The relative stability of 2Σ+ and 2Π states in the systems CSi−, Si2− SiP and P2+ as predicted in MRD-CI calculations

Theoretical study on ten Λ-S states of Si2(-) anion: potential energy curves, spectroscopy and spin-orbit couplings

The potential energy curves (PECs) of seventeen Ω states generated from the ten Λ-S states of the Si2(-) anion are studied in detail using an ab initio quantum chemical method for the first time. The PECs are calculated for internuclear separations from 0.10 to 1.20 nm by the complete active space self-consistent field method, which is followed by the internally contracted multireference configuration interaction approach with the Davidson modification. The spin-orbit coupling is accounted for by the Breit-Pauli Hamiltonian. Core-valence correlation and scalar relativistic corrections are considered. Core-valence correlation corrections are included using an aug-cc-pCVTZ basis set. Scalar relativistic correction calculations are made with the third-order Douglas-Kroll Hamiltonian approximation at the level of a cc-pV5Z basis set. Obvious effect of core-valence correlation corrections on the PECs is observed, in particular for the two lowest (2)Πu and (2)Σg(+) Λ-S states. All the PECs are extrapolated to the complete basis set limit. The lowest (2)Πu Λ-S state is found to be the ground state of Si2(-) anion. The convergence observations of present calculations are made and the convergent behavior is discussed with respect to the basis set and level of theory. The effects of core-electron correlations on the energy splitting are studied by the all-electron aug-cc-pCVTZ basis set. Using these PECs, the spectroscopic parameters of Λ-S and Ω states involved are determined. The vibrational manifolds are evaluated for each Λ-S and Ω state of non-rotation Si2(-) anion. It shows that the spectroscopic parameters and molecular constants of ten Λ-S and seventeen Ω states reported here can be expected to be reliable predicted ones.

Theoretical Study of the Vertical Electronic Spectra, Electron-Spin g-Factors and Hyperfine Coupling Constants of the (C 2v) Cyclic Radicals C3 +, Si3 +, C3 − and Si3 −

The vertical transition energies and Δg-values of cyclic C3 +, Si3 + (X 2 B 2), C3 − (12 A 1, metastable) and Si3 − (X 2 A 1) are studied with ROHF MOs, multireference (MRDCI) wavefunctions and 6-311+G(2d) basis sets. Si3 − has at least eight bound states. For each radical, the in-plane components of the g shift, Δgyy and Δgzz (with gii = g e + Δgii ), have similar negative values (about −1200 ppm for C3 +, C3 − and about −8000 ppm for Si3 +, Si3 −). The Δgxx ’s are larger in magnitude, negative for C3 +, Si3 + (−11000 and −72000 ppm) but positive for C3 −, Si3 − (8000 and 165000 ppm). The hyperfine coupling constants A iso, A dip are evaluated with ab initio CISD, QCISD, CCD, MP4SDQ methods as well as with density functional theory (DFT) methods (SVWN, B3LYP, B3PW91, PW91PW91), using a spin-unrestricted formalism; several basis sets are considered. Both approaches give internally consistent A iso’s and A dip’s for each X3 − center, with the (positive) s-spin-density at each basal atom (average A iso (MHz) = 75 for C3 − and −25 for Si3 −) being smaller than at the apical center (275 and −95 MHz). The results are less satisfactory for X3 +. All treatments agree, more or less, in a larger (positive) s-density at the basal atoms (A iso(MHz) from 300 to 700 for C3 +, and −80 to −150 for Si3 +), whereas the s-contribution at the apical center depends on the method: for C3 +, it is positive with ab initio but negative with DFT, whereas for Si3 + the opposite trend is found; as well, A dip(X3 +) shows unusually large discrepancies for each center. For these radicals, literature values of A iso and A dip are not available, either from experimental or multireference ab initio studies, to allow for comparisons with our results.