13C nuclear magnetic relaxation studies at 62 MHz

Abstract

An apparatus suitable to measure 13C relaxation times TI and Ta in liquids at 62 MHz is described. The required field of 58 kG is generated by a superconducting magnet. Results of TX measurements on a variety of liquids, including benzene, derivatives of benzene, saturated rings, CSn and others are reported. They are discussed in terms of dipole-dipole interactions, spin-rotation interactions and anisotropic chemical shifts, the latter of which turn out to play only a minor role. Intramolecular dipole-dipole interactions are found to provide by far the most important spin-lattice relaxation mechanism, whenever protons are bound directly to the carbons under investigation, even when the samples contain dissolved oxygen.

Nitrogen-15 NMR of pyridine in high magnetic: Fields

“…It has been shown (7,16) that the 13C relaxation rates for ring carbons are solely determined in very good approximation by intramolecular DDI between the carbon and the directly attached proton. This holds even at a frequency of 61 MHz, where our measurements were performed.…”

“…The sample was degassed by several freezepump-thaw cycles. The NMR probes operating at 14, 30, and 32 MHz as well as the measurement procedure for the liquid phase have been described before (7,16,17), At 32 MHz, the corresponding magnetic field for 15N is about 74.2 kG. Measurements in the liquid phase were made down to -60°C showing that the sample can be supercooled far below the melting point (-42°C).…”

“…For '£)u, we then get the expression [16] Now the correlation times can be eliminated by multiplication of the individual relaxation rates:…”

Molecular motion in liquid toluene from a study of 13C and 2D relaxation times

“…The intramolecular dipole-dipole interaction of 13C with the covalently bound deuteron accounts for mOre than 90 /~ of the total dipolar relaxation rate. This follows simply from intra-and intermolecular atomic distances (2). This means that the relevant directions for the couplings that relax DC and 2D, respectively, are identieal.…”

“…For protons, however, both intra-and intermolecular dipole--dipole interactions major contributions to the relaxation rate. For 13e, on the other hand, the intermolecular dipole-dipole interaction can be safely neglected for carbons directly bound to a hydrogen atom (2). Another relaxation mechanism which has been demonstrated molecular reorientation.…”