NMR studies of viomycin

The ¹H and ¹³C NMR spectra of antibiotic Viomycin sulphate and its constituent aminoacids in H₂O and D₂O were examined in a wide range of pH values. The spectra were analysed by proton spin-spin decoupling experiments and measurements of the relative intensities of the resonance and chemical shift values. Exchange experiments were also carried out and an almost complete assignment of the resonances of the spectra was possible. The data obtained are discussed in terms of possible structures for Viomycin and the presence of intramolecular hydrogen bonds. Some information about the conformation of the molecule is also given. A tentative assignment of the ¹³C spectrum of Viomycin is given and the experimental ¹³C chemical shift values compare well with the theoretical ones evaluated by Grant's rule.     

A nuclear magnetic resonance study of three isomeric dinaphthothiophenes

The ¹H and ¹³C NMR spectra of three isomeric dinaphthothiophenes are recorded. Chemical shift assignments were made on the basis of substituent chemical shift (SCS) effect arguments, coupling considerations, proton—proton auto-correlated homonulcear (COSY) two-dimensional spectroscopy, and proton—carbon heteronuclear correlation (HETCOR) two-dimensional spectroscopy. Some previously published ¹H chemical shifts and coupling constants are compared with our data and some divergences are noted.     

The study of longifolene by two-dimensional NMR spectroscopy

The complete assignment of the ¹³C NMR spectrum of longifolene was achieved from double quantum coherence measurements, while combined evaluation of a ¹H? ¹³C heteronuclear chemical shift correlation diagram and a homonuclear ¹H J-resolved diagram provided all proton chemical shifts. Conformational information on the seven-membered ring of the tricyclic sesquiterpene was obtained from proton chemical shift considerations.     

Carbon-13 nuclear magnetic resonance studies on high spin iron(III) porphyrins

Carbon-13 NMR studies on a series of high spin iron(III) porphyrins, namely tetraphenylporphyrin iron(III) halides [Fe(TPP) X, X=Cl, Br, I] in CDCl₃ solution are reported. As expected the¹³C shifts are found to be an order of magnitude larger than the corresponding proton shifts. The dipolar contribution, which is quite important for the proton NMR, becomes much less significant for the¹³C shifts. No systematic variation in the¹³C shift across the series is observed, except for the meso-carbon which shows a small but gradual decrease in going from the chloro to the iodo complex. The¹³C shift for the various carbon atoms of the porphyrin ligand shows interesting pattern which is discussed in terms of spin delocalisation mechanisms.     

Identification of N-methylpyrazole analog isomers by 13C NMR

Using several N-methyl pyrazole analogs the utility of ¹³C nmr in determining isomeric structures was examined. The chemical shift assignments of the pyrazole carbons and thus the isomeric structures were determined using a combination of proton coupled and proton decoupled ¹³C nmr spectra.     

Hetero-“Tickling” am A3X-System von 13CH312CN,eine Bestimmungsmethode hoher Genauigkeit von Methylkohlenstoffverschiebungen

Heteronuclear spin tickling experiments in the degenerate A₃X spin system of a ¹³CH₃ group have proved to be suitable for accurate carbon shift determinations in methyl groups. A weak perturbation of one of the ¹³C quadruplet lines splits both proton doublet lines into a sub-triplet with the relative signal strengths depending on whether an outer or inner ¹³C line is irradiated. Deviation from exact resonance with one of the ¹³C transitions modifies the relative intensities, and finally further splitting of the outer subtriplet lines occurs in accordance with theory. The amount of this additional splitting is equal to the difference between the double resonance frequency and the ¹³C transition frequency to be measured. Signal distortions observed when the double resonance spectra are recorded in the INDOR sweep mode have to be ascribed to the fact that the tickling effect competes with the generation of transient nutations.     

Multinuclear two-dimensional NMR: Assignments of natural abundance polypeptide 13C, 1H and 15N chemical shifts and demonstration of isomer interconversion

The polypeptide carbobenzoxy-glycyl-L -prolyl-L -leucyl-L -alanyl-L -proline (0.2 M in DMSO-d₆) was investigated using ¹³C, ¹H and ¹⁵N NMR in natural abundance at 4.7 tesla. The existence of cis–trans-Gly-Pro and -Ala-Pro bonds permits up to four isomers, and all four were observed (in a 60:30:7:3 ratio). ¹³C shifts of the proline β-CH₂ resonances are consistent only with the 60% form being trans–trans. The 30% form is either trans–cis or cis–trans (order as above) and was tentatively assigned as cis-trans on the basis of relaxation behavior. Refocused INEPT studies aided the ¹³C assignments. The ¹⁵N data were obtained using both NOE and INEPT excitation, with signals evident for the three major isomers. The spectra were analysed by starting from the ¹³C data, which were assigned based on known regularities in peptide spectra. A ¹³C? ¹H heteronuclear two-dimensional chemical shift correlation experiment allowed direct assignment of proton shifts for major and minor isomers. The NH proton shifts were assigned by running a homonuclear two-dimensional chemical shift correlation experiment and noting the correlation with the previously assigned α-CH protons. The ¹⁵N resonances were then assigned from a ¹⁵N? ¹H heteronuclear two-dimensional chemical shift correlation experiment, relating the ¹⁵N signals directly to the NH proton resonances. Isomer interconversion between the two major isomers was demonstrated by performing a magnetization transfer homonuclear 2D experiment. Off-diagonal intensity was noted relating the major and minor isomer alanine NH proton, as well as for the major and minor isomer leucine NH protons.     

