Excited state minimum

Accordingly, the reaction path then proceeds via the Ag excited state on the excited state PES until the conical intersection region is reached, passing through an excited state minimum. At the conical intersection, the molecule drops down to the ground... 

The chemical significance of conical intersections is that they provide sites of unit efficiency for return from an excited electronic state to the ground state. It turns out that the probability of (nonradiative) hopping between two electronic states is inversely dependent on the energy gap between them. So the return from the excited-state minimum to the ground-state maximum in Figure 21.4 would be a... 

The photophysical properties of cytosine are very similar to those of uracil (Section 10.3.1). The global tttt excited state minimum structure is characterized by a large H(5)C(5)C(6)H(6) dihedral angle of 66° [41, 42] and a C(5)C(6) bond length... 

Examples of photoreactions may be found among nearly all classes of organic compounds. From a synthetic point of view a classification by chromo-phore into the photochemistry of carbonyl compounds, enones, alkenes, aromatic compounds, etc., or by reaction type into photochemical oxidations and reductions, eliminations, additions, substitutions, etc., might be useful. However, photoreactions of quite different compounds can be based on a common reaction mechanism, and often the same theoretical model can be used to describe different reactions. Thus, theoretical arguments may imply a rather different classification, based, for instance, on the type of excited-state minimum responsible for the reaction, on the number and arrangement of centers in the reaction complex, or on the number of active orbitals per center. (Cf. Michl and BonaCid-Kouteck, 1990.)... 

Common spectroscopic techniques test small portions of the ground and/or excited state PES either around the gs minimum (IR and non-resonant Raman spectra, electronic absorption spectra.) or in the proximity of the excited state minimum (steady-state fluorescence). These spectra are then satisfactorily described in the best harmonic approximation, a local harmonic approach that approximates the PES with parabolas whose curvatures match the exact curvatures calculated at the specific position of interest [78]. Anharmonicity in this approach manifests itself with the dependence of harmonic frequencies and relaxation energies on the actual nuclear configuration [79]. Along these lines we predicted softened (hardened) vibrational frequencies for the ground (excited) state [74], amplified and p-dependent infrared and Raman intensities [68, 74], different Frank-Condon... 

The formation of benzvalene is formally an x[2 + 2] cyclo-addition. The S, (Bju) reaction path from benzene toward prefulvene starts at an excited-state minimum with symmetry and proceeds over a transition state to the geometry of prefulvene, where it enters a funnel in S, due to an S,-So conical intersection and continues on the Sg surface, mostly back to benzene, but in part on to benzvalene (Palmer et al., 1993 Sobolewski et al., 1993). At prefulvene geometries, Sg has a flat biradicaloid region of high energy with very shallow minima whose exact location depends on calcula-tional details (Kato, 1988 Palmer, et al., 1993, Sobolewski et al., 1993). Fulvene has been proposed to be formed directly from prefulvene or via secondary isomerization of benzvalene (Bryce-Smith and Gilbert, 1976). Calculations support the former pathway with a carbene intermediate (Dreyerand Klessinger, 1995). 

The ground state potential curve (state A, Fig. 7) shows a potential barrier in the vicinity of the excited state minimum. The dotted parts of branches b and c beyond point I correspond to the potential curves of the doubly excited configurations. Going from DHP to I requires an activation energy of 23 kcal/mole which is lowered to ca. 18 kcal/mole if the C.l. depression of state A is taken into account ) Going from cis-stilbene to 1 along the thermal path is very strongly endothermic... 

Emission from the exciplex will occur according to the Franck-Condon principle i.e., vertically from the excited-state minimum (no change of the nuclear configuration during the emission process). The separation of M and N in the ex-... 

In this first approach we follow Kasha s rule, that is, we consider that photochemical reactivity starts from the minimum of the lowest excited-state surface and focus on how the products are formed from there. Later we will discuss how the excited-state minimum can be reached from the FC region. There the molecnle may go through a crossing between two excited states or a state switch along an avoided crossing. The concepts we introduce now will also be useful to discuss these issues. 

Species Paths from the FC Geometry to the Excited-State Minimum... 

We now turn to the connection between the FC region and the lowest excited-state minimum that forms the starting point for reactivity. In the simplest case, the minimum of the spectroscopic state (the state that absorbs the excitation energy) and the lowest excited-state minimum (the minimum on the Si surface) are the same. When the spectroscopic state is a higher state, the initial decay will involve crossings... 

We see from this model that the crystal field strength determines the horizontal displacement A = - Qg of the T2 excited state minimum and the Stokes... 

Figure 9 Two-dimensional potential energy surfaces for tra 5 -Re02(vinylimidazole)4Cl along the 0=Re=0 coordinate and a low-frequency Re-N(imidazole) mode. Elliptical contours are observed for the harmonic potential (top), the bottom potential includes coupling between normal modes and electronic states and is flattened in the area below the excited state minimum, indicated by the dot.

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