Conical intersections, photochemical systems

Conical intersections, introduced over 60 years ago as possible efficient funnels connecting different elecbonically excited states [1], are now generally believed to be involved in many photochemical reactions. Direct laboratory observation of these subsurfaces on the potential surfaces of polyatomic molecules is difficult, since they are not stationary points . The system is expected to pass through them veiy rapidly, as the transition from one electronic state to another at the conical intersection is very rapid. Their presence is sunnised from the following data [2-5] ... 

The H4 system is the prototype for many four-elecbon reactions [34]. The basic tetrahedral sfructure of the conical intersection is preserved in all four-electron systems. It arises from the fact that the four electrons are contributed by four different atoms. Obviously, the tefrahedron is in general not a perfect one. This result was found computationally for many systems (see, e.g., [37]). Robb and co-workers [38] showed that the structure shown (a tetraradicaloid conical intersection) was found for many different photochemical transformations. Having the form of a tetrahedron, the conical intersection can exist in two enantiomeric structures. However, this feature is important only when chiral reactions are discussed. 

INORGANIC COMPLEXES. The cis-trans isomerization of a planar square form of a rt transition metal complex (e.g., of Pt " ) is known to be photochemically allowed and themrally forbidden [94]. It was found experimentally [95] to be an inhamolecular process, namely, to proceed without any bond-breaking step. Calculations show that the ground and the excited state touch along the reaction coordinate (see Fig. 12 in [96]). Although conical intersections were not mentioned in these papers, the present model appears to apply to these systems. 

As a last example of a molecular system exhibiting nonadiabatic dynamics caused by a conical intersection, we consider a model that recently has been proposed by Seidner and Domcke to describe ultrafast cis-trans isomerization processes in unsaturated hydrocarbons [172]. Photochemical reactions of this type are known to involve large-amplitode motion on coupled potential-energy surfaces [169], thus representing another stringent test for a mixed quantum-classical description that is complementary to Models 1 and II. A number of theoretical investigations, including quantum wave-packet studies [163, 164, 172], time-resolved pump-probe spectra [164, 181], and various mixed... 

In our final case study we shall focus on the photoinduced mer to fac isomerization reaction recently observed by Harkins and Peters (109). We have investigated this system in detail (110), and it provides a nice example of a general conical intersection in an inorganic photochemical problem, i.e., one that is not imposed by symmetry via a Jahn-Teller degeneracy. 

When the molecule contains hetero atoms such as nitrogen or oxygen one may want to include also lone-pair orbitals of rr-type in the active space. Note, however, that c —> tt excitations are of another symmetry than tt tt excitations for planar systems. One can therefore often use a different active space for these two types of excitations. The CASSCF method is frequently used to study photochemical processes that involve conical intersections, intersystem crossings, etc. where simpler approaches, as for example, time-dependent (TD) DFT do not work well. Here, one is only interested in the lower excited states of different spin-multiplicities and the demands on the active space are not so high. 

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