Carbon-13 nuclear magnetic resonance studies on dichloropyridoquinolines

Carbon-13 NMR chemical shifts are reported for six angular and one linear dichloropyridoquinolines in CDCl₃. The chemical shift assignments have been made using model compounds, fully coupled spectra, selective proton decoupling and results from lanthanide shift studies. Chlorine substituent effects are compared to those reported for chloroquinolines. The effect of the heteroaromatic nitrogen atoms on ¹³C? ¹H coupling constants in these polycyclic systems are compared to those reported for pyridine systems.     

Nuclear magnetic resonance of phosphorus compounds: 9—The NMR spectra of phosphirane

The 100 MHz proton and 40.4 MHz ³¹P NMR spectra of phosphirane (1) have been recorded at ?20 °C and analysed iteratively to yield coupling constants and chemical shifts. The 22.6 MHz ¹³C spectrum of 1 was recorded at 0 °C and analyzed. The ³¹P chemical shift of 1 was measured as 40 467 515.97 ± 0.08 Hz relative to TMS as 100 000 000.00 Hz. The geminal P-C-H couplings in 1 are opposite in sign and of different magnitudes (+16.14 and ?2.64 Hz); the P? H coupling (+158.34 Hz) is smaller than that in any other organic phosphine. These observations are discussed and correlated with the geometry of 1. The electronic structure of the strained ring of 1 is discussed in terms of a localized valence bond approach.     

Carbon-13 chemical shifts of bicyclic compounds

¹³C NMR absorption spectra of 50 bicyclic hydrocarbons, alcohols and ketones have been measured, in addition to some terpenes. The ¹³C chemical shifts are approximately additive for similar compounds and can be used for the determination of molecular structure; they differ for endo- and exo-isomers, just as in proton spectra. These quite regular and predictable ¹³C shift differences are much larger and are caused by the 1,4-nonbonded interaction between atoms heavier than hydrogen, not by magnetic anisotropy effects.     

Conformational study of methyl esters of some aliphatic erythro- and threo-dichlorocarboxylic acids

The average conformations of methyl esters of some aliphatic erythro- and threo-dichlorocarboxylic acids in dilute carbon tetrachloride solutions have been determined from the vicinal proton–proton coupling constants and ¹H NMR shifts. The ¹³C shift differences between the erythro and threo forms are compared and discussed with regard to the differences in the average conformations.     

Long range optimized two-dimensional proton-carbon chemical shift correlation. Application in the total assignment of the 1H- and 13C-NMR spectra of 9-methylphenanthro[4,3-a]dibenzothiophene

The utilization of the long range optimized proton-carbon chemical shift correlation experiment in the total assignment of the proton and carbon nmr spectra of 9-methylphenanthro[4,3-a]dibenzothiophene is described. The experiment was employed in concert with conventional proton-carbon chemical shift correlation, heteronuclear relayed coherence transfer and broad band homonuclearly decoupled proton-carbon chemical shift correlation experiments. The experiments in combination offer a useful alternative to the ¹³C-¹³C double quantum coherence experiments, providing significantly better sensitivity and nearly the same capabilities.     

Cyclometallated compounds II. Proton and carbon-13 nuclear magnetic resonance spectral assignments of cyclopalladated compounds

The ¹H and ¹³C NMR spectra of nineteen cyclopalladated acetylacetonate complexes are reported. Definitve spectral assignments are made on the basis of selective proton decoupling experiments, difference NOE spectra and both homo- and hetero-nuclear two-dimensional correlation spectroscopy. The Pd(acac) substituent is shown to induce characteristic chemical shift changes in both proton and carbon spectra. These effects, however, vary from those of differently substituted palladium and other metal substituents.     

The diphenyliodonium-2-carboxylates: An NMR structure investigation

Diphenyliodonium-2-carboxylate, phenyl 4-methylphenyliodonium-2-carboxylate and phenyl 5-methyl-phenyliodonium-2-carboxylate have been examined by ¹H and ¹³C NMR spectroscopy. Each compound showed one proton considerably upfield from the others. From the two substituted compounds this was deduced to be H-6 on the ring containing the carboxylate group. Shift reagents and relaxation measurements were used to make the ¹³C chemical shift assignments. These data are most consistent with a cyclic, neutral structure for these iodonium carboxylates. The iodine is in the center of a trigonal bipyramid with the unsubstituted phenyl and carboxylate group apical, which places the C-6 proton in the shielding region of the adjacent phenyl group.     

