unity, leaving pure antiphase magnetization of the proton with respect to carbon as in Equation (1).

INEPT product-operator transformation for Equation 1

This antiphase magnetization is converted into antiphase magnetization of ¹³C with respect to ¹H by the two simultaneous 90° pulses. During acquisition C,H spin coupling develops again, forming an in-phase ¹³C magnetization, which however is multiplied by γ_H, and thus we obtain a proton-enhanced ¹³C signal. This signal appears in antiphase due to the sine term in Equation (2).

INEPT product-operator transformation for Equation 2

However, there occurs an additional contribution to the signal from the ¹³C magnetization. I_Cz is first inverted by the 180° ¹³C pulse and then converted into transverse magnetization by the 90° ¹³C pulse. It develops C,H spin coupling during acquisition, giving an in-phase signal due to the cosine term in Equation (3).

INEPT carbon-magnetization contribution in Equation 3

This signal has the intensity ratio of 1:1, whereas the signal obtained in Equation (2) has the intensity ratio 4:(−4), superposition yields the intensity ratio 5:(−3) as observed in spectrum a. The phase cycle of the INEPT sequence eliminates all signal contributions stemming from initial carbon magnetization; thus in spectrum b a 4:(−4) doublet is seen. Furthermore, all signals from quaternary carbon atoms are suppressed.

In the case of the polarization of ¹³C nuclei by ¹H (γ_H/γ_C ≈ 4) the theoretical relative intensities of the multiplets obtained in one scan are as follows:

CH	5	−3
CH₂	9	2	−7
CH₃	13	15	−9	−11

This distortion of multiplets is a drawback of the INEPT sequence. Therefore the sequences INEPT+ (see Exp. 6.6) and DEPT (see Exp. 6.9) were developed.

8. Own Observations