PMR spectra of dimers of cyclopentadienyl compounds

The PMR spectra of cyclopentadine endo-dimer, its 1y-acetoxy-derivative and the dimer of spiro-heptadiene have been thoroughly studied. The assignment is proved by the use of spindecoupling experiments and the INDOR-technique. The relative signs of several coupling constants were found and used for the assignment. General trends in the proton chemical shifts and coupling constants are discussed for the derivatives of the cyclopentadiene dimer and for the substituted cyclopentadiene dimers.     

NMR study of 4-membered rings. VIII—The sign of the four-bond couplings in adducts of cyclopentenone with vic-dichloroethylenes

The proton spectra of the four stereoisomers prepared by photocycloaddition of 2-cyclopentenone to cis and trans dichloroethylene have been analysed using tickling and INDOR techniques in order to obtain the sign of the cross-ring four-bond coupling constants in the cyclobutane ring. These parameters are correlated with the relative orientation of the protons involved. The complete NMR analysis of the seven-spin system of the cycloadduct endo-7-phenoxybicyclo[3.2.0]hept-2-en-6-one has been carried out and all the four-bond coupling constants have been obtained and discussed. The sign of the cross-ring coupling in the cyclobutanone ring has been determined.     

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.     

Studies on olefins. IX—proton NMR study of some CH2CR1R2 and CHR3CR1R2 alkenes

The proton NMR spectra of 2,3,4,4-tetramethyl-3-t-butylpent-1-ene rotarners have been completely assigned by low temperature NOE measurements. Chemical shifts and cis and trans allylic coupling constants are unambiguously determined. It is shown that other 2-substituted propenes can be assigned on the basis of the coupling constants, but not from the chemical shift data. In 1,2-disubstituited propenes, however, the transoid coupling constant falls in the range of the cisoid coupling constant values of the 2-substituted propenes. Coupling constants cannot, therefore, be used in the 1,2-disubstituted propene series as a criterion for determining structure.     

NMR experiments on acetals—XXXIX: Conformational insights through NMR studies at 300 MHz of 2-methyl-4-halogenomethyl-1,3-dioxolanes

NMR spectra of cis- and trans-2-methyl-4-halogeno-methyl-1,3-dioxolanes have been analysed at 300 MHz. Some of the extracted parameters facilitate easy distinction between these 1,3-dioxolanes and the corresponding structurally isomeric 2-Methyl-5-halogeno-1,3-dioxanes. Criteria enabling configurational assignments to be made for the cis-trans isomers of the dioxolane series are tested. The Me-2 group causes an upfield shift (0·2 to 0·3 ppm) of a trans proton at position 5, but the reversed shift for the corresponding cis proton. This competes with, or even overwhelms the effect of the CH₂X-4 substituent, which by virtue of its pronounced preferential rotameric orientation and in comparison with a simple Me-group, has no large upfield effect on the shift of the syn-adjacent proton. Shift criteria and coupling constants J_{H-4, H-5} in cis- and trans derivatives allow further conformational insights into these 1,3-dioxolanes.     

Isomerization of chemically activated bicyclo[3.1.0]hex-2-ene and related C6H8 isomers

Singlet methylene was reacted with cyclopentadiene to give chemically activated bicyclo[3.1.0]hex-2-ene (BCH). The rate of isomerization of BCH to 1,4-cyclohexadiene, 1,3-cyclohexadiene, cis-1,3,5-hexatriene, and l-methylcyclopentadiene is compared with calculated rate constants using the RRKM theory and measured or estimated thermal Arrhenius parameters. Subsequent isomerizations of the C₆H₈ products are also measured and calculated. These include 1,4-cyclohexadiene to benzene and the reversible reactions between 1,3-cyclohexadiene, cis-1,3,5-hexatriene, and trans-1,3,5-hexatriene. The results provide new data for several of these reactions which have not been observed in thermal studies. Agreement between the observed and calculated rates using the strong collision assumption is satisfactory except for the trans-1,3,5-hexatriene to cis-1,3,5-hexatriene reaction.     

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.     

Spectral analysis of the high-resolution proton-coupled carbon-13 NMR spectra of 1,3,2-dithiarsolanes

The high-resolution proton-coupled ¹³C NMR spectra of 2-chloro- and 2-phenyl-4-methyl-1,3,2-dithiarsolane and 2-chloro- and 2-phenyl-1,3,2-dithiarsolane have been obtained and completely analysed. The spectral analysis has provided all the ¹³C chemical shifts and ¹³C? ¹H coupling constants, including their relative signs. The results of this work are adequately interpreted in terms of two rapidly equilibrating half-chair conformers with the 4-methyl pseudo-axial or pseudo-equatorial. The cis isomers appear to be conformationally biased towards the form with the 4-methyl pseudo-equatorial. The two one-bond ¹³C? ¹H coupling constants of the methylene group are distinctly different. For both the cis and trans isomers, the largest one-bond coupling always involves the methylene proton situated syn to the substituent at arsenic. The observed variation in the geminal and vicinal ¹³C? ¹H coupling constants are discussed in terms of a stereospecific inductive effect of the sulphur hetero-atoms and conformational factors.     

Stereochemical characterization of some mono- and diaryl substituted ethylene oxides by NMR spectroscopy

NMR spectra of several styrene, stilbene and stilbazole oxides have been determined, and chemical shifts and coupling constants have been correlated with cis-and trans-configurations. Assignments have been made for all protons, and double resonance technique and ¹³C? H coupling constants have been used in some particular cases. An explanation is proposed for the observation that chemical shifts of oxirane protons are higher for cis than for trans isomers.     

