Note that the Sy operator is replaced by the —2SxI2 operator because it undergoes /-coupling evolution with respect to H2 for a period of exactly 1/(2/), arriving at the antiphase state on the —X axis. We can view this product operator as a mixture of ZQC and DQC involving 13C and H1 (more precisely, {DQ}X+{ZQ}X), which is antiphase with respect to the spin state of H2. That is, if H2 is in the a state, we multiply by 1/2 (to get {DQ}X+{ZQ}X) and if H2 is in the p state, we multiply by —1/2 (to get —{DQ}1 — {ZQ}X).

The effect of the XH pulse of width © can be calculated using the sine and cosine terms for each of the proton operators:

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