The complete-active-space SCF and CASMP2 methods are used to characterize th low-lying excited singlet and triplet states of diacetylene.With the exception of the 1 ~1#DELTA#_u state,all excited states considered are found to have geometrical structures distorted away from linear.Three of the triplet states are predicted to have non-planar C_2-symmetry structures.The other states are predicated to be planar,existing as both cis (C_(2v)) and trans (C_(2h))isomeric forms.Vibrational frequencies are calculated for the low-lying electronically excited states,and vertical and adiabatic excitation energies are repored.
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