TILTED-AXIS PRECESSION AND PHASE-SENSITIVE DETECTION OF NUCLEAR MAGNETIZATION

The precession of nuclear magnetization about tilted axes in the rotat ing frame and its consequences for phase-sensitive NMR detection are e xamined in detail in this work, Expressions for the frequencies and ph ases of a NMR spectrum acquired during tilted-axis precession are deri ved, and utilized to show how phase-cycling schemes that eliminate und esired artifacts in such spectra can be determined, The explicit use o f 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 res ults that confirm the effectiveness of the phase cycles and illustrate their applicability under conditions of practical interest are report ed. An analysis of artifacts not totally eliminated by these phase-cyc ling schemes is described. The source of these residual artifacts in m ultiple-pulse applications is shown to be large deviations of the effe ctive offset Hamiltonian parameters from canonical values calculated u sing zeroth-order coherent-averaging theory. These deviations are an i ntrinsic characteristic of multiple-pulse line-narrowing techniques, a nd are not eradicated even when the experiments are performed under id eal conditions. The consequences of this shortcoming of the zeroth-ord er theory for the interpretation of multiple-pulse spectra are discuss ed. (C) 1996 Academic Press, Inc.