Substituent effect of the nitro group on alkanes in carbon-13 n.m.r. spectroscopy

Linear correlation coefficients were obtained for the substituent effect of the nitro group in primary nitroalkanes. The solvent effect of chloroform is also discussed, as well as the influence of the scalar relaxation on the half-width of α-carbons in ¹³C n.m.r. spectroscopy.     

A carbon-13 n.m.r. study of sulfur-donor ligand ortho-metalated iron carbonyl complexes

Carbon-13 n.m.r. data are reported for a series of sulfur-donor ligand ortho-metalated iron carbonyl complexes derived from thiobenzophenones. The chemical shift δ153·5–161·4 for the aromatic carbon σ-bonded to iron is readily identified, and occurs in the same region as the signal for the corresponding carbon in a related nitrogen-donor ligand complex. The benzylic carbon gives a signal at δ62·8–63·3, subject to the nature of the complex.     

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.     

Concerning carbon-13 n.m.r. spectroscopy of 5β-hydroxylated phytoecdysones

The ¹³C n.m.r. spectra of the 5β-hydroxylated phytoecdysones polypodine B, muristerone A and kaladasterone are presented and briefly discussed together with the spectrum of makisterone A. Comparisons with previously reported spectra of ecdysone, ecdysterone and poststerone are made and allow a correction for the C-20/C-24 assignment for ecdysone.     

Carbon-13 n.m.r. investigation on the nitrogen methylation of the mono- and diazanaphthalenes

The ¹³C n.m.r. spectra of the N-methylated mono- and diazanaphthalenes have been recorded and analysed. It has been shown that N-methylation as well as N-protonation in cinnoline occur predominantly at the β-nitrogen atom. N-methylation and N-protonation show a similar effect on the ¹³C chemical shifts.     

Silicon-29 and carbon-13 n.m.r. studies of organosilicon chemistry. VII—trimethylsilylated silicates

Silicon-29 n.m.r. was used to study the products from trimethylsilylation of minerals. Chemical shifts are reported for four model compounds, viz. the monomer, dimer, linear trimer (from natrolite) and cyclic tetramer (from laumontite). Information is then obtained from the products of trimethylsilylation of the anhydrous calcium silicates wollastonite and pseudo-wollastonite. In the latter case it is shown that the principal products are the four model compounds, and percentage yields are obtained for these.     

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.     

Electronic effects in some 3-substituted-2-cyclohexenones: A study by carbon-13 n.m.r. spectroscopy

The carbon-13 n.m.r. spectra of some 3-substituted-2-cyclohexenones show very wide variations in chemical shift at C-3 that correlate with Pauling electronegativity χ, even wider variations at C-2 that correlate with the Taft resonance parameter σ_Rº and almost no variation at C-1. These very different effects are shown to be consistent with the electronic distribution for these and related compounds revealed by other spectroscopic techniques. Reasons for the insensitivity of carbon-13 n.m.r. to configurational and conformational variation in simple enamino-ketones are discussed.     

13C n.m.r. investigation on the first and second nitrogen protonation in the diazanaphthalenes

The ¹³C chemical shifts of the diazanaphthalenes have been recorded as a function of the pH value, providing classical titration curves. From these curves the pK₁ and pK₂ values have been determined taking into account the activity coefficients. The changes in ¹³C chemical shift under the influence of nitrogen protonation (Δδ) can be described by two protonation parameter sets.     

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.     

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.     

Carbon-13 n.m.r. studies of methylquinolines and methylisoquinolines

Spectra are reported for all seven methylquinolines, nine dimethylquinolines, six methylisoquinolines and one dimethylisoquinoline. Substituent effects for the α, ortho, meta, para, vinylogous para and peri positions are reported for the monomethylcompounds. Additivity of these substituent effects is demonstrated for the dimethyl compounds which also exhibit a vicinal dimethyl steric effect.     

Carbon-13 n.m.r. characterization of indenyllithium in solution

¹³C N.m.r. chemical shifts obtained for indenyl- and cyclopentadienyllithium in several, mostly ethereal, solvents are presented. The chemical shift changes induced by varying the solvent are assumed to probe the actual π-electron density distribution, thus providing an insight into the structure of the ion-pair. The increase in the estimated π-electron densities obtained for the protonated aromatic carbons of indenyllithium by increasing the coordinating ability of the solvent gives evidence of a change of the ion-pair equilibrium towards solvent-separated species. The average chemical shifts for indenyl- and cyclopentadienyllithium were independent of the choice of solvent, a fact which supports the ion-pair model in all studied media.     

Carbon-13 n.m.r. analysis of selected strychnos alkaloids

The carbon-13 chemical shifts of strychnine and ten closely related analogues have been assigned and confirmed in several cases using selective proton decoupling techniques. Where additivity relationships are used the importance of having data on an array of structural analogues is emphasized. Quaternization at N-19 exerts long range polarization effects throughout the molecular framework. The effects of N-oxide formation on the carbon-13 shifts of 5 to 9-membered ring heterocycles and strychnine are discussed as aids to structural assignment. A Zwitterionic structure for strychnic acid is deduced.     

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 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.     

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.     

Carbon-13 n.m.r. study of naturally occurring xanthones

The ¹³C n.m.r. spectra of 36 naturally occurring xanthones are reported and all chemical shifts assigned. The shifts in substituted xanthones can be predicted from substituent effects evaluated for simple derivatives. The agreement between calculated and observed shifts decreases as the extent of substitution in the xanthone increases.