13C n.m.r. chemical shifts of carbodiimides

The ¹³C n.m.r. chemical shifts of the sp-hybridized carbons in dialkylcarbodiimides have values of δc ? 140. These shifts are compared with those of similarly hybridized carbons occuring in other classes of compounds.     

Temperature effects on 13C n.m.r. chemical shifts of normal alkanes and some line r and branched 1-alkenes

¹³C n.m.r. chemical shifts of a number of 1,1-disubstituted ethylenes are presented. Moreover, effects of changing temperatures on the ¹³C n.m.r. chemical shifts of some of these compounds as well as of three normal alkanes are given. These variations in chemical shifts are attributed to varying amounts of sterically induced shifts in the different conformational equilibria. In addition to the well-known 1,4 interaction between two alkyl groups shielding effects on the carbon atoms of the connecting bonds are also proposed. No definite explanation of this effect is presented at this time. It is further shown that no simple correlations exist between ¹³C n.m.r. chemical shifts and calculated total charge densities at this level. Instead, the experimental results in 1-alkenes are rationalized by assuming a linear dependence of the ¹³C n.m.r. chemical shifts of C-1 and C-2 via rehybridizations on changes in bond angles for small skeletal deformations caused by steric interactions. These changes in geometries, as well as conformational energies in three 1-alkenes, were calculated by means of VFF calculations. Finally. upfield shifts for both C-2 and C-4 are proposed for those conformations of 1-alkenes in which the C-3? C-4 group interacts with the p_z-orbital of C-2.     

Diamagnetic contribution and substituent effects on 13C n.m.r. spectra

Correction of measured chemical shifts by subtracting the diamagnetic contributions caused by certain substituents on various carbons of a molecule leads to data that allow a reasonable interpretation of substituent effects on chemical shifts in ¹³C n.m.r. spectroscopy. It is shown that both paramagnetic and diamagnetic terms increase on all carbons of a molecule when the atomic number of a substituent increases down the same group of the periodic table. An explanation of α-, β-, and γ- effects for methyl substitution using the same procedure is attempted.     

13C n.m.r. of organosulphur compounds: II—13C chemical shifts and conformational analysis of methyl substituted thiacyclohexanes

¹³C n.m.r. spectra of methyl substituted thianes and thianium cations have been determined. The magnitude of the ¹³C substituent effects of an equatorial methyl group or of a gem-dimethyl grouping appear to depend in a systematic way on whether the carbon atom concerned is adjacent to, or removed from, the heteroatom. The shieldings are discussed in relation to the conformational properties of the thiane ring. Moreover, the average ¹³C substituent parameters obtained from conformationally biased systems are applied to potentially mobile systems to assess the position of the conformational equilibrium.     

13C n.m.r. spectra of some polychloroalkenes

¹³C n.m.r. spectra have been measured for thirty-two polychloroalkenes including (i) monosubstituted compounds CH₂?CHCCl_nH_2?nX, where ? X stands for ? H, ? Cl, alkyl, and trisubstituted alkenes CCl₂?CHAlk, none of which form geometric isomers; (ii) disubstituted compounds RCH?CHR′; (iii) and (iv) trisubstituted compounds of the types RCCl?CHR′ and CHCl?CClR, respectively. Compounds (ii) to (iv) represent either individual isomers or mixtures of the Z and E forms. In the case of compounds (ii) and (iii), the ordering of chemical shifts is δ_E > δ_Z for the sp²-carbon atoms and δ_E < δ_z for the adjacent tetrahedral ones. On the contrary, the signals of the sp²-carbon atoms of compounds (iv) obey the rule δ_E < δ_z. The effect of vinyl and allyl groups as substituents on the ¹³C chemical shifts of chlorine-containing groups is discussed. The dependence of the sp²-carbon spin–spin coupling constants J(¹³C? ¹H) on the number of chlorinated substituents in the molecule is also considered.     

