EXCITED-STATE CIS-TRANS ISOMERIZATION OF CIS-HEXATRIENE - A CAS-SCF COMPUTATIONAL STUDY

Two different reaction pathways for the cis-trans isomerization of cis-hexa-1,3,5-triene in its first excited (2(1)A(g)) state have been determined using the CAS-SCF (six-orbital/six-electron active space) and CAS-SCF/MP2 methods with 4-31G and DZ+d basis sets. Intrinsic reaction coordinate calculations demonstrate that these pathways correspond to c/t and Z/E interconversion of the initial- cis-hexatriene (tZt) isomer. However, these two isomerization processes do not terminate on the excited-state potential energy surface since both the c/t and Z/E pathways lead to ''products'' which are located at only one-third of the way along the ''expected'' (i.e., 180 degrees rotation) reaction coordinates. The two excited-state ''product'' wells, which are both entered by overcoming small barriers (4 and 6 kcal mol(-1)), correspond to two different Born-Oppenheimer violation regions centered on two low-lying conical intersection points. Thus, while an excited-state cis-hexatriene molecule can easily initiate a cis-trans isomerization process, this process can be completed only on the ground-state potential energy surface after passage through a conical intersection where a fast, radiationless decay is possible. The existence of these nonadiabatic reaction pathways is consistent with the available experimental data on the photochemistry and photophysics of cis-hexatrienes.