AB-INITIO STUDIES OF ELECTRONICALLY EXCITED CARBON-DISULFIDE

The excited electronic states of carbon disulfide (CS2) are examined t heoretically by exploiting the CIS and CIS-MP2 configuration interacti on methods in conjunction with extensive sets of basis functions (e.g. , 6-311(+)G()). At their respective equilibrium geometries, the lowes t-lying states of CS2 are predicted to have symmetry labels and electr onic energies given by X(1) Sigma(g)(+) < a(3)B(2) < b(3)A(2)(R) < A(1 )A(2) < (BB2)-B-1(V) < c(3)B(2) < d(3)A(2) < C(1)A(2), where the lette rs in parentheses refer to established spectroscopic designations for the R and V absorption systems. The bent b(3)A(2)(R) and c(3)B(2) stat es are found to correlate with a degenerate (3) Delta(u) level in the linear molecule. Analogous Renner-Teller effects in the (1) Delta(u), level give rise to (1)A(2) and B-1(2) potential surfaces, the latter o f which correlates to the well-studied (BB2)-B-1(V) state. The presenc e of an unexpected crossing between the (1) Delta(u), and (1) Sigma(u) (-) curves of linear CS2 makes definitive assignment for the other mem ber of this Renner-Teller doublet difficult, with an apparent reversal of relative energy ordering encountered as a function of the C-S bond distance. The implications of this effect, as well as the influence e xerted by neighboring electronic manifolds (e.g., the hitherto unobser ved d(3)A(2) surface which supports spin-orbit allowed electric dipole transitions from the X(1) Sigma(g)(+) ground state), are discussed in terms of recent studies performed on the near-ultraviolet photochemis try and photophysics of CS2. While ab initio properties predicted for the b(3)A(2)(R) State are in good accord with previous spectroscopic m easurements, the calculated equilibrium geometry, barrier to linearity , and vibrational frequencies for the (BB2)-B-1(V) potential surface d iffer significantly from experimental observations.