Bonding and singlet–triplet gap of silicon trimer: Effects of protonation and attachment of alkali metal cations

We revisit the singlet–triplet energy gap (Δ E ST ) of silicon trimer and evaluate the gaps of its derivatives by attachment of a cation (H + , Li + , Na + , and K + ) using the wavefunction‐based methods including the composite G4, coupled‐cluster theory CCSD(T)/CBS, CCSDT and CCSDTQ, and CASSCF/CASPT2 (for Si 3 ) computations. Both 1 A 1 and 3A 2 'states of Si 3 are determined to be degenerate. An intersystem crossing between both states appears to be possible at a point having an apex bond angle of around α  = 68 ± 2° which is 16 ± 4 kJ/mol above the ground state. The proton, Li + and Na + cations tend to favor the low‐spin state, whereas the K + cation favors the high‐spin state. However, they do not modify significantly the Δ E ST . The proton affinity of silicon trimer is determined as PA(Si 3 ) = 830 ± 4 kJ/mol at 298 K. The metal cation affinities are also predicted to be LiCA(Si 3 ) = 108 ± 8 kJ/mol, NaCA(Si 3 ) = 79 ± 8 kJ/mol and KCA(Si 3 ) = 44 ± 8 kJ/mol. The chemical bonding is probed using the electron localization function, and ring current analyses show that the singlet three‐membered ring Si 3 is, at most, nonaromatic. Attachment of the proton and Li + cation renders it anti‐aromatic. © 2015 Wiley Periodicals, Inc.