15N chemical shifts and one bond 15N?1H coupling constants in simple amides

An indirect method is employed for determining the ¹⁵N parameters at the natural abundance level in a series of simple acyclic and cyclic amides. The one bond coupling constant, ¹J(¹⁵N¹H), and the ¹⁵N chemical shift are measured as a function of the carbonyl substituent group or the ring size and the nature of the solvent (CCl₄ or H₂O). These ¹⁵N parameters are related to the amide bond structure, the nitrogen configuration and possible intermolecular hydrogen bonding (amide-amide or amide-water).     

15N NMR spectroscopy 24—chemical shifts and coupling constants of α-amino acid N-carboxyanhydrides and related heterocycles

The chemical shifts of amino acid N-carboxyanhydrides (NCAs) and cyclic or linear urethanes are less sensitive to solvent effects than those of amides and lactams. The values of the one-bond ¹⁵N? ¹H coupling constants depend on the solvent and are 5-8 Hz larger than those of ureas and amides. The ¹⁵N? ¹³C coupling constant of the N? CO group is also unusually high, while that of the N—CH group lies within the range known for N-acylated aliphatic amines. The one-bond ¹⁵N? ¹³C coupling constant was found to be insensitive to conformational changes.     

1H and 13C NMR chemical shifts and spin-spin coupling constants in trans- and cis-decalins

The NMR parameters characterizing the spectra of trans- and cis-decalins were determined from theoretical calculations and experimental spectra. The calculated values of the shielding constants are in good agreement with the measured chemical shifts, with a small but noticeable difference in accuracy for the bridgehead atoms. Of all the spin-spin coupling constants, only most of (1)J(C,C) and (1)J(C,H) values could be extracted from the spectra, and the corresponding computed values are in good agreement with experiment. It appears that the applied density functional theory (DFT) approach overestimates slightly the J(C,C) coupling and underestimates the differences between one-bond (1)J(C,H) coupling constants. For all these constants [J(C,C), J(C,H) and J(H,H)] through one to three bonds, which could not be obtained experimentally, the predicted values are in good agreement with the general rules relating spin-spin coupling to the number and spatial arrangement of the intervening bonds.     

1H and 13C chemical shifts and coupling constants of lupane. Application of two-dimensional NMR techniques

2D NMR techniques (J-resolved ¹³C, ¹³C? ¹³C correlated, ¹H? ¹³C correlated) were used to gather more chemical shift and coupling information on the pentacyclic triterpene, lupane. They confirm and complete ¹³C assignments made earlier, and corroborate the constitution and major configurational details of lupane. Lupane is a parent compound and spectral reference in the study of sedimental demethylated triterpenes.     

The effect of cyclopropyl groups on chemical shifts and coupling constants

Some shielding and deshielding effects by an adjacent cyclopropyl ring and the effect of the ring on geminal coupling constants α- to the ring are reported and discussed.     

19F chemical shifts, coupling constants and conformational preferences in monosubstituted perfluoroparacyclophanes

In the process of studying the chemistry of perfluoro[2.2]paracyclophanes (PFPCs), a novel class of compounds, it became necessary to identify some disubstituted products. To achieve this goal, we characterize in this work some monosubstituted PFPCs, identifying their (19)F-(19) F coupling patterns, and establishing a methodology for the assignment of their (19)F chemical shifts. The pattern of coupling constants indicates a skewed geometry in which the upper deck moves towards or away from the substituent, depending on the substituent electron-donor character and size. Quantum chemical calculations, performed at the HF/6-311 + G(d,p)//B3LYP/EPR-III level of theory, confirmed the conformations inferred from coupling constants and reproduced well the values of the couplings. Transmission mechanisms for the FC term of four- and five-bond (19)F-(19) F couplings are discussed in detail. Understanding the conformational preferences of PFPCs and how they are reflected by the coupling constants facilitates the assignment of (19)F chemical shifts in monosubstituted PFPCs and the identification of the disubstituted products.     

Correlation between 29Si-15N spin-spin coupling constants and the 15N-NMR chemical shifts in silatranes

A nonlinear correlation has been found between the ²⁹Si-¹⁵N spin-spin coupling constants and the ¹⁵N-NMR chemical shifts in silatranes. The dependence has been used to calculate corrections to the spin-spin coupling constant for the electronegativity of a substituent on the silicon atom. This has allowed the previously obtained data on the length and order of the N Si coordinate bond for silatranes in solutions to be refined.Translated from Khimiya Geterotisiklicheskikh Soedinenii, No. 5, pp. 701–703, May, 1989.     