Spectroscopic studies on olefins—III: NMR of cis- and trans- disubstituted olefins

The proton NMR spectra of four cis/trans pairs of 1,2-disubstituted olefins have been analysed. The observed trends of the olefinic, vicinal and allylic proton-proton coupling constants are discussed in terms of rehybridization at the sp² carbon atoms. It is found that in these substances the trans-allylic couplings are generally more negative than the cis-allylic couplings contrary to Barfield's¹ theoretical prediction but in agreement with some earlier experimental data. Steric hindrance between cisoid substituents is cited as the probable origin of this end related trends.     

A study of 5-membered ring conformations by the temperature dependence of the vicinal spin–spin coupling constants. Conformations of cis- and trans-isomers of 4-benzoylamino-3-acetoxythiophane

Conformational equilibrium in cis- and trans-4-benzoylamino-3-acetoxythiophanes has been analysed on the basis of the temperature dependence of the vicinal spin–spin coupling constants. The conformations of the compounds have been established. It has been shown that in problems of this kind the real accuracy of the experiments corresponds to a wide range of spin-spin coupling constant values and differences between the conformer energies. To narrow the range, additional data have been used, such as the temperature dependence of other vicinal coupling constants in the same molecule, angular dependence of the vicinal constants, etc.     

Conformational equilibria in isomeric methylcyclohexanols studied by 13C NMR spectroscopy

The conformational equilibrium constants for isomeric methylcyclohexanols (cis- and trans-1,2-, 1,3- and 1,4-methylcyclohexanols) have been determined from peak area measurements in the completely proton decoupled low temperature ¹³C NMR spectra of the individual conformers. The ¹³C chemical shifts are discussed in terms of the additive model.     

Measurements of Long-Range Homonuclear Coupling Constants

Coupling constants in proton systems provide access to useful structural information. Several methods have been proposed to measure these constants in high-resolution spectra, but many of them are not well suited when the coupling constants are comparable to the spectral linewidth. In such a case the measurement of the apparent splitting, obtained from conventional NMR or from two-dimensional correlation spectroscopy (COSY), can cause a miscalculation of the true coupling constant value. In this work, data processing for extracting small coupling constants is described. Signals are obtained from spin-echo experiments and analysed in the time domain in such a way that couplings are apparently multiplied by n+1, where n is positive. Small coupling constants in 4-methyl-1,3-dioxane were obtained by this method. © 1997 John Wiley & Sons, Ltd.     

Die Kernresonanzspektren von N-substituierten 2.6-Dimethylmorpholinen

The NMR spectra of N-substituted 2,6-dimethylmorpholines after separation into cis- and trans-isomers by VPC have been investigated. The spectral parameters (chemical shift and coupling constants) are evaluated by a mathematical analysis. The spectra of the cis-compounds show some special features so far unexplainable. The difference of the chemical shift of the protons in 3-position of the cis-isomers depends strongly on the nature of the N-substituent.     

Rates of Proton Abstraction from Conjugated Dienes in the Gas Phase

The rates of proton transfer from 2, 4-hexadiene, 1, 3-cycloheptadiene, cyclopentadiene and acetone to t-butoxide have been measured in the gas phase using pulsed-ion-cyclotron-resonance spectroscopy. The rate constants are (units of 10^?10 cm³ molecule^?1 s^?1): 2.7 ± 0.4, 3.8 ± 0.4, 6.1 ± 0.7 and 10.8 ± 1.5, respectively. These results are analyzed in terms of the properties of the encounter complex and reaction transition states. The reaction profile for t-butoxide + cyclopentadiene is modeled using RRKM theory and an estimate for the central barrier height is obtained.     

The proton spectrum including 13C-satellites of π-benzenechromium tricarbonyl in a nematic solvent

The proton spectrum of π-benzenechromium tricarbonyl with ¹³C-satellites due to ring- and carbonyl-carbons at natural abundance has been investigated in a nematic solvent. The structural data are found to be in agreement with those from electron and neutron diffraction measurements. The proton–proton indirect coupling constants have also been determined.     

Distortion analysis of severely flattened rings

The NMR spectra of the nonaromatic protons in several benzo-substituted five- and six-membered rings have been analyzed at 60- and 90-MHz. The vicinal coupling constants from the? CH₂? CH₂? fragments give a quantitative assessment of the degree of ring flattening. The distortions are discussed in terms of the structural components of the molecules.     

Conformational study of methyl esters of some aliphatic erythro- and threo-dichlorocarboxylic acids

The average conformations of methyl esters of some aliphatic erythro- and threo-dichlorocarboxylic acids in dilute carbon tetrachloride solutions have been determined from the vicinal proton–proton coupling constants and ¹H NMR shifts. The ¹³C shift differences between the erythro and threo forms are compared and discussed with regard to the differences in the average conformations.     

13C?13C coupling constants in diacetylene (1,3-butadiyne) and its bis(triethylsilyl) derivative

Completely ¹³C-labelled diacetylene and its bis(triethylsilyl) derivative have been synthesized and all the possible spin–spin couplings between the acetylenic carbon nuclei have been determined from their Fourier transform ¹³C NMR spectra. J(CC) values in diacetylene have been also computed by means of the finite perturbation INDO method. Carbon–proton coupling constants in diacetylene have been determined from the spectrum at natural abundance. It has been established that J(CC) values across the triple bond in diacetylene and bis(triethylsilyl)diacetylene are greater than in acetylene and triethylsilylacetylene, respectively. The increase is interpreted in terms of σ- and π-electronic changes which occur with the coupling of two isolated triple bonds into a dimer-like system. All CC coupling constants are greater in diacetylene than in bis(triethylsilyl)diacetylene, which indicates that strong (p→d)π interaction takes place in the latter compound.