In contrast to ethylene, in the acetylene-derived adducts the barriers for the internal rotation and singlet-triplet MECPs are much higher. Moreover, these species have stable closed-shell singlet diradical (its structure is close to MECP). Because the singlet geometry is MECP-like, the difference in MECP heights in the acetylene-and propyne-derived species correlates with the adiabatic singlet-triplet gaps.