The first 90° pulse aligns the z-magnetization vector along the −y direction. Chemical shift evolution before acquisition can be neglected, as the two 180° carbon pulses only change the sign of the operator, and the 90° pulse in the −y direction has no effect on the y-magnetization, thus the receiver detects a negative signal.

For a CH group we start with γ_HI_Hz, which is transformed into transverse proton magnetization by the first 90° proton pulse, giving Equation (2). During the following two delays d2, C,H spin coupling evolves, which is not refocused due to the simultaneous 180° proton and carbon pulses. Since we set τ equal to d2 = 1/4J and use two d2 periods in this sequence, the cosine term becomes zero and the sine term unity. Chemical shift evolution of the protons is refocused by the 180° pulse. Thus, at the end of the second delay τ we find antiphase magnetization of the ¹H with respect to the ¹³C.

The two simultaneous 90° proton and carbon pulses perform the polarization transfer by transforming −2γ_HI_HxI_Cz into antiphase magnetization of the carbon with respect to the proton. In the final part of the sequence an observable in-phase ¹³C magnetization is generated. Note that this magnetization bears the factor γ_H, and is thus four times stronger than a normal carbon signal. To choose a suitable d3 value, one has to compromise for CH, CH₂, and CH₃. For a CH group we find that with d3 close to 1/2J the sine term disappears, leaving a positive ¹³C signal at the receiver, giving Equation (3).

In summary, the sequence can be viewed as a refocused INEPT with an additional ¹³C pulse at the beginning.

8. Own Observations