Bonding and Singlet-Triplet Gap of Silicon Trimer: Effects of Protonation and Attachment of Alkali Metal Cations

We revisit the singlet-triplet energy gap (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 (3) 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 =68 +/- 2 degrees 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 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. (c) 2015 Wiley Periodicals, Inc.