13C Dipolar Couplings Research Articles

Structure and Molecular Ordering of a Nematic Liquid Crystal Studied by Natural-Abundance Double-Quantum 13C NMR

We have studied the mesogenic molecule 4-n-pentyl-4‘-cyanobiphenyl (5CB) in the nematic phase by double-quantum 13C nuclear magnetic resonance (NMR). The experiment detects selectively the very rare isotopomers containing natural pairs of 13C spins. The 13C−13C dipolar couplings were interpreted in terms of the structure and orientational order of the cyanobiphenyl fragment. Different strategies were employed to analyze the internal motion of the core moiety. This is the first investigation of molecular conformation and order which exploits magnetic 13C−13C dipolar couplings in a non-labeled liquid crystal.

Study of phospholipid structure by 1H, 13C, and 31P dipolar couplings from two-dimensional NMR

Various motionally averaged 31P-1H, 13C-1H, 1H-1H, and 31P-13C dipolar couplings were measured for natural-abundance and unoriented phosphocholine in the L alpha phase. The couplings were obtained and assigned by a variety of advanced and partly novel two-dimensional solid-state NMR experiments. Whereas 31P-1H and 31P-13C dipolar couplings provide long-range structural constraints, geminal 1H-1H couplings and the signs of 13C-1H couplings are important new elements in a segmental order-tensor analysis of the lipid headgroup and glycerol backbone. The implications of these measured dipolar couplings for the conformational exchange of the lipid headgroup and the bending of the headgroup from the glycerol backbone are discussed. These dipolar couplings are also analyzed semiquantitatively in terms of the segmental order tensor.

Spin density description of rotational-echo double-resonance, transferred-echo double-resonance and two-dimensional transferred-echo double-resonance solid state nuclear magnetic resonance

The spin density matrix formalism has been applied to rotational-echo double-resonance (REDOR), transferred-echo double-resonance (TEDOR) and two-dimensional (2D) TEDOR experiments in order to obtain an expression for the signal intensities. TEDOR spectra of 15N-labeled glycine were measured with different dipolar evolution times. 2D-TEDOR spectra were measured of doubly labeled glycine-2- 13C, 15N and of 15N-labeled glycine. Both the TEDOR and the 2D-TEDOR spectra were readily obtained although the 2D-TEDOR experiment on 15N-labeled glycine used a lot of machine time. Even though the 15N-1- 13C dipolar coupling is relatively small (200 Hz), the 1-C resonance can still be observed.

Extraction of 1H1H and 1H13C dipolar couplings from spectra acquired in inhomogeneous magnetic fields

AbstractExperimental procedures are presented for the extraction of 1H1H and 1H13C dipolar couplings from molecules oriented in high magnetic fields without a requirement for high field homogeneity. These couplings impose restrictions on structure in a manner similar to NOE distances and can serve as useful additional constraints for defining structure. The experimental procedures involve pulse sequences which refocus the effects of magnetic field inhomogeneities but which retain dipolar couplings, scalar couplings and, in cases of chemically distinct groups, chemical shift differences. The methods are important for the extension of experiments to higher fields and more complex media where field homogeneity is hard to maintain. The experiments are illustrated using samples containing sodium acetate‐99% 13C2 or alanine‐99% 13C3 dissolved in a nematic liquid crystal and oriented in an inhomogeneous magnetic field. Coupling constants obtained from the studies are in good agreement with couplings extracted from one‐dimensional spectra obtained in homogeneous fields.

The interaction constants in 13C and 2H nuclear magnetic resonance relaxation studies

2H and 13C nuclear magnetic resonance relaxation data are presented for specifically deuterated and protonated sodium hexanoate, respectively. Analysis of the data yields the ratio between the 13C dipolar and 2H quadrupolar coupling constants. This ratio differs significantly from the value obtained from commonly used values for the constants. The 13C coupling constant is then evaluated theoretically and consequently the 2H quadrupole constant is obtained as well. Finally, some 2H and C NMR relaxation data for a sodium octanoate/water system are analyzed using the “traditional” values of the coupling constants and the values presented in this work.

Molecular and structural information from 14N–13C dipolar couplings manifested in high resolution 13C NMR spectra of solids

The dipolar coupling between 14N and 13C is not suppressed by magic angle sample spinning because the relatively large 14N quadrupole interaction shifts the axis of quantization of the 14N spins away from the direction of the applied field. The resulting 13C resonance line shapes are influenced by the sign, magnitude, and asymmetry parameter of the 14N quadrupole coupling tensor; the internuclear distance; the magnitude of the applied magnetic field; and the orientation of the internuclear vector in the principal axis system of the electric field gradient. It is demonstrated that one or more of these parameters can be determined from the 13C NMR data if the others are known by virtue of single crystal studies or molecular symmetry.

Homogeneous and inhomogeneous nuclear spin echoes in organic solids: Adamantane

Under conditions of proton dipolar decoupling, the 13C magnetization in adamantane has been further subjected to the following separate coherent averaging procedures: magic angle mechanical spinning, Carr-Purcell, Carr-Purcell-Meiboom-Gill, and multiple-pulse (solid echo) sequences. A comparison of the respective widths of the 13C NMR response demonstrates that in adamantane the 13C 13C intermolecular dipolar coupling is approximately 40 Hz; all but the Carr-Purcell sequence reduced this, albeit with varying efficiencies. It is suggested that in an organic solid with substantial molecular motion in the kilohertz range, the 13C 13C dipolar coupling can limit spectral resolution even under magic angle sample spinning.