The electronic states of propyne studied by optical (VUV) absorption, near-threshold electron energy-loss (EEL) spectroscopy and ab initio multi-reference configuration interaction calculations

The VUV absorption and EELS spectra of propyne (CH₃CCH) have been reinvestigated, and reassigned in the light of multi-reference multi-root CI calculations of quadruple zeta quality containing both valence and Rydberg type functions. While the Rydberg states were considered as linear, and hence studied as vertical excitations, the known bending of valence states in alkynes led to a parallel investigation of the equilibrium structures of the lowest singlet and triplet valence states of cis and trans type in C_s symmetry, by GVB calculations. Some of the rotamers from these bent states are clearly saddle-points. The calculated vibration frequencies of the ground state have been compared with experiment.