The first singlet excited states (S-1) which control the ultrafast (i.e. su
bpicosecond) photochemistry of 2-cis-penta-2,4-dieniminium cation (2-cis-C5
H6NH2+), all-trans-hexa-1,3,5-triene (all-trans-HT) and cyclohexa-1,3-diene
(CHD) have been investigated using ab initio MCSCF and multireference MP2
theories. The structure of the corresponding potential energy surfaces (PES
s) has been characterized by computing novel unconstrained and symmetry-con
strained minimum-energy paths (MEP) starting from Franck-Condon and S-2/S-1
conical intersection points on S-1. Furthermore, analytical frequency comp
utations have been used to produce quantitative information on the surface
curvature.
We show that the S-1 energy surface is characterized by two domains, region
I and region II. Region I controls the initial acceleration of the excited
state molecule. In contrast, region II is a low-lying region of S-1 and co
ntrols the evolution towards fully efficient decay to the ground state. The
energy surface structure indicates that the double-bond isomerization of 2
-cis-C5H6NH2+ and all-trans-MT and the ring-opening of CMD are prototypes o
f three classes of barrierless reactions characterized by a different excit
ed state dynamics. In 2-cis-C5H6NK2+ and, more loosely, in all-trans-HT the
initial relaxation results in the production of a totally symmetric S-1 tr
ansient. The following triggering of the S-1 -->S-0 decay requires energy r
edistribution along a symmetry-breaking (torsional) mode leading to an S-2/
S-1 conical intersection (CI). In contrast, the shape of region I of CHD in
dicates that an almost direct (i.e. impulsive) motion towards an asymmetric
S-1/S-0 CI occurs upon initial relaxation. Previously reported and novel s
emi-classical trajectory computations and the available experimental eviden
ce seem to support these conclusions.