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.     

Orientation of the unshared electron pair of the nitrogen atom in N-substituted 2,5-dimethyl-4-piperidinones

The axial orientation of the unshared electron pair of the nitrogen atom in cis isomers of N-substituted [R=H, CH₃, CH₂C₆H₅, or C(CH₃)₃] 2,5-dimethyl-4-piperidinones was proven by using the stereospecificity of the direct ¹³C-¹³C spin-spin coupling constants, vicinal ¹³C-¹H spin-spin coupling constants, and two-dimensional nuclear-Overhauser-effect spectroscopy (NOESY).Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 512–513, April, 1989.     

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.     

Deuterium-induced 13C nuclear magnetic resonance isotope shifts and 13C?2H couplings for signal assignments and determination of deuteration site in cyclooctanone

Unambiguous signal assignments in the ¹³C spectra of monodeuterated cyclooctanones derived from the lead tetraacetate or silver oxide/bromine oxidations of the corresponding 1-monodeutero-alcohols have been obtained from deuterium-induced ¹³C isotope shifts and geminal and vicinal ¹³C? ²H spin-spin coupling constants. The label in the ketones is shown to be in position 5.     

Refinement of the angular dependence of the peptide vicinal NHCαH coupling constant

The refined dependence of the peptide NHCαH vicinal coupling constant on the dihedral angle θ have been derived on the basis of the accumulated experimental data. The mean permissible values (in Hz) are approximated by ³J_NHCH = 9·4 cos² θ - 1·1 cos θ + 0·4 An analogous relationship for the sum of two vicinal NHCαH₂ coupling constants in the glycyl residue have been calculated from the above dependence. Measurements on N-methylacetamide in various solvents and in the presence of an alkali salt showed the vicinal constant NHCH to vary by not more than ± 3%. Some of the other proposed ³J_NHCH(θ) dependencies give too low values for the cis-oriented NH and CαH bonds. This may be due to the fact that in these correlations the data for compounds with cis-amide bonds have been used for 0° ? θ ? 90° region of the dependence.     

NMR characterization of α- and β-mannopyranurono-2,6-lactones

Determining the stereochemistry at the anomeric position of glycosides is imperative in chemical synthesis. As for 1,2-cis-β-glycosides such as β-mannosides, ¹³C-¹H coupling constants, ¹J_CHs, are conventionally used when glycosides are in an ordinary chair conformation. In this work, we searched for appropriate criteria to determine the stereochemistry of products in a recently developed glycosylation reaction using 2,6-lactones. As a result, we found that α-glycosides exhibited vicinal coupling constants of around 3.0?Hz?at the anomeric proton, as well as a long-range coupling between protons at C-1 and C-5, whereas β-glycosides had coupling constants of 1.2?Hz?at most. These figures are expected to be useful for future assignments. In addition, the observations obtained from this study revealed the conformation of glycosides with a 2,6-lactone moiety.     

Elektronenstruktur und NMR-Parameter von substituierten 5-Phenyl-2,4-pentadiensäuren

The ¹H and ¹³C NMR chemical shifts as well as vicinal HH coupling constants of substituted 5-phenyl-2,4-pentadienoic acids Ar? CH?CH? CR?CR? COOH are reported and discussed in connection with the molecular structure. The ¹³C chemical shift values show an alternation along the chain and can be linearly correlated to the π-electron charge densities as calculated by use of the PPP-method. The effect of para-substituents and solvents upon the ¹³C chemical shifts can be described in terms of the mutual atom-atom polarizabilities.     

C,H coupling constants of propene

The proton coupled ¹³C NMR spectrum of propene has been analysed at 100.6 and 25.2 MHz and a complete set of C,H coupling constants is reported. In particular, vicinal and one-bond coupling constants are discussed with respect to substituent effects of alkyl groups. An incremental method allows us to predict ¹J(CH) in more highly methyl-substituted alkenes.     

