13C NMR Lineshapes Research Articles

Dynamic properties of commensurate and incommensurate spin density waves as probed by 13C NMR

We have measured 13C NMR lineshape and nuclear spin lattice relaxation in the spin-density-wave (SDW) state of (TMTTF) 2Br and (TMTSF) 2PF 6. The incommensurability of the SDW in (TMTSF) 2PF 6 is confirmed, and we show that the phason mode is dominant in the relaxation. However, we found that (TMTTF) 2Br is commensurate and estimated that the SDW amplitude is small. No phason contribution to the relaxation is observed in this compound. We discuss some implications for theory.

(13C,2H) residual dipolar and indirect spin–spin coupling effects in high-resolution13C nuclear magnetic resonance spectra of solids

We report high-resolution solid-state 13C NMR spectra of urea and thiourea inclusion compounds containing deuteriated guest molecules. The spectra (recorded under conditions of magic-angle sample spinning and high-power 1H decoupling at several magnetic fields from 4.7 to 11.7 T) for CD, CD2 and CD3 groups are triplets, pentets and heptets, respectively. These multiplets arise from the effects of residual dipolar interaction and indirect spin–spin coupling between 13C and 2H. A full analysis of these lineshapes is presented, and the effect of molecular motion is discussed. It is shown that the 13C NMR lineshape for deuteriated materials which are in a rapid motion regime (with respect to the 2H NMR timescale) can be rationalized by considering the motionally averaged 2H quadrupole coupling constant 〈χ〉.

Urea exchange across the human erythrocyte membrane measured using 13C NMR lineshape analysis.

The 13C NMR spectrum of 13C-urea in a suspension of human red cells of reduced mean cell volume was observed to contain partially resolved resonances arising from the intra- and extracellular populations of the compound. It was shown that at 25 degrees C and a magnetic field strength of 9.4 T, the rate of exchange of urea between the intra- and extracellular populations was such that the NMR lineshape was sensitive to a change in the rate of 13C-urea exchange, induced either by the addition of the urea transport inhibitor phloretin, or by the addition of 12C-urea. Total lineshape analysis of 13C NMR spectra of 13C-urea in red cell suspensions containing different concentrations of 12C-urea resulted in a weighted mean estimate for the Km and Vmax for urea equilibrium exchange from three experiments of 44 +/- 18 mM and 3.1 +/- 0.6 x 10(-8) mol cm-2 s-1, respectively (the errors denote the weighted mean standard deviations). These estimates of Km and Vmax were significantly lower than previous values reported in the literature and determined using other techniques.

Isotope effects in the nmr spectra of nitromethane

The high-resolution 1H, 13C, 14N, 15N, and 17O NMR spectra of some isotopomeric nitromethanes have been obtained and all possible coupling constants have been measured. The one-bond 17O 15N coupling constant has been found for the first time. The program QUADR has been applied to analyze the 13C 14N multiplets in the 13C NMR lineshapes modified by the 14N quadrupole effects. 2 D 1 H isotope effects on some coupling constants and 2 D 1 H , 13 C 12 C , and 15N 14N isotope shifts for 1H, 13C, and 15N nuclei have been also established.

Diiron μ-vinylcarbyne complexes have unusually low barriers to vinyl rotation because conjugation is maintained throughout rotation

Rotational Barriers of diiron μ-vinylcarbyne complexes were measured by temperature dependent 1H and 13C NMR line shape analysis. The planar vinylcarbyne ligand in [C 5H 5(CO)Fe] 2(μ-CO)[μ-C- E-CHCHC 6H 4- p-N(CH 3) 2] +BF 4 − ( 5) was found to possess three measurable rotational barriers. Rotation of the entire vinylcarbyne ligand has the lowest barrier to rotation, Δ G ‡ 218 10.6(1) kcal mol −1. Rotation of the N(CH 3) 2 group has a barrier of Δ G ‡ 210 10.9(1) kcal mol −1, and rotation of the aryl group has a barrier of Δ G ‡ 275 13.0(1) kcal mol −1. Fenske-Hall molecular orbital calculations reveal that diiron vinylcarbyne complexes have low barriers to ligand rotation because the bridging carbyne carbon has orthogonal p orbitals which can accept electron density from the vinyl unit throughout rotation.

13C NMR and force field investigations of hydrindane conformations

Abstract13C NMR shifts of trans‐ and cis‐annelated bicyclo[4.3.0]nonanes with substituents R in position 8 (R  H, OH, Cl, Br) and 1‐hydroxy derivatives were analysed on the basis of force field calculated torsional angles using Allinger's MM1 program. Shielding increments for the 6 membered ring agree with corresponding cyclohexane values within ± 0.8 ppm maximal deviation. 13C NMR line shape analysis with cis‐hydrindane between 148 and 180 K yielded ΔH* = 37.0 ± 0.4 kJ mol−1 and ΔS* = 28 J mol−1 K−1 for the topomerization. The force field calculated reaction profile showed ΔH* = 37 kJ mol−1, in close agreement.

NMR spectra of systems with restricted motion: Cross-linked polymer gels

Swollen cross-linked polystyrene, and a parallel solution of linear polystyrene in CCl 4, were studied by 1H NMR, both conventional and with magic angle rotation (MAR-NMR), and by 13C NMR lineshape, T 1 and NOEF. The 1H MAR-NMR spectra of the cross-linked polymer were found to be almost identical to conventional 1H NMR spectra of the linear polymer in a broad temperature range. Also, the 13C T 1 and NOE parameters were equal for the cross-linked and linear polymer indicating little difference in the dynamics of rapid internal motions. The characteristic “super-Lorentzian” lineshape of conventional 1H NMR spectra of the cross-linked polymer gel is analyzed in terms of a convolution of a residual dipolar broadening function G( ν − ν 0) with a narrow lineshape function S( ν − ν 0)defined by rapid internal motions.