Yu.E.Chernysh,V.S.Tsygankov,L.K. Ochotnikova,B.S.Lukyanov
Rostov State University,
344090, Rostov Don, Russia
e-mail: vst@krinc.rnd.runnet.ru
In our previous papers [1-3] a novel technique concerning the suppression of coherence transfer effects was demonstrated on the basis of MUltiplet-Selective EXcitation (MUSEX) of spin systems. Hereinafter it will be referred to as the MUSEX EXSY experiment including both COrrelation SpectroscopY (COSY) and EXchange SpectroscopY (EXSY) methods, in which all sorts of nonselective and multiplet-selective pulse combination are introduced. There are 32 combinations in such an experiment, with each specific problem decided by a sequence of this one. Here we will discuss only observable terms for the two coupled spins as a result of calculating them on the basis of the product operator formalism [4] for all the above - mentioned sequences as follows:
A. [900 - t 1 - 900 (A) - t m - 900 (A) - t 2 ] .
It should be noted that the first pair of pulses in this sequence is in fact the MUSEX COSY experiment [2] where after the second pulse, which is selective with respect to A nucleus, the remaining transverse components continue to precess with the same frequences as in the period "t1" and longitudinal ones undergo exchange processes (if they occur) during tm . Then both of them are recorded by the third selective 900 pulse. The density operator at the following stages can be described as follows s0,,s1 ,s2 from [1] and s3 , s4 from [2]. FT transformation of which results in 2D spectrum as their superposition (Fig.1). The spectrum seems to be useful for investigating exchange processes proceeding in multi-spin systems, especialy in those with overlapping and crowded spectra for the spectra are quite sparse with respect to cross-peaks. Therefore, it can be used to obtain better evidence for the dynamic behaviour of the molecule of interest. So the pattern of dia- and cross-peaks is also suitable for observing exchange processes. After excitation of any random exchange processes, the exchange lines seem to appear within dia-peaks, in other words their square structure tends to be restored.
B. [900 (A) - t 1 - 900 - t m - 900 (A) - t 2 ] .
For the AB system the observable terms can be represented like the A case with this scheme.
C. [900 (A) - t 1 - 900 (A) - t m - 900 - t 2 ] .
It should be noted that two selective pulses under ALternating EXcitation (ALEX) of A/B nuclei (that is one pulse is 90 (A/B) and the other is 900 (B/A)) offer almost the same 2D spectra as those above covered, with the pattern of diagonal and cross-peaks being distinguished.
All of the sequences can be run as the
entire MUSEX EXSY experiment, which possesses the following
advantages over the standard EXSY sequence.
Fig.1 Resulting spectrum for ( NSP - SP(A) -
SP(A))n . Here NSP - a nonselective pulse, SP(A) - a
selective pulse for A nucleus