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The vertical transition energies and Δ g -values of cyclic C 3  + , Si 3  +  ( X  2  B  2 ), C 3  −  (1 2  A  1 , metastable) and Si 3  −  ( X  2  A  1 ) are studied with ROHF MOs, multireference (MRDCI) wavefunctions and 6-311+G(2 d ) basis sets. Si 3  −  has at least eight bound states. For each radical, the in-plane components of the  g  shift, Δ g yy   and Δ g zz   (with  g ii   =  g  e  + Δ g ii  ), have similar negative values (about −1200 ppm for C 3  + , C 3  −  and about −8000 ppm for Si 3  + , Si 3  − ). The Δ g xx  ’s are larger in magnitude, negative for C 3  + , Si 3  +  (−11000 and −72000 ppm) but positive for C 3  − , Si 3  −  (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 X 3  −  center, with the (positive)  s -spin-density at each basal atom (average  A  iso  (MHz) = 75 for C 3  −  and −25 for Si 3  − ) being smaller than at the apical center (275 and −95 MHz). The results are less satisfactory for X 3  + . All treatments agree, more or less, in a larger (positive)  s -density at the basal atoms ( A  iso (MHz) from 300 to 700 for C 3  + , and −80 to −150 for Si 3  + ), whereas the  s -contribution at the apical center depends on the method: for C 3  + , it is positive with  ab initio  but negative with DFT, whereas for Si 3  +  the opposite trend is found; as well,  A  dip (X 3  + ) 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.

zeitschrift für physikalische chemie

Theoretical Study of the Vertical Electronic Spectra, Electron-Spin  g -Factors and Hyperfine Coupling Constants of the ( C  2v ) Cyclic Radicals C 3  + , Si 3  + , C 3  −  and Si 3  −