A classification of polyenes into 4L + 2- and 4L-classes on the basis of Coulson's bond orders and information theory and its application to the interpretation of electrocyclic reactions

TY - JOUR

T1 - A classification of polyenes into 4L + 2- and 4L-classes on the basis of Coulson's bond orders and information theory and its application to the interpretation of electrocyclic reactions

AU - Fratev, F.

AU - Enchev, V.

AU - Karadakov, P.

AU - Castaño, O.

PY - 1984

Y1 - 1984

N2 - It is shown that in the framework of the π-electron approximation even polyenes can be unambiguously divided into 4L- and 4L +2- classes. The classification scheme is based upon the sign alternation of the bond-order between the first and the last atoms and upon the different information content of the bond-order distribution for 4L- and 4L +2- polyenes, respectively. The classification has been shown to hold for the four lowest-lying electronic states: the ground state 1A g−, the spectroscopically permitted excited state 1B u+, the spectroscopically forbidden state 1A g−, and the lowest triplet state 3B u+ at all levels of sophistication in the π-electron approximation, ranging from the simple Hückel Hamiltonian to the PPP-multi-CI approach. It is shown that the proposed classification is valid also for heteropolyenes and polyenelike/heteropolyenelike fragments from complex organic molecules. The classification has been shown to be useful for the theoretical interpretation of thermo- and photoelectrocyclic reactions. The sign of the bond order between the first and the last atoms (or the average information content) determines unambiguously the topology of the forming cyclic transition state and, thence, the course and the mechanism of these reactions. The results obtained indirectly support the hypothesis that the lowest excited states in butadiene and hexatriene are of the type 1B u+.

AB - It is shown that in the framework of the π-electron approximation even polyenes can be unambiguously divided into 4L- and 4L +2- classes. The classification scheme is based upon the sign alternation of the bond-order between the first and the last atoms and upon the different information content of the bond-order distribution for 4L- and 4L +2- polyenes, respectively. The classification has been shown to hold for the four lowest-lying electronic states: the ground state 1A g−, the spectroscopically permitted excited state 1B u+, the spectroscopically forbidden state 1A g−, and the lowest triplet state 3B u+ at all levels of sophistication in the π-electron approximation, ranging from the simple Hückel Hamiltonian to the PPP-multi-CI approach. It is shown that the proposed classification is valid also for heteropolyenes and polyenelike/heteropolyenelike fragments from complex organic molecules. The classification has been shown to be useful for the theoretical interpretation of thermo- and photoelectrocyclic reactions. The sign of the bond order between the first and the last atoms (or the average information content) determines unambiguously the topology of the forming cyclic transition state and, thence, the course and the mechanism of these reactions. The results obtained indirectly support the hypothesis that the lowest excited states in butadiene and hexatriene are of the type 1B u+.

U2 - 10.1002/qua.560260605

DO - 10.1002/qua.560260605

M3 - Article

SN - 1097-461X

VL - 26

SP - 993

EP - 1015

JO - INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY

JF - INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY

IS - 6

ER -