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 π‐electron part of s‐trans‐1,3‐butadiene has been expanded into a set of localized bonding schemes and their weights. These bonding schemes are close to the resonance structures used in organic chemistry. The expansion technique has been applied to both the electronic ground state and the electronically first‐excited singlet and triplet π,π* states. The manifolds of large‐weight bonding schemes represent approximate resonance hybrids for the ground and the singlet and triplet π,π* states of s‐trans‐1,3‐butadiene. These resonance hybrids, obtained by theory alone, permit a qualitative rationalization of a significant part of the known singlet and triplet photochemistry. Eine CASSCF‐Wellenfunktion für den π‐Elektronenteil des s‐trans‐1,3‐Butadiens wurde in einen Satz von lokalisierten Bindungschemata und deren Gewichte entwickelt. Diese Bindungsschemata sind den Resonanzstrukturen der organischen Chemie sehr ähnlich. Das Entwicklungsverfahren wendeten wir auf den Grundzustand und die ersten angeregten Singulett‐ und Triplett‐π,π*‐Zustände an. Die Sätze von Bindungsschemata, die Schemata mit großen Gewichten enthalten, stellen ungefähre Resonanzhybride für den Grundzustand und die Singulett‐ und Triplett‐π,π*‐Zustände von s‐trans‐1,3‐Butadien dar. Diese Resonanzhybride, erhalten allein durch Theorie, erlauben es einen großen Teil der bekannten Photochemie des s‐trans‐1,3‐Butadiens qualitativ zu erklären. Electronically excited molecules entering photoreactions are approximately described by organic photochemists through the classic tools of organic resonance structures and their weights. A procedure is suggested for obtaining resonance structures and their weights from a computed CASSCF wave function. The theoretically derived resonance structures (shown here) and their weights accord with a large part of the known photochemistry of s‐trans‐1,3‐butadiene.