On the assignment of the carbon-13 NMR spectrum of bilirubin

An individual assignment of certain peaks in the proton and ¹³C NMR spectra of bilirubin and of bilirubin dimethyl ester is described. The assignment was achieved through the spectra of vinylneoxanthobilirubinic acid and by off-resonance decoupled ¹³C spectra of bilirubin dimethyl ester. The signals due to the endo and exo vinyl groups were separately assigned. This assignment allows a rational interpretation of previously obtained spin-lattice relaxation times of the vinyl carbon peaks. The two vinyl groups are found to differ considerably in their motional correlation time.     

Complete assignment of the carbon-13 NMR spectrum of chlorophyl a

The ¹³C NMR spectrum (at natural abundance) of monomeric chlorophyll α in acetone-d₆ has been recorded to re-examine the assignments of the low field (aromatic-olefinic) region of the spectrum. The assignments, made by the examination of the fully coupled spectrum and by the use of long-range selective ¹H decoupling (LSPD) with low-power irradiation, were compared with those of the previous reports. The results of the present work clarify the ambiguities previously encountered in the assignment of the 10a-ester, 7c-propionyl, P-2-phytyl, 2b-vinyl, γ- and β-methine carbon atoms, as well as the β-pyrrolic carbon-6 and α-pyrrolic carbons ?16 and ?17 of chlorophyll α. Reassignment of the three last carbons was found necessary. Knowledge of the chemical shifts of these carbon atoms was considered to be particularly valuable, as it yields relevant information on the delocalized π electron system which is crucial for the function of chlorophyll in photosynthesis.     

The 13C NMR spectrum of abietic acid and its methyl ester

The assignment of lines in the ¹³C NMR spectrum of abietic acid and its methyl ester have been made using carbon T₁ values and lanthanide induced shift effects. Structure–NMR parameter values are briefly discussed.     

Topics in carbon-13 NMR spectroscopy

The salts [NMe₄]₂[Ni₅(CO)₁₂], [NMe₄]₂[Ni₆(CO)₁₂], [NMe₄]₂[Ni₁₂(CO)₂₁H₂] and [NEt₄]₃[Ni₁₂(CO)₂₁H] in acetone are efficient catalysts for the polymerisation of acetylene.     

A carbon-13 NMR study of phenyl-substituted cyclophosphazenes

The ¹³C NMR spectra of the cyclotriphosphazenes N₃P₃CI_6?n Ph_n (n=2, 3, 4, 6) and the tetrameric derivatives 2,2,6,6-N₄P₄CI₄Ph₄ and N₄P₄Ph₈ have been recorded. The carbon chemical shifts can be largely explained on the basis of concomitant mesomeric electron release from the benzene ring and inductive electron withdrawal by phosphorus. The magnitude of ¹J(P, C-1) for the non-geminal compounds is ?40 Hz higher than that observed for the geminal compounds. Some aspects of the NMR data are also discussed with reference to the X-ray crystal structures of several phenyl-substituted cyclotri- and cyclotetra-phosphazenes.     

A carbon-13 NMR study of benzonorbornene isomers

The carbon-13 NMR spectra of a series of exo-and endo-epimers of 2-substituted benzonorbornene (2-substituent = OH, OCHO, Br, NH₂, NHMe or NMe₂) have been examined. The spectra are readily assigned by comparison with the coupled and off-resonance proton decoupled spectra as well as by the use of shift reagents. Interesting and diagnostic features of the spectra allow the distinction between epimers.     

The 13C NMR spectrum of coumarandione

A re-examination of the ¹³C NMR spectra of coumarandione and isatin with the aid of proton-coupled ¹³C NMR spectra and selective proton decoupling experiments is reported.     

The carbon-13 and proton NMR spectra of nicotine in aqueous media

The carbon-13 and proton nmr spectra of nicotine in aqueous solution have been ivnestigated. The changes resulting from alteration of pD level are detailed. In the case of the carbon-13 spectrum, these changes are essentially independent of the acid present. Changes in previously published assignments for carbons (2) and (6) and protons (2) and (6) are presented.     

