Effect of convection and B1 inhomogeneity on singlet relaxation experiments

Nuclear spin singlet lifetimes can often exceed the T₁ length scales by a large factor. This property makes them suitable for polarization storage. The measurement of such long lifetimes itself can become challenging due to the influence of even very weak relaxation mechanisms. Here we show that a judicious choice of the singlet-to-triplet conversion method is highly important in order to achieve reliable singlet relaxation measurements. In particular, we identify thermal convection, in connection with B₁ field gradients, asa significant apparent decay mechanism, which limits the ability to measure the true singlet state lifetimes. Highly B₁-compensated broadband singlet excitation/detection sequences are shown to minimize the influence of macroscopic molecular motion and B₁ inhomogeneity.

Correction: Singlet lifetime measurements in an all-proton chemically equivalent spin system by hyperpolarization and weak spin lock transfers

Correction for ‘Singlet lifetime measurements in an all-proton chemically equivalent spin system by hyperpolarization and weak spin lock transfers’ by Y. Zhang et al., Phys. Chem. Chem. Phys., 2015, 17, 24370–24375.

Singlet lifetime measurements in an all-proton chemically equivalent spin system by hyperpolarization and weak spin lock transfers

Hyperpolarized singlet states provide the opportunity for polarization storage over periods significantly longer than T1. Here, we show how the singlet state in a chemically equivalent proton spin system can be revealed by a weak power spin-lock. This procedure allowed the measurement of the lifetimes of the singlet state in protic solvents. The contributions of different intra- and intermolecular relaxation mechanisms to singlet lifetimes are investigated with this procedure.

High-resolution NMR of quadrupolar nuclei using mixed multiple-quantum coherences

A multiple-quantum magic angle spinning (MQMAS) NMR experiment of quadrupolar nuclei is demonstrated, which uses two different multiple quantum coherences in t(1) to refocus the quadrupolar broadening. This experiment has the potential of achieving improved resolution over current techniques.

Thermal convection currents in NMR: flow profiles and implications for coherence pathway selection

Rayleigh-Benard convection currents are visualized in a vertical cylindrical tube by means of magnetic resonance imaging. Axially antisymmetric flow, multiple vertical rolls, and twisted node planes are observed. The flow can also be induced by strong RF irradiation. Its effects on the coherence pathways in NMR experiments employing field gradients are discussed. Copyright 2000 Academic Press.

Unwanted signal leakage in excitation sculpting with single axis gradients

Excitation sculpting (T-L. Hwang and A. J. Shaka, J. Magn. Reson. A 112, 275-279 (1995)) used for solvent suppression and selective excitation in NMR bases its success on the ability to remove baseline and phase errors created by the application of selective rf pulses. This is achieved by the application of two pulsed field gradient (PFG) echoes in sequence. It is essential that the two pairs of PFGs select the coherence transfer steps independently of each other, which is conveniently achieved if they are applied along orthogonal spatial axes. Here, the much more common case where both PFG pairs must be applied along a single axis is investigated. This is shown to lead to complications for certain ratios of PFG strengths. The original theory of excitation sculpting is restated in the spherical basis for convenience. Some of the effects can only be explained by invoking the dipolar demagnetizing field.

Mapping the B1 field distribution with nonideal gradients in a high-resolution NMR spectrometer

To understand the behavior of many NMR experiments, it is important to determine the spatial distribution of the B1 field. In this paper, we show how this distribution can be mapped independently of spin density, coil responsiveness, and nonlinearities of the B0 field gradients. As a by-product we obtain a map of the (possibly nonlinear) spatial variation of the B0 field gradients used in the imaging procedure.

Efficient simulation of coherence transfer pathway selection by phase cycling and pulsed field gradients in NMR

The selection of well-defined coherence transfer pathways is an essential feature of all but the simplest NMR and EPR pulse sequences. This selection can be achieved by phase cycling and by pulsed field gradients. The properties of the RF-pulses (flip angle, offset effects, inhomogeneity) and transport phenomena (diffusion, flow) in conjunction with gradients cause a weighting of the different coherence transfer pathways. We present a method by which the selection process can be simulated efficiently and visualized easily. In its basic form it involves straightforward matrix manipulations without reference to the density matrix and the particular spin system. This method is implemented in a MATLAB program, called CCCP (Complete Calculation of Coherence Pathways). Copyright 1998 Academic Press.