Studies in tertiary amine oxides part II—carbon-13 nuclear magnetic resonance spectra of selected acetylenic amines,their N-oxides and the Meisenheimer rearrangement products

Carbon-13 NMR chemical shifts and ¹J(CH), ²J(CH) and ³J(CH) coupling constants of selected saturated nitrogen heterocyclic molecules containing the acetylenic moiety have been determined. These NMR parameters have also been determined for the corresponding N-oxides and the Meisenheimer rearrangement products, the O-allenylhydroxylamines. The effect of the N-oxidation on the chemical shifts of the ring and the acetylenic carbon atoms is discussed.     

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.     

15N NMR studies of amino acids and their reaction products with formaldehyde

Natural abundance ¹⁵N NMR spectroscopy has been used to investigate the effect of pH on the ¹⁵N chemical shifts of lysine and of ε-hydroxymethyllysine. A computer calcualtion which fits the chemical shifts of both α-and ε-nitrogen atoms versus pH has been used to predict the pK_a values. ¹⁵N chemical shifts and some ¹J(¹⁵NH) values of some other amino acids and of their reaction products with formaldehyde are also reported.     

Carbon-13 nuclear magnetic resonance chemical shifts and 13C-199Hg coupling constants for symmetrical dialkylmercurials

The carbon-13 chemical shifts and coupling constants (J[¹³C? ¹⁹⁹Hg]) have been determined for a series of eleven symmetrically substituted organomercurials. Empirical substituent parameters can be calculated which correlate observed and predicted chemical shifts for dialkylmercurials.     

Studies in nuclear magnetic resonance—XII: Complete analysis of the proton spectra of tert-butylcyohexane-3,3,4,4,5,5-d6 and -3,3,5,5-d4

Accurate parameters for the two title compounds have been obtained by the aid of a spectral subtraction technique in conjunction with analysis by LAOCN3. It is observed that J_{1eq, 6eq} and J_{1ax, 2ax} are abnormally large and small, respectively, and the significance of these and other coupling constants are discussed in terms of the probable geometry of tert-butylcyclohexane. The tert-butyl group affects the chemical shifts of the 4-protons, indicating that tert-butylcyclohexyl derivatives are poor models for fixed conformers of monosubstituted cyclohexanes. The deuterium isotope effects on proton chemical shifts appear to be influenced by the same factors as proton-proton coupling constants.     

Correlations between molecular parameters and the spin–spin coupling constants of vicinal protons: Criticism of the karplus equations

Conformationally rigid systems such as xylopyranose 1,2,4-ortho esters ( 1a ) and ( 1b ) and 10-methoxy-6-aza-isoadamantane ( 2 ), for which the identity of conformations both in the crystalline state and in solution can reasonably be assumed, provide good models for the study of experimental correlations between the spin–spin coupling constants (J) of the vicinal protons and the dihedral bond angles (DBA) determined from an X-ray study in the H? C? C? H bond system. The Karplus equations and their modifications (with and without corrections for the electronegativity of adjacent groups) were found to be unable to provide satisfactory correlations between these parameters. Optimum coefficients for the equations connecting the J and DBA values with corrections for electronegativity were calculated by the least squares method. The same procedure was used to obtain an equation connecting the J with DBA values using the sum of the chemical shifts as a measure of the electronic factors affecting the J ∝ DBA dependence. The accuracy of the equation thus obtained lies within ±18% as opposed to ±48% for the original Karplus equations. A similar correlation was obtained for the angles between the intercrossing lines formed by the directions of the vicinal C? H bonds (ILA). When combined with the sum of chemical shifts, ILA provides a better correlation with the coupling constants J than the conventional parameter DBA.     

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.     

13C-NMR-Spektren von Cyclopropenen und Benzocyclopropenen

The substituent parameters of methyl-, halogen-, methoxycarbonyl- and phenyl-substituted cyclopropenes as well as of cyclopropenone and diphenylcyclopropenone are determined from the ¹³C chemical shifts. The influence of the carbonyl group on the chemical shifts and charge densities is studied in the cyclopropenones. The interaction between the cyclopropene nucleus and the annulated benzene ring in benzocyclopropenes is evaluated with the aid of the ¹³C NMR spectra of di- and tetraphenyl-substituted benzocyclopropene derivatives and of dimethyl 1H-cyclopropa[a]naphthalene-1,1-dicarboxylate. The J values are shown to be better indicators of structural changes than the δ values. α-Effects of substituents give a good correlation with Hammett σ₁-values.     

