Excitation Of Multiple-quantum Coherences Research Articles

Application of amplitude-modulated radiofrequency fields to the magic-angle spinning NMR of spin- [formula omitted] nuclei

We report pulse sequences for the sensitivity enhancement of magic-angle spinning and multiple-quantum magic-angle spinning spectra of spin- 7 2 systems. Sensitivity enhancement is obtained with the use of fast amplitude-modulated (FAM) radiofrequency pulses. In one-dimensional magic-angle spinning experiments, signal enhancement of 3 is obtained by a FAM pulse followed by a soft 90° pulse. In two-dimensional multiple-quantum magic-angle spinning experiments, FAM pulses are used for both the excitation of multiple-quantum coherences and for their conversion into observable single-quantum coherences. The observed signal enhancements are 2.2 in 3Q experiments, 3.1 in 5Q experiments, and 4.1 in 7Q experiments, compared to the conventional two-pulse scheme. The pulse schemes are demonstrated on the 45Sc NMR of Sc 2(SO 4) 3 · 5H 2O and the 139La NMR of LaAlO 3. We also demonstrate the generation of FAM pulses by double-frequency irradiation.

Selective Correlation of Amide Groups to Glycine Alpha Protons and of Arginine Guanidine Groups to Delta Protons in Proteins by Multiple Quantum Spectroscopy

A new 3D pulse sequence correlates backbone amide proton and nitrogen with alpha proton resonances selectively for glycine residues in a fully doubly labeled (15N,13C) protein. The excitation of multiple quantum coherences provides optimized resolution and sensitivity. Degenerate alpha proton groups can be promptly recognized. Correlation of guanidine NH groups to delta protons of arginine side chains is also obtained.

Excitation of multiple quantum coherences by a single nonselective radiofrequency pulse due to the breakdown of the high-field approximation

Multiple quantum coherences are created by a single nonselective pulse from the equilibrium density matrix when the high-field approximation breaks down. In this situation the equilibrium density matrix also contains bilinear spin operators. In the case of a two spin system having scalar as well as dipolar coupling, both double and zero quantum coherences are excited whereas in the presence of scalar coupling alone, only the zero quantum coherence is created, with no double quantum coherence. Flip angle dependence of the multiple quantum coherences is discussed.

Cascades of Selective Pulses on Connected Single-Quantum Transitions Leading to the Selective Excitation of Multiple-Quantum Coherences

A symmetric cascade of selective pulses applied on connected transitions leads to the excitation of a selected multiple-quantum coherence by a well-defined angle. This cascade selectively operates on the subspace of the multiple-quantum coherence and acts as a generator of rotation selectively on the multiple-quantum subspace. Single-transition operator algebra has been used to explain these experiments. Experiments have been performed on two- and three-spin systems. It is shown that such experiments can be utilized to measure the relaxation times of selected multiple-quantum coherences or of a specifically prepared initial longitudinal state of the spin system.

Experimental Aspects of Chirp NMR Spectroscopy

Chirp pulses have been used to excite transverse magnetization over bandwidths that are about 50 times wider than the amplitude of the RF field. Experimental profiles are shown of the amplitudes and phases as a function of offset. The efficiency of refocusing by chirp pulses over large bandwidths is proven by experiment. It is shown that RF inhomogeneity leads to an attenuation of the echo intensity but does not perturb its phase. The excitation of multiple-quantum coherences and their reconversion into single-quantum magnetization by chirp pulses is demonstrated experimentally for pairs of carbon-13 nuclei with large chemical-shift differences. The performance of chirp double-quantum excitation methods is compared with that of techniques using monochromatic composite pulses. Various improvements are discussed, such as the smoothing of the RF amplitude profile at the edges of the sweep and the use of phase cycles for the selection of coherence-transfer pathways. The requirements of chirp pulses are compared with those of hard pulses in terms of peak RF power.

Multiple quantum NMR excitation with a one-quantum hamiltonian

Excitation of multiple quantum coherence in dipolar coupled spin systems is usually accomplished with a two-quantum multiple pulse sequence which can be time reversed by means of a 90° phase shift. The application of such an excitation scheme to a spin system in thermal equilibrium excites only even orders of multiple quantum coherence. We demonstrate here time reversible pulse sequences that excite all orders of coherence by creating a pure one-quantum average hamiltonian. We also describe pulse schemes which can be used to create pure one- or two-quantum average hamiltonians with variable scaling between +1 and −1. These excitation schemes are relevant to the study of spin clustering by multiple quantum NMR.