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

We revisit the singlet–triplet energy gap (ΔEST) 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 Si3) computations. Both 1A1 and 3 A2' states of Si3 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 ΔEST. The proton affinity of silicon trimer is determined as PA(Si3) = 830 ± 4 kJ/mol at 298 K. The metal cation affinities are also predicted to be LiCA(Si3) = 108 ± 8 kJ/mol, NaCA(Si3) = 79 ± 8 kJ/mol and KCA(Si3) = 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 Si3 is, at most, nonaromatic. Attachment of the proton and Li+ cation renders it anti‐aromatic. © 2015 Wiley Periodicals, Inc. Both 1A1 and 3A′2 states of Si3 are degenerate. The H+, Li+, and Na+ cations favor the singlet state, whereas K+ favors the triplet. Some thermochemical parameters are predicted: PA(Si3) = 830 ± 4, LiCA(Si3) =108 ± 8, NaCA(Si3) = 79 ± 8, and KCA(Si3) = 44 ± 8 kJ/mol. The ring current shows that the singlet three‐membered Si3 ring is, at most, nonaromatic. Attachment of H+ and Li+ renders it anti‐aromatic.