Novel methods for characterizing a decoupler channel using "undetectable" quantum coherences

Exact solutions for the effect of time-independent RF pulses on any initial configuration of an IS J-coupled system demonstrate that on-resonance CW decoupling yields signals whose frequency depends on RF field strength and homogeneity. These signals are enhanced starting with "undetectable" antiphase and multiple quantum coherences, which can also produce centerband intensity to mimic the signal from decoupled S-x. Conversely, these coherences can be generated from S-x using a low-power pulse, B-1 = J/2, of length (root 2J)(-1), dubbed a "90(J) pulse" since it is the selective equivalent of ((2J)(-1)-90[1]). Utilizing 90(J) pulses, new characterization-of-decoupler (COD) pulse sequences can determine the performance of an insensitive I-spin channel by observing large signals from either antiphase or multiple quantum coherences with the S-spin channel, allowing, in minutes rather than hours: (i) frequency calibration to an accuracy of 0.1 Hz; (ii) measurement of RF amplitudes over a 500-fold variation; and (iii) mapping of RF homogeneity along the sample axis with a single 1D B-1 spectrum. These 90(J) coherence transfer pulses are of potential general use for selective spectroscopy, (C) 1999 Academic Press.