AB-INITIO CHARACTERIZATION OF ELECTRONICALLY EXCITED-STATES IN HIGHLY UNSATURATED-HYDROCARBONS

The results of extensive electronic structure calculations performed for a series of molecules: (CC)nH, H(CC)nH+ for n=2,3,4 and for (CC)3H- and H(CC)3H- are presented. The theoretical scheme is based on the complete-active-space self-consistent-field (CASSCF) method, with remaining dynamic electron correlation effects added in the subsequent step with the use of the second-order perturbation theory with the CASSCF wave function as the reference state (CASPT2). The results support the hypothesis of Fulara et al [Nature 336, 439 (1993)] that the 2Π→2Π electronic transitions in the series of CnH radicals might be responsible for some of the "diffuse interstellar bands" observed in the visible and near-infrared spectra of stars located on the far side of diffuse interstellar clouds. © 1995 American Institute of Physics.