Investigation of substituent effects of chalcones by 13C n.m.r. spectroscopy

¹³C chemical shifts for 23 para- and meta-substituted chalcones of the types 1 and 2 have been determined. The aromatic shieldings are compared with previous results for other aromatic derivatives. Correlations of the ¹³C chemical shifts of vinyl carbons and carbonyl carbons as well as ring carbons with Hammett σ parameters, π electron densities and the reactivity parameters of Swain and Lupton provide a consistent picture of electronic effects transmitted through the carbon framework of the compounds studied.     

13C n.m.r. studies of organosilanes: II—substituent chemical shift parameters for alkoxysilanes

The ¹³C n.m.r. spectra of forty alkoxysilanes of the general type X_nSi(OR)_4–n (X = CH₃, C₆H₅, H; R = CH₃, C₂H₅, n-C₃H₇, i-C₃H₇, n-C₄H₉, i-C₄H₉, s-C₄H₉, n-C₅H₁₁, CH(CH₃)(C₆H₅), C₆H₅) have been recorded and assigned. The chemical shifts of the α-carbon resonances of the alkoxy groups are shown to depend on both the nature of the alkoxy group and the number and type of substituents on the silicon. Regression analyses of the data give empirical substituent chemical shift (SCS) parameters for the silyl substituents. The β-carbon resonances are shown to be dependent on the presence of the silyl group, but not the specific silyl substituents.     

13C n.m.r. studies of some phenanthrene and fluorene derivatives

The ¹H and ¹³C spectra of fluorene, fluorenone, phenanthrene and their 4-methyl and 4,5-dimethyl derivatives have been examined. To complete the analyses for fluorenone and 4-methylfluorenone, ¹H spectra were recorded at 270 MHz. The results from the ¹H spectra permitted unequivocal assignments for the protonated aryl carbons by selective proton decoupling. A consistent set of assignments for the quaternary carbons was obtained through consideration of the dominant relaxation processes operative at these centres. This series of compounds was examined to investigate the shielding effects produced by the close approach of methyl groups separated by five bonds for comparison with the contrasting trends found for methyl carbons separated by three and four bonds. The results indicate that the relative orientation of the methyl groups is an extremely critical factor governing their shieldings and those of neighboring centres.     

13C n.m.r. investigation on the nitrogen methylation of some azabenzenes

The ¹H and ¹³C n.m.r. spectra of N-methylated pyridine, pyridazine, pyrimidine and pyrazine and N,N-dimethylated pyrimidine and pyrazine have been recorded and analysed. The change in the ¹³C chemical shifts under the influence of N-methylation (Δδ) in the diazabenzenes could be predicted by the Δδ values of pyridine. A comparison of the Δδ values of N-methylation with those of N-protonation showed that both reactions have a similar effect.     

13C n.m.r. spectra of some pyrylium salts and related compounds

The ¹³C chemical shifts of several alkyl and phenyl substituted pyrylium perchlorates, together with related pyridine and pyridinium salts, are reported. The shifts in the isoelectronic series benzene, pyridine, pyrylium cation correlate well with charge densities calculated by INDO MO theory. Charge densities also account for the shift changes found at C-3, C-4 and C-5 for protonation of pyridine and 2,4,6-trimethylpyridine. The shift changes observed on protonation for C-2 and C-6, along the series pyridine, 2,4,6-trimethylpyridine and 2,4,6-triphenylpyridine can only be rationalized by consideration of both charge density and π-bond order changes. The effects of alkyl substitution on the shifts of the pyrylium cations are not accounted for by charge density changes. Empirical correlations of these shifts with literature data for the alkylbenzenes and the shifts of the phenyl substituted 6-membered heterocycles are discussed.     

13C n.m.r. spectra of cyclopropenes

¹³C n.m.r. chemical shifts and one bond ¹³C, ¹H coupling constants for cyclopropene and its 1- and 3-methyl derivatives as well as for methyl cyclopropane have been measured. The data for cyclopropene are 108·9 ppm and 228·2 Hz, 2·3 ppm and 167·0 Hz in the olefinic and allylic position, respectively. Substituent effects for the methyl group are discussed.     

