The structure of protonated HCP: A classical or non-classical ion?

Ab initio calculations have been carried out at MP2 with the 6-31+G(d,p), 6-311+G(d,p), aug’-cc-pVDZ, aug’-cc-pVTZ, aug-cc-pVTZ, and aug’-cc-pVQZ basis sets, and at MP4, CCSD, and CCSD(T) with the aug’-cc-pVTZ basis, to probe the potential surfaces for equilibrium structures of HCPH+. All levels of theory indicate that protonation occurs at C, with the formation of π-protonated Cs(π) or C2v structures. The non-classical Cs(π) structure is usually preferred, although the potential surfaces are relatively flat in this region.