1H, 13C and 77Se NMR spectra of substituted selenoanisoles

The ¹H, ¹³C and ⁷⁷Se chemical shifts and the ¹J[C(Me)H(Me)], ^1.2J(SeC) and ²J(SeH) coupling constants in 14 para- or meta-substituted selenoanisoles, R? C₆H₄? Se? CH₃, have been measured and the dependence of these parameters on the electronic effects of the substituent R is discussed. A significant (up to 6 ppm) deviation from additivity of the substituent influence on the shielding of the ¹³C ring carbons has been found.     

Carbon-13 nuclear magnetic resonance spectra of some methyl and phenyl substituted 2H-azirines

Carbon-13 n.m.r. spectra have been obtained for some methyl and phenyl substituted 2H-azirines. The higher field resonance of C-2 than that of the corresponding aziridine carbon is interpreted in terms of ring strain. Substituent effects on the chemical shifts of the azirine ring carbons are discussed. A set of additivity parameters for the methyl and phenyl groups are obtained which can be used for the calculation of the chemical shifts of the azirine ring carbons. The substituent effect of an azirine ring on the chemical shift of benzene is also discussed in comparison with those of some other substituents. A high degree of s character (48.5%) in the exocyclic orbital of C-3 is indicated by a large J(¹³C-3,H) value (242.5 Hz).     

31P-NMR-Daten heterocyclischer phosphine. Diastereomerenzuordnung an 1,3-Oxa-, 1,3-Aza- und 1,3-thiaphospholanen

The diastereomers of 16 1,3-oxa-, 1,3-aza- and 1,3- thiaphospholanes were assigned by means of the coupling constants ²J(P? C? H) and ³J(P? C? CH₃) and the linewidths of the ³¹P signals and ¹H chemical shifts of CH₃ groups. It is shown that the change in the ³¹P chemical shifts allows the estimation of the relative configuration in these compounds.     

Configuration of functional trifluoromethylated dienoates,aryldienoates and trienoates

The configuration of certain trifluoromethylated functional dienoates, aryldienoates and trienoates is presented by the measurement of their ¹³C NMR and ¹⁹F NMR chemical shifts, and their ³ J(C–F), ⁴J(H–F) and through‐space ⁵J(H–F) coupling constants. Copyright © 2002 John Wiley & Sons, Ltd.     

15N chemical shifts as a conformational probe in enaminones A variable temperature study at natural isotope abundance

The high sensitivity of ¹⁵N shielding to the displacement of the lone pair electrons makes it a useful conformational probe for remote parts of a conjugated molecule. Thus, the chemical shifts are observed for different rotamers of enaminones in the slow exchange limit. The interpretation of the ¹⁵N chemical shifts in terms of the non-planarity of the E, s-E rotamers is in accord with ¹³C chemical shifts and ¹J(CH) coupling constants.     

A statistical study of the additivities of substituent effects in the 13C NMR chemical shifts of hydroxy- and chloro-substituted benzenes

The ¹³C NMR spectra of six hydroxybenzenes, all chlorobenzenes, all chlorophenols and eight chlorocatechols are measured and assigned. The additivity of the substituent effects and the usefulness of some corrective parameters are studied with regression analysis. The order of the chemical shifts is most efficiently predicted by the simplest substituent effect model, containing only the direct effects of the substituents, although the 95% confidence limits of the calculated shifts are as high as 5.6 ppm. If the chemical shifts need to be predicted within the measuring errors (approximately 0.05–0.10 ppm, in the present data), the number of necessary corrections is very impractical. The correction parameters are found to be independent of the solvent if no conformational effects are operative. These corrections, or the non-additivity parameters, are thus theoretically interesting. Approximate ¹J(CH) couplings are also reported and the additivity of substitution effects on these parameters are discussed.     

