Carbonyl 13C Research Articles

A nitrogen-15 spin-lattice relaxation study of alumichrome

15N-NMR spectra of a 0.07 M solution of 99.2% 15N-enriched alumichrome in [ 2H 6]Me 2SO were recorded at 10.13 MHz. The signals of all the nine nitrogens in this molecule were well resolved. The chemical shifts of the six peptide backbone resonances show good agreement with values previously determined on the basis of indirect 1H-{ 15N} detection. The three metal-bound side chain hydroxamate nitrogens are partially inequivalent in the 15N spectrum, which is consistent with a distorted octahedral configuration of the ligands. This observation is complementary to the chemical shift pattern exhibited by the carbonyl 13C resonances of the hydroxamate groups, providing support for the hypothesis of electron density shifts (charge relay) between N and CO in amides. The 1H- 15N dipolar and scalar spin-coupling interactions could be distinguished and measured by recording undecoupled spectra and { 1H}- 15N decoupled spectra under various gated 1H-noise irradiation routines. The ornithyl and glycyl amide 15N resonances are clearly distinguishable from their different patterns of heteronuclear spin-coupling to one or two 1H α's respectively. The 15N longitudinal relaxation times, T 1 and the 15N-{ 1H} nuclear Overhauser enhancements were measured. The Overhauser data clearly showed that 1H- 15N dipole-dipole interactions are the dominant relaxation mechanisms for all the nitrogen nuclei, the relaxation times for the protonated amide nitrogens being approximately 20 times shorter than those for the non-protonated hydroxamate nitrogens. The rotational correlation time calculated from the 15N T 1 data under the assumption of isotropic rotational tumbling, τ r ≈ 3.8 · 10 −10 s, is in good agreement with previously reported values, τ r ≈ 4.1 · 10 −10 s, estimated from 13C spin-lattice relaxation experiments at 25.1 and 90.5 MHz. Relative to the triornithyl region of the peptide backbone, the triglycyl amides exhibit somewhat longer relaxation times. Upon replacement of the amide protons by deuterons, the peptidyl resonances in alumichrome were broadened and could be detected only after prolonged spectrum accumulation in the absence of 1H irradiation. 1H noise decoupling resulted in almost complete nulling of the deuterated amide nitrogens' resonances. This study clearly shows the present advantage of 15N-isotope enrichment for structural investigations of peptides and proteins by 15N-NMR spectroscopy.

Electron spin resonance of di-t-butyl-ketyl alkali metal ion pairs

Alkali metal radical ion pairs were prepared by reacting di-t-butyl ketone with the whole series of the alkali metals. Two different types of spectra were recorded, one showing hyperfine splitting by one alkali nucleus and one with splitting produced by two counter ions. The latter, though attributed in the literature to a biradical species, is shown to be due to ionic aggregates formed by a ketyl radical, two alkali cations and possibly a diamagnetic anion. Linewidth alternation effects in the alkali multiplets indicate that the alkali nuclei do not occupy equivalent positions and that they are subjected to an intramolecular motion which interchange the two positions. The carbonyl 13C hyperfine splitting constants are shown to be strongly dependent on the counter ion.

NMR Spectra of π-Cyclopentadienyl Manganese Tricarbonyl in Nematic and Isotropic Solvents

Abstract 1H-NMR spectra including 13C-H satellites in the natural abundance of 13C in π-cyclopentadienyl manganese tricarbonyl dissolved in a nematic solvent are reported. In addition, the 13C-NMR spectrum of the compound in the natural abundance of 13C, in an isotropic medium is studied. Values of the direct and the indirect HH and 13CH couplings are given. Information on the molecular geometry of the ring skeleton thus obtained is discussed. Contributions of the molecular vibrations to the observed dipolar couplings are estimated. The direct dipolar coupling between the carbonyl 13C and the ring protons are used to derive the distance between the plants containing these carbons and the cyclopentadienyl ring.

Solvent dependence of 13CH and HH coupling in acetone and dimethyl sulfoxide

AbstractThe coupling constants J13CH and JHH for acetone and dimethyl sulfoxide were measured in a variety of solvents. J13CH for acetone was found to vary by up to 1.5% and JHH by up to 30%. The magnitudes of these variations are in almost linear relationship with the carbonyl 13C and 17O chemical shifts in the same solvent. A reason for this is sought in polar structures caused by the formation of hydrogen bonds and favoured by the polarity of the solvent. J13CH for dimethyl sulfoxide also varies with the solvent but JHH remains constant.