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.     

NMR investigations on alanyl-[15% 13C, 95% 15N]-proline: 15N chemical shifts and 13C?15N coupling constants

The dipeptide alanylproline has been prepared with the proline residue both ¹³C (15%) and ¹⁵N (95%) enriched. ¹⁵N NMR spectra of alanylproline reveal signals for both possible conformations—cis and trans—of the dipeptide backbone in solution. Different pK values for both conformers are obtained from the pH dependence of the ¹⁵N chemical shifts using a least square programme based on the Henderson–Hasselbach equation. These different values are discussed in terms of interaction between the α-amino group and the carboxylate group and between the carboxylate oxygen and the carbonyl oxygen of the dipeptide via hydrogen bonding. Further evidence for these interactions is obtained from the pH dependence of the ratio of the ¹⁵N NMR signal intensities of the two conformers. One, two or three bonded ¹³C? ¹⁵N coupling constants measured in the ¹³C NMR high resolution spectra have different values in the cis and trans isomers of alanylproline and thus indicate different geometry in the pyrrolidine ring.     

19F coupling constants and chemical shifts in some dimethylamino and alkoxy derivatives of tellurium hexafluoride,cis TeF4XY

The spectra of A₂BC spin systems provided by the ¹⁹F nuclei in cis (RO) (MeO)TeF₄ and (RO) (Me₂N)TeF₄ (R = Me, Et, Pr, i-Pr) have been recorded and analysed. The geminal coupling constants ²J(F,F) range from 137–174 Hz and trends in the ¹⁹F chemical shifts permit complete assignment of the resonances. Stereospecific coupling between ¹⁹F and the protons of the N-methylamino groups is also observed.     

15N NMR chemical shifts of ring substituted benzonitriles

15N chemical shifts in an extensive series of para (15) and meta (15) as well as ortho (8) substituted benzonitriles, X-C6H4-CN, were measured in deuteriochloroform solutions, using three different methods of referencing. The standard error of the average chemical shift was less than 0.03 ppm in most cases. The results are discussed for both empirical correlations with substituent parameters and quantum chemical calculations. The 15N chemical shifts calculated at the GIAO/B3LYP/6-31 + G*//B3LYP/6-31 + G* level reproduce the experimental values well, and include nitrogen atoms in the substituent groups (range of 300 ppm with slope 0.98 and R = 0.998, n = 43). The 15N shifts in hydroxybenzonitriles are affected by interaction with the OH group. Therefore, these derivatives are excluded from the correlation analysis. The resultant 15N chemical shift correlates well with substituent constants, both in the simple Hammett or DSP relationships and the 13C substituent-induced chemical shifts of the CN carbon.     

Carbon-13 NMR spectroscopy of heterocyclic compounds—II : A 20 MHz study of chemical shifts and carbon-proton coupling constants for coumarin and some bromocoumarins

Complete assignments of chemical shifts and extensive assignments of carbon-proton coupling constants are presented for coumarin through a study also involving detailed measurements on 3-bromo-, 6-bromo-, 7-bromo-, 3,6-dibromo- and 3,6,8-tribromo-coumarin.     

Carbon-13 chemical shifts and coupling constants in cis- and trans-1,2-di-tert-butylethylene

The spectral parameters of the title compounds were redetermined to resolve discrepancies between data reported in the literature.     

13C NMR chemical shifts of some highly-branched acyclic compounds

A study has been made of the ¹³C chemical shifts of a number of acyclic alkanes, alkenes, nitriles and ketones which contain quaternary carbon atoms. Similar data have also been obtained for the series of compounds involved in the synthesis of triisopropylacetic acid. Substituent effects and steric factors in these highly substituted compounds are discussed in relation to the chemical shifts.     

Characterization of coupling constants to nitrogen. Carbon-13 chemical shifts and carbon–nitrogen coupling constants of substituted benzaldoximes

Carbon-13 chemical shifts and ¹³C?¹⁵N coupling constants of substituted benzaldoximes and of two non-aromatic oximes have been determined. The coupling constants display little variation as a function of the nature or position of substitution, which suggests that perturbations in the π electron distribution have little effect. The values are compared with the markedly different ones reported for mesitaldoxime, and, in conjunction with the observed solvent effects, are discussed within the Schulman-Venanzi framework for assessing the role of the lone-pair.