A two-dimensional nuclear overhauser experiment with pure absorption phase in four quadrants☆

The NMR signals of isotopically or chemically dilute nuclear spins S in solids can be enhanced by repeatedly transferring polarization from a more abundant species I of high abundance (usually protons) to which they are coupled. The gain in power sensitivity as compared with conventional observation of the rare spins approaches NII(I+1)γI2/NSS(S+1)γS2, or ∼ 103 for S = 13C, I = 1H in organic solids. The transfer of polarization is accomplished by any of a number of double resonance methods. High‐frequency resolution of the S ‐spin signal is obtained by decoupling of the abundant spins. The experimental requirements of the technique are discussed and a brief comparison of its sensitivity with other procedures is made. Representative applications and experimental results are mentioned.

Proton‐enhanced NMR of dilute spins in solids

A two-dimensional nuclear overhauser enhancement (2d noe) experiment for the elucidation of complete proton-proton cross-relaxation networks in biological macromolecules

https://spin.niddk.nih.gov/bax/lit/508/52.pdf

Practical aspects of two-dimensional transverse NOE spectroscopy

Two-dimensional N.M.R. spectroscopy is applied to the elucidation of cross relaxation pathways in liquids. The theory underlying two dimensional studies of cross relaxation and of transient nuclear Overhauser effects is developed. The influence of the correlation time of the molecular random process is investigated. It is found that in the limit of short correlation times (extreme narrowing limit) weak negative cross-peaks are observed. However, for long correlation times (spin diffusion limit) strong positive cross-peaks can be obtained. The technique appears particularly promising for the study of cross relaxation in macromolecules. Examples of intra- and intermolecular cross relaxation in the extreme narrowing limit are presented.

Elucidation of cross relaxation in liquids by two-dimensional N.M.R. spectroscopy

A new general technique for the investigation of exchange processes in molecular systems is proposed and demonstrated. Applications comprise the study of chemical exchange, of magnetization transfer by inter‐ and intramolecular relaxation in liquids, and of spin diffusion and cross‐relaxation processes in solids.

Investigation of exchange processes by two‐dimensional NMR spectroscopy

A discussion is given of the effects of a pair of pulses of rf magnetic field (at the NMR frequency) on the Zeeman and dipolar energies of the spin system in a solid. Zeeman order is partly transformed into dipolar order when the two pulses have different rf phases. A theoretical calculation provides a very simple relation between the efficiency of this transfer of energy (as a function of the characteristics of the pulses and their time separation) and the magnitude and shape of the dipolar component of the free-precession signal. This prediction has been verified quantitatively in the case of the ${\mathrm{F}}^{19}$ spins in a Ca${\mathrm{F}}_{2}$ crystal with the large magnetic field in a [100] direction. A maximum efficiency of transfer of Zeeman energy into dipolar energy of 56% has been obtained with a $\frac{\ensuremath{\pi}}{2}$ phase shift between a first $\frac{\ensuremath{\pi}}{2}$ pulse and a second $\frac{\ensuremath{\pi}}{4}$ pulse, separated by a time of the order of ${T}_{2}$. (An ideal adiabatic demagnetization in the rotating frame would have an efficiency of 100%.) The experimental results also indicate that the ${\mathrm{F}}^{19}$ spin system reaches internal quasiequilibrium in a time of the order of a few ${T}_{2}$ after the pulse pair. It is also shown that the use of a pair of rf pulses (both of the same rf phase this time) leads to a new method for measuring the complete shape of free-precession signals, which avoids the usual difficulties due to the finite recovery time of the observation circuit.

Nuclear Magnetic Resonance in Solids: Thermodynamic Effects of a Pair of rf Pulses

Investigation of exchange processes by two-dimensional nmr spectroscopy

Superoperators in Magnetic Resonance

The standard theory of NMR relaxation in liquids (with molecular motion described as a classical Brownian motion, and including intermolecular spin–spin couplings) is re‐examined, taking great care not to drop significant contributions from the dipolar coupling between distant molecules. This results in ‘‘modified Bloch–Redfield equations’’ for the spins in a single molecule, valid at all spin temperatures, which contain both the usual relaxation terms and a coupling of each spin with a classical average dipolar field. Delicate issues raised in this derivation, like the neglect of quantum correlations between spins on different molecules at (repeated) initial times, are discussed with the help of exact calculations (for all spin temperatures) performed on a simplified model which includes equal couplings between all N spins of a system. The same model is used to compare the merits of different forms of ‘‘high temperature’’ approximation. We also propose an iterative scheme for solving the ‘‘modified Bloch...

Jean Jeener

Papers

Investigation of exchange processes by two‐dimensional NMR spectroscopy

Nuclear Magnetic Resonance in Solids: Thermodynamic Effects of a Pair of rf Pulses

Investigation of exchange processes by two-dimensional nmr spectroscopy

Superoperators in Magnetic Resonance

Unified derivation of the dipolar field and relaxation terms in the Bloch-Redfield equations of liquid NMR