Composite pulse with offset modulation

In this section, three experiments are going to be discussed. Two of them, a broadband inversion and a Hahn spin echo, are well-known in the rotating frame. They need to meet the requirement of the phase coherence in PIPs in order to work properly in the Eigenframe. The third is a composite pulse with offset modulation. 

The two PIPs constitute an offset modulated composite pulse for use with a 900 MHz NMR instrument. As a result, the centre of 13C is subjected to two consecutive 2ir rotations of opposite offsets, one positive and the other negative, but of the same (x) phases in the two different Eigenframes as shown in Fig. 23. In the vicinity of the centre of the 13C , the sum of the two rotation angles can be expressed as... 

The first of these arises when the long spin-lock pulse acts in an analogous fashion to the last 90" pulse of the COSY experiment so causing coherence transfer between J-coupled spins. The resulting peaks display the usual antiphase COSY peak stmcture and tend to be weak so are of least concern. A far greater problem arises from TOCSY transfers which arise because the spin-lock period in ROESY is similar to that used in the TOCSY experiment (Section 5.7). This may, therefore, also induce coherent transfers between J-coupled spins when these experience similar rf fields, that is, when the Hartmann-Hahn matching condition is satisfied. Since the ROESY spin-lock is not modulated (i.e. not a composite pulse sequence), this match is restricted to mutually coupled spins with similar chemical shift offsets or to those with equal but opposite... 

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