We revisit the singlet-triplet energy gap (ΔE(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 Si3) computations. Both (1)A1 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 ΔE(ST). 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.
Keywords: CCSD(T)/CBS; CCSDT; CCSDTQ; G4 method; electron localization function; metal cation affinities; proton affinity; ring current; silicon trimer; singlet-triplet gap.
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