Tilted-axis precession and phase-sensitive detection of nuclear magnetization

The precession of nuclear magnetization about tilted axes in the rotating frame and its consequences for phase-sensitive NMR detection are examined in detail in this work, Expressions for the frequencies and phases of a NMR spectrum acquired during tilted-axis precession are derived, and utilized to show how phase-cycling schemes that eliminate undesired artifacts in such spectra can be determined, The explicit use of these expressions is demonstrated in a calculation of efficient new phase cycles for three line-narrowing experiments (MREV-8, BR-24, and C-24) exemplifying precession of nuclear magnetization about axes not aligned with the static held direction, Experimental and simulated results that confirm the effectiveness of the phase cycles and illustrate their applicability under conditions of practical interest are reported. An analysis of artifacts not totally eliminated by these phase-cycling schemes is described. The source of these residual artifacts in multiple-pulse applications is shown to be large deviations of the effective offset Hamiltonian parameters from canonical values calculated using zeroth-order coherent-averaging theory. These deviations are an intrinsic characteristic of multiple-pulse line-narrowing techniques, and are not eradicated even when the experiments are performed under ideal conditions. The consequences of this shortcoming of the zeroth-order theory for the interpretation of multiple-pulse spectra are discussed. (C) 1996 Academic Press, Inc.