Conformational study of phosphoserine in aqueous solutions. II—1H n.m.r. results

The 270 MHz n.m.r. spectra of phosphoserine (PSer) have been measured and completely analysed for seven pD values. The resulting vicinal coupling constants ³J(HαHβ) and ³J(PHβ) are used to discuss the conformations of PSer with respect to the (Hα)? Cα? Cβ? (O) and (Cα)? Cβ? O? (P) bond axes. The all-trans conformer predominates for all pD values, with its fractional population being minimal at pD values close to neutrality. The Cβ? O bond rotamer populations agree reasonably well with those obtained from ¹³C investigations.     

The 13C,H coupling constants in structural and conformational analysis: II The correlation of 3J(HCCH) and 3J(CH3CCH) in ethylenes and their methyl derivatives

The vicinal ³J(C H₃C?CH ) coupling constants were determined for a number of propylene derivatives and compared with the ³J(H C?CH ) couplings of the corresponding ethylenes. A linear regression analysis yielded the correlation ³J(CH) = 0.46 ׳ J(HH)+1.58 Hz, the correlation coefficient being 0.956.     

Application of 1H and 13C NMR to the structural elucidation of tetrahydroquinoxalines condensed with five-membered heterocycles

¹H and ¹³C NMR spectral data for 21 N-methyltetrahydroquinoxalines annelated with furan, pyrrole, imidazole or thiazole rings are reported. Unambiguous assignments of the ring junction ¹³C resonances were made on the basis of selective decoupling experiments and with the aid of one-bond and long-range ¹³C–¹H coupling constants. The effects of five-membered heterocycles on the ¹H and ¹³C chemical shifts of the ring junction hydrogen and carbon atoms are considered. Values of one-bond ¹J(CH) and vicinal ³J(HH) coupling constants between the ring junction protons are also discussed as a diagnostic means for structural elucidation of tetrahydroquinoxalines condensed with five-membered heterocycles.     

1H and 13C NMR studies on 1,3,2-dioxarsolanes

The ¹H and ¹³C NMR spectra of a series of 1,3,2-dioxarsolanes have been obtained at 2.1 T and some at 9.4 T. The chemical shifts and spin-spin coupling constants have been obtained from complete spectral analyses of the ¹H and proton-coupled ¹³C spectra. The spectral data are interpreted on the basis of two rapidly interconverting half-chair conformers with a pseudoaxial substituent at arsenic. Unique assignment of syn/anti or cis/trans geometries have been made from ¹H or ¹³C NMR spectroscopy alone. The syn and trans isomers of the 4-methyl- and 4,5-dimethyl-1,3,2-dioxarsolanes, respectively, appear to be conformationally biased towards the forms with pseudoequatorial methyl groups. The general trends in the geminal and vicinal ¹H? ¹H and ¹³C? ¹H coupling constants are interpreted in terms of stereospecific, electronegativity and lone-pair effects of the oxygen heteroatoms and conformational factors. The NMR data on the 1,3,2-dioxarsolanes are discussed with reference to related 1,3-dithoxa- and 1,3-dithia- five-membered rings with As, P, S or C at the 2-position.     

13C?13C vicinal coupling constants of sulphonium cations

The ¹³C? ¹³C vicinal coupling constants of cyclic sulphonium cations, with the labelled carbon bound to the positively charged sulphur atom, cannotbe related to the dihedral angle ? by a Karplus type correlation.     

Conformational analysis of N-vinyl-2-phenylpyrrole

A conformational analysis of N-vinyl-2-phenylpyrrole was undertaken according to the results from experimental measurements and nonempirical quantum-chemical calculation of the high-level ¹³C–¹H and ¹³C–¹³C spin-spin coupling constants. Angular relationships were established in the direct and vicinal J _C,C and J _C,H constants in the N-vinylpyrrole fragment, making it possible to use them in stereochemical investigations of the vinyl derivatives of pyrrole. Dedicated to Boris Aleksandrovich Trofimov on his 70th jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 35–42, January, 2009.     

