Geometrical relaxation in the excited singlet states of propylene

The consequences of the twist around the double bond in propylene for the properties of its low lying excited singlet states have been investigated by the ab initio large-scale multireference configuration interaction method (MRD-CI). A substantial increase in the dipole moments of the S₁ and S₂ excited states was found for a large interval of the twist angel θ = 50–130°. The variation of the VB covalent VB ionic contributions to the correlated wavefunctions of these two states a function of twisting has been analyzed. The connection with the occurence of an avoided crossing of the two excited singlets near the twist angel θ = 75°, which results in no change in dipole moment directions, is pointed out. The existence of destructive or constructive interference between acceptor and donor substitution has been investigated on the example of the pyramidalization at one of the vinylic C atoms. A competition of opposing effects matrix can invert the dipole moment direction in the excited states. Preliminary investigation of the nonadiabatic coupling elements indicates that the “sudden polarization” effect willnot disappear through vibronic coupling, and that the return of excited molecules to the ground electronic state will not be immediate.