The electronic spectrum of linear and rhombic C-4

Large-scale multi-reference configuration interaction calculations are empl oyed in order to calculate the vertical electronic spectrum of C-4 in its l inear and rhombic geometrical conformations up to 6.5 eV. The recently meas ured transitions of linear C-4 at 0.93 eV ((IIu)-I-3 <-- X(3)Sigma(g)(-)) a nd at 3.26 eV ((3)Sigma(u)(-) <-- X(3)Sigma(g)(-)) are confirmed. The first allowed transition of rhombic C-4 is calculated at 2.15 eV (B-1(1u) <-- X( 1)A(g)) and this transition should help to identify rhombic C-4 since there are no transitions of linear C-4 in this energy region. (C) 2000 Elsevier Science B.V. All rights reserved.