J pulses for multiplet-selective NMR

Exact product operator solutions have been obtained for the evolution of weakly coupled spin-1/2 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 (t(p) = [root 2J](-1) s), low-power (B-1 = J/2 Hz), constant amplitude pulse applied on resonance to one spin in an IS system completely interconverts the spinstates S-z <----> 2s(x)I(z) and S-x <----> 2S(z)I(z) when the RF is applied to the S spins, and interconverts S-x <----> 2S(y)I(y) 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 degrees 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 5 pulse, the 90(J) variety, which accomplishes the third spin state transformation listed above. 5 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 5 modulation during RF irradiation. (C) 1999 Academic Press.