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 carbon-13 NMR spectrum of dimethoxybilirubin dimethyl ester

The ¹³C NMR spectrum of dimethoxybilirubin dimethyl ester (1) is reported. Methoxyvinylneoxanthobilirubinic acid methyl ester, a compound containing one endo-vinylpyrromethene unit, was prepared and used in the assignment of the spectrum of 1.     

On the assignments of carbon-13 NMR spectra of cyclic nucleotides

Two conflicting assignments have been proposed for the ¹³C NMR signals of 3′,5′-cyclic nucleotides. This communication describes selective decoupling experiments for several cyclic nucleotides which provide the correct assignment based on unambiguous experimental evidence.     

On the assignment of the 13C-NMR spectrum of phenothiazine

The assignment of the ¹³C-nmr spectrum of phenothiazine was made by comparison with the ¹³C-nmr spectrum of 1,9-dideuteriophenothiazine, synthesized via repeated lithiations and subsequent deuterations of phenothiazine.     

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 assignment study of 3,5-diarylisoxazoles

The assignment of the carbon-13 nmr resonances of 3-(aryl)-5-phenylisoxazoles (I) and 3-phenyl-5-(aryl)-isoxazoles (II) in dimethylsulfoxide-d₆ has been made. The assignments were made by using model compounds, chemical shift arguments, coupling with fluorine substituents, and shifts resulting from quaternization of the isoxazole ring. A carbon-13 nmr method for distinguishing between I and II which can be produced simultaneously by certain synthetic methods is presented.     

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.     

Studies in tertiary amine oxides. III—carbon-13 NMR assignment of N-alkynyl cyclic amines

Carbon-13 NMR spectra of 24 N-alkynyl cyclic amines were analysed. The effect of substituting the acetylenic proton by n-butyl, tert-butyl and phenyl groups was determined.     

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.     

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).     

Temperature effects on the carbon-13 NMR shifts of selected compounds

The effects of temperature on the ¹³C NMR shifts of benzene, cyclohexane, pyridine, cyclohexene, neopentane, hexane, and heptane have been determined from +10 to +70°C. In addition, the temperature shift of ²D in D₂O was found to be +10.4 × 10^?3 ppm/°C. All shifts appear to be linear functions of temperature within experimental error.     

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.     

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.