AbstractThe vertical transition energies and Δg-values of cyclic C3+, Si3+ (X2B2), C3− (12A1, metastable) and Si3− (X2A1) 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 = ge + 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 Aiso, Adip 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 Aiso’s and Adip’s for each X3− center, with the (positive) s-spin-density at each basal atom (average Aiso (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 (Aiso(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, Adip(X3+) shows unusually large discrepancies for each center. For these radicals, literature values of Aiso and Adip are not available, either from experimental or multireference ab initio studies, to allow for comparisons with our results.
Performance of the DLPNO-CCSD and recent DFT methods in the calculation of isotropic and dipolar contributions to 14N hyperfine coupling constants of nitroxide radicals
