Fig. . shows the field dependences of C signals obtained for all considered isotopologues according to protocol 1 (Fig. ). Previous works showed that for the three-spin system of two protons and single C there are four LACs, two at positive fields and two at negative fields, situated symmetrically with respect to zero field . For 13 C2-maleic acid, LACs occur at fields -292 nT, -137 nT, +137 nT, and +292 nT. For protons initially polarized in their singlet state and for positive fields, only the LAC at +292 nT that induces mixing of 𝑆 0 ′ 𝛼 and 𝑇 + ′ 𝛽 states (as discussed in the previous section) is relevant. The other, lower field LACs corresponds to crossing of 𝑇 0 ′ 𝛼 and 𝑇 + ′ 𝛽 states and becomes operative only when performing adiabatic field sweep from negative to positive values. The single active LAC in this case yields a smooth field dependence with a single maximum. The situation for 13 C1-maleate is similar, but the field dependence is much broader, which reflects the broader LAC region due to strong C-H coupling in this isotopologue. The experimentally observed field dependences are in qualitative agreement with calculations, but the transfer efficiency in 13 C1-maleic acid was significantly lower than in 13 C2-maleic acid. This difference is probably caused by much faster relaxation of the 13 C1 nucleus than C2 due to stronger dipolar interaction with the nearby proton.