An 1H n.m.r. study of complexation and self-association processes in the ternary system: tert-butyl alcohol–acetone–cyclohexane

The hydroxyl proton chemical shifts of tert-butyl alcohol, dissolved in acetone and in binary mixtures of acetone with cyclohexane, have been measured as functions of concentration at temperatures of 20, 35, 50 and 60 °C. Using both the equilibrium constant and the quasi-lattice methods for the interpretation of experimental data, the thermodynamic parameters of self-association and complexation have been determined, and their dependences on the solvent composition have been analysed. The chemical shifts of the hydroxyl proton obtained in the complexes and the thermodynamic parameters of complexation of tert-butyl alcohol with acetone-d₆ proved to be the same as for the tert-butyl alcohol–acetone system.     

NMR studies in the heterocyclic series. XVIII—Carbon-13 NMR study of azolium salts. Attempted correlation of chemical shifts with calculated charge densities

The carbon-13 chemical shifts and the ¹J(CH) coupling constants for 18 azolium salts (di-N-methyl-pyrazolium, -indazolium, -benzimidazolium and -benzotriazolium iodides) have been determined and assigned. The chemical shifts are discussed as a function of substituent effects, positive charge introduction and total electronic density (calculated by the CNDO/2 method). The general problem of correlations between chemical shifts and total charge densities in azoles is discussed. A statistical approach shows that these correlations are of poor quality.     

Study of the intrinsic and conformational solvent effects on the chemical shift of the aldehydic proton in furfural and thiophenealdehyde

The chemical shifts of the aldehydic proton in furfural, thiophenealdehyde and benzaldehyde have been measured in fourteen solvents. The correlation of the chemical shifts of thiophenealdehyde and benzaldehyde is excellent (r = 0·996) while it is lower for furfural–benzaldehyde (r = 0·956). The long range coupling constant J_α5 of furiurai has been measured in twelve solvents and the rotameric mole fractions determined. The chemical shifts of individual rotamers are calculated and shown to correlate with benzaldehyde, (r = 0·992; 0·993). Only one rotamer is predominant for thiophenaldehyde in all solvents. The intrinsic solvent effects of the three aldehydes are similar.     

Carbon-13 nuclear magnetic resonance spectroscopy—VII:para-substituted fluorobenzenes

C-13 and F-19 NMR spectra of seventeen para-substituted fluorobenzenes were measured and the chemical shifts as well as coupling constants with respect to substituents were analysed. The chemical shifts of the fluorine, the C₁ and the C₂ atoms were found to depend on the total electron densities. In the case of the C₃ atom, the chemical shifts seem to depend on π-electron densities rather than the total electron densities. The present calculations also indicate that the chemical shift of the C₄ atom depends mainly on σ-electron densities due to the inductive effects of substituents. The strongest factor influencing the coupling constant, ⁿJ(C? F), is also considered to be the π-electron densities on the carbon atoms. In the case of the direct couplings, ¹J(C? F), the π-bond orders are important.     

Structures and NMR Parameters of 1,2-Diphenylcyclobutanes

A new reaction scheme for obtaining cis and trans 1,2-diphenylcyclobutane is described. Using ¹H-NMR at 600 MHz, full spectral assignment was made for both isomers, obtaining all J coupling constants and chemical shifts. NMR results on cis and trans 1,2-diphenylcyclobutane are compared with the vicinal coupling constants obtained by the Barfield–Smith equations from the literature internal and dihedral angles of cyclobutane. In the trans isomer, in agreement with previous results on halo-cyclobutanes, the conformation with the phenyls in the pseudo-di-equatorial position is strongly preferred. On the contrary, the cis isomer fluctuates between the two equivalent conformations: phenyl pseudo-axial and pseudo-equatorial.     

15N and 13C n.m.r. study of azimines and azoxybenzenes

¹⁵N chemical shifts, ¹J(NN), ¹J(NC) and ²J(NC) coupling constants have been used to prove the open chain structure and configuration of cis- and trans-2,3-diphenyl-1-phthalimidoazimines. For comparison, the corresponding data of the iso-π-electronic cis- and trans-azoxybenzenes are also reported and discussed.