Triplet CH3+, SiH3+, C2H5+, SiCH5+, and Si2H5+ systems are studied with var
ious correlated quantum mechanical methods. Only the triplet methyl and sil
yl cations have a single minimum. The other systems have from two to four m
inima. The best singlet-triplet energy difference predictions for the most
stable isomers are: methyl cation 87 kcal/mol, silyl cation 71 kcal/mol, et
hyl 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-t
riplet gaps are rationalized by regarding the effect of protonation, methyl
ation and silylation on the energy differences between singly occupied MOs
in triplet methylene and triplet silylene. (C) 2001 Elsevier Science B.V. A
ll rights reserved.