A theoretical study of protonation of triatomic silicon–carbon compounds

Ab initio molecular orbital methods are employed to study the low‐lying states of C 3 H + , SiC 2 H + , Si 2 CH + , and Si 3 H + . Special attention is paid to a comparative study between C 3 H + and Si 3 H + . In both cases a 3 B 2 state is found to lie the lowest at the HF level, although inclusion of correlation effects favor a linear structure ( 1 Σ + state) for C 3 H + , which lies 25 kcal/mol below the 3 B 2 state at the MP 4 level, and a bent structure ( 1 A ′ state) for Si 3 H + , which lies just 2 kcal/mol below the 3 B 2 state. The proton affinities of C 3 , SiC 2 , Si 2 C, and Si 3 are estimated at different levels of theory. Both protonation at carbon and silicon atoms are considered for SiC 2 and Si 2 C. It is found that C 3 comparatively has a low proton affinity. On the other hand, Si 3 has a relatively high proton affinity compared with the protonation at silicon atom for both SiC 2 and Si 2 C. These results are discussed on the basis of electronic structure arguments.