THE (X)OVER-TILDE(2)PI AND (A)OVER-TILDE(2)SIGMA STATES OF FCN- AB-INITIO CALCULATIONS AND SIMULATION OF THE HE-I PHOTOELECTRON-SPECTRA OF FCN AND CLCN( AND CLCN+ )

Geometry optimization and harmonic vibrational frequency calculations at the CASSCF, MP2 and CCSD(T) levels with basis sets up to 6-311G(2df ) quality were carried out on the (X) over tilde(1) Sigma(+) states of FCN and ClCN and the (X) over tilde(2)II and (A) over tilde(2) Sigma( +) states of their cations. Adiabatic ionization energies were calcula ted up to the CCSD(T)/6-311G(3df)//CCSD(T)/6-311G(2d) level. Some B3LY P calculations were performed also for the ground states of the neutra l molecules and the cations. Franck-Condon simulations were performed for the first two bands in the He I photoelectron spectra of FCN and C lCN by employing the ab initio computed geometries and frequencies. By comparing the observed and the simulated spectra obtained from differ ent CN and CX (X = F or Cl) ionic bond lengths chosen on the basis of the nb initio computed values, the following structural parameters are obtained for the two lowest-lying states of FCN+ and ClCN+ (the metho d of deriving the uncertainties is described): FCN+ (X) over tilde(2) Pi: r(CF) = 1.205 +/- 0.005 Angstrom; r(CN) = 1.223 +/- 0.002 Angstrom (A) over tilde(2) Sigma(+): r(CF) = 1.215 +/- 0.002 Angstrom; r(CN) = 1.137 +/- (0.006) Angstrom ClCN+ (X) over tilde(2) Pi: r(CCl) = 1.556 +/- (0.006) Angstrom; r(CN) = 1.214 +/- 0.002 Angstrom (A) over tilde (2) Sigma(+): r(CCl) = 1.589 +/- 0.003 Angstrom; r(CN) = 1.148 +/- (0. 006) Angstrom.