Developments in two dimensional NMR spectroscopy of multiple quantum coherences of couples spins in liquids

Abstract

The pathway description provides a straightforward method of computing flip-angle dependence of various multiple quantum coherences in the 2D MQ and the 2D COSY spectroscopy. Using this flip-angle dependence it is possible to distinguish various connectivity classes of the coherences detected via the single quantum transitions. Monitoring the amplitudes of peaks in ±? quadrants allows distinction between coherence transfer between participating spins and non-participating spins. It is shown that, in general, transfer of a multiple quantum coherence to a connected single quantum transition takes place via a lower order pathway. The intensity of such transitions can be enhanced compared to others by the use of a flip angle of the coherence transfer pulse lower than 90°. In COSY spectroscopy, all the connected transitions take lower order paths compared to unconnected transitions and can be exclusively detected by the use of a low flip angle. On the other hand, use of a large flip angle (? = 150°) allows selective detection of only unconnected transitions and elimination of diagonal peaks. The observed MQ artifacts in COSY spectra are of a rather general nature and can occur whenever strongly coupled spins are present in the sample. Furthermore, these artifacts cannot be completely cancelled by the usual phase cycling procedures (24), since the carry-over effects will exist between experiments having different t? values in the COSY algorithm. The artifacts can be removed by waiting sufficiently long between experiments, i.e., by making RD > 5T?. However, this imposes severe constraints on experimental time and will lead to impractically long experimental times. An important conclusion of this analysis is that since RD is usually of the order of T?, the initial conditions fluctuate between different t? experiments resulting in spread of not only MQ artifacts into t? noise but also noise along single quantum resonances and thus contributes in general to t? noise. This source of noise has not been identified earlier and is also not easy to remove.