Proton to carbon-13 INEPT in solid-state NMR spectroscopy

A refocused INEPT through-bond coherence transfer technique is demonstrated for NMR of rigid organic solids and is shown to provide a valuable building block for the development of NMR correlation experiments in biological solids. The use of efficient proton homonuclear dipolar decoupling in combination with a direct spectral optimization procedure provides minimization of the transverse dephasing of coherences and leads to very efficient through-bond (1)H-(13)C INEPT transfer for crystalline organic compounds. Application of this technique to 2D heteronuclear correlation spectroscopy leads to up to a factor of 3 increase in sensitivity for a carbon-13 enriched sample in comparison to standard through-bond experiments and provides excellent selectivity for one-bond transfer. The method is demonstrated on a microcrystalline sample of the protein Crh (2 x 10.4 kDa).     

1H and 13C NMR of platinum porphyrins—III. Platinum deuteroporphyrin IX dimethyl ester

¹H and ¹³C NMR spectra of the title compound are reported and peaks assigned by comparison with the free-base porphyrin. Long-range spin—spin coupling is observed between platinum and the macrocyclic protons and carbon atoms. Meso-protons are assigned from line-broadening observed for three of the peaks, possibly caused by residual coupling of these protons to the pyrrole β-protons. Allylic coupling of the pyrrole β-protons to adjacent methyl groups is also noted, and supports the tautomeric model of an 18-atom delocalisation pathway around the macrocycle. The ¹H NMR spectrum of the title compound is more concentration-dependent than that of the free-base porphyrin and probably reflects additional macrocyclic interactions via the platinum atoms.     

Synthesis and carbon-13 NMR analysis of some organoselenium compounds

Carbon-13 chemical shift assignments are reported for a series of 10 organoselenium compounds of the type PhSeL.In the course of this study, hydroalumination and bromination of alkynylselenium compounds was performed for the first time, and compared with the rate of hydroalumination and bromination of the corresponding alkynylsilane.The phenyl carbon shifts reflect ground state electron density changes. The selenium atom transmits electronic effects when the selenium atom acts as a link or bridge between two conjugative groups. For alkynylphenyl selenide, a polarization of charge within the triple bond occurs.     

A carbon-13 NMR spectroscopic study of ferrocenylmethyl carbonium ions

The ¹³C NMR spectra of three ferrocenylmethyl carbonium ions and of the alcohols from which they were derived have been obtained. The chemical shifts and coupling constants of the carbonium ions are explicable in terms of a structure that has the positive charge extensively delocalized throughout the entire molecule, with a fulvenelike structure for the substituted cyclopentadienyl ring.     

Observation of steric hindrance in heteraferrocenophanes by carbon-13 NMR

The ¹³C NMR spectra of stereoisomeric trans- and cis-1,3-dimethyl-2-oxa[3](1, 1′)ferrocenophanes, 1,2,3-trithia[3](1, 1′)ferrocenophane and 1,3-dithia[3](1, 1′)ferrocenophane have been recorded and analysed. The presence of steric hindrance in some of these compounds was confirmed and the conformations of the molecules were determined.     

A carbon-13 NMR study of some sulfur-containing organic compounds

Carbon-13 NMR spectra have been obtained and analyzed for a series of saturated linear and cyclic sulfur-containing organic molecules. For the large cyclic polythiaethers whose proton spectra are broad and unresolved, it is found that carbon-13 spectra are sharp and well resolved. The resonances of all carbons fall within the expected chemical shift ranges. Based upon the comparative resolution in the spectra of large and small cyclic species, it is concluded that whatever conformational motion may exist within the rings must be occurring very rapidly on the NMR time scale.     

The NMR spectrum and conformation of 2,3-difluoropropionic acid ethyl ester

The ¹H and ¹⁹F NMR spectra of the CH₂FCHF group in FCH₂CHFCO₂C₂H₅ were analysed on the basis of an ABCXY spin system. The non-equivalence of the methylene protons is discussed in terms of rotational isomerism. It is concluded that the dominant rotamer is probably considerably distorted from a perfectly staggered orientation. The variation of ³J_HF with substituent electronegativity is reviewed.