A comparative study between para‐aminophenyl and ortho‐aminophenyl benzothiazoles using NMR and DFT calculations

Ortho‐substituted and para‐substituted aminophenyl benzothiazoles were synthesised and characterised using NMR spectroscopy. A comparison of the proton chemical shift values reveals significant differences in the observed chemical shift values for the NH protons indicating the presence of a hydrogen bond in all ortho‐substituted compounds as compared to the para compounds. The presence of intramolecular hydrogen bond in the ortho amino substituted aminophenyl benzothiazole forces the molecule to be planar which may be an additional advantage in developing these compounds as Alzheimer's imaging agent because the binding to amyloid fibrils prefers planar compounds. The splitting pattern of the methylene proton next to the amino group also showed significant coupling to the amino proton consistent with the notion of the existence of slow exchange and hydrogen bond in the ortho‐substituted compounds. This is further verified by density functional theory calculations which yielded a near planar low energy conformer for all the o‐aminophenyl benzothiazoles and displayed a hydrogen bond from the amine proton to the nitrogen of the thiazole ring. A detailed analysis of the ¹H, ¹³C and ¹⁵N NMR chemical shifts and density functional theory calculated structures of the compounds are described. Copyright © 2014 John Wiley & Sons, Ltd.     

Melamine–formaldehyde resins: Constitutional characterization by fourier transform 13C-NMR spectroscopy

The random chemical structures of melamine–formaldehyde resins, including methylated melamine–formaldehyde resins and urea–melamine formaldehyde resins, were investigated by ¹³C-NMR spectroscopy (Fourier transform). All the combined formaldehydes, methylol and methyl ether groups, methylene structures, and dimethylene ether structures were assigned. A ¹³C chemical shift of methylene carbon occurred by substitution of other constituents of the methylene group for a proton of the adjacent monosubstituted nitrogen atom, as shown in a ¹³C-NMR spectrum of urea–formaldehyde resins. It was found that the chemical shift of each corresponding carbon of both melamine resins and urea–melamine resins was almost superimposed with that of urea resins.     

Carbon-13 NMR chemical shifts in some substituted 1,2,4-triazol-3-ones

Carbon-13 NMR data for 15 substituted 1,2,4-triazol-3-ones are presented and discussed with regard to enolization in the neutral molecules. The coupling constant and chemical shift data show that the proton at N-2 is not exchanging rapidly in the DMSO-d₆ solvent. Using D₂O? OD^? as a solvent, it is found that the C-3 and C-5 resonances are shifted downfield by nearly the same amount, suggesting that the proton at N-4 is being removed. Enolization in the neutral molecules does not occur to any significant extent.     

13c-chemische verschiebungen und substituenteneffekte in methylazulenen

The ¹³C chemical shifts of all possible monomethyl azulenes and of some polymethyl derivatives have been determined by the Pulse Fourier Transform technique and have been assigned on the basis of ¹³C,H coupling constants and/or splitting patterns taken from proton coupled ¹³C NMR spectra. The shift differences observed in the monomethyl derivatives as compared to azulene itself are additive as far as sterically unperturbed polymethyl derivatives are concerned and can be viewed on as substituent constants. As is shown by the results of HMO- and EHT-calculations the substituent induced shifts in monomethyl azulenes without sterical hindrance can be correlated with charge density changes, with the exception of the β -effect, which is proportional to the bond length resp. the π bond order.     

NMR spectroscopy of organophosphorus compounds II: Stereochemistry of [1-(2H-azirin-2-yl)alkyl]phosphonates

Summary The conformation and relative configuration of [1-(2H-azirin-2-yl)alkyl]phosphonates (2) has been established by thorough investigation of some characteristic representatives of the series by¹H,¹³C,¹⁵N, and³¹P NMR spectroscopy. It is shown that the chemical shift of the proton located to the phosphonate group can be used as a criterion for the discrimination and stereochemical assignment of diastereoisomers. NMR spectroscopic features of the compounds are discussed in terms of structural relationships.Dedicated to Univ.-Prof. Dr.K. Schlögl with the best wishes to his 70^th birthday     

Synthesis and 1H-, 13C-NMR analysis of some substituted 1,2,3,4-tetrahydroisoquinolines

Synthesis and ¹H-, ¹³C-nmr analysis of some substituted 3-phenyl-1,2,3,4-tetrahydroisoquinolines are reported. Spectroscopy assignments of hydrogen and carbon resonances were made on the basis of standard chemical shift theory, comparison with reference compounds, attached proton test and fully coupled ¹³C-nmr spectra. Data showed that at room temperature two conformers predominated for the 1,3-disubstituted and 1,2,3-trisubstituted isoquinolines.