Two different reaction pathways for the cis-trans isomerization of cis-hexa-1, 3, 5-triene in its first excited (21Ag) 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° rotation) reaction coordinates. The two excited-state “products” 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.
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Olivucci et al. (1994) studied this question.