OFF-RESONANCE IRRADIATION EFFECT IN STEADY-STATE NMR SATURATION-TRANSFER

The off-resonance irradiation effect (spill-over effect), occurring in steady-state NMR saturation-transfer experiments, is studied theoreti cally and experimentally for a two-spin system in chemical exchange, w hen a contralateral irradiation is applied to record the reference spe ctrum. The relevant parameter chosen for this study is the saturation- transfer ratio. It is defined as the ratio between the value of one ex changing magnetization, obtained when saturating the other, and that o f the same magnetization measured when applying a contralateral irradi ation. The theoretical study is carried out via a model based on the B loch equations modified for chemical exchange and expressed in a doubl y tilted single rotating frame. The saturation-transfer ratio is expre ssed as a function of the saturating RF field magnitude. It is shown t hat the RF field applied off-resonance during the acquisition of the r eference spectrum does not correct the experimental saturation-transfe r ratio for the spill-over effect. In fact, the saturation-transfer ra tio increases with the magnitude of this held. This result is qualitat ively explained by the consequence of the effective magnetic fields' r elative orientations upon the amount of exchanged magnetization. The v alidity of the theoretical description is tested experimentally with a solution of N,N-dimethylacetamide in which chemical exchange arises f rom internal hindered rotation. An experimental protocol is proposed t o detect spill-over and correct it when necessary. The way to describe spill-over theoretically when more than two spins interact by chemica l exchange and/or dipolar coupling is also given. (C) 1997 Academic Pr ess.