Nuclear Overhauser Effects and 13C Relaxation Times in 13C–{H} Double Resonance Spectra

Abstract

Articles you may be interested inHydration in polymer studied through magic angle spinning nuclear magnetic resonance and heteronuclear 13C{1H} Overhauser enhancement spectroscopy: Crossrelaxation and location of water in poly(acrylamide)The effect of proton decoupling upon carbon-13 magnetic resonance spectra is treated in detail with the Solomon formulation for multiply irradiated spin systems. The factors affecting the nuclear Overhauser enhancement are discussed in terms of competition between the dipole-dipole relaxation mechanism essential to the Overhauser effect and other relaxation processes. Using a density matrix formulation, it is exhibited that the maximum Overhauser effect (achieved when the dipolar mechanism dominates) is independent of the number of hydrogen atoms interacting with a relaxing carbon-13 nucleus. On the other hand, if the molecular tumbling motion is isotropic and the dipole-dipole mechanism dominates the relaxation process, then to a first approximation Tl can be shown to depend inversely upon the number of directly bonded hydrogen atoms. Expressions for treating an AMX three-spin system are given also, and the effect of the third spin is discussed. The highly symmetric adamantane molecule provides an excellent test case for the validity of the theoretical conclusions as it is an isotropic tumbler because of the rigidity and its tetrahedral symmetry. Furthermore, it has two nonequivalent carbon positions having different numbers of hydrogen atoms. The experimental Overhauser enhancement was found to be the same for both carbons and the T1's for these two carbons, measured with the adiabatic rapid passage technique, reflected the two-to-one factor expected for CH and CH. groups, respectively. Minor deviations are explained with small contributions from vicinal hydrogens. Having exhibited the validity of the theoretical treatment for adarnantane, it becomes evident that exceptions may be interpreted in terms of deviations from isotropic tumbling motion. As most molecules lack the symmetry to be isotropic in rotational diffusional processes, this experimental technique offers an important way for studying anisotropic molecular motion in liquids.

Conformational analysis of methyl β-cellobioside by ROESY NMR spectroscopy and MD simulations in combination with the CROSREL method

“…In addition, a fully relaxed 13C NMR spectrum was recorded with a repetition time of 2.82 s in order to determine the initial magnetization. The interpretation of the TI values in terms of rotational mobility was according to ref 11. Briefly, for carbohydrates in solution, the 13C relaxation is governed by dipolar coupling between covalently bonded 'H and 13C spins.…”

Nuclear Magnetic Relaxation and the Biological Membrane

“…For l3C, at least as far as Tl is concerned, relaxation of nuclei with directly bonded protons is thought to be predominantly intramolecular (Kuhlmann et al, 1970). For protons, however, both intramolecular and intermolecular contributions are expected to be important, and the observed relaxation times will be given by the sum of these contributions, thus…”

“…The dominant mechanism for Tl relaxation of l3C nuclei with directly bonded protons is generally the dipolar interaction between the carbon nuclei and these protons (Kuhlmann et al, 1970). For the nonprotonated carbonyl carbon in dipalmitoyllecithin, this efficient relaxation mechanism is not available, and a long Tl value is observed.…”

Dynamical dimer structure and liquid structure of fatty acids in their binary liquid mixture: dodecanoic and 3-phenylpropionic acids system

“…Reciprocal effective correlation time, 1/τ c , for the rotational movement of the carbon atoms in the LA and PPA molecules in their liquid mixture was calculated from the 13 C NMR spin-lattice relaxation time T 1 by assuming that T 1 of a protonated carbon is overwhelmingly dominated by dipole-dipole interaction with the attached protons (Kuhlmann et al, 1970;Farrar and Becker, 1971). Owing to the restriction of rotational and segmental mobilities (Hamilton et al, 1974), however, 1/τ c , is a semi-quantitative measure of the segmental movement of the molecule and expressed as: where h is Planck's constant and γ C and γ H are the gyromagnetic ratio of 13 C and 1 H, respectively.…”