13C chemical shifts of some model olefins

An extensive carbon-13 nuclear magnetic resonance study of selected model olefins dissolved in deuteriochloroform has been carried out under standardized conditions. Assignments of the chemical shifts have been made. The influence of the nature of the solvent and the effect of changing the concentration of the solute have been investigated. The results are intended to provide a practical aid for the analysis of olefinic materials.     

13C n.m.r. spectra of some thiobenzamides. Correlation between 13C and 1H n.m.r. spectra and signal assignments of syn and anti CH2 groups

¹³C n.m.r. spectra of a series of N,N-disubstituted thioamides have been recorded and signal assignments were performed. Separate signals are observed for methylene groups fixed on the nitrogen atom. Since the carbon atom syn to the thiocarbonyl sulfur resonates at higher field than the anti carbon, the syn-anti assignment in ¹H n.m.r. is easily obtained by selective double irradiation. This method, which is rapid and reliable, affords a rather general solution to the interesting problem of resonance assignments in tertiary amides and thioamides (and in analogous molecules such as oximes and nitrosamines).     

Carbon-13 n.m.r. spectroscopy of some Strychnos alkaloids

¹³C N.m.r. spectra have been determined for strychnine and a series of fourteen derivatives. The assignments are based on previously established data and on off-resonance decoupled spectra. The shifts resulting from the alterations in molecular structure are discussed and explained, in part, as a consequence of conformational changes. This detailed study indicates that changes are required in some previously published assignments.     

13C n.m.r. spectra of quercetin and rutin

The noise-decoupled and gated-decoupled ¹³C n.m.r. spectra of quercetin and rutin are studied and assignments are proposed. The previous assignments of carbons 2 and 4′, carbons 5 and 9, carbons 6 and 8 have been reversed.     

13C n.m.r. chemical shifts in aliphatic alcohols and the γ-shift caused by hydroxyl and methyl groups

Empirical correlations for the prediction of the chemical shifts of the carbon atoms in aliphatic alcohols are presented. Structural and conformational effects contributing to the ¹³C chemical shifts of γ-carbons are discussed.     

13C n.m.r. spectra of 2-substituted pyrimidines

¹³C n.m.r. chemical shifts and carbon-proton coupling constants of 2-substituted pyrimidines are reported. The carbon chemical shifts are correlated with π-electron densities. Substituents which cause deshielding at the directly bound carbon (e.g. NH₂, OCH₃ and F) exert a more powerful effect in the benzene series than in the pyridine or pyrimidine series. The carbon-proton coupling constants do not correlate with the electronegativity of the substituents. Carbon-proton coupling constants and proton-proton coupling constants over the same number of bonds do not obey the Karabatsos relationship. The changes in the carbon-proton coupling constants in 2(1H)-pyrimidinone and 2(1H)-pyrimidinethione which accompany anion and cation formation are reported.     

13C n.m.r. spectra of N,N-dialkylamides

Carbon-13 n.m.r. spectra of formic, acetic, propionic and butyric acid amides with N,N-di-n-alkyl substituents have been completely assigned with the aid of extensive double resonance experiments. The data obtained were used to study long range steric effects on chemical and solvent shifts.     

A 13C n.m.r. study of the charge-transfer complexes between o-chloranil and some aromatic electron donors

The ¹³C n.m.r. spectra of complexes between o-chloranil and aromatic electron donors were studied. Complexation leads to a general diamagnetic shift of the ¹³C n.m.r. signals for the acceptor (o-chloranil), but for signals from the ¹³C nuclei in the donors both diamagnetic and paramagnetic shifts are found. These phenomena are thought to be the result of competing anisotropy and charge-migration effects. Charge migration in o-chloranil complexes appears to be more important than in corresponding 1,3,5-trinitrobenzene complexes.