FORCE-FIELDS FOR ULTRAFAST PHOTOCHEMISTRY - THE S-2 (1B(U))-]S-1(2A(G))-]S-0 (1A(G)) REACTION-PATH FOR ALL-TRANS-HEXA-1,3,5-TRIENE

High-level ab initio quantum chemical computations (MC-SCF and multire ference Moller-Plesset perturbation theory) have been used to investig ate the composite relaxation path on three different singlet electroni c states of an isolated all-trans-hexa-l,3,5-triene (trans-HT) molecul e: the spectroscopic B-u (valence-ionic) state, the lower lying dark ( i.e. symmetry forbidden covalent) 2A(g) state (S-1), and finally the g round state (S-0). Our results support the hypothesis that IVR (intern al vibrational energy redistribution) from totally symmetric to non-to tally symmetric modes must control the dynamics of ultrafast decay in short all-trans polyenes. The salient features of the reaction path ar e as follows: (a) Motion out of the S-2 FC region and the subsequent r elaxation along the S-1 energy surface lies within the space of totall y symmetric deformations of the trans-HT molecular backbone. (b) The t riggering of fast S-1 -->S-0 radiationless decay requires a non-totall y symmetric deformation of the molecular backbone along a nearly barri erless (+/-2 kcal mol(-1)) path. (c) The molecular structure at the S- 1-->S-0 decay channel (i.e. at the S-1/S-0 crossing point) and its sub sequent evolution on the relaxation path which develops along the S-0 energy surface indicate that reactant back-formation must be the favor ed process.