J Pulses for Multiplet-Selective NMR

Abstract

Exact product operator solutions have been obtained for the evolution of weakly coupled spin-12 ImSn systems during arbitrary RF irradiation of one spin. These solutions, which completely characterize the nature of J-coupling modulation during RF pulses, show that significant exchange occurs between single-spin magnetization and two-spin product operator states when the RF field strength is comparable to the coupling. In particular, a long (tp = [2J]−1 s), low-power (B1 = J/2 Hz), constant amplitude pulse applied on resonance to one spin in an IS system completely interconverts the spinstates Sz ↔ 2SxIz and Sx ↔ 2SzIz when the RF is applied to the S spins, and interconverts Sx ↔ 2SyIy in 100% yield when the RF is applied to the I spins. Thus, these “J pulses,” which select a bandwidth approximately equal to J Hz, may replace any combination of a (2J)−1 delay period and a consecutive hard 90° pulse in any polarization transfer or multiple quantum sequence. Although these rectangular pulses are highly frequency selective, in general they increase the replaced (2J)−1 period by only a modest 40%, a time saving of a factor of 5 compared to existing pulses exhibiting the same selectivity. In favorable cases, there is no increase in duration of a pulse sequence using a particular type of J pulse, the 90J variety, which accomplishes the third spin state transformation listed above. J pulses will be advantageous for systems subject to rapid signal loss from relaxation and more generally for the enhanced operation of pulse sequences via the use of J modulation during RF irradiation.