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. 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. 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).     

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.     

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.     

N.m.r. studies on aromatic compounds. I—13C n.m.r. spectra of some mono- and disulpho-substituted hydroxycarboxylic acids

Carbon-13 n.m.r. spectra of 3-hydroxy-4-sulpho-2-naphthoic, 3-hydroxy-5-sulpho-2-naphthoic, 3-hydroxy-7-sulpho-2-naphthoic, 5-sulphosalicylic, 3-hydroxy-5,7-disulpho-2-naphthoic, 1-hydroxy-4,7-disulpho-2-naphthoic, and 3,5-disulphosalicylic acids were recorded with and without proton noise-decoupling. Analyses of the spectra were carried out for all compounds except 3-hydroxy-5-sulpho-2-naphthoic acid which dimerized. The fine splitting caused by long-range coupling was used in identifying the lines of the ¹³C n.m.r. spectra.     

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.     

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.     

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. 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.     

A 13C n.m.r. study of the B6 vitamins and of their aldimine derivatives

The vitamins, pyridoxine, pyridoxal, pyridoxamine, pyridoxal-5′-phosphate and pyridoxamine-5′-phosphate, have been studied in aqueous solution over a pH range of 2–12 by ¹³C nuclear magnetic resonance spectroscopy. Resonance assignments are made primarily by the spin–spin coupling constants of carbons with protons and with phosphorus. The proton–carbon coupling constants show a marked conformational dependence in the hemiacetal form of pyridoxal. Furthermore, the H-6? C-5 coupling constant in the vitamins is much smaller than the corresponding constant in pyridine. This may be due either to an effect of the C-5 substituent in vitamins or to a different electronic configuration of the zwitterionic hydroxypyridine ring. The addition of manganese to a solution of pyridoxal phosphate causes line broadenings consistent with the interaction of the metal ion with this vitamin at the formyl and phenolic oxygens. The chemical shifts of the aromatic carbons of pyridoxine have been calculated, as a function of pH, by summing shielding parameters which were estimated empirically from pyridine derivatives. The calculated shifts agree well with the experimental data for C-3, C-5 and C-6, less well for C-2, and poorly for C-4. The deviation from additivity for C-4 indicates a preferred orientation for the 4-hydroxymethyl substituent caused by internal hydrogen bonding between the substituents at C-3 and C-4. Evidence is presented for the existence of the free aldehyde form of pyridoxal at alkaline pH. Aldimine complexes of pyridoxal and pyridoxal phosphate with amines and amino acids have also been studied. Characteristic chemical shift changes caused by both pyridinium and aldimine nitrogen deprotonations are seen. Additionally, the chemical shifts of carbons of the pyridine ring are dependent upon the structure of the imine, especially when the aldimine nitrogen is protonated. We conclude that this dependency is due to steric effects in an aldimine complex which is constrained by internal hydrogen bonding. We also discuss the merits of carbons 3 and 4 as possible sites of cofactor labeling for enzymatic studies.     

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.     

Carbon-13 n.m.r. studies of some saturated 1,2-oxazepine derivatives

The influence of incorporating an group, an oxygen atom and the fragment in a saturated 7-membered ring system on carbon-13 n.m.r. chemical shifts is examined, and also, the influence of the dioxolane ring moiety on the ¹³C chemical shifts in these ring systems. Substituent effects are generally additive except in cases where the ring is heavily substituted with methyl groups. A large upfield steric shift (γ effect) of 7–8 ppm is observed in two of the derivatives. An example of long range nonequivalence is also observed. Assignments of the ¹³C n.m.r. spectra have been made by comparison with model compounds, and from proton coupled ¹³C n.m.r. spectra. The synthesis of several new compounds is described.     

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.     

13C n.m.r. spectroscopy: Comparison of the spectra of some dimeric Catharanthus alkaloids and their derivatives

The ¹³C n.m.r. spectra of some dimeric Catharanthus alkaloids are reported and assigned. Methods devised to aid in the assignment of resonances in complex molecules are described. The ¹³C n.m.r. spectra of several derivatives of vinblastine are discussed.     

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.     

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.