The structure and stability of classical and bridged C2H(Formula presented.) is reinvestigated. The SCF and CEPA-PNO computations performed with flexible s and p basis sets including two d-sets on carbon confirm our previous results. We find the protonated acetylene structure to be more stable than the vinyl cation by 3.5–4 kcal/mol. The energy barrier for the interconversion of these two structures is at most a few tenths of a kcal/mol. The equilibrium SCF geometries of Weber et al.  are affected insignificantly by further optimization at the CEPA-PNO level. Several structures for the interaction of C2H(Formula presented.) with HF have been investigated at the SCF level. With our largest basis set which includes a complete set of polarization functions we find a remarkable levelling of the stabilities of most of the structures. In these cases the stabilization energy ΔE ranges from −10 to −13 kcal/mol.
- Interaction of CH(Formula presented.) with HF