Improvement of NMR experiments by employing semiselective half-Gaussian-shaped pulses

Half-Gaussian-shaped pulses yield an almost ideal excitation profile, if the relatively broad dispersion component is eliminated by a simple nonselective 90° purge pulse. In contrast to Gaussian-shaped pulses, the half-Gaussian excitation scheme does not create significant amounts of antiphase components, but leads almost quantitatively to in-phase magnetization of the excited signal. This feature is especially useful in experiments where a semiselective excitation is followed by a mixing of in-phase components (e.g., in NOESY, ROESY, or TOCSY type experiments). In such a case even signals with large or numerous couplings give rise to a very efficient transfer. In addition, the magnetization vectors in the transverse plane show an almost perfect alignment after semiselective excitation with a purged half-Gaussian pulse. Thus it can be employed advantageously in all experiments where a semiselective excitation is directly followed by the t₁ evolution period. In such experiments the use of Gaussian pulses leads to very large first-order phase gradients, the correction of which causes severe distortions of the baseline. The alternative use of purged half-Gaussian-shaped pulses results only in very small phase gradients, comparable to those in experiments with nonselective excitation.