Triplet states of carbenium and silylium cations

Triplet CH3+, SiH3+, C2H5+, SiCH5+, and Si2H5+ systems are studied with various correlated quantum mechanical methods. Only the triplet methyl and silyl cations have a single minimum. The other systems have from two to four minima. The best singlet–triplet energy difference predictions for the most stable isomers are: methyl cation 87 kcal/mol, silyl cation 71 kcal/mol, ethyl cation 101 kcal/mol, methylsilyl cation 73 kcal/mol (all CCSD(T)/TZ2PF) and disilyl cation 48 kcal/mol (MP2/TZ2PF). These changes in the singlet–triplet gaps are rationalized by regarding the effect of protonation, methylation and silylation on the energy differences between singly occupied MOs in triplet methylene and triplet silylene.