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 −

Abstract

Abstract 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.