The photochemistry of 1,3-butadiene rationalized by means of theoretical resonance structures and their weights

A complete active-space self-consistent-field wave function for the pi-elec tron part of s-trans-1,3-butadiene has been expanded into a set of localize d bonding schemes and their weights. These bonding schemes are close to the resonance structures used in organic chemistry. The expansion technique ha s been applied to both the electronic ground state and the electronically f irst-excited singlet and tripret pi,pi* states. The manifolds of large-weig ht bonding schemes represent approximate resonance hybrids for the ground a nd the singlet and triplet pi,pi* states of s-trans-1,3-butadiene. These re sonance hybrids, obtained by theor, alone, permit a qualitative rationaliza tion of a significant part of the known singlet and triplet photochemistry.