Hochauflösungs-13C-NMR-Spektroskopie. I—Analyse der Spektren des 1,2,4- und 1,3,5-Triazins sowie ihrer Methyl- und Phenylderivate

Using the pulse Fourier transform technique, the high resolution carbon-13 NMR spectra of 1,2,4- and 1,3,5-triazine and their methyl derivatives as well as the ¹H noise decoupled spectra of phenyl- and methylphenyl substituted triazines, have been recorded and analysed. The chemical shifts of the polysubstituted 1,2,4-triazines normally show additivity; deviations observed with phenyl derivatives are ascribed to steric effects. The ¹³C? H coupling constants are compared to those of benzene, pyridine, the diazines and disubstituted ethylenes. The strong influence on the coupling constants exerted by number and position of the nitrogen atoms is revealed by the observation that geminal couplings in 1,2,4-triazine are larger than vicinal ones, in contrast to the usual sequence.     

High resolution 13C NMR spectroscopy V—structure dependence of the 13C,H coupling constants in aceheptylene,azulene and 5-azaazulene

In order to study the structure dependence of the ¹³C,H coupling constants in polycyclic conjugated π electron systems with 5-and 7-membered rings, the ¹H coupled ¹³C NMR spectra of aceheptylene, azulene and 5-azaazulene have been analysed. The results are discussed, including published data of monocyclic reference compounds, and are compared to theoretical values calculated by the FPT-INDO method. It is shown that, first, the geminal ¹³C,H couplings in the unsaturated 5-membered carbocycles are predominantly dependent on CC bond lengths. Second, the vicinal ¹³C,H coupling constants in 5- as well as in 7-membered unsaturated carbocyclic rings can be linearly related to the lengths of the central CC bonds. Third, the vicinal inter-ring couplings show a large variation and give information on ring junction, thus being of special value in structure determination and are governed mainly by the bond angles. Fourth, the calculated ¹³C,H couplings across one and two bonds are totally useless for practical purposes; those across three bonds, however, at least reflect the experimental trends.     

Assignment of indirect 13C, 1H couplings in the 13C NMR spectra of some purine and pyrimidine nucleosides and their analogues by long-range selective 1H decoupling

The assignments of the long-range ¹³C, ¹H coupling constants in the ¹³C NMR spectra of the base moieties of several purine and pyrimidine nucleosides and their analogues were established by the application of long-range selective ¹H decoupling with low-power ¹H irradiation. The ³J values between the carbons and protons of the pyrimidine ring in these compounds were larger than the ²J values. The conformational preference of these nucleosides in solution was shown to be predominantly anti from the value of the vicinal ¹³C, ¹H coupling constants between H-1′ and the base carbons through the glycosidic bond.     

Carbon-13 NMR of some representative reactive small ring nitrogen heterocycles

The PFT ¹³C NMR spectra of some representative 1-azirines are discussed. Two striking features are the large ¹³C? ¹H spin coupling constants and the diagnostic chemical shift of C₃.     

NMR spectral studies of quinoline derivatives. Long range 13C?1H coupling constants in methylquinoline derivatives

Long range ¹³C? ¹H coupling constants are reported for a number of methylquinoline derivatives. The results have been compared with those observed in the pyridine series and correlated with ¹H? ¹H coupling constants.     

1H NMR parameters of the N-terminal 13-residue C-peptide of ribonuclease in aqueous solution

The ¹H NMR chemical shifts and the spin-spin coupling constants of the non-exchangeable protons of the N-terminal 13-residue C-peptide of ribonuclease A, obtained by cleavage of the enzyme with cyanogen bromide, have been measured in a 5 mM solution in D₂O (pH 3.0, 24°C) at 360 MHz. The titration parameters for end groups (Lys-1 and homo-Ser-13) and side chains (Lys-1, Glu-2, Lys-7, Glu-9 and His-12) have been determined. The chemical shifts, their temperature coefficients and the vicinal coupling constants, ³J(HNCH-α), for the exchangeable NH protons have been measured in a 5 mM solution in D₂O/H₂O (1:9 v/v) at pH 3.0. An assignment of observed signals to individual residue protons based on characteristic shifts, standard double resonance experiments, spectral simulations and titration shifts is proposed. All experimental evidence indicates that under the conditions studied the C-peptide is in a random coil form.