependence of ²J(CC) on bond angle variation and on methyl substitution is evaluated, and it is shown that ²J(CC) depends linearly on the bond orbital s-character product of the terminal carbon atoms, whereas no systematic relationship with the hybridization of the central carbon atom was obtained. Alkyl group substituent effects are found to be additive. The coupling constants of a number of cyclobutane derivatives are discussed on the basis of these structural relationships; it is shown that the experimental findings can be interpreted quite consistently by assuming a dual-pathway coupling mechanism.     

13C NMR spectra and carbon-proton coupling constants of various methylangelicins

The ¹³C-nmr spectra of various methyl derivatives of angelicin are reported. The assignment of chemical shifts for all the C atoms has been achieved by using carbon-proton coupling constants, nuclear Overhauser effect consideration and shift effects caused by the introduction of methyl groups on various positions of the angelicin nucleus. Substituent effects on ¹³C chemical shifts and carbon-proton coupling constants are discussed.     

Acetyl Methyl Torsion in N‐Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry

The gas‐phase structures and parameters describing acetyl methyl torsion of N‐ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier‐transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C₁ symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm^?1 of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The ¹⁴N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values.     

Carbon-13 NMR studies of monohydroxylated and monochlorinated derivatives of Z- and E-p-menthanes

J(¹³C¹H) coupling constants for some methyl- and aminopyrimidines have been determined by ¹³C NMR. Both the one-bond and long-bond and long-range coupling constants follow general trends which can be summarized in a few simple rules. In particular, the ³J(C-i,H) coupling constants between a ring carbon C-i and the ring protons are larger than the ²J(C-i,H) coupling constants. The opposite is observed for the couplings between the ring carbons and the methyl protons: ³J(C,Me). These general rules are very useful for the assignment of resonances in complex ¹³C spectra of pyrimidines and seem to be valid for other 6-membered aromatic nitrogen heterocycles. Furthermore, the additivity of substituent effects on ¹J (CH) for monosubstituted pyrimidines allows the estimation of ¹J (CH) for polysubstituted pyrimidines with a very good accuracy.     

Etudes RMN en série hétérocyclique. XV—Résonance Magnétique Nucléaire de 1H et de 19F de quelques fluoro-2 benzothiazoles

Chemical shifts and coupling constants of five 2-fluorobenzothiazoles 6-substituted by Me, H, F, Cl and NO₂ are given. A large cross-ring coupling constant (J₂₆ = 10·4 Hz) is found in the 2,6-difluorobenzothiazole.     

I. Etude de l'Influence de la Nature et de l'Orientation d'un Substituant sur les Constantes de Couplage en Série Tertiobutyl-4 Cyclohexanonique par RMN à Haut Champ

The 250 MHz proton spectra of six α-halogeno-4-tert-butylcyclohexanones have been recorded and the dependence of the coupling constants upon the nature, the position, the orientation and the number of substituents (C?O, Cl, t-Bu) has been examined. The results obtained are interpreted in terms of geometrical deformation or substituent effects.     

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-Spektroskopische Untersuchungen an methylsubstituierten Cyclohexen-(4)-dicarbonsäure-(1,2)-anhydriden

Chemical shifts and coupling constants of methyl substituted 4-cyclohexene-1,2-dicarboxylic anhydrides were determined. The NMR data demonstrate the boat conformation of the cis-anhydrides. The orientation of the cis-anhydride ring relative to the cyclohexene boat depends on the constitution and configuration of the compounds, and can be deduced from coupling constants and δ-values of the H₁- and H₂-protons. Methyl groups in the neighbourhood of these protons shift the resonance frequencies dependent upon molecular conformation.     

Geminal C,H spin coupling in substituted alkenes

Two-bond C,H coupling constants for the coupling pathways C_?C? H_ (type I) and (type II) have been studied in a variety of mono-, di- and trisubstituted ethylenes. For ²J(CH) of type I, additive increments have been determined for 20 substituents, which allow a satisfactory calculation of coupling constants, including relative signs, in di? and trisubstituted ethylenes. Several applications for E/Z isomers are presented. In cyclic compounds a positive coupling increment, δJ, is observed in going from 6? to 5?membered ring compounds.     

Stereospecificity of <Superscript>13</Superscript>C shielding constants in acetone azine as a tool for configurational assignment of ketone azines

According to the ¹³C NMR data, the chemical shift of the methyl carbon atom in acetone azine in the trans position with respect to the lone electron pair on the neighboring nitrogen atom is lower by 7 ppm than that of the methyl carbon atom in the cis position. The corresponding direct ¹³C-¹³C coupling constant for the trans-methyl groups is lower by 10 Hz as compared to the cis-methyl groups. The experimental spectral data are reproduced well by nonempirical quantum-chemical calculations. The observed stereospecificity of ¹³C chemical shifts and ¹³C-¹³C coupling constants may be used as an effective tool in configurational analysis of various ketone azines.     

Protonenresonanz-Spektren des CycIohexa-3,5-dien-1-on-Systems (ortho-Dienone)

Proton magnetic resonance spectra of a scries of ortho-dienones have been analyzed using multiple resonance and computer techniques. The sequence of the chemical shifts are in agreement with the calculated π-electron distribution. The coupling constants of the ring protons are of like sign, whereas those of the methyl protons with the ring protons alternate in sign with the number of intervening bonds.     

Equation de Karplus appliquée à des systèmes sp2–sp3

A method of best fitting for the calculation of vicinal coupling constants is presented for sp²-sp³ systems. Results for 68 vicinal coupling constants from 32 different products are presented. We obtain the best Karplus-type equation to date with a dispersion factor of 31,01%.     

Conformational preferences of 2-methoxy-acetophenone

On the basis of the presence or absence of long-range spin–spin coupling constants between side–chain and ring nuclei in 2-methoxyacetophenone, some literature ambiguities about the conformational preferences of the side-chains in this compound can be resolved. The long-range coupling between the methoxy protons and the ring proton ortho to the methoxy group, ⁵J(H, CH₃)o, is (?)0.28 ± 0.02 Hz, as expected for a conformation in which the methoxy group lies in the benzene plane and cis to H-3. The methyl protons of the acetyl group do not couple to H-6, implying that this methyl group does not approach H-6 closely. However, the ¹³C nucleus of this methyl group couples by +0.4 Hz to H-5 and not to H-3. This stereospecific five-bond coupling implies that the acetyl group predominantly prefers an arrangement in which the carbonyl group lies trans to the other substituent, as would be expected electrostatically. Large twists out of the ring plane are not consistent with the observed couplings.     

13C-NMR spectra and carbon-proton coupling constants of variously annulated furocoumarins

The ¹³C-nmr spectra of variously annulated methylfurocoumarins are reported. The assignments of chemical shifts for all the C resonances has been achieved by using carbon-proton coupling constants, relaxation efficiency considerations and shift effects caused by the introduction of methyl groups at various positions of the furocoumarin nucleus. Substituent effects on ¹³C chemical shifts and carbon-proton coupling constants are discussed.     

Harmoniques des vibrations de valence C?H dans les alcanes et les halogénures de t-butyle

The first and second harmonic C? H and C? D valence vibrations of alkanes and t-butyl halides have been measured. A model is proposed for the methyl and methylene groups, which allows the calculation of the anharmonicity constants of every vibrational mode and of the couplings between the modes. The already known large values (?100 to ?150 cm^?1) for the anharmonicity constants that couple the symmetrical with the asymmetrical modes are confirmed. As compared with lower harmonics combination bands become predominant for the higher ones; they allow the application of a diatomic model to estimate higher harmonics